

Network Working Group                                          C. Groves
Request for Comments: 3525                                   M. Pantaleo
Obsoletes: 3015                                              LM Ericsson
Category: Standards Track                                    T. Anderson
                                                              Consultant
                                                               T. Taylor
                                                         Nortel Networks
                                                                 Editors
                                                               June 2003


                  Gateway Control Protocol Version 1

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2003).  All Rights Reserved.

Abstract

   This document defines the protocol used between elements of a
   physically decomposed multimedia gateway, i.e., a Media Gateway and a
   Media Gateway Controller.  The protocol presented in this document
   meets the requirements for a media gateway control protocol as
   presented in RFC 2805.

   This document replaces RFC 3015.  It is the result of continued
   cooperation between the IETF Megaco Working Group and ITU-T Study
   Group 16.  It incorporates the original text of RFC 3015, modified by
   corrections and clarifications discussed on the Megaco
   E-mail list and incorporated into the Study Group 16 Implementor's
   Guide for Recommendation H.248.  The present version of this document
   underwent  ITU-T Last Call as Recommendation H.248 Amendment 1.
   Because of ITU-T renumbering, it was published by the ITU-T as
   Recommendation H.248.1 (03/2002), Gateway Control Protocol Version 1.

   Users of this specification are advised to consult the H.248 Sub-
   series Implementors' Guide at http://www.itu.int/itudoc/itu-
   t/com16/implgd for additional corrections and clarifications.


Groves, et al.              Standards Track                     [Page 1]

RFC 3525                Gateway Control Protocol               June 2003


Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

Table of Contents

   1 Scope.........................................................5
     1.1 Changes From RFC 3015.....................................5
     1.2 Differences From ITU-T Recommendation H.248.1 (03/2002)...5
   2 References....................................................6
     2.1 Normative references......................................6
     2.2 Informative references....................................9
   3 Definitions..................................................10
   4 Abbreviations................................................11
   5 Conventions..................................................12
   6 Connection model.............................................13
     6.1 Contexts.................................................16
     6.2 Terminations.............................................17
       6.2.1 Termination dynamics.................................21
       6.2.2 TerminationIDs.......................................21
       6.2.3 Packages.............................................22
       6.2.4 Termination properties and descriptors...............23
       6.2.5 Root Termination.....................................25
   7 Commands.....................................................26
     7.1 Descriptors..............................................27
       7.1.1 Specifying parameters................................27
       7.1.2 Modem descriptor.....................................28
       7.1.3 Multiplex descriptor.................................28
       7.1.4 Media descriptor.....................................29
       7.1.5 TerminationState descriptor..........................29
       7.1.6 Stream descriptor....................................30
       7.1.7 LocalControl descriptor..............................31
       7.1.8 Local and Remote descriptors.........................32
       7.1.9 Events descriptor....................................35
       7.1.10 EventBuffer descriptor..............................38
       7.1.11 Signals descriptor..................................38
       7.1.12 Audit descriptor....................................40
       7.1.13 ServiceChange descriptor............................41
       7.1.14 DigitMap descriptor.................................41
       7.1.15 Statistics descriptor...............................46
       7.1.16 Packages descriptor.................................47
       7.1.17 ObservedEvents descriptor...........................47
       7.1.18 Topology descriptor.................................47
       7.1.19 Error Descriptor....................................50
     7.2 Command Application Programming Interface................50
       7.2.1 Add..................................................51


Groves, et al.              Standards Track                     [Page 2]

RFC 3525                Gateway Control Protocol               June 2003


       7.2.2 Modify...............................................52
       7.2.3 Subtract.............................................53
       7.2.4 Move.................................................55
       7.2.5 AuditValue...........................................56
       7.2.6 AuditCapabilities....................................59
       7.2.7 Notify...............................................60
       7.2.8 ServiceChange........................................61
       7.2.9 Manipulating and Auditing Context Attributes.........65
       7.2.10 Generic Command Syntax..............................66
     7.3 Command Error Codes......................................66
   8 Transactions.................................................66
     8.1 Common parameters........................................68
       8.1.1 Transaction Identifiers..............................68
       8.1.2 Context Identifiers..................................68
     8.2 Transaction Application Programming Interface............69
       8.2.1 TransactionRequest...................................69
       8.2.2 TransactionReply.....................................69
       8.2.3 TransactionPending...................................71
     8.3 Messages.................................................72
   9 Transport....................................................72
     9.1 Ordering of Commands.....................................73
     9.2 Protection against Restart Avalanche.....................74
   10 Security Considerations.....................................75
     10.1 Protection of Protocol Connections......................75
     10.2 Interim AH scheme.......................................76
     10.3 Protection of Media Connections.........................77
   11 MG-MGC Control Interface....................................78
     11.1 Multiple Virtual MGs....................................78
     11.2 Cold start..............................................79
     11.3 Negotiation of protocol version.........................79
     11.4 Failure of a MG.........................................80
     11.5 Failure of an MGC.......................................81
   12 Package definition..........................................82
     12.1 Guidelines for defining packages........................82
       12.1.1 Package.............................................83
       12.1.2 Properties..........................................84
       12.1.3 Events..............................................85
       12.1.4 Signals.............................................85
       12.1.5 Statistics..........................................86
       12.1.6 Procedures..........................................86
     12.2 Guidelines to defining Parameters to Events and Signals.86
     12.3 Lists...................................................87
     12.4 Identifiers.............................................87
     12.5 Package registration....................................88
   13 IANA Considerations.........................................88
     13.1 Packages................................................88
     13.2 Error codes.............................................89
     13.3 ServiceChange reasons...................................89


Groves, et al.              Standards Track                     [Page 3]

RFC 3525                Gateway Control Protocol               June 2003


   ANNEX A  Binary encoding of the protocol.......................90
     A.1 Coding of wildcards......................................90
     A.2 ASN.1 syntax specification...............................92
     A.3 Digit maps and path names...............................111
   ANNEX B Text encoding of the protocol.........................113
     B.1 Coding of wildcards.....................................113
     B.2 ABNF specification......................................113
     B.3 Hexadecimal octet coding................................127
     B.4 Hexadecimal octet sequence..............................127
   ANNEX C Tags for media stream properties......................128
     C.1 General media attributes................................128
     C.2 Mux properties..........................................130
     C.3 General bearer properties...............................130
     C.4 General ATM properties..................................130
     C.5 Frame Relay.............................................134
     C.6 IP......................................................134
     C.7 ATM AAL2................................................134
     C.8 ATM AAL1................................................136
     C.9 Bearer capabilities.....................................137
     C.10 AAL5 properties........................................147
     C.11 SDP equivalents........................................148
     C.12 H.245..................................................149
   ANNEX D Transport over IP.....................................150
     D.1 Transport over IP/UDP using Application Level Framing ..150
       D.1.1 Providing At-Most-Once functionality................150
       D.1.2 Transaction identifiers and three-way handshake.....151
       D.1.3 Computing retransmission timers.....................152
       D.1.4 Provisional responses...............................153
       D.1.5 Repeating Requests, Responses and Acknowledgements..153
     D.2 Using TCP...............................................155
       D.2.1 Providing the At-Most-Once functionality............155
       D.2.2 Transaction identifiers and three-way handshake.....155
       D.2.3 Computing retransmission timers.....................156
       D.2.4 Provisional responses...............................156
       D.2.5 Ordering of commands................................156
   ANNEX E  Basic packages.......................................157
     E.1 Generic.................................................157
     E.2 Base Root Package.......................................159
     E.3 Tone Generator Package..................................161
     E.4 Tone Detection Package..................................163
     E.5 Basic DTMF Generator Package............................166
     E.6 DTMF detection Package..................................167
     E.7 Call Progress Tones Generator Package...................169
     E.8 Call Progress Tones Detection Package...................171
     E.9 Analog Line Supervision Package.........................172
     E.10 Basic Continuity Package...............................175
     E.11 Network Package........................................178
     E.12 RTP Package............................................180


Groves, et al.              Standards Track                     [Page 4]

RFC 3525                Gateway Control Protocol               June 2003


     E.13 TDM Circuit Package....................................182
   APPENDIX I EXAMPLE CALL FLOWS (INFORMATIVE)...................184
     A.1 Residential Gateway to Residential Gateway Call.........184
       A.1.1 Programming Residential GW Analog Line Terminations
             for Idle Behavior...................................184
       A.1.2 Collecting Originator Digits and Initiating
             Termination.........................................186
   APPENDIX II  Changes From RFC 3015............................195
   Intellectual Property Rights..................................210
   Acknowledgments...............................................211
   Authors' Addresses............................................212
   Full Copyright Statement......................................213

1  Scope

   The present document, which is identical to the published version of
   ITU-T Recommendation H.248.1 (03/2002) except as noted below, defines
   the protocols used between elements of a physically decomposed
   multimedia gateway.  There are no functional differences from a
   system view between a decomposed gateway, with distributed sub-
   components potentially on more than one physical device, and a
   monolithic gateway such as described in ITU-T Recommendation H.246.
   This document does not define how gateways, multipoint control units
   or interactive voice response units (IVRs) work.  Instead it creates
   a general framework that is suitable for these applications.

   Packet network interfaces may include IP, ATM or possibly others.
   The interfaces will support a variety of Switched Circuit Network
   (SCN) signalling systems, including tone signalling, ISDN, ISUP, QSIG
   and GSM.  National variants of these signalling systems will be
   supported where applicable.

1.1 Changes From RFC 3015

   The differences between this document and RFC 3015 are documented in
   Appendix II.

1.2 Differences From ITU-T Recommendation H.248.1 (03/2002)

   This document differs from the corresponding ITU-T publication in the
   following respects:

   -  Added IETF front matter in place of the corresponding ITU-T
      material.

   -  The ITU-T summary is too H.323-specific and has been omitted.


Groves, et al.              Standards Track                     [Page 5]

RFC 3525                Gateway Control Protocol               June 2003


   -  The IETF conventions have been stated as governing this document.
      As discussed in section 5 below, this gives slightly greater
      strength to "should" requirements.

   -  The Scope section (just above) has been edited slightly to suit
      its IETF context.

   -  Added normative references to RFCs 2026 and 2119.

   -  Figures 4, 5, and 6 show the centre of the context for greater
      clarity.  Also added Figure 6a showing an important additional
      example.

   -  Added a paragraph in section 7.1.18 which was approved in the
      Implementor's Guide but lost inadvertently in the ITU-T approved
      version.

   -  This document incorporates corrections to the informative examples
      in Appendix I which also appear in H.248.1 version 2, but which
      were not picked up in H.248.1 (03/2002).

   -  This document includes a new Appendix II listing all the changes
      from RFC 3015.

   -  This document includes an Acknowledgements section listing the
      authors of RFC 3015 but also many other people who contributed to
      the development of the Megaco/H.248.x protocol.

   -  Moved the Intellectual Property declaration to its usual place in
      an IETF document and added a reference to declarations on the IETF
      web site.

2  References

   The following ITU-T Recommendations and other references contain
   provisions which, through reference in this text, constitute
   provisions of this RFC.  At the time of publication, the editions
   indicated were valid.  All Recommendations and other references are
   subject to revision; all users of this RFC are therefore encouraged
   to investigate the possibility of applying the most recent edition of
   the Recommendations and other references listed below.  A list of the
   currently valid ITU-T Recommendations is regularly published.

2.1   Normative references

   -  ITU-T Recommendation H.225.0 (1999), Call signalling protocols and
      media stream packetization for packet-based multimedia
      communication systems.


Groves, et al.              Standards Track                     [Page 6]

RFC 3525                Gateway Control Protocol               June 2003


   -  ITU-T Recommendation H.235 (1998), Security and encryption for
      H-Series (H.323 and other H.245-based) multimedia terminals.

   -  ITU-T Recommendation H.245 (1998), Control protocol for multimedia
      communication.

   -  ITU-T Recommendation H.246 (1998), Interworking of H-series
      multimedia terminals with H-series multimedia terminals and
      voice/voiceband terminals on GSTN and ISDN.

   -  ITU-T Recommendation H.248.8 (2002), H.248 Error Codes and Service
      Change Reasons.

   -  ITU-T Recommendation H.323 (1999), Packet-based multimedia
      communication systems.

   -  ITU-T Recommendation I.363.1 (1996), B-ISDN ATM adaptation layer
      (AAL) specification: Type 1 AAL.

   -  ITU-T Recommendation I.363.2 (1997), B-ISDN ATM adaptation layer
      (AAL) specification: Type 2 AAL.

   -  ITU-T Recommendation I.363.5 (1996), B-ISDN ATM adaptation layer
      (AAL) specification: Type 5 AAL.

   -  ITU-T Recommendation I.366.1 (1998), Segmentation and Reassembly
      Service Specific Convergence Sublayer for the AAL type 2.

   -  ITU-T Recommendation I.366.2 (1999), AAL type 2 service specific
      convergence sublayer for trunking.

   -  ITU-T Recommendation I.371 (2000), Traffic control and congestion
      control in B-ISDN.

   -  ITU-T Recommendation Q.763 (1999), Signalling System No. 7 - ISDN
      user part formats and codes.

   -  ITU-T Recommendation Q.765.5 (2001), Application transport
      mechanism - Bearer independent call control (BICC).

   -  ITU-T Recommendation Q.931 (1998), ISDN user-network interface
      layer 3 specification for basic call control.

   -  ITU-T Recommendation Q.2630.1 (1999), AAL type 2 signalling
      protocol (Capability Set 1).


Groves, et al.              Standards Track                     [Page 7]

RFC 3525                Gateway Control Protocol               June 2003


   -  ITU-T Recommendation Q.2931 (1995), Digital Subscriber Signalling
      System No. 2 (DSS2) - User-Network Interface (UNI) - Layer 3
      specification for basic call/connection control.

   -  ITU-T Recommendation Q.2941.1 (1997), Digital Subscriber
      Signalling System No. 2 - Generic identifier transport.

   -  ITU-T Recommendation Q.2961.1 (1995), Additional signalling
      capabilities to support traffic parameters for the tagging option
      and the sustainable call rate parameter set.

   -  ITU-T Recommendation Q.2961.2 (1997), Additional traffic
      parameters: Support of ATM transfer capability in the broadband
      bearer capability information element.

   -  ITU-T Recommendation Q.2965.1 (1999), Digital subscriber
      signalling system No. 2 - Support of Quality of Service classes.

   -  ITU-T Recommendation Q.2965.2 (1999), Digital subscriber
      signalling system No. 2 - Signalling of individual Quality of
      Service parameters.

   -  ITU-T Recommendation V.76 (1996), Generic multiplexer using V.42
      LAPM-based procedures.

   -  ITU-T Recommendation X.213 (1995), Information technology - Open
      Systems Interconnection - Network service definition plus
      Amendment 1 (1997), Addition of the Internet protocol address
      format identifier.

   -  ITU-T Recommendation X.680 (1997), Information technology -
      Abstract Syntax Notation One (ASN.1): Specification of basic
      notation.

   -  ITU-T Recommendation X.690 (1997), Information Technology - ASN.1
      Encoding Rules: Specification of Basic Encoding Rules (BER),
      Canonical Encoding Rules (CER) and Distinguished Encoding Rules
      (DER).

   -  ATM Forum (1996), ATM User-Network Interface (UNI) Signalling
      Specification - Version 4.0.

   [RFC 1006] Rose, M. and D. Cass, "ISO Transport Service on top of the
              TCP, Version 3", STD 35, RFC 1006, May 1987.

   [RFC 2026] Brander, S., "The Internet Standards Process -- Revision
              3", BCP 9, RFC 2026, October 1996.


Groves, et al.              Standards Track                     [Page 8]

RFC 3525                Gateway Control Protocol               June 2003


   [RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC 2234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", RFC 2234, November 1997.

   [RFC 2327] Handley, M. and V. Jacobson, "SDP: Session Description
              Protocol", RFC 2327, April 1998.

   [RFC 2402] Kent, S. and R. Atkinson, "IP Authentication Header", RFC
              2402, November 1998.

   [RFC 2406] Kent, S. and R. Atkinson, "IP Encapsulating Security
              Payload (ESP)", RFC 2406, November 1998.

2.2   Informative references

   -  ITU-T Recommendation E.180/Q.35 (1998), Technical characteristics
      of tones for the telephone service.

   -  CCITT Recommendation G.711 (1988), Pulse Code Modulation (PCM) of
      voice frequencies.

   -  ITU-T Recommendation H.221 (1999), Frame structure for a 64 to
      1920 kbit/s channel in audiovisual teleservices.

   -  ITU  T Recommendation H.223 (1996), Multiplexing protocol for low
      bit rate multimedia communication.

   -  ITU-T Recommendation H.226 (1998), Channel aggregation protocol
      for multilink operation on circuit-switched networks

   -  ITU-T Recommendation Q.724 (1998), Signalling procedures.

   -  ITU-T Recommendation Q.764 (1999), Signalling system No. 7 - ISDN
      user part signalling procedures.

   -  ITU-T Recommendation Q.1902.4 (2001), Bearer independent call
      control protocol - Basic call procedures.

   [RFC 768]  Postel, J., "User Datagram Protocol", STD 6, RFC 768,
              August 1980.

   [RFC 791]  Postel, J., "Internet Protocol", STD 5, RFC 791, September
              1981.

   [RFC 793]  Postel, J., "Transmission Control Protocol", STD 7, RFC
              793, September 1981.


Groves, et al.              Standards Track                     [Page 9]

RFC 3525                Gateway Control Protocol               June 2003


   [RFC 1661] Simpson, W., Ed., "The Point-to-Point Protocol (PPP)", STD
              51, RFC 1661, July 1994.

   [RFC 1889] Schulzrinne, H., Casner, S., Frederick, R. and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", RFC 1889, January 1996.

   [RFC 1890] Schulzrinne, H. and G. Fokus, "RTP Profile for Audio and
              Video Conferences with Minimal Control",  RFC 1890,
              January 1996.

   [RFC 2401] Kent, S. and R. Atkinson, "Security Architecture for the
              Internet Protocol", RFC 2401, November 1998.

   [RFC 2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, December 1998.

   [RFC 2543] Handley, M., Schulzrinne, H., Schooler, E. and J.
              Rosenberg, "SIP: Session Initiation Protocol", RFC 2543,
              March 1999.

   [RFC 2805] Greene, N., Ramalho, M. and B. Rosen, "Media Gateway
              Control Protocol Architecture and Requirements", RFC 2805,
              April 2000.

3  Definitions

   This document defines the following terms:

   Access gateway:
   A type of gateway that provides a User-Network Interface (UNI) such
   as ISDN.

   Descriptor:
   A syntactic element of the protocol that groups related properties.
   For instance, the properties of a media flow on the MG can be set by
   the MGC by including the appropriate descriptor in a command.

   Media Gateway (MG):
   The media gateway converts media provided in one type of network to
   the format required in another type of network.  For example, a MG
   could terminate bearer channels from a switched circuit network
   (e.g., DS0s) and media streams from a packet network (e.g., RTP
   streams in an IP network).  This gateway may be capable of processing
   audio, video and T.120 alone or in any combination, and will be
   capable of full duplex media translations.  The MG may also play
   audio/video messages and perform other IVR functions, or may perform
   media conferencing.


Groves, et al.              Standards Track                    [Page 10]

RFC 3525                Gateway Control Protocol               June 2003


   Media Gateway Controller (MGC):
   Controls the parts of the call state that pertain to connection
   control for media channels in a MG.

   Multipoint Control Unit (MCU):
   An entity that controls the setup and coordination of a multi-user
   conference that typically includes processing of audio, video and
   data.

   Residential gateway:
   A gateway that interworks an analogue line to a packet network.  A
   residential gateway typically contains one or two analogue lines and
   is located at the customer premises.

   SCN FAS signalling gateway:
   This function contains the SCN Signalling Interface that terminates
   SS7, ISDN or other signalling links where the call control channel
   and bearer channels are collocated in the same physical span.

   SCN NFAS signalling gateway:
   This function contains the SCN Signalling Interface that terminates
   SS7 or other signalling links where the call control channels are
   separated from bearer channels.

   Stream:
   Bidirectional media or control flow received/sent by a media gateway
   as part of a call or conference.

   Trunk:
   A communication channel between two switching systems such as a DS0
   on a T1 or E1 line.

   Trunking gateway:
   A gateway between SCN network and packet network that typically
   terminates a large number of digital circuits.

4  Abbreviations

   This RFC document uses the following abbreviations:

   ALF   Application Layer Framing

   ATM   Asynchronous Transfer Mode

   CAS   Channel Associated Signalling

   DTMF  Dual Tone Multi-Frequency


Groves, et al.              Standards Track                    [Page 11]

RFC 3525                Gateway Control Protocol               June 2003


   FAS   Facility Associated Signalling

   GSM   Global System for Mobile communications

   GW    GateWay

   IANA  Internet Assigned Numbers Authority (superseded by Internet
         Corporation for Assigned Names and Numbers - ICANN)

   IP    Internet Protocol

   ISUP  ISDN User Part

   IVR   Interactive Voice Response

   MG    Media Gateway

   MGC   Media Gateway Controller

   NFAS  Non-Facility Associated Signalling

   PRI   Primary Rate Interface

   PSTN  Public Switched Telephone Network

   QoS   Quality of Service

   RTP   Real-time Transport Protocol

   SCN   Switched Circuit Network

   SG    Signalling Gateway

   SS7   Signalling System No. 7

5  Conventions

   In the H.248.1 Recommendation, "SHALL" refers to a mandatory
   requirement, while "SHOULD" refers to a suggested but optional
   feature or procedure.  The term "MAY" refers to an optional course of
   action without expressing a preference.  Note that these definition
   are overridden in the present document by the RFC 2119 conventions
   stated at the beginning of this document.  RFC 2119 has a more
   precise definition of "should" than is provided by the ITU-T.


Groves, et al.              Standards Track                    [Page 12]

RFC 3525                Gateway Control Protocol               June 2003


6  Connection model

   The connection model for the protocol describes the logical entities,
   or objects, within the Media Gateway that can be controlled by the
   Media Gateway Controller.  The main abstractions used in the
   connection model are Terminations and Contexts.

   A Termination sources and/or sinks one or more streams.  In a
   multimedia conference, a Termination can be multimedia and sources or
   sinks multiple media streams.  The media stream parameters, as well
   as modem, and bearer parameters are encapsulated within the
   Termination.

   A Context is an association between a collection of Terminations.
   There is a special type of Context, the null Context, which contains
   all Terminations that are not associated to any other Termination.
   For instance, in a decomposed access gateway, all idle lines are
   represented by Terminations in the null Context.

   Following is a graphical depiction of these concepts.  The diagram of
   Figure 1 gives several examples and is not meant to be an
   all-inclusive illustration.  The asterisk box in each of the Contexts
   represents the logical association of Terminations implied by the
   Context.


Groves, et al.              Standards Track                    [Page 13]

RFC 3525                Gateway Control Protocol               June 2003


         +------------------------------------------------------+
         |Media Gateway                                         |
         | +-------------------------------------------------+  |
         | |Context                          +-------------+ |  |
         | |                                 | Termination | |  |
         | |                                 |-------------| |  |
         | |  +-------------+             +->| SCN Bearer  |<---+->
         | |  | Termination |   +-----+   |  |   Channel   | |  |
         | |  |-------------|   |     |---+  +-------------+ |  |
       <-+--->| RTP Stream  |---|  *  |                      |  |
         | |  |             |   |     |---+  +-------------+ |  |
         | |  +-------------+   +-----+   |  | Termination | |  |
         | |                              |  |-------------| |  |
         | |                              +->| SCN Bearer  |<---+->
         | |                                 |   Channel   | |  |
         | |                                 +-------------+ |  |
         | +-------------------------------------------------+  |
         |                                                      |
         |                                                      |
         |                    +------------------------------+  |
         |   (NULL Context)   |Context                       |  |
         |  +-------------+   |              +-------------+ |  |
         |  | Termination |   | +-----+      | Termination | |  |
         |  |-------------|   | |     |      |-------------| |  |
         |  | SCN Bearer  |   | |  *  |------| SCN Bearer  |<---+->
         |  |   Channel   |   | |     |      |   Channel   | |  |
         |  +-------------+   | +-----+      +-------------+ |  |
         |                    +------------------------------+  |
         |                                                      |
         |                                                      |
         | +-------------------------------------------------+  |
         | |Context                                          |  |
         | |  +-------------+                +-------------+ |  |
         | |  | Termination |   +-----+      | Termination | |  |
         | |  |-------------|   |     |      |-------------| |  |
       <-+--->| SCN Bearer  |---|  *  |------| SCN Bearer  |<---+->
         | |  |   Channel   |   |     |      |   Channel   | |  |
         | |  +-------------+   +-----+      +-------------+ |  |
         | +-------------------------------------------------+  |
         | ___________________________________________________  |
         +------------------------------------------------------+

            Figure 1: Examples of Megaco/H.248 Connection Model


Groves, et al.              Standards Track                    [Page 14]

RFC 3525                Gateway Control Protocol               June 2003


   The example in Figure 2 shows an example of one way to accomplish a
   call-waiting scenario in a decomposed access gateway, illustrating
   the relocation of a Termination between Contexts.  Terminations T1
   and T2 belong to Context C1 in a two-way audio call.  A second audio
   call is waiting for T1 from Termination T3.  T3 is alone in Context
   C2.  T1 accepts the call from T3, placing T2 on hold.  This action
   results in T1 moving into Context C2, as shown in Figure 3.

         +------------------------------------------------------+
         |Media Gateway                                         |
         | +-------------------------------------------------+  |
         | |Context C1                                       |  |
         | |  +-------------+                +-------------+ |  |
         | |  | Term. T2    |   +-----+      | Term. T1    | |  |
         | |  |-------------|   |     |      |-------------| |  |
       <-+--->| RTP Stream  |---|  *  |------| SCN Bearer  |<---+->
         | |  |             |   |     |      |   Channel   | |  |
         | |  +-------------+   +-----+      +-------------+ |  |
         | +-------------------------------------------------+  |
         |                                                      |
         | +-------------------------------------------------+  |
         | |Context C2                                       |  |
         | |                                 +-------------+ |  |
         | |                    +-----+      | Term. T3    | |  |
         | |                    |     |      |-------------| |  |
         | |                    |  *  |------| SCN Bearer  |<---+->
         | |                    |     |      |   Channel   | |  |
         | |                    +-----+      +-------------+ |  |
         | +-------------------------------------------------+  |
         +------------------------------------------------------+

     Figure 2: Example Call Waiting Scenario / Alerting Applied to T1


Groves, et al.              Standards Track                    [Page 15]

RFC 3525                Gateway Control Protocol               June 2003


         +------------------------------------------------------+
         |Media Gateway                                         |
         | +-------------------------------------------------+  |
         | |Context C1                                       |  |
         | |  +-------------+                                |  |
         | |  | Term. T2    |   +-----+                      |  |
         | |  |-------------|   |     |                      |  |
       <-+--->| RTP Stream  |---|  *  |                      |  |
         | |  |             |   |     |                      |  |
         | |  +-------------+   +-----+                      |  |
         | +-------------------------------------------------+  |
         |                                                      |
         | +-------------------------------------------------+  |
         | |Context C2                                       |  |
         | |  +-------------+                +-------------+ |  |
         | |  | Term. T1    |   +-----+      | Term. T3    | |  |
         | |  |-------------|   |     |      |-------------| |  |
       <-+--->| SCN Bearer  |---|  *  |------| SCN Bearer  |<---+->
         | |  |   Channel   |   |     |      |   Channel   | |  |
         | |  +-------------+   +-----+      +-------------+ |  |
         | +-------------------------------------------------+  |
         +------------------------------------------------------+

          Figure 3. Example Call Waiting Scenario / Answer by T1

6.1   Contexts

   A Context is an association between a number of Terminations.  The
   Context describes the topology (who hears/sees whom) and the media
   mixing and/or switching parameters if more than two Terminations are
   involved in the association.

   There is a special Context called the null Context.  It contains
   Terminations that are not associated to any other Termination.
   Terminations in the null Context can have their parameters examined
   or modified, and may have events detected on them.

   In general, an Add command is used to add Terminations to Contexts.
   If the MGC does not specify an existing Context to which the
   Termination is to be added, the MG creates a new Context.  A
   Termination may be removed from a Context with a Subtract command,
   and a Termination may be moved from one Context to another with a
   Move command.  A Termination SHALL exist in only one Context at a
   time.


Groves, et al.              Standards Track                    [Page 16]

RFC 3525                Gateway Control Protocol               June 2003


   The maximum number of Terminations in a Context is a MG property.
   Media gateways that offer only point-to-point connectivity might
   allow at most two Terminations per Context.  Media gateways that
   support multipoint conferences might allow three or more Terminations
   per Context.

6.1.1 Context attributes and descriptors

   The attributes of Contexts are:

   -  ContextID.

   -  The topology (who hears/sees whom).

      The topology of a Context describes the flow of media between the
      Terminations within a Context.  In contrast, the mode of a
      Termination (send/receive/...) describes the flow of the media at
      the ingress/egress of the media gateway.

   -  The priority is used for a Context in order to provide the MG with
      information about a certain precedence handling for a Context.
      The MGC can also use the priority to control autonomously the
      traffic precedence in the MG in a smooth way in certain
      situations (e.g., restart), when a lot of Contexts must be handled
      simultaneously.  Priority 0 is the lowest priority and a priority
      of 15 is the highest priority.

   -  An indicator for an emergency call is also provided to allow a
      preference handling in the MG.

6.1.2 Creating, deleting and modifying Contexts

   The protocol can be used to (implicitly) create Contexts and modify
   the parameter values of existing Contexts.  The protocol has commands
   to add Terminations to Contexts, subtract them from Contexts, and to
   move Terminations between Contexts.  Contexts are deleted implicitly
   when the last remaining Termination is subtracted or moved out.

6.2   Terminations

   A Termination is a logical entity on a MG that sources and/or sinks
   media and/or control streams.  A Termination is described by a number
   of characterizing Properties, which are grouped in a set of
   Descriptors that are included in commands.  Terminations have unique
   identities (TerminationIDs), assigned by the MG at the time of their
   creation.


Groves, et al.              Standards Track                    [Page 17]

RFC 3525                Gateway Control Protocol               June 2003


   Terminations representing physical entities have a semi-permanent
   existence.  For example, a Termination representing a TDM channel
   might exist for as long as it is provisioned in the gateway.
   Terminations representing ephemeral information flows, such as RTP
   flows, would usually exist only for the duration of their use.

   Ephemeral Terminations are created by means of an Add command.  They
   are destroyed by means of a Subtract command.  In contrast, when a
   physical Termination is Added to or Subtracted from a Context, it is
   taken from or to the null Context, respectively.

   Terminations may have signals applied to them (see 7.1.11).
   Terminations may be programmed to detect Events, the occurrence of
   which can trigger notification messages to the MGC, or action by the
   MG.  Statistics may be accumulated on a Termination.  Statistics are
   reported to the MGC upon request (by means of the AuditValue command,
   see 7.2.5) and when the Termination is taken out of the call it is
   in.

   Multimedia gateways may process multiplexed media streams.  For
   example, Recommendation H.221 describes a frame structure for
   multiple media streams multiplexed on a number of digital 64 kbit/s
   channels.  Such a case is handled in the connection model in the
   following way.  For every bearer channel that carries part of the
   multiplexed streams, there is a physical or ephemeral "bearer
   Termination".  The bearer Terminations that source/sink the digital
   channels are connected to a separate Termination called the
   "multiplexing Termination".  The multiplexing termination is an
   ephemeral termination representing a frame-oriented session.  The
   MultiplexDescriptor for this Termination describes the multiplex used
   (e.g., H.221 for an H.320 session) and indicates the order in which
   the contained digital channels are assembled into a frame.

   Multiplexing terminations may be cascades (e.g., H.226 multiplex of
   digital channels feeding into a H.223 multiplex supporting an H.324
   session).

   The individual media streams carried in the session are described by
   StreamDescriptors on the multiplexing Termination.  These media
   streams can be associated with streams sourced/sunk by Terminations
   in the Context other than the bearer Terminations supporting the
   multiplexing Termination.  Each bearer Termination supports only a
   single data stream.  These data streams do not appear explicitly as
   streams on the multiplexing Termination and they are hidden from the
   rest of the context.

   Figures 4, 5, 6, and 6a illustrate typical applications of the
   multiplexing termination and Multiplex Descriptor.


Groves, et al.              Standards Track                    [Page 18]

RFC 3525                Gateway Control Protocol               June 2003


                  +-----------------------------------+
                  | Context     +-------+             |
                 +----+         |       |             |
   Circuit 1 -|--| TC1|---------+ Tmux  |             |
              |  +----+ (Str 1) |       |  Audio    +-----+
              |   |             |       +-----*-----+     |-----
              |  +----+         | H.22x | Stream 1  |     |
   Circuit 2 -|--| TC2|---------+ multi-|           | TR1 |
              |  +----+ (Str 1) | plex  |           |(RTP)|
              |   |             |       |  Video    |     |
              |  +----+         |       +-----*-----+     |-----
   Circuit 3 -|--| TC3|---------+       | Stream 2  |     |
              /  +----+ (Str 1) |       |           +-----+
             /    |             +-------+             |
            /     +-----------------\-----------------+
   Audio, video, and control         \
   signals are carried in frames    Tmux is an ephemeral with two
   spanning the circuits.           explicit Stream Descriptors
                                    and a Multiplex Descriptor.

      Figure 4: Multiplexed Termination Scenario - Circuit to Packet
              (Asterisks * denote the centre of the context)

                    Context
                  +--------------------------------------+
                  |       +-------+        +-------+     |
                 +----+   |       |        |       |   +----+
   Circuit 1 ----| TC1|---+ Tmux1 |  Audio | Tmux2 +---| TC4|---
                 +----+   |       +---*----+       |   +----+
                  |       |       |  Str 1 |       |     |
                 +----+   | H.22x |        | H.22x |   +----+
   Circuit 2 ----| TC2|---+ multi-|        | multi-+---| TC5|---
                 +----+   | plex  |        | plex  |   +----+
                  |       |       |  Video |       |     |
                 +----+   |       +---*----+       |   +----+
   Circuit 3 ----| TC3|---+       |  Str 2 |       +---| TC6|---
                 +----+   |       |        |       |   +----+
                  |       +-------+        +-------+     |
                  +-----------------\-----/--------------+
                                     \   /
             Tmux1 and Tmux2 are ephemerals each with two
            explicit Stream Descriptors and a Multiplex Descriptor.

      Figure 5: Multiplexed Termination Scenario - Circuit to Circuit
              (Asterisks * denote the centre of the context)


Groves, et al.              Standards Track                    [Page 19]

RFC 3525                Gateway Control Protocol               June 2003


                  +-----------------------------------+
                  | Context     +-------+             |
                 +----+         |       |             |
   Circuit 1 -|--| TC1|---------+ Tmux  |             |
              |  +----+ (Str 1) |       |  Audio    +-----+
              |   |             |       +-----*-----+ TR1 |-----
              |  +----+         | H.22x | Stream 1  |(RTP)|
   Circuit 2 -|--| TC2|---------+ multi-|           +-----+
              |  +----+ (Str 1) | plex  |             |
              |   |             |       |  Video    +-----+
              |  +----+         |       +-----*-----+ TR2 |-----
   Circuit 3 -|--| TC3|---------+       | Stream 2  |(RTP)|
              /  +----+ (Str 1) |       |           +-----+
             /    |             +-------+             |
            /     +-----------------\-----------------+
   Audio, video, and control         \ Tmux is an ephemeral with two
   signals are carried in frames    explicit Stream Descriptors and
   spanning the circuits.           and a Multiplex Descriptor.

      Figure 6: Multiplexed Termination Scenario - Single to Multiple
                               Terminations
              (Asterisks * denote the centre of the context)

            Context
          +---------------------------------------------+
          |       +-------+       +-------+             |
   Cct 1 +----+   |       |       |       | Audio     +-----+
     ----| TC1|---+ Tmux1 |       | Tmux2 +-----*-----| TR1 |-----
         +----+   |       |       |       | Stream 1  |(RTP)|
          |       |       | Data  |       |           +-----+
   Cct 2 +----+   | H.226 +-------+ H.223 |             |
     ----| TC2|---+ multi-|(Str 1)| multi-| Control   +-----+
         +----+   | plex  |       | plex  +-----*-----+ Tctl|-----
          |       |       |       |       | Stream 3  +-----+
   Cct 3 +----+   |       |       |       |             |
     ----| TC3|---+       |       |       |           +-----+
         +----+   |       |       |       +-----*-----+ TR2 |-----
          |       +-------+       |       |  Video    |(RTP)|
          |                       +-------+ Stream 2  +-----+
          |                                             |
          +---------------------------------------------+
        Tmux1 has a Multiplex Descriptor and a single data stream.
        Tmux2 has a Multiplex Descriptor with a single bearer and
        three explicit Stream Descriptors.

    Figure 6a: Multiplexed Termination Scenario - Cascaded Multiplexes
              (Asterisks * denote the centre of the context)
     Note: this figure does not appear in Rec.  H.248.1


Groves, et al.              Standards Track                    [Page 20]

RFC 3525                Gateway Control Protocol               June 2003


   Terminations may be created which represent multiplexed bearers, such
   as an ATM AAL Type 2 bearer.  When a new multiplexed bearer is to be
   created, an ephemeral Termination is created in a Context established
   for this purpose.  When the Termination is subtracted, the
   multiplexed bearer is destroyed.

6.2.1 Termination dynamics

   The protocol can be used to create new Terminations and to modify
   property values of existing Terminations.  These modifications
   include the possibility of adding or removing events and/or signals.
   The Termination properties, and events and signals are described in
   the ensuing subclauses.  An MGC can only release/modify Terminations
   and the resources that the Termination represents which it has
   previously seized via, e.g., the Add command.

6.2.2 TerminationIDs

   Terminations are referenced by a TerminationID, which is an arbitrary
   schema chosen by the MG.

   TerminationIDs of physical Terminations are provisioned in the Media
   Gateway.  The TerminationIDs may be chosen to have structure.  For
   instance, a TerminationID may consist of trunk group and a trunk
   within the group.

   A wildcarding mechanism using two types of wildcards can be used with
   TerminationIDs.  The two wildcards are ALL and CHOOSE.  The former is
   used to address multiple Terminations at once, while the latter is
   used to indicate to a media gateway that it must select a Termination
   satisfying the partially specified TerminationID.  This allows, for
   instance, that a MGC instructs a MG to choose a circuit within a
   trunk group.

   When ALL is used in the TerminationID of a command, the effect is
   identical to repeating the command with each of the matching
   TerminationIDs.  The use of ALL does not address the ROOT
   termination.  Since each of these commands may generate a response,
   the size of the entire response may be large.  If individual
   responses are not required, a wildcard response may be requested.  In
   such a case, a single response is generated, which contains the UNION
   of all of the individual responses which otherwise would have been
   generated, with duplicate values suppressed.  For instance, given a
   Termination Ta with properties p1=a, p2=b and Termination Tb with


Groves, et al.              Standards Track                    [Page 21]

RFC 3525                Gateway Control Protocol               June 2003


   properties p2=c, p3=d, a UNION response would consist of a wildcarded
   TerminationId and the sequence of properties p1=a, p2=b,c and p3=d.
   Wildcard response may be particularly useful in the Audit commands.

   The encoding of the wildcarding mechanism is detailed in Annexes A
   and B.

6.2.3 Packages

   Different types of gateways may implement Terminations that have
   widely differing characteristics.  Variations in Terminations are
   accommodated in the protocol by allowing Terminations to have
   optional Properties, Events, Signals and Statistics implemented by
   MGs.

   In order to achieve MG/MGC interoperability, such options are grouped
   into Packages, and typically a Termination realizes a set of such
   Packages.  More information on definition of packages can be found in
   clause 12.  An MGC can audit a Termination to determine which
   Packages it realizes.

   Properties, Events, Signals and Statistics defined in Packages, as
   well as parameters to them, are referenced by identifiers (Ids).
   Identifiers are scoped.  For each package, PropertyIds, EventIds,
   SignalIds, StatisticsIds and ParameterIds have unique name spaces and
   the same identifier may be used in each of them.  Two PropertyIds in
   different packages may also have the same identifier, etc.

   To support a particular package the MG must support all properties,
   signals, events and statistics defined in a package.  It must also
   support all Signal and Event parameters.  The MG may support a subset
   of the values listed in a package for a particular Property or
   Parameter.

   When packages are extended, the properties, events, signals and
   statistics defined in the base package can be referred to using
   either the extended package name or the base package name.  For
   example, if Package A defines event e1, and Package B extends Package
   A, then B/e1 is an event for a termination implementing Package B. By
   definition, the MG MUST also implement the base Package, but it is
   optional to publish the base package as an allowed interface.  If it
   does publish  A, then A would be reported on the Package Descriptor
   in AuditValue as well as B, and event A/e1 would be available on a
   termination.  If the MG does not publish A, then only B/e1 would be
   available.  If published through AuditValue, A/e1 and B/e1 are the
   same event.


Groves, et al.              Standards Track                    [Page 22]

RFC 3525                Gateway Control Protocol               June 2003


   For improved interoperability and backward compatibility, an MG MAY
   publish all Packages supported by its Terminations, including base
   Packages from which extended Packages are derived.  An exception to
   this is in cases where the base packages are expressly "Designed to
   be extended only".

6.2.4 Termination properties and descriptors

   Terminations have properties.  The properties have unique
   PropertyIDs. Most properties have default values, which are
   explicitly defined in this protocol specification or in a package
   (see clause 12) or set by provisioning.  If not provisioned
   otherwise, the properties in all descriptors except TerminationState
   and LocalControl default to empty/"no value" when a Termination is
   first created or returned to the null Context.  The default contents
   of the two exceptions are described in 7.1.5 and 7.1.7.

   The provisioning of a property value in the MG will override any
   default value, be it supplied in this protocol specification or in a
   package.  Therefore if it is essential for the MGC to have full
   control over the property values of a Termination, it should supply
   explicit values when ADDing the Termination to a Context.
   Alternatively, for a physical Termination the MGC can determine any
   provisioned property values by auditing the Termination while it is
   in the NULL Context.

   There are a number of common properties for Terminations and
   properties specific to media streams.  The common properties are also
   called the Termination state properties.  For each media stream,
   there are local properties and properties of the received and
   transmitted flows.

   Properties not included in the base protocol are defined in Packages.
   These properties are referred to by a name consisting of the
   PackageName and a PropertyId.  Most properties have default values
   described in the Package description.  Properties may be read-only or
   read/write.  The possible values of a property may be audited, as can
   their current values.  For properties that are read/write, the MGC
   can set their values.  A property may be declared as "Global" which
   has a single value shared by all Terminations realizing the package.
   Related properties are grouped into descriptors for convenience.

   When a Termination is added to a Context, the value of its read/write
   properties can be set by including the appropriate descriptors as
   parameters to the Add command.  Similarly, a property of a
   Termination in a Context may have its value changed by the Modify
   command.


Groves, et al.              Standards Track                    [Page 23]

RFC 3525                Gateway Control Protocol               June 2003


   Properties may also have their values changed when a Termination is
   moved from one Context to another as a result of a Move command.  In
   some cases, descriptors are returned as output from a command.

   In general, if a Descriptor is completely omitted from one of the
   aforementioned Commands, the properties in that Descriptor retain
   their prior values for the Termination(s) upon which the Command
   acts.  On the other hand, if some read/write properties are omitted
   from a Descriptor in a Command (i.e., the Descriptor is only
   partially specified), those properties will be reset to their default
   values for the Termination(s) upon which the Command acts, unless the
   package specifies other behavior.  For more details, see clause 7.1
   dealing with the individual Descriptors.

   The following table lists all of the possible descriptors and their
   use.  Not all descriptors are legal as input or output parameters to
   every command.

   Descriptor name  Description

   Modem            Identifies modem type and properties when
                    applicable

   Mux              Describes multiplex type for multimedia
                    Terminations (e.g., H.221, H.223, H.225.0) and
                    Terminations forming the input mux

   Media            A list of media stream specifications (see 7.1.4)

   TerminationState Properties of a Termination (which can be defined
                    in Packages) that are not stream specific

   Stream           A list of remote/local/localControl descriptors for
                    a single stream

   Local            Contains properties that specify the media flows
                    that the MG receives from the remote entity.

   Remote           Contains properties that specify the media flows
                    that the MG sends to the remote entity.

   LocalControl     Contains properties (which can be defined in
                    packages) that are of interest between the MG and
                    the MGC.

   Events           Describes events to be detected by the MG and what
                    to do when an event is detected.


Groves, et al.              Standards Track                    [Page 24]

RFC 3525                Gateway Control Protocol               June 2003


   EventBuffer      Describes events to be detected by the MG when
                    Event Buffering is active.

   Signals          Describes signals (see 7.1.11) applied  to
                    Terminations.

   Audit            In Audit commands, identifies which information is
                    desired.

   Packages         In AuditValue, returns a list of Packages realized
                    by Termination.

   DigitMap         Defines patterns against which sequences of a
                    specified set of events are to be matched so they
                    can be reported as a group rather than singly.

   ServiceChange    In ServiceChange, what, why service change
                    occurred, etc.

   ObservedEvents   In Notify or AuditValue, report of events observed.

   Statistics       In Subtract and Audit, report of Statistics kept on
                    a Termination.

   Topology         Specifies flow directions between Terminations in a
                    Context.

   Error            Contains an error code and optionally error text;
                    it may occur in command replies and in Notify
                    requests.

6.2.5 Root Termination

   Occasionally, a command must refer to the entire gateway, rather than
   a Termination within it.  A special TerminationID, "Root" is reserved
   for this purpose.  Packages may be defined on Root.  Root thus may
   have properties, events and statistics (signals are not appropriate
   for root).  Accordingly, the root TerminationID may appear in:

   -  a Modify command - to change a property or set an event

   -  a Notify command - to report an event

   -  an AuditValue return - to examine the values of properties and
      statistics implemented on root

   -  an AuditCapability - to determine what properties of root are
      implemented


Groves, et al.              Standards Track                    [Page 25]

RFC 3525                Gateway Control Protocol               June 2003


   -  a ServiceChange - to declare the gateway in or out of service.

   Any other use of the root TerminationID is an error.  Error code
   410 - Incorrect identifier shall be returned in these cases.

7  Commands

   The protocol provides commands for manipulating the logical entities
   of the protocol connection model, Contexts and Terminations.
   Commands provide control at the finest level of granularity supported
   by the protocol.  For example, Commands exist to add Terminations to
   a Context, modify Terminations, subtract Terminations from a Context,
   and audit properties of Contexts or Terminations.  Commands provide
   for complete control of the properties of Contexts and Terminations.
   This includes specifying which events a Termination is to report,
   which signals/actions are to be applied to a Termination and
   specifying the topology of a Context (who hears/sees whom).

   Most commands are for the specific use of the Media Gateway
   Controller as command initiator in controlling Media Gateways as
   command responders.  The exceptions are the Notify and ServiceChange
   commands: Notify is sent from Media Gateway to Media Gateway
   Controller, and ServiceChange may be sent by either entity.  Below is
   an overview of the commands; they are explained in more detail in
   7.2.

   1) Add - The Add command adds a Termination to a Context.  The Add
      command on the first Termination in a Context is used to create a
      Context.

   2) Modify - The Modify command modifies the properties, events and
      signals of a Termination.

   3) Subtract - The Subtract command disconnects a Termination from its
      Context and returns statistics on the Termination's participation
      in the Context.  The Subtract command on the last Termination in a
      Context deletes the Context.

   4) Move - The Move command atomically moves a Termination to another
      Context.

   5) AuditValue - The AuditValue command returns the current state of
      properties, events, signals and statistics of Terminations.

   6) AuditCapabilities - The AuditCapabilities command returns all the
      possible values for Termination properties, events and signals
      allowed by the Media Gateway.


Groves, et al.              Standards Track                    [Page 26]

RFC 3525                Gateway Control Protocol               June 2003


   7) Notify - The Notify command allows the Media Gateway to inform the
      Media Gateway Controller of the occurrence of events in the Media
      Gateway.

   8) ServiceChange - The ServiceChange command allows the Media Gateway
      to notify the Media Gateway Controller that a Termination or group
      of Terminations is about to be taken out of service or has just
      been returned to service.  ServiceChange is also used by the MG to
      announce its availability to a MGC (registration), and to notify
      the MGC of impending or completed restart of the MG.  The MGC may
      announce a handover to the MG by sending it a ServiceChange
      command.  The MGC may also use ServiceChange to instruct the MG to
      take a Termination or group of Terminations in or out of service.

   These commands are detailed in 7.2.1 through 7.2.8.

7.1   Descriptors

   The parameters to a command are termed Descriptors.  A descriptor
   consists of a name and a list of items.  Some items may have values.
   Many Commands share common descriptors.  This subclause enumerates
   these descriptors.  Descriptors may be returned as output from a
   command.  In any such return of descriptor contents, an empty
   descriptor is represented by its name unaccompanied by any list.
   Parameters and parameter usage specific to a given Command type are
   described in the subclause that describes the Command.

7.1.1 Specifying parameters

   Command parameters are structured into a number of descriptors.  In
   general, the text format of descriptors is
   DescriptorName=<someID>{parm=value, parm=value, ...}.

   Parameters may be fully specified, overspecified or underspecified:

   1) Fully specified parameters have a single, unambiguous value that
      the command initiator is instructing the command responder to use
      for the specified parameter.

   2) Underspecified parameters, using the CHOOSE value, allow the
      command responder to choose any value it can support.

   3) Overspecified parameters have a list of potential values.  The
      list order specifies the command initiator's order of preference
      of selection.  The command responder chooses one value from
      the offered list and returns that value to the command initiator.


Groves, et al.              Standards Track                    [Page 27]

RFC 3525                Gateway Control Protocol               June 2003


   If a required descriptor other than the Audit descriptor is
   unspecified (i.e., entirely absent) from a command, the previous
   values set in that descriptor for that Termination, if any, are
   retained.  In commands other than Subtract, a missing Audit
   descriptor is equivalent to an empty Audit descriptor.  The Behaviour
   of the MG with respect to unspecified parameters within a descriptor
   varies with the descriptor concerned, as indicated in succeeding
   subclauses.  Whenever a parameter is underspecified or overspecified,
   the descriptor containing the value chosen by the responder is
   included as output from the command.

   Each command specifies the TerminationId the command operates on.
   This TerminationId may be "wildcarded".  When the TerminationId of a
   command is wildcarded, the effect shall be as if the command was
   repeated with each of the TerminationIds matched.

7.1.2 Modem descriptor

   The Modem descriptor specifies the modem type and parameters, if any,
   required for use in e.g., H.324 and text conversation.  The
   descriptor includes the following modem types: V.18, V.22, V.22 bis,
   V.32, V.32 bis, V.34, V.90, V.91, Synchronous ISDN, and allows for
   extensions.  By default, no Modem descriptor is present in a
   Termination.

7.1.3 Multiplex descriptor

   In multimedia calls, a number of media streams are carried on a
   (possibly different) number of bearers.  The multiplex descriptor
   associates the media and the bearers.  The descriptor includes the
   multiplex type:

   -  H.221;

   -  H.223;

   -  H.226;

   -  V.76;

   -  possible extensions,

   and a set of TerminationIDs representing the multiplexed bearers, in
   order.  For example:

      Mux = H.221{ MyT3/1/2, MyT3/2/13, MyT3/3/6, MyT3/21/22}


Groves, et al.              Standards Track                    [Page 28]

RFC 3525                Gateway Control Protocol               June 2003


7.1.4 Media descriptor

   The Media descriptor specifies the parameters for all the media
   streams.  These parameters are structured into two descriptors: a
   TerminationState descriptor, which specifies the properties of a
   Termination that are not stream dependent, and one or more Stream
   descriptors each of which describes a single media stream.

   A stream is identified by a StreamID.  The StreamID is used to link
   the streams in a Context that belong together.  Multiple streams
   exiting a Termination shall be synchronized with each other.  Within
   the Stream descriptor, there are up to three subsidiary descriptors:
   LocalControl, Local, and Remote.  The relationship between these
   descriptors is thus:

   Media descriptor
      TerminationState Descriptor
      Stream descriptor
         LocalControl descriptor
         Local descriptor
         Remote descriptor

   As a convenience, LocalControl, Local, or Remote descriptors may be
   included in the Media descriptor without an enclosing Stream
   descriptor.  In this case, the StreamID is assumed to be 1.

7.1.5 TerminationState descriptor

   The TerminationState descriptor contains the ServiceStates property,
   the EventBufferControl property and properties of a Termination
   (defined in Packages) that are not stream specific.

   The ServiceStates property describes the overall state of the
   Termination (not stream specific).  A Termination can be in one of
   the following states: "test", "out of service", or "in service".  The
   "test" state indicates that the Termination is being tested.  The
   state "out of service" indicates that the Termination cannot be used
   for traffic.  The state "in service" indicates that a Termination can
   be used or is being used for normal traffic.  "in service" is the
   default state.


Groves, et al.              Standards Track                    [Page 29]

RFC 3525                Gateway Control Protocol               June 2003


   Values assigned to Properties may be simple values
   (integer/string/enumeration) or may be underspecified, where more
   than one value is supplied and the MG may make a choice:

   -  Alternative Values - multiple values in a list, one of which must
      be selected

   -  Ranges - minimum and maximum values, any value between min and max
      must be selected, boundary values included

   -  Greater Than/Less Than - value must be greater/less than specified
      value

   -  CHOOSE Wildcard - the MG chooses from the allowed values for the
      property

   The EventBufferControl property specifies whether events are buffered
   following detection of an event in the Events descriptor, or
   processed immediately.  See 7.1.9 for details.

7.1.6 Stream descriptor

   A Stream descriptor specifies the parameters of a single
   bidirectional stream.  These parameters are structured into three
   descriptors: one that contains Termination properties specific to a
   stream and one each for local and remote flows.  The Stream
   Descriptor includes a StreamID which identifies the stream.  Streams
   are created by specifying a new StreamID on one of the Terminations
   in a Context.  A stream is deleted by setting empty Local and Remote
   descriptors for the stream with ReserveGroup and ReserveValue in
   LocalControl set to "false" on all Terminations in the Context that
   previously supported that stream.

   StreamIDs are of local significance between MGC and MG and they are
   assigned by the MGC.  Within a Context, StreamID is a means by which
   to indicate which media flows are interconnected: streams with the
   same StreamID are connected.

   If a Termination is moved from one Context to another, the effect on
   the Context to which the Termination is moved is the same as in the
   case that a new Termination were added with the same StreamIDs as the
   moved Termination.


Groves, et al.              Standards Track                    [Page 30]

RFC 3525                Gateway Control Protocol               June 2003


7.1.7 LocalControl descriptor

   The LocalControl descriptor contains the Mode property, the
   ReserveGroup and ReserveValue properties and properties of a
   Termination (defined in Packages) that are stream specific, and are
   of interest between the MG and the MGC.  Values of properties may be
   underspecified as in 7.1.1.

   The allowed values for the mode property are send-only, receive-only,
   send/receive, inactive and loop-back. "Send" and "receive" are with
   respect to the exterior of the Context, so that, for example, a
   stream set to mode=sendOnly does not pass received media into the
   Context.  The default value for the mode property is "Inactive".
   Signals and Events are not affected by mode.

   The boolean-valued Reserve properties, ReserveValue and ReserveGroup,
   of a Termination indicate what the MG is expected to do when it
   receives a Local and/or Remote descriptor.

   If the value of a Reserve property is True, the MG SHALL reserve
   resources for all alternatives specified in the Local and/or Remote
   descriptors for which it currently has resources available.  It SHALL
   respond with the alternatives for which it reserves resources.  If it
   cannot not support any of the alternatives, it SHALL respond with a
   reply to the MGC that contains empty Local and/or Remote descriptors.
   If media begins to flow while more than a single alternative is
   reserved, media packets may be sent/received on any of the
   alternatives and must be processed, although only a single
   alternative may be active at any given time.

   If the value of a Reserve property is False, the MG SHALL choose one
   of the alternatives specified in the Local descriptor (if present)
   and one of the alternatives specified in the Remote descriptor (if
   present).  If the MG has not yet reserved resources to support the
   selected alternative, it SHALL reserve the resources.  If, on the
   other hand, it already reserved resources for the Termination
   addressed (because of a prior exchange with ReserveValue and/or
   ReserveGroup equal to True), it SHALL release any excess resources it
   reserved previously.  Finally, the MG shall send a reply to the MGC
   containing the alternatives for the Local and/or Remote descriptor
   that it selected.  If the MG does not have sufficient resources to
   support any of the alternatives specified, it SHALL respond with
   error 510 (insufficient resources).

   The default value of ReserveValue and ReserveGroup is False.  More
   information on the use of the two Reserve properties is provided in
   7.1.8.


Groves, et al.              Standards Track                    [Page 31]

RFC 3525                Gateway Control Protocol               June 2003


   A new setting of the LocalControl Descriptor completely replaces the
   previous setting of that descriptor in the MG.  Thus, to retain
   information from the previous setting, the MGC must include that
   information in the new setting.  If the MGC wishes to delete some
   information from the existing descriptor, it merely resends the
   descriptor (in a Modify command) with the unwanted information
   stripped out.

7.1.8 Local and Remote descriptors

   The MGC uses Local and Remote descriptors to reserve and commit MG
   resources for media decoding and encoding for the given Stream(s) and
   Termination to which they apply.  The MG includes these descriptors
   in its response to indicate what it is actually prepared to support.
   The MG SHALL include additional properties and their values in its
   response if these properties are mandatory yet not present in the
   requests made by the MGC (e.g., by specifying detailed video encoding
   parameters where the MGC only specified the payload type).

   Local refers to the media received by the MG and Remote refers to the
   media sent by the MG.

   When text encoding the protocol, the descriptors consist of session
   descriptions as defined in SDP (RFC 2327).  In session descriptions
   sent from the MGC to the MG, the following exceptions to the syntax
   of RFC 2327 are allowed:

   -  the "s=", "t=" and "o=" lines are optional;

   -  the use of CHOOSE is allowed in place of a single parameter value;
      and

   -  the use of alternatives is allowed in place of a single parameter
      value.

   A Stream Descriptor specifies a single bi-directional media stream
   and so a single session description MUST NOT include more than one
   media description ("m=" line).  A Stream Descriptor may contain
   additional session descriptions as alternatives.  Each media stream
   for a termination must appear in distinct Stream Descriptors.  When
   multiple session descriptions are provided in one descriptor, the
   "v=" lines are required as delimiters; otherwise they are optional in
   session descriptions sent to the MG.  Implementations shall accept
   session descriptions that are fully conformant to RFC 2327.  When
   binary encoding the protocol the descriptor consists of groups of
   properties (tag-value pairs) as specified in Annex C.  Each such
   group may contain the parameters of a session description.


Groves, et al.              Standards Track                    [Page 32]

RFC 3525                Gateway Control Protocol               June 2003


   Below, the semantics of the Local and Remote descriptors are
   specified in detail.  The specification consists of two parts.  The
   first part specifies the interpretation of the contents of the
   descriptor.  The second part specifies the actions the MG must take
   upon receiving the Local and Remote descriptors.  The actions to be
   taken by the MG depend on the values of the ReserveValue and
   ReserveGroup properties of the LocalControl descriptor.

   Either the Local or the Remote descriptor or both may be:

   1) unspecified (i.e., absent);

   2) empty;

   3) underspecified through use of CHOOSE in a property value;

   4) fully specified; or

   5) overspecified through presentation of multiple groups of
      properties and possibly multiple property values in one or more of
      these groups.

   Where the descriptors have been passed from the MGC to the MG, they
   are interpreted according to the rules given in 7.1.1, with the
   following additional comments for clarification:

   a) An unspecified Local or Remote descriptor is considered to be a
      missing mandatory parameter.  It requires the MG to use whatever
      was last specified for that descriptor.  It is possible that there
      was no previously specified value, in which case the descriptor
      concerned is ignored in further processing of the command.

   b) An empty Local (Remote) descriptor in a message from the MGC
      signifies a request to release any resources reserved for the
      media flow received (sent).

   c) If multiple groups of properties are present in a Local or Remote
      descriptor or multiple values within a group, the order of
      preference is descending.

   d) Underspecified or overspecified properties within a group of
      properties sent by the MGC are requests for the MG to choose one
      or more values which it can support for each of those properties.
      In case of an overspecified property, the list of values is in
      descending order of preference.

   Subject to the above rules, subsequent action depends on the values
   of the ReserveValue and ReserveGroup properties in LocalControl.


Groves, et al.              Standards Track                    [Page 33]

RFC 3525                Gateway Control Protocol               June 2003


   If ReserveGroup is True, the MG reserves the resources required to
   support any of the requested property group alternatives that it can
   currently support.  If ReserveValue is True, the MG reserves the
   resources required to support any of the requested property value
   alternatives that it can currently support.

   NOTE - If a Local or Remote descriptor contains multiple groups of
   properties, and ReserveGroup is True, then the MG is requested to
   reserve resources so that it can decode or encode the media stream
   according to any of the alternatives.  For instance, if the Local
   descriptor contains two groups of properties, one specifying
   packetized G.711 A-law audio and the other G.723.1 audio, the MG
   reserves resources so that it can decode one audio stream encoded in
   either G.711 A-law format or G.723.1 format.  The MG does not have to
   reserve resources to decode two audio streams simultaneously, one
   encoded in G.711 A-law and one in G.723.1.  The intention for the use
   of ReserveValue is analogous.

   If ReserveGroup is true or ReserveValue is True, then the following
   rules apply:

   -  If the MG has insufficient resources to support all alternatives
      requested by the MGC and the MGC requested resources in both Local
      and Remote, the MG should reserve resources to support at least
      one alternative each within Local and Remote.

   -  If the MG has insufficient resources to support at least one
      alternative within a Local (Remote) descriptor received from the
      MGC, it shall return an empty Local (Remote) in response.

   -  In its response to the MGC, when the MGC included Local and Remote
      descriptors, the MG SHALL include Local and Remote descriptors for
      all groups of properties and property values it reserved resources
      for.  If the MG is incapable of supporting at least one of the
      alternatives within the Local (Remote) descriptor received from
      the MGC, it SHALL return an empty Local (Remote) descriptor.

   -  If the Mode property of the LocalControl descriptor is RecvOnly,
      SendRecv, or LoopBack, the MG must be prepared to receive media
      encoded according to any of the alternatives included in its
      response to the MGC.

   If ReserveGroup is False and ReserveValue is False, then the MG
   SHOULD apply the following rules to resolve Local and Remote to a
   single alternative each:

   -  The MG chooses the first alternative in Local for which it is able
      to support at least one alternative in Remote.


Groves, et al.              Standards Track                    [Page 34]

RFC 3525                Gateway Control Protocol               June 2003


   -  If the MG is unable to support at least one Local and one Remote
      alternative, it returns Error 510 (Insufficient Resources).

   -  The MG returns its selected alternative in each of Local and
      Remote.

   A new setting of a Local or Remote descriptor completely replaces the
   previous setting of that descriptor in the MG.  Thus, to retain
   information from the previous setting, the MGC must include that
   information in the new setting.  If the MGC wishes to delete some
   information from the existing descriptor, it merely resends the
   descriptor (in a Modify command) with the unwanted information
   stripped out.

7.1.9 Events descriptor

   The EventsDescriptor parameter contains a RequestIdentifier and a
   list of events that the Media Gateway is requested to detect and
   report.  The RequestIdentifier is used to correlate the request with
   the notifications that it may trigger.  Requested events include, for
   example, fax tones, continuity test results, and on-hook and off-hook
   transitions.  The RequestIdentifier is omitted if the
   EventsDescriptor is empty (i.e., no events are specified).

   Each event in the descriptor contains the Event name, an optional
   streamID, an optional KeepActive flag, and optional parameters.  The
   Event name consists of a Package Name (where the event is defined)
   and an EventID.  The ALL wildcard may be used for the EventID,
   indicating that all events from the specified package have to be
   detected.  The default streamID is 0, indicating that the event to be
   detected is not related to a particular media stream.  Events can
   have parameters.  This allows a single event description to have some
   variation in meaning without creating large numbers of individual
   events.  Further event parameters are defined in the package.

   If a digit map completion event is present or implied in the
   EventsDescriptor, the EventDM parameter is used to carry either the
   name or the value of the associated digit map.  See 7.1.14 for
   further details.

   When an event is processed against the contents of an active Events
   Descriptor and found to be present in that descriptor ("recognized"),
   the default action of the MG is to send a Notify command to the MGC.
   Notification may be deferred if the event is absorbed into the
   current dial string of an active digit map (see 7.1.14).  Any other
   action is for further study.  Moreover, event recognition may cause
   currently active signals to stop, or may cause the current Events
   and/or Signals descriptor to be replaced, as described at the end of


Groves, et al.              Standards Track                    [Page 35]

RFC 3525                Gateway Control Protocol               June 2003


   this subclause.  Unless the Events Descriptor is replaced by another
   Events Descriptor, it remains active after an event has been
   recognized.

   If the value of the EventBufferControl property equals LockStep,
   following detection of such an event, normal handling of events is
   suspended.  Any event which is subsequently detected and occurs in
   the EventBuffer descriptor is added to the end of the EventBuffer (a
   FIFO queue), along with the time that it was detected.  The MG SHALL
   wait for a new EventsDescriptor to be loaded.  A new EventsDescriptor
   can be loaded either as the result of receiving a command with a new
   EventsDescriptor, or by activating an embedded EventsDescriptor.

   If EventBufferControl equals Off, the MG continues processing based
   on the active EventsDescriptor.

   In the case of an embedded EventsDescriptor being activated, the MG
   continues event processing based on the newly activated
   EventsDescriptor.

     NOTE 1 - For purposes of EventBuffer handling, activation of an
     embedded EventsDescriptor is equivalent to receipt of a new
     EventsDescriptor.

   When the MG receives a command with a new EventsDescriptor, one or
   more events may have been buffered in the EventBuffer in the MG.  The
   value of EventBufferControl then determines how the MG treats such
   buffered events.

   Case 1

   If EventBufferControl equals LockStep and the MG receives a new
   EventsDescriptor, it will check the FIFO EventBuffer and take the
   following actions:

   1) If the EventBuffer is empty, the MG waits for detection of events
      based on the new EventsDescriptor.

   2) If the EventBuffer is non-empty, the MG processes the FIFO queue
      starting with the first event:

      a) If the event in the queue is in the events listed in the new
         EventsDescriptor, the MG acts on the event and removes the
         event from the EventBuffer.  The time stamp of the Notify shall
         be the time the event was actually detected.  The MG then waits
         for a new EventsDescriptor.  While waiting for a new
         EventsDescriptor, any events detected that appear in the


Groves, et al.              Standards Track                    [Page 36]

RFC 3525                Gateway Control Protocol               June 2003


         EventsBufferDescriptor will be placed in the EventBuffer.  When
         a new EventsDescriptor is received, the event processing will
         repeat from step 1.

      b) If the event is not in the new EventsDescriptor, the MG SHALL
         discard the event and repeat from step 1.

   Case 2

   If EventBufferControl equals Off and the MG receives a new
   EventsDescriptor, it processes new events with the new
   EventsDescriptor.

   If the MG receives a command instructing it to set the value of
   EventBufferControl to Off, all events in the EventBuffer SHALL be
   discarded.

   The MG may report several events in a single Transaction as long as
   this does not unnecessarily delay the reporting of individual events.

   For procedures regarding transmitting the Notify command, refer to
   the appropriate annex or Recommendation of the H.248 sub-series for
   specific transport considerations.

   The default value of EventBufferControl is Off.

     NOTE 2 - Since the EventBufferControl property is in the
     TerminationStateDescriptor, the MG might receive a command that
     changes the EventBufferControl property and does not include an
     EventsDescriptor.

   Normally, recognition of an event shall cause any active signals to
   stop.  When KeepActive is specified in the event, the MG shall not
   interrupt any signals active on the Termination on which the event is
   detected.

   An event can include an Embedded Signals descriptor and/or an
   Embedded Events descriptor which, if present, replaces the current
   Signals/Events descriptor when the event is recognized.  It is
   possible, for example, to specify that the dial-tone Signal be
   generated when an off-hook Event is recognized, or that the dial-tone
   Signal be stopped when a digit is recognized.  A media gateway
   controller shall not send EventsDescriptors with an event both marked
   KeepActive and containing an embedded SignalsDescriptor.


Groves, et al.              Standards Track                    [Page 37]

RFC 3525                Gateway Control Protocol               June 2003


   Only one level of embedding is permitted.  An embedded
   EventsDescriptor SHALL NOT contain another embedded EventsDescriptor;
   an embedded EventsDescriptor MAY contain an embedded
   SignalsDescriptor.

   An EventsDescriptor received by a media gateway replaces any previous
   Events descriptor.  Event notification in process shall complete, and
   events detected after the command containing the new EventsDescriptor
   executes, shall be processed according to the new EventsDescriptor.

   An empty Events Descriptor disables all event recognition and
   reporting.  An empty EventBuffer Descriptor clears the EventBuffer
   and disables all event accumulation in LockStep mode: the only events
   reported will be those occurring while an Events Descriptor is
   active.  If an empty Events Descriptor is activated while the
   Termination is operating in LockStep mode, the events buffer is
   immediately cleared.

7.1.10   EventBuffer descriptor

   The EventBuffer descriptor contains a list of events, with their
   parameters if any, that the MG is requested to detect and buffer when
   EventBufferControl equals LockStep (see 7.1.9).

7.1.11   Signals descriptor

   Signals are MG generated media such as tones and announcements as
   well as bearer-related signals such as hookswitch.  More complex
   signals may include a sequence of such simple signals interspersed
   with and conditioned upon the receipt and analysis of media or
   bearer-related signals.  Examples include echoing of received data as
   in Continuity Test package.  Signals may also request preparation of
   media content for future signals.

   A SignalsDescriptor is a parameter that contains the set of signals
   that the Media Gateway is asked to apply to a Termination.  A
   SignalsDescriptor contains a number of signals and/or sequential
   signal lists.  A SignalsDescriptor may contain zero signals and
   sequential signal lists.  Support of sequential signal lists is
   optional.

   Signals are defined in packages.  Signals shall be named with a
   Package name (in which the signal is defined) and a SignalID.  No
   wildcard shall be used in the SignalID.  Signals that occur in a
   SignalsDescriptor have an optional StreamID parameter (default is 0,
   to indicate that the signal is not related to a particular media
   stream), an optional signal type (see below), an optional duration
   and possibly parameters defined in the package that defines the


Groves, et al.              Standards Track                    [Page 38]

RFC 3525                Gateway Control Protocol               June 2003


   signal.  This allows a single signal to have some variation in
   meaning, obviating the need to create large numbers of individual
   signals.

   Finally, the optional parameter "notifyCompletion" allows a MGC to
   indicate that it wishes to be notified when the signal finishes
   playout.  The possible cases are that the signal timed out (or
   otherwise completed on its own), that it was interrupted by an event,
   that it was halted when a Signals descriptor was replaced, or that it
   stopped or never started for other reasons.  If the notifyCompletion
   parameter is not included in a Signals descriptor, notification is
   generated only if the signal stopped or was never started for other
   reasons.  For reporting to occur, the signal completion event (see
   E.1.2) must be enabled in the currently active Events descriptor.

   The duration is an integer value that is expressed in hundredths of a
   second.

   There are three types of signals:

   -  on/off - the signal lasts until it is turned off;

   -  timeout - the signal lasts until it is turned off or a specific
      period of time elapses;

   -  brief - the signal will stop on its own unless a new Signals
      descriptor is applied that causes it to stop; no timeout value is
      needed.

   If a signal of default type other than TO has its type overridden to
   type TO in the Signals descriptor, the duration parameter must be
   present.

   If the signal type is specified in a SignalsDescriptor, it overrides
   the default signal type (see 12.1.4).  If duration is specified for
   an on/off signal, it SHALL be ignored.

   A sequential signal list consists of a signal list identifier and a
   sequence of signals to be played sequentially.  Only the trailing
   element of the sequence of signals in a sequential signal list may be
   an on/off signal.  The duration of a sequential signal list is the
   sum of the durations of the signals it contains.

   Multiple signals and sequential signal lists in the same
   SignalsDescriptor shall be played simultaneously.

   Signals are defined as proceeding from the Termination towards the
   exterior of the Context unless otherwise specified in a package.


Groves, et al.              Standards Track                    [Page 39]

RFC 3525                Gateway Control Protocol               June 2003


   When the same Signal is applied to multiple Terminations within one
   Transaction, the MG should consider using the same resource to
   generate these Signals.

   Production of a Signal on a Termination is stopped by application of
   a new SignalsDescriptor, or detection of an Event on the Termination
   (see 7.1.9).

   A new SignalsDescriptor replaces any existing SignalsDescriptor.  Any
   signals applied to the Termination not in the replacement descriptor
   shall be stopped, and new signals are applied, except as follows.
   Signals present in the replacement descriptor and containing the
   KeepActive flag shall be continued if they are currently playing and
   have not already completed.  If a replacement signal descriptor
   contains a signal that is not currently playing and contains the
   KeepActive flag, that signal SHALL be ignored.  If the replacement
   descriptor contains a sequential signal list with the same identifier
   as the existing descriptor, then

   -  the signal type and sequence of signals in the sequential signal
      list in the replacement descriptor shall be ignored; and

   -  the playing of the signals in the sequential signal list in the
      existing descriptor shall not be interrupted.

7.1.12   Audit descriptor

   The Audit descriptor specifies what information is to be audited.
   The Audit descriptor specifies the list of descriptors to be
   returned.  Audit may be used in any command to force the return of
   any descriptor containing the current values of its properties,
   events, signals and statistics even if that descriptor was not
   present in the command, or had no underspecified parameters.
   Possible items in the Audit descriptor are:

      Modem
      Mux
      Events
      Media
      Signals
      ObservedEvents
      DigitMap
      Statistics
      Packages
      EventBuffer


Groves, et al.              Standards Track                    [Page 40]

RFC 3525                Gateway Control Protocol               June 2003


   Audit may be empty, in which case, no descriptors are returned.  This
   is useful in Subtract, to inhibit return of statistics, especially
   when using wildcard.

7.1.13   ServiceChange descriptor

   The ServiceChangeDescriptor contains the following parameters:

      .  ServiceChangeMethod
      .  ServiceChangeReason
      .  ServiceChangeAddress
      .  ServiceChangeDelay
      .  ServiceChangeProfile
      .  ServiceChangeVersion
      .  ServiceChangeMGCId
      .  TimeStamp
      .  Extension

   See 7.2.8.

7.1.14   DigitMap descriptor

7.1.14.1 DigitMap definition, creation, modification and deletion

   A DigitMap is a dialing plan resident in the Media Gateway used for
   detecting and reporting digit events received on a Termination.  The
   DigitMap descriptor contains a DigitMap name and the DigitMap to be
   assigned.  A digit map may be preloaded into the MG by management
   action and referenced by name in an EventsDescriptor, may be defined
   dynamically and subsequently referenced by name, or the actual
   digitmap itself may be specified in the EventsDescriptor.  It is
   permissible for a digit map completion event within an Events
   descriptor to refer by name to a DigitMap which is defined by a
   DigitMap descriptor within the same command, regardless of the
   transmitted order of the respective descriptors.

   DigitMaps defined in a DigitMapDescriptor can occur in any of the
   standard Termination manipulation Commands of the protocol.  A
   DigitMap, once defined, can be used on all Terminations specified by
   the (possibly wildcarded) TerminationID in such a command.  DigitMaps
   defined on the root Termination are global and can be used on every
   Termination in the MG, provided that a DigitMap with the same name
   has not been defined on the given Termination.  When a DigitMap is
   defined dynamically in a DigitMap descriptor:

   -  A new DigitMap is created by specifying a name that is not yet
      defined.  The value shall be present.


Groves, et al.              Standards Track                    [Page 41]

RFC 3525                Gateway Control Protocol               June 2003


   -  A DigitMap value is updated by supplying a new value for a name
      that is already defined.  Terminations presently using the
      digitmap shall continue to use the old definition; subsequent
      EventsDescriptors specifying the name, including any
      EventsDescriptor in the command containing the DigitMap
      descriptor, shall use the new one.

   -  A DigitMap is deleted by supplying an empty value for a name that
      is already defined.  Terminations presently using the digitmap
      shall continue to use the old definition.

7.1.14.2 DigitMap Timers

   The collection of digits according to a DigitMap may be protected by
   three timers, viz. a start timer (T), short timer (S), and long timer
   (L).

   1) The start timer (T) is used prior to any digits having been
      dialed.  If the start timer is overridden with the value set to
      zero (T=0), then the start timer shall be disabled.  This implies
      that the MG will wait indefinitely for digits.

   2) If the Media Gateway can determine that at least one more digit is
      needed for a digit string to match any of the allowed patterns in
      the digit map, then the interdigit timer value should be set to a
      long (L) duration (e.g., 16 seconds).

   3) If the digit string has matched one of the patterns in a digit
      map, but it is possible that more digits could be received which
      would cause a match with a different pattern, then instead of
      reporting the match immediately, the MG must apply the short timer
      (S) and wait for more digits.

   The timers are configurable parameters to a DigitMap.  Default values
   of these timers should be provisioned on the MG, but can be
   overridden by values specified within the DigitMap.

7.1.14.3 DigitMap Syntax

   The formal syntax of the digit map is described by the DigitMap rule
   in the formal syntax description of the protocol (see Annex A and
   Annex B).  A DigitMap, according to this syntax, is defined either by
   a string or by a list of strings.  Each string in the list is an
   alternative event sequence, specified either as a sequence of digit
   map symbols or as a regular expression of digit map symbols.  These
   digit map symbols, the digits "0" through "9" and letters "A" through
   a maximum value depending on the signalling system concerned, but
   never exceeding "K", correspond to specified events within a package


Groves, et al.              Standards Track                    [Page 42]

RFC 3525                Gateway Control Protocol               June 2003


   which has been designated in the Events descriptor on the Termination
   to which the digit map is being applied.  (The mapping between events
   and digit map symbols is defined in the documentation for packages
   associated with channel-associated signalling systems such as DTMF,
   MF, or R2.  Digits "0" through "9" MUST be mapped to the
   corresponding digit events within the signalling system concerned.
   Letters should be allocated in logical fashion, facilitating the use
   of range notation for alternative events.)

   The letter "x" is used as a wildcard, designating any event
   corresponding to symbols in the range "0"-"9".  The string may also
   contain explicit ranges and, more generally, explicit sets of
   symbols, designating alternative events any one of which satisfies
   that position of the digit map.  Finally, the dot symbol "." stands
   for zero or more repetitions of the event selector (event, range of
   events, set of alternative events, or wildcard) that precedes it.  As
   a consequence of the third timing rule above, inter-event timing
   while matching a terminal dot symbol uses the short timer by default.

   In addition to these event symbols, the string may contain "S" and
   "L" inter-event timing specifiers and the "Z" duration modifier.  "S"
   and "L" respectively indicate that the MG should use the short (S)
   timer or the long (L) timer for subsequent events, overriding the
   timing rules described above.  If an explicit timing specifier is in
   effect in one alternative event sequence, but none is given in any
   other candidate alternative, the timer value set by the explicit
   timing specifier must be used.  If all sequences with explicit timing
   controls are dropped from the candidate set, timing reverts to the
   default rules given above.  Finally, if conflicting timing specifiers
   are in effect in different alternative sequences, the long timer
   shall be used.

   A "Z" designates a long duration event: placed in front of the
   symbol(s) designating the event(s) which satisfy a given digit
   position, it indicates that that position is satisfied only if the
   duration of the event exceeds the long-duration threshold.  The value
   of this threshold is assumed to be provisioned in the MG.

7.1.14.4 DigitMap Completion Event

   A digit map is active while the Events descriptor which invoked it is
   active and it has not completed.  A digit map completes when:

   -  a timer has expired; or

   -  an alternative event sequence has been matched and no other
      alternative event sequence in the digit map could be matched
      through detection of an additional event (unambiguous match); or


Groves, et al.              Standards Track                    [Page 43]

RFC 3525                Gateway Control Protocol               June 2003


   -  an event has been detected such that a match to a complete
      alternative event sequence of the digit map will be impossible no
      matter what additional events are received.

   Upon completion, a digit map completion event as defined in the
   package providing the events being mapped into the digit map shall be
   generated.  At that point the digit map is deactivated.  Subsequent
   events in the package are processed as per the currently active event
   processing mechanisms.

7.1.14.5 DigitMap Procedures

   Pending completion, successive events shall be processed according to
   the following rules:

   1) The "current dial string", an internal variable, is initially
      empty.  The set of candidate alternative event sequences includes
      all of the alternatives specified in the digit map.

   2) At each step, a timer is set to wait for the next event, based
      either on the default timing rules given above or on explicit
      timing specified in one or more alternative event sequences.  If
      the timer expires and a member of the candidate set of
      alternatives is fully satisfied, a timeout completion with full
      match is reported.  If the timer expires and part or none of any
      candidate alternative is satisfied, a timeout completion with
      partial match is reported.

   3) If an event is detected before the timer expires, it is mapped to
      a digit string symbol and provisionally added to the end of the
      current dial string.  The duration of the event (long or not long)
      is noted if and only if this is relevant in the current symbol
      position (because at least one of the candidate alternative event
      sequences includes the "Z" modifier at this position in the
      sequence).

   4) The current dial string is compared to the candidate alternative
      event sequences.  If and only if a sequence expecting a
      long-duration event at this position is matched (i.e., the event
      had long duration and met the specification for this position),
      then any alternative event sequences not specifying a long
      duration event at this position are discarded, and the current
      dial string is modified by inserting a "Z" in front of the symbol
      representing the latest event.   Any sequence expecting a long-
      duration event at this position but not matching the observed
      event is discarded from the candidate set.  If alternative event
      sequences not specifying a long duration event in the given


Groves, et al.              Standards Track                    [Page 44]

RFC 3525                Gateway Control Protocol               June 2003


      position remain in the candidate set after application of the
      above rules, the observed event duration is treated as irrelevant
      in assessing matches to them.

   5) If exactly one candidate remains and it has been fully matched, a
      completion event is generated indicating an unambiguous match.  If
      no candidates remain, the latest event is removed from the current
      dial string and a completion event is generated indicating full
      match if one of the candidates from the previous step was fully
      satisfied before the latest event was detected, or partial match
      otherwise.  The event removed from the current dial string will
      then be reported as per the currently active event processing
      mechanisms.

   6) If no completion event is reported out of step 5, processing
      returns to step 2.

7.1.14.6 DigitMap Activation

   A digit map is activated whenever a new Event descriptor is applied
   to the Termination or embedded Event descriptor is activated, and
   that Event descriptor contains a digit map completion event.  The
   digit map completion event contains an eventDM field in the requested
   actions field.  Each new activation of a digit map begins at step 1
   of the above procedure, with a clear current dial string.  Any
   previous contents of the current dial string from an earlier
   activation are lost.

   A digit map completion event that does not contain an eventDM field
   in its requested actions field is considered an error.  Upon receipt
   of such an event in an EventsDescriptor, a MG shall respond with an
   error response, including Error 457 - Missing parameter in signal or
   event.

7.1.14.7 Interaction Of DigitMap and Event Processing

   While the digit map is activated, detection is enabled for all events
   defined in the package containing the specified digit map completion
   event.  Normal event behaviour (e.g., stopping of signals unless the
   digit completion event has the KeepActive flag enabled) continues to
   apply for each such event detected, except that:

   -  the events in the package containing the specified digit map
      completion event other than the completion event itself are not
      individually notified and have no side-effects unless separately
      enabled; and


Groves, et al.              Standards Track                    [Page 45]

RFC 3525                Gateway Control Protocol               June 2003


   -  an event that triggers a partial match completion event is not
      recognized and therefore has no side effects until reprocessed
      following the recognition of the digit map completion event.

7.1.14.8 Wildcards

   Note that if a package contains a digit map completion event, then an
   event specification consisting of the package name with a wildcarded
   ItemID (Property Name) will activate a digit map; to that end, the
   event specification must include an eventDM field according to
   section 7.1.14.6.  If the package also contains the digit events
   themselves, this form of event specification will cause the
   individual events to be reported to the MGC as they are detected.

7.1.14.9 Example

   As an example, consider the following dial plan:

   0                      Local operator

   00                     Long-distance operator

   xxxx                   Local extension number (starts with 1-7)

   8xxxxxxx               Local number

   #xxxxxxx               Off-site extension

   *xx                    Star services

   91xxxxxxxxxx           Long-distance number

   9011 + up to 15 digits International number


   If the DTMF detection package described in E.6 is used to collect the
   dialed digits, then the dialing plan shown above results in the
   following digit map:

    (0| 00|[1-7]xxx|8xxxxxxx|Fxxxxxxx|Exx|91xxxxxxxxxx|9011x.)

7.1.15   Statistics descriptor

   The Statistics Descriptor provides information describing the status
   and usage of a Termination during its existence within a specific
   Context.  There is a set of standard statistics kept for each
   Termination where appropriate (number of octets sent and received for


Groves, et al.              Standards Track                    [Page 46]

RFC 3525                Gateway Control Protocol               June 2003


   example).  The particular statistical properties that are reported
   for a given Termination are determined by the Packages realized by
   the Termination.  By default, statistics are reported when the
   Termination is Subtracted from the Context.  This behaviour can be
   overridden by including an empty AuditDescriptor in the Subtract
   command.  Statistics may also be returned from the AuditValue
   command, or any Add/Move/Modify command using the Audit descriptor.

   Statistics are cumulative; reporting Statistics does not reset them.
   Statistics are reset when a Termination is Subtracted from a Context.

7.1.16   Packages descriptor

   Used only with the AuditValue command, the PackageDescriptor returns
   a list of Packages realized by the Termination.

7.1.17   ObservedEvents descriptor

   ObservedEvents is supplied with the Notify command to inform the MGC
   of which event(s) were detected.  Used with the AuditValue command,
   the ObservedEventsDescriptor returns events in the event buffer which
   have not been Notified.  ObservedEvents contains the
   RequestIdentifier of the EventsDescriptor that triggered the
   notification, the event(s) detected, optionally the detection time(s)
   and any parameters of the observed event.  Detection times are
   reported with a precision of hundredths of a second.

7.1.18   Topology descriptor

   A Topology descriptor is used to specify flow directions between
   Terminations in a Context.  Contrary to the descriptors in previous
   subclauses, the Topology descriptor applies to a Context instead of a
   Termination.  The default topology of a Context is that each
   Termination's transmission is received by all other Terminations.
   The Topology descriptor is optional to implement.  An MG that does
   not support Topology descriptors, but receives a command containing
   one, returns Error 444 Unsupported or unknown descriptor, and
   optionally includes a string containing the name of the unsupported
   Descriptor ("Topology") in the error text in the error descriptor.

   The Topology descriptor occurs before the commands in an action.  It
   is possible to have an action containing only a Topology descriptor,
   provided that the Context to which the action applies already exists.


Groves, et al.              Standards Track                    [Page 47]

RFC 3525                Gateway Control Protocol               June 2003


   A Topology descriptor consists of a sequence of triples of the form
   (T1, T2, association).  T1 and T2 specify Terminations within the
   Context, possibly using the ALL or CHOOSE wildcard.  The association
   specifies how media flows between these two Terminations as follows.

   -  (T1, T2, isolate) means that the Terminations matching T2 do not
      receive media from the Terminations matching T1, nor vice versa.

   -  (T1, T2, oneway) means that the Terminations that match T2 receive
      media from the Terminations matching T1, but not vice versa.  In
      this case use of the ALL wildcard such that there are Terminations
      that match both T1 and T2 is not allowed.

   -  (T1, T2, bothway) means that the Terminations matching T2 receive
      media from the Terminations matching T1, and vice versa.  In this
      case it is allowed to use wildcards such that there are
      Terminations that match both T1 and T2.  However, if there is a
      Termination that matches both, no loopback is introduced.

   CHOOSE wildcards may be used in T1 and T2 as well, under the
   following restrictions:

   -  the action (see clause 8) of which the topology descriptor is part
      contains an Add command in which a CHOOSE wildcard is used;

   -  if a CHOOSE wildcard occurs in T1 or T2, then a partial name SHALL
      NOT be specified.

   The CHOOSE wildcard in a Topology descriptor matches the
   TerminationID that the MG assigns in the first Add command that uses
   a CHOOSE wildcard in the same action.  An existing Termination that
   matches T1 or T2 in the Context to which a Termination is added, is
   connected to the newly added Termination as specified by the Topology
   descriptor.

   If a termination is not mentioned within a Topology Descriptor, any
   topology associated with it remains unchanged.  If, however, a new
   termination is added into a context its association with the other
   terminations within the context defaults to bothway, unless a
   Topology Descriptor is given to change this (e.g., if T3 is added to
   a context with T1 and T2 with topology (T3, T1, oneway) it will be
   connected bothway to T2).

   Figure 7 and the table following it show some examples of the effect
   of including topology descriptors in actions.  In these examples it
   is assumed that the topology descriptors are applied in sequence.


Groves, et al.              Standards Track                    [Page 48]

RFC 3525                Gateway Control Protocol               June 2003


     +------------------+  +------------------+  +------------------+
     |      +----+      |  |      +----+      |  |      +----+      |
     |      | T2 |      |  |      | T2 |      |  |      | T2 |      |
     |      +----+      |  |      +----+      |  |      +----+      |
     |       ^  ^       |  |          ^       |  |          ^       |
     |       |  |       |  |          |       |  |          |       |
     |    +--+  +--+    |  |          +---+   |  |          +--+    |
     |    |        |    |  |              |   |  |             |    |
     |    v        v    |  |              v   |  |             |    |
     | +----+    +----+ |  | +----+    +----+ |  | +----+    +----+ |
     | | T1 |<-->| T3 | |  | | T1 |<-->| T3 | |  | | T1 |<-->| T3 | |
     | +----+    +----+ |  | +----+    +----+ |  | +----+    +----+ |
     +------------------+  +------------------+  +------------------+
     1. No Topology Desc.   2. T1, T2, Isolate    3. T3, T2, Oneway

     +------------------+  +------------------+  +------------------+
     |      +----+      |  |      +----+      |  |      +----+      |
     |      | T2 |      |  |      | T2 |      |  |      | T2 |      |
     |      +----+      |  |      +----+      |  |      +----+      |
     |          |       |  |          ^       |  |       ^  ^       |
     |          |       |  |          |       |  |       |  |       |
     |          +--+    |  |          +---+   |  |    +--+  +--+    |
     |             |    |  |              |   |  |    |        |    |
     |             v    |  |              v   |  |    v        v    |
     | +----+    +----+ |  | +----+    +----+ |  | +----+    +----+ |
     | | T1 |<-->| T3 | |  | | T1 |<-->| T3 | |  | | T1 |<-->| T3 | |
     | +----+    +----+ |  | +----+    +----+ |  | +----+    +----+ |
     +------------------+  +------------------+  +------------------+
     4. T2, T3 oneway      5. T2, T3 bothway     6. T1, T2 bothway

     Note: the direction of the arrow indicates the direction of flow.

                       Figure 7: Example topologies

   Topology Description

   1 No topology descriptors    When no topology descriptors are
                                included, all Terminations have a
                                bothway connection to all other
                                Terminations.

   2 T1, T2 Isolate             Removes the connection between T1 and
                                T2.  T3 has a bothway connection with
                                both T1 and T2.  T1 and T2 have bothway
                                connection to T3.


Groves, et al.              Standards Track                    [Page 49]

RFC 3525                Gateway Control Protocol               June 2003


   3 T3, T2 oneway              A oneway connection from T3 to T2 (i.e.,
                                T2 receives media flow from T3).  A
                                bothway connection between T1 and T3.

   4 T2, T3 oneway              A oneway connection between T2 to T3.
                                T1 and T3 remain bothway connected.

   5 T2, T3 bothway             T2 is bothway connected to T3.  This
                                results in the same as 2.

   6 T1, T2 bothway (T2, T3     All Terminations have a bothway
     bothway and T1, T3         connection to all other Terminations.
     bothway may be implied or
     explicit).

   A oneway connection must be implemented in such a way that the other
   Terminations in the Context are not aware of the change in topology.

7.1.19   Error Descriptor

   If a responder encounters an error when processing a transaction
   request, it must include an error descriptor in its response.  A
   Notify request may contain an error descriptor as well.

   An error descriptor consists of an IANA-registered error code,
   optionally accompanied by an error text.  H.248.8 contains a list of
   valid error codes and error descriptions.

   An error descriptor shall be specified at the "deepest level" that is
   semantically appropriate for the error being described and that is
   possible given any parsing problems with the original request.  An
   error descriptor may refer to a syntactical construct other than
   where it appears.  For example, Error descriptor 422 - Syntax Error
   in Action, could appear within a command even though it refers to the
   larger construct - the action - and not the particular command within
   which it appears.

7.2   Command Application Programming Interface

   Following is an Application Programming Interface (API) describing
   the Commands of the protocol.  This API is shown to illustrate the
   Commands and their parameters and is not intended to specify
   implementation (e.g., via use of blocking function calls).  It
   describes the input parameters in parentheses after the command name
   and the return values in front of the Command.  This is only for
   descriptive purposes; the actual Command syntax and encoding are


Groves, et al.              Standards Track                    [Page 50]

RFC 3525                Gateway Control Protocol               June 2003


   specified in later subclauses.  The order of parameters to commands
   is not fixed.  Descriptors may appear as parameters to commands in
   any order.  The descriptors SHALL be processed in the order in which
   they appear.

   Any reply to a command may contain an error descriptor; the API does
   not specifically show this.

   All parameters enclosed by square brackets ([. . .]) are considered
   optional.

7.2.1 Add

   The Add Command adds a Termination to a Context.

     TerminationID
     [,MediaDescriptor]
     [,ModemDescriptor]
     [,MuxDescriptor]
     [,EventsDescriptor]
     [,SignalsDescriptor]
     [,DigitMapDescriptor]
     [,ObservedEventsDescriptor]
     [,EventBufferDescriptor]
     [,StatisticsDescriptor]
     [,PackagesDescriptor]
       Add( TerminationID
        [, MediaDescriptor]
        [, ModemDescriptor]
        [, MuxDescriptor]
        [, EventsDescriptor]
        [, EventBufferDescriptor]
        [, SignalsDescriptor]
        [, DigitMapDescriptor]
        [, AuditDescriptor]
        )

   The TerminationID specifies the Termination to be added to the
   Context.  The Termination is either created, or taken from the null
   Context.  If a CHOOSE wildcard is used in the TerminationID, the
   selected TerminationID will be returned.  Wildcards may be used in an
   Add, but such usage would be unusual.  If the wildcard matches more
   than one TerminationID, all possible matches are attempted, with
   results reported for each one.  The order of attempts when multiple
   TerminationIDs match is not specified.

   The optional MediaDescriptor describes all media streams.


Groves, et al.              Standards Track                    [Page 51]

RFC 3525                Gateway Control Protocol               June 2003


   The optional ModemDescriptor and MuxDescriptor specify a modem and
   multiplexer if applicable.  For convenience, if a Multiplex
   descriptor is present in an Add command and lists any Terminations
   that are not currently in the Context, such Terminations are added to
   the Context as if individual Add commands listing the Terminations
   were invoked. If an error occurs on such an implied Add, error 471 -
   Implied Add for Multiplex failure shall be returned and further
   processing of the command shall cease.

   The EventsDescriptor parameter is optional.  If present, it provides
   the list of events that should be detected on the Termination.

   The EventBufferDescriptor parameter is optional.  If present, it
   provides the list of events that the MG is requested to detect and
   buffer when EventBufferControl equals LockStep.

   The SignalsDescriptor parameter is optional.  If present, it provides
   the list of signals that should be applied to the Termination.

   The DigitMapDescriptor parameter is optional.  If present, it defines
   a DigitMap definition that may be used in an EventsDescriptor.

   The AuditDescriptor is optional.  If present, the command will return
   descriptors as specified in the AuditDescriptor.

   All descriptors that can be modified could be returned by MG if a
   parameter was underspecified or overspecified.  ObservedEvents,
   Statistics, and Packages, and the EventBuffer descriptors are
   returned only if requested in the AuditDescriptor.

   Add SHALL NOT be used on a Termination with a serviceState of
   "OutofService".

7.2.2 Modify

   The Modify Command modifies the properties of a Termination.

     TerminationID
     [,MediaDescriptor]
     [,ModemDescriptor]
     [,MuxDescriptor]
     [,EventsDescriptor]
     [,SignalsDescriptor]
     [,DigitMapDescriptor]
     [,ObservedEventsDescriptor]
     [,EventBufferDescriptor]
     [,StatisticsDescriptor]
     [,PackagesDescriptor]


Groves, et al.              Standards Track                    [Page 52]

RFC 3525                Gateway Control Protocol               June 2003


      Modify( TerminationID
         [, MediaDescriptor]
         [, ModemDescriptor]
         [, MuxDescriptor]
         [, EventsDescriptor]
         [, EventBufferDescriptor]
         [, SignalsDescriptor]
         [, DigitMapDescriptor]
         [, AuditDescriptor]
         )

   The TerminationID may be specific if a single Termination in the
   Context is to be modified.  Use of wildcards in the TerminationID may
   be appropriate for some operations.  If the wildcard matches more
   than one TerminationID, all possible matches are attempted, with
   results reported for each one.  The order of attempts when multiple
   TerminationIDs match is not specified.  The CHOOSE option is an
   error, as the Modify command may only be used on existing
   Terminations.

   For convenience, if a Multiplex Descriptor is present in a Modify
   command, then:

   -  if the new Multiplex Descriptor lists any Terminations that are
      not currently in the Context, such Terminations are added to the
      context as if individual commands listing the Terminations were
      invoked.

   -  if any Terminations listed previously in the Multiplex Descriptor
      are no longer present in the new Multiplex Descriptor, they are
      subtracted from the context as if individual Subtract commands
      listing the Terminations were invoked.

   The remaining parameters to Modify are the same as those to Add.
   Possible return values are the same as those to Add.

7.2.3 Subtract

   The Subtract Command disconnects a Termination from its Context and
   returns statistics on the Termination's participation in the Context.

     TerminationID
     [,MediaDescriptor]
     [,ModemDescriptor]
     [,MuxDescriptor]
     [,EventsDescriptor]
     [,SignalsDescriptor]
     [,DigitMapDescriptor]


Groves, et al.              Standards Track                    [Page 53]

RFC 3525                Gateway Control Protocol               June 2003


     [,ObservedEventsDescriptor]
     [,EventBufferDescriptor]
     [,StatisticsDescriptor]
     [,PackagesDescriptor]
      Subtract(TerminationID
         [, AuditDescriptor]
         )

   TerminationID in the input parameters represents the Termination that
   is being subtracted.  The TerminationID may be specific or may be a
   wildcard value indicating that all (or a set of related) Terminations
   in the Context of the Subtract Command are to be subtracted.  If the
   wildcard matches more than one TerminationID, all possible matches
   are attempted, with results reported for each one.  The order of
   attempts when multiple TerminationIDs match is not specified.

   The use of CHOOSE in the TerminationID is an error, as the Subtract
   command may only be used on existing Terminations.

   ALL may be used as the ContextID as well as the TerminationId in a
   Subtract, which would have the effect of deleting all Contexts,
   deleting all ephemeral Terminations, and returning all physical
   Terminations to Null Context.  Subtract of a termination from the
   Null Context is not allowed.

   For convenience, if a multiplexing Termination is the object of a
   Subtract command, then any bearer Terminations listed in its
   Multiplex Descriptor are subtracted from the context as if individual
   Subtract commands listing the Terminations were invoked.

   By default, the Statistics parameter is returned to report
   information collected on the Termination or Terminations specified in
   the Command.  The information reported applies to the Termination's
   or Terminations' existence in the Context from which it or they are
   being subtracted.

   The AuditDescriptor is optional.  If present, the command will return
   only those descriptors as specified in the AuditDescriptor, which may
   be empty.  If omitted, the Statistics descriptor is returned, by
   default.  Possible return values are the same as those to Add.

   When a provisioned Termination is Subtracted from a Context, its
   property values shall revert to:

   -  the default value, if specified for the property and not
      overridden by provisioning;

   -  otherwise, the provisioned value.


Groves, et al.              Standards Track                    [Page 54]

RFC 3525                Gateway Control Protocol               June 2003


7.2.4 Move

   The Move Command moves a Termination to another Context from its
   current Context in one atomic operation.  The Move command is the
   only command that refers to a Termination in a Context different from
   that to which the command is applied.  The Move command shall not be
   used to move Terminations to or from the null Context.

     TerminationID
     [,MediaDescriptor]
     [,ModemDescriptor]
     [,MuxDescriptor]
     [,EventsDescriptor]
     [,SignalsDescriptor]
     [,DigitMapDescriptor]
     [,ObservedEventsDescriptor]
     [,EventBufferDescriptor]
     [,StatisticsDescriptor]
     [,PackagesDescriptor]
      Move( TerminationID
         [, MediaDescriptor]
         [, ModemDescriptor]
         [, MuxDescriptor]
         [, EventsDescriptor]
         [, EventBufferDescriptor]
         [, SignalsDescriptor]
         [, DigitMapDescriptor]
         [, AuditDescriptor]
         )

   The TerminationID specifies the Termination to be moved.  It may be
   wildcarded, but CHOOSE shall not be used in the TerminationID.  If
   the wildcard matches more than one TerminationID, all possible
   matches are attempted, with results reported for each one.  The order
   of attempts when multiple TerminationIDs match is not specified.  The
   Context to which the Termination is moved is indicated by the target
   ContextId in the Action.  If the last remaining Termination is moved
   out of a Context, the Context is deleted.

   The Move command does not affect the properties of the Termination on
   which it operates, except those properties explicitly modified by
   descriptors included in the Move command.  The AuditDescriptor with
   the Statistics option, for example, would return statistics on the
   Termination just prior to the Move.  Possible descriptors returned
   from Move are the same as for Add.


Groves, et al.              Standards Track                    [Page 55]

RFC 3525                Gateway Control Protocol               June 2003


   For convenience, if a multiplexing Termination is the object of a
   Move command, then any bearer Terminations listed in its Multiplex
   Descriptor are also moved as if individual Move commands listing the
   Terminations were invoked.

   Move SHALL NOT be used on a Termination with a serviceState of
   "OutofService".

7.2.5 AuditValue

   The AuditValue Command returns the current values of properties,
   events, signals and statistics associated with Terminations.

   TerminationID
   [,MediaDescriptor]
   [,ModemDescriptor]
   [,MuxDescriptor]
   [,EventsDescriptor]
   [,SignalsDescriptor]
   [,DigitMapDescriptor]
   [,ObservedEventsDescriptor]
   [,EventBufferDescriptor]
   [,StatisticsDescriptor]
   [,PackagesDescriptor]
     AuditValue(TerminationID,
      AuditDescriptor
      )

   TerminationID may be specific or wildcarded.  If the wildcard matches
   more than one TerminationID, all possible matches are attempted, with
   results reported for each one.  The order of attempts when multiple
   TerminationIDs match is not specified.  If a wildcarded response is
   requested, only one command return is generated, with the contents
   containing the union of the values of all Terminations matching the
   wildcard.  This convention may reduce the volume of data required to
   audit a group of Terminations.  Use of CHOOSE is an error.

   The appropriate descriptors, with the current values for the
   Termination, are returned from AuditValue.  Values appearing in
   multiple instances of a descriptor are defined to be alternate values
   supported, with each parameter in a descriptor considered
   independent.

   ObservedEvents returns a list of events in the EventBuffer.  If the
   ObservedEventsDescriptor is audited while a DigitMap is active, the
   returned ObservedEvents descriptor also includes a digit map
   completion event that shows the current dial string but does not show
   a Termination method.


Groves, et al.              Standards Track                    [Page 56]

RFC 3525                Gateway Control Protocol               June 2003


   EventBuffer returns the set of events and associated parameter values
   currently enabled in the EventBufferDescriptor.  PackagesDescriptor
   returns a list of packages realized by the Termination.
   DigitMapDescriptor returns the name or value of the current DigitMap
   for the Termination.  DigitMap requested in an AuditValue command
   with TerminationID ALL returns all DigitMaps in the gateway.
   Statistics returns the current values of all statistics being kept on
   the Termination.   Specifying an empty Audit descriptor results in
   only the TerminationID being returned.  This may be useful to get a
   list of TerminationIDs when used with wildcard.  Annexes A and B
   provide a special syntax for presenting such a list in condensed
   form, such that the AuditValue command tag does not have to be
   repeated for each TerminationID.

   AuditValue results depend on the Context, viz. specific, null, or
   wildcarded.  (Note that ContextID ALL does not include the null
   Context.)  The TerminationID may be specific, or wildcarded.

   The following are examples of what is returned in case the context
   and/or the termination is wildcarded and a wildcarded response has
   been specified.

   Assume that the gateway has 4 terminations: t1/1, t1/2, t2/1 and
   t2/2.  Assume that terminations t1/* have implemented packages aaa
   and bbb and that terminations t2/* have implemented packages ccc and
   ddd.  Assume that Context 1 has t1/1 and t2/1 in it and that Context
   2 has t1/2 and t2/2 in it.

   The command:

     Context=1{AuditValue=t1/1{Audit{Packages}}}

   Returns:

     Context=1{AuditValue=t1/1{Packages{aaa,bbb}}}

   The command:

     Context=*{AuditValue=t2/*{Audit{Packages}}}

   Returns:

     Context=1{AuditValue=t2/1{Packages{ccc,ddd}}},
     Context=2{AuditValue=t2/2{Packages{ccc,ddd}}}

   The command:

     Context=*{W-AuditValue=t1/*{Audit{Packages}}}


Groves, et al.              Standards Track                    [Page 57]

RFC 3525                Gateway Control Protocol               June 2003


   Returns:

     Context=*{W-AuditValue=t1/*{Packages{aaa,bbb}}}

   Note: A wildcard response may also be used for other commands such as
   Subtract.

   The following illustrates other information that can be obtained with
   the AuditValue Command:

   ContextID TerminationID Information Obtained

   Specific  wildcard      Audit of matching Terminations in a Context

   Specific  specific      Audit of a single Termination in a Context

   Null      Root          Audit of Media Gateway state and events

   Null      wildcard      Audit of all matching Terminations in the
                            null Context

   Null      specific      Audit of a single Termination outside of any
                            Context

   All       wildcard      Audit of all matching Terminations and the
                            Context to which they are associated

   All       Root          List of all ContextIds (the ContextID list
                            should be returned by using multiple action
                            replies, each containing a ContextID from
                            the list)

   All       Specific      (Non-null) ContextID in which the
                            Termination currently exists


Groves, et al.              Standards Track                    [Page 58]

RFC 3525                Gateway Control Protocol               June 2003


7.2.6 AuditCapabilities

   The AuditCapabilities Command returns the possible values of
   properties, events, signals and statistics associated with
   Terminations.

     TerminationID
     [,MediaDescriptor]
     [,ModemDescriptor]
     [,MuxDescriptor]
     [,EventsDescriptor]
     [,SignalsDescriptor]
     [,ObservedEventsDescriptor]
     [,EventBufferDescriptor]
     [,StatisticsDescriptor]
      AuditCapabilities(TerminationID,
         AuditDescriptor
         )

   The appropriate descriptors, with the possible values for the
   Termination are returned from AuditCapabilities.  Descriptors may be
   repeated where there are multiple possible values.  If a wildcarded
   response is requested, only one command return is generated, with the
   contents containing the union of the values of all Terminations
   matching the wildcard.  This convention may reduce the volume of data
   required to audit a group of Terminations.

   Interpretation of what capabilities are requested for various values
   of ContextID and TerminationID is the same as in AuditValue.

   The EventsDescriptor returns the list of possible events on the
   Termination together with the list of all possible values for the
   EventsDescriptor Parameters.  EventBufferDescriptor returns the same
   information as EventsDescriptor.  The SignalsDescriptor returns the
   list of possible signals that could be applied to the Termination
   together with the list of all possible values for the Signals
   Parameters.  StatisticsDescriptor returns the names of the statistics
   being kept on the termination.  ObservedEventsDescriptor returns the
   names of active events on the Termination.  DigitMap and Packages are
   not legal in AuditCapability.


Groves, et al.              Standards Track                    [Page 59]

RFC 3525                Gateway Control Protocol               June 2003


   The following illustrates other information that can be obtained with
   the AuditCapabilties Command:

   ContextID TerminationID Information Obtained

   Specific  wildcard      Audit of matching Terminations in a Context

   Specific  specific      Audit of a single Termination in a Context

   Null      Root          Audit of MG state and events

   Null      wildcard      Audit of all matching Terminations in the
                            Null Context

   Null      specific      Audit of a single Termination outside of any
                            Context

   All       wildcard      Audit of all matching Terminations and the
                            Context to which they are associated

   All       Root          Same as for AuditValue

   All       Specific      Same as for AuditValue

7.2.7 Notify

   The Notify Command allows the Media Gateway to notify the Media
   Gateway Controller of events occurring within the Media Gateway.

     TerminationID
      Notify(TerminationID,
         ObservedEventsDescriptor,
         [ErrorDescriptor]
         )

   The TerminationID parameter specifies the Termination issuing the
   Notify Command.  The TerminationID shall be a fully qualified name.

   The ObservedEventsDescriptor contains the RequestID and a list of
   events that the Media Gateway detected in the order that they were
   detected.  Each event in the list is accompanied by parameters
   associated with the event and optionally an indication of the time
   that the event was detected.  Procedures for sending Notify commands
   with RequestID equal to 0 are for further study.

   Notify Commands with RequestID not equal to 0 shall occur only as the
   result of detection of an event specified by an Events descriptor
   which is active on the Termination concerned.


Groves, et al.              Standards Track                    [Page 60]

RFC 3525                Gateway Control Protocol               June 2003


   The RequestID returns the RequestID parameter of the EventsDescriptor
   that triggered the Notify Command.  It is used to correlate the
   notification with the request that triggered it.  The events in the
   list must have been requested via the triggering EventsDescriptor or
   embedded events descriptor unless the RequestID is 0 (which is for
   further study).

   The ErrorDescriptor may be sent in the Notify Command as a result of
   Error 518 - Event buffer full.

7.2.8 ServiceChange

   The ServiceChange Command allows the Media Gateway to notify the
   Media Gateway Controller that a Termination or group of Terminations
   is about to be taken out of service or has just been returned to
   service.  The Media Gateway Controller may indicate that
   Termination(s) shall be taken out of or returned to service.   The
   Media Gateway may notify the MGC that the capability of a Termination
   has changed.  It also allows a MGC to hand over control of a MG to
   another MGC.

   TerminationID,

     [ServiceChangeDescriptor]
      ServiceChange ( TerminationID,
         ServiceChangeDescriptor
         )

   The TerminationID parameter specifies the Termination(s) that are
   taken out of or returned to service.  Wildcarding of Termination
   names is permitted, with the exception that the CHOOSE mechanism
   shall not be used.  Use of the "Root" TerminationID indicates a
   ServiceChange affecting the entire Media Gateway.

   The ServiceChangeDescriptor contains the following parameters as
   required:

   -  ServiceChangeMethod
   -  ServiceChangeReason
   -  ServiceChangeDelay
   -  ServiceChangeAddress
   -  ServiceChangeProfile
   -  ServiceChangeVersion
   -  ServiceChangeMgcId
   -  TimeStamp


Groves, et al.              Standards Track                    [Page 61]

RFC 3525                Gateway Control Protocol               June 2003


   The ServiceChangeMethod parameter specifies the type of ServiceChange
   that will or has occurred:

   1) Graceful - indicates that the specified Terminations will be taken
      out of service after the specified ServiceChangeDelay; established
      connections are not yet affected, but the Media Gateway Controller
      should refrain from establishing new connections and should
      attempt to gracefully tear down existing connections on the
      Termination(s) affected by the serviceChange command.  The MG
      should set Termination serviceState at the expiry of
      ServiceChangeDelay or the removal of the Termination from an
      active Context (whichever is first), to "out of service".

   2) Forced - indicates that the specified Terminations were taken
      abruptly out of service and any established connections associated
      with them may be lost.  For non-Root terminations, the MGC is
      responsible for cleaning up the Context (if any) with which the
      failed Termination is associated.  At a minimum the Termination
      shall be subtracted from the Context.  The Termination
      serviceState should be "out of service".  For the root
      termination, the MGC can assume that all connections are lost on
      the MG and thus can consider that all the terminations have been
      subtracted.

   3) Restart - indicates that service will be restored on the specified
      Terminations after expiration of the ServiceChangeDelay.  The
      serviceState should be set to "inService" upon expiry of
      ServiceChangeDelay.

   4) Disconnected - always applied with the Root TerminationID,
      indicates that the MG lost communication with the MGC, but it was
      subsequently restored to the same MGC (possibly after trying other
      MGCs on a pre-provisioned list).  Since MG state may have changed,
      the MGC may wish to use the Audit command to resynchronize its
      state with the MG's.

   5) Handoff - sent from the MGC to the MG, this reason indicates that
      the MGC is going out of service and a new MGC association must be
      established.  Sent from the MG to the MGC, this indicates that the
      MG is attempting to establish a new association in accordance with
      a Handoff received from the MGC with which it was previously
      associated.

   6) Failover - sent from MG to MGC to indicate the primary MG is out
      of service and a secondary MG is taking over.  This serviceChange
      method is also sent from the MG to the MGC when the MG detects
      that MGC has failed.


Groves, et al.              Standards Track                    [Page 62]

RFC 3525                Gateway Control Protocol               June 2003


   7) Another value whose meaning is mutually understood between the MG
      and the MGC.

   The ServiceChangeReason parameter specifies the reason why the
   ServiceChange has or will occur.  It consists of an alphanumeric
   token (IANA registered) and, optionally, an explanatory string.

   The optional ServiceChangeAddress parameter specifies the address
   (e.g., IP port number for IP networks) to be used for subsequent
   communications.  It can be specified in the input parameter
   descriptor or the returned result descriptor.  ServiceChangeAddress
   and ServiceChangeMgcId parameters must not both be present in the
   ServiceChangeDescriptor or the ServiceChangeResultDescriptor.  The
   ServiceChangeAddress provides an address to be used within the
   Context of the association currently being negotiated, while the
   ServiceChangeMgcId provides an alternate address where the MG should
   seek to establish another association.  Note that the use of
   ServiceChangeAddress is not encouraged.  MGCs and MGs must be able to
   cope with the ServiceChangeAddress being either a full address or
   just a port number in the case of TCP transports.

   The optional ServiceChangeDelay parameter is expressed in seconds.
   If the delay is absent or set to zero, the delay value should be
   considered to be null.  In the case of a "graceful"
   ServiceChangeMethod, a null delay indicates that the Media Gateway
   Controller should wait for the natural removal of existing
   connections and should not establish new connections.  For "graceful"
   only, a null delay means the MG must not set serviceState "out of
   service" until the Termination is in the null Context.

   The optional ServiceChangeProfile parameter specifies the Profile (if
   any) of the protocol supported.  The ServiceChangeProfile includes
   the version of the profile supported.

   The optional ServiceChangeVersion parameter contains the protocol
   version and is used if protocol version negotiation occurs (see
   11.3).

   The optional TimeStamp parameter specifies the actual time as kept by
   the sender.  As such, it is not necessarily absolute time according
   to, for example, a local time zone - it merely establishes an
   arbitrary starting time against which all future timestamps
   transmitted by a sender during this association shall be compared.
   It can be used by the responder to determine how its notion of time
   differs from that of its correspondent.  TimeStamp is sent with a
   precision of hundredths of a second.


Groves, et al.              Standards Track                    [Page 63]

RFC 3525                Gateway Control Protocol               June 2003


   The optional Extension parameter may contain any value whose meaning
   is mutually understood by the MG and MGC.

   A ServiceChange Command specifying the "Root" for the TerminationID
   and ServiceChangeMethod equal to Restart is a registration command by
   which a Media Gateway announces its existence to the Media Gateway
   Controller.  The Media Gateway may also announce a registration
   command by specifying the "Root" for the TerminationID and
   ServiceChangeMethod equal to Failover when the MG detects MGC
   failures.  The Media Gateway is expected to be provisioned with the
   name of one primary and optionally some number of alternate Media
   Gateway Controllers.  Acknowledgement of the ServiceChange Command
   completes the registration process, except when the MGC has returned
   an alternative ServiceChangeMgcId as described in the following
   paragraph.  The MG may specify the transport ServiceChangeAddress to
   be used by the MGC for sending messages in the ServiceChangeAddress
   parameter in the input ServiceChangeDescriptor.  The MG may specify
   an address in the ServiceChangeAddress parameter of the ServiceChange
   request, and the MGC may also do so in the ServiceChange reply.  In
   either case, the recipient must use the supplied address as the
   destination for all subsequent transaction requests within the
   association.  At the same time, as indicated in clause 9, transaction
   replies and pending indications must be sent to the address from
   which the corresponding requests originated.  This must be done even
   if it implies extra messaging because commands and responses cannot
   be packed together.  The TimeStamp parameter shall be sent with a
   registration command and its response.

   The Media Gateway Controller may return a ServiceChangeMgcId
   parameter that describes the Media Gateway Controller that should
   preferably be contacted for further service by the Media Gateway.  In
   this case the Media Gateway shall reissue the ServiceChange command
   to the new Media Gateway Controller.  The MGC specified in a
   ServiceChangeMgcId, if provided, shall be contacted before any
   further alternate MGCs.  On a HandOff message from MGC to MG, the
   ServiceChangeMgcId is the new MGC that will take over from the
   current MGC.

   The return from ServiceChange is empty except when the Root
   terminationID is used.  In that case it includes the following
   parameters as required:

   -  ServiceChangeAddress, if the responding MGC wishes to specify a
      new destination for messages from the MG for the remainder of the
      association;

   -  ServiceChangeMgcId, if the responding MGC does not wish to sustain
      an association with the MG;


Groves, et al.              Standards Track                    [Page 64]

RFC 3525                Gateway Control Protocol               June 2003


   -  ServiceChangeProfile, if the responder wishes to negotiate the
      profile to be used for the association;

   -  ServiceChangeVersion, if the responder wishes to negotiate the
      version of the protocol to be used for the association.

   The following ServiceChangeReasons are defined.  This list may be
   extended by an IANA registration as outlined in 13.3.

      900 Service Restored
      901 Cold Boot
      902 Warm Boot
      903 MGC Directed Change
      904 Termination malfunctioning
      905 Termination taken out of service
      906 Loss of lower layer connectivity (e.g., downstream sync)
      907 Transmission Failure
      908 MG Impending Failure
      909 MGC Impending Failure
      910 Media Capability Failure
      911 Modem Capability Failure
      912 Mux Capability Failure
      913 Signal Capability Failure
      914 Event Capability Failure
      915 State Loss

7.2.9 Manipulating and Auditing Context Attributes

   The commands of the protocol as discussed in the preceding subclauses
   apply to Terminations.  This subclause specifies how Contexts are
   manipulated and audited.

   Commands are grouped into actions (see clause 8).  An action applies
   to one Context.  In addition to commands, an action may contain
   Context manipulation and auditing instructions.

   An action request sent to a MG may include a request to audit
   attributes of a Context.  An action may also include a request to
   change the attributes of a Context.

   The Context properties that may be included in an action reply are
   used to return information to a MGC.  This can be information
   requested by an audit of Context attributes or details of the effect
   of manipulation of a Context.


Groves, et al.              Standards Track                    [Page 65]

RFC 3525                Gateway Control Protocol               June 2003


   If a MG receives an action which contains both a request to audit
   context attributes and a request to manipulate those attributes, the
   response SHALL include the values of the attributes after processing
   the manipulation request.

7.2.10   Generic Command Syntax

   The protocol can be encoded in a binary format or in a text format.
   MGCs should support both encoding formats.  MGs may support both
   formats.

   The protocol syntax for the binary format of the protocol is defined
   in Annex A.  Annex C specifies the encoding of the Local and Remote
   descriptors for use with the binary format.

   A complete ABNF of the text encoding of the protocol per RFC 2234 is
   given in Annex B.  SDP is used as the encoding of the Local and
   Remote descriptors for use with the text encoding as modified in
   7.1.8.

7.3   Command Error Codes

   Errors consist of an IANA registered error code and an explanatory
   string.  Sending the explanatory string is optional.  Implementations
   are encouraged to append diagnostic information to the end of the
   string.

   When a MG reports an error to a MGC, it does so in an error
   descriptor.  An error descriptor consists of an error code and
   optionally the associated explanatory string.

   H.248.8 contains the error codes supported by Recommendations in the
   H.248 sub-series.

8  Transactions

   Commands between the Media Gateway Controller and the Media Gateway
   are grouped into Transactions, each of which is identified by a
   TransactionID.  Transactions consist of one or more Actions.  An
   Action consists of a non-empty series of Commands, Context property
   modifications, or Context property audits that are limited to
   operating within a single Context.  Consequently, each Action
   typically specifies a ContextID.  However, there are two
   circumstances where a specific ContextID is not provided with an
   Action.  One is the case of modification of a Termination outside of
   a Context.  The other is where the controller requests the gateway to
   create a new Context.  Figure 8 is a graphic representation of the
   Transaction, Action and Command relationships.


Groves, et al.              Standards Track                    [Page 66]

RFC 3525                Gateway Control Protocol               June 2003


      +----------------------------------------------------------+
      | Transaction x                                            |
      |  +----------------------------------------------------+  |
      |  | Action 1                                           |  |
      |  | +---------+  +---------+  +---------+  +---------+ |  |
      |  | | Command |  | Command |  | Command |  | Command | |  |
      |  | |    1    |  |    2    |  |    3    |  |    4    | |  |
      |  | +---------+  +---------+  +---------+  +---------+ |  |
      |  +----------------------------------------------------+  |
      |                                                          |
      |  +----------------------------------------------------+  |
      |  | Action 2                                           |  |
      |  | +---------+                                        |  |
      |  | | Command |                                        |  |
      |  | |    1    |                                        |  |
      |  | +---------+                                        |  |
      |  +----------------------------------------------------+  |
      |                                                          |
      |  +----------------------------------------------------+  |
      |  | Action 3                                           |  |
      |  | +---------+  +---------+  +---------+              |  |
      |  | | Command |  | Command |  | Command |              |  |
      |  | |    1    |  |    2    |  |    3    |              |  |
      |  | +---------+  +---------+  +---------+              |  |
      |  +----------------------------------------------------+  |
      +----------------------------------------------------------+

               Figure 8: Transactions, Actions and Commands

   Transactions are presented as TransactionRequests.  Corresponding
   responses to a TransactionRequest are received in a single reply,
   possibly preceded by a number of TransactionPending messages (see
   8.2.3).

   Transactions guarantee ordered Command processing.  That is, Commands
   within a Transaction are executed sequentially.  Ordering of
   Transactions is NOT guaranteed - transactions may be executed in any
   order, or simultaneously.

   At the first failing Command in a Transaction, processing of the
   remaining Commands in that Transaction stops.  If a command contains
   a wildcarded TerminationID, the command is attempted with each of the
   actual TerminationIDs matching the wildcard.  A response within the
   TransactionReply is included for each matching TerminationID, even if
   one or more instances generated an error.  If any TerminationID
   matching a wildcard results in an error when executed, any commands
   following the wildcarded command are not attempted.


Groves, et al.              Standards Track                    [Page 67]

RFC 3525                Gateway Control Protocol               June 2003


   Commands may be marked as "Optional" which can override this
   behaviour - if a command marked as Optional results in an error,
   subsequent commands in the Transaction will be executed.  If a
   command fails, the MG shall as far as possible restore the state that
   existed prior to the attempted execution of the command before
   continuing with command processing.

   A TransactionReply includes the results for all of the Commands in
   the corresponding TransactionRequest.  The TransactionReply includes
   the return values for the Commands that were executed successfully,
   and the Command and error descriptor for any Command that failed.

   TransactionPending is used to periodically notify the receiver that a
   Transaction has not completed yet, but is actively being processed.

   Applications SHOULD implement an application level timer per
   transaction.  Expiration of the timer should cause a retransmission
   of the request.  Receipt of a Reply should cancel the timer.  Receipt
   of Pending should restart the timer.

8.1   Common parameters

8.1.1 Transaction Identifiers

   Transactions are identified by a TransactionID, which is assigned by
   sender and is unique within the scope of the sender.  A response
   containing an error descriptor to indicate that the TransactionID is
   missing in a request shall use TransactionID 0 in the corresponding
   TransactionReply.

8.1.2 Context Identifiers

   Contexts are identified by a ContextID, which is assigned by the
   Media Gateway and is unique within the scope of the Media Gateway.
   The Media Gateway Controller shall use the ContextID supplied by the
   Media Gateway in all subsequent Transactions relating to that
   Context.  The protocol makes reference to a distinguished value that
   may be used by the Media Gateway Controller when referring to a
   Termination that is currently not associated with a Context, namely
   the null ContextID.

   The CHOOSE wildcard is used to request that the Media Gateway create
   a new Context.

   The MGC may use the ALL wildcard to address all Contexts on the MG.
   The null Context is not included when the ALL wildcard is used.


Groves, et al.              Standards Track                    [Page 68]

RFC 3525                Gateway Control Protocol               June 2003


   The MGC shall not use partially specified ContextIDs containing the
   CHOOSE or ALL wildcards.

8.2   Transaction Application Programming Interface

   Following is an Application Programming Interface (API) describing
   the Transactions of the protocol.  This API is shown to illustrate
   the Transactions and their parameters and is not intended to specify
   implementation (e.g., via use of blocking function calls).  It will
   describe the input parameters and return values expected to be used
   by the various Transactions of the protocol from a very high level.
   Transaction syntax and encodings are specified in later subclauses.

8.2.1 TransactionRequest

   The TransactionRequest is invoked by the sender.  There is one
   Transaction per request invocation.  A request contains one or more
   Actions, each of which specifies its target Context and one or more
   Commands per Context.

     TransactionRequest(TransactionId {
         ContextID {Command ... Command},
            . . .
         ContextID {Command ... Command } })

   The TransactionID parameter must specify a value for later
   correlation with the TransactionReply or TransactionPending response
   from the receiver.

   The ContextID parameter must specify a value to pertain to all
   Commands that follow up to either the next specification of a
   ContextID parameter or the end of the TransactionRequest, whichever
   comes first.

   The Command parameter represents one of the Commands mentioned in 7.2
   (Command Application Programming Interface).

8.2.2 TransactionReply

   The TransactionReply is invoked by the receiver.  There is one reply
   invocation per transaction.  A reply contains one or more Actions,
   each of which must specify its target Context and one or more
   Responses per Context.  The TransactionReply is invoked by the
   responder when it has processed the TransactionRequest.


Groves, et al.              Standards Track                    [Page 69]

RFC 3525                Gateway Control Protocol               June 2003


   A TransactionRequest has been processed:

   -  when all actions in that TransactionRequest have been processed;
      or

   -  when an error is encountered in processing that
      TransactionRequest, except when the error is in an optional
      command.

   A command has been processed when all descriptors in that command
   have been processed.

   A SignalsDescriptor is considered to have been processed when it has
   been established that the descriptor is syntactically valid, the
   requested signals are supported and they have been queued to be
   applied.

   An EventsDescriptor or EventBufferDescriptor is considered to have
   been processed when it has been established that the descriptor is
   syntactically valid, the requested events can be observed, any
   embedded signals can be generated, any embedded events can be
   detected, and the MG has been brought into a state in which the
   events will be detected.

     TransactionReply(TransactionID {
         ContextID { Response ... Response },
            . . .
         ContextID { Response ... Response } })

   The TransactionID parameter must be the same as that of the
   corresponding TransactionRequest.

   The ContextID parameter must specify a value to pertain to all
   Responses for the action.  The ContextID may be specific, all or
   null.

   Each of the Response parameters represents a return value as
   mentioned in 7.2, or an error descriptor if the command execution
   encountered an error.  Commands after the point of failure are not
   processed and, therefore, Responses are not issued for them.

   An exception to this occurs if a command has been marked as optional
   in the Transaction request.  If the optional command generates an
   error, the transaction still continues to execute, so the Reply
   would, in this case, have Responses after an Error.

   Section 7.1.19 Error Descriptor specifies the generation of error
   descriptors.  The text below discusses several individual cases.


Groves, et al.              Standards Track                    [Page 70]

RFC 3525                Gateway Control Protocol               June 2003


   If the receiver encounters an error in processing a ContextID, the
   requested Action response will consist of the Context ID and a single
   error descriptor, 422 - Syntax Error in Action.

   If the receiver encounters an error such that it cannot determine a
   legal Action, it will return a TransactionReply consisting of the
   TransactionID and a single error descriptor, 422 - Syntax Error in
   Action.  If the end of an action cannot be reliably determined but
   one or more commands can be parsed, it will process them and then
   send 422 - Syntax Error in Action as the last action for the
   transaction.  If the receiver encounters an error such that is cannot
   determine a legal Transaction, it will return a TransactionReply with
   a null TransactionID and a single error descriptor (403 - Syntax
   Error in TransactionRequest).

   If the end of a transaction cannot be reliably determined and one or
   more Actions can be parsed, it will process them and then return 403
   - Syntax Error in Transaction as the last action reply for the
   transaction.  If no Actions can be parsed, it will return 403 -
   Syntax Error in TransactionRequest as the only reply.

   If the terminationID cannot be reliably determined, it will send 442
   - Syntax Error in Command as the action reply.

   If the end of a command cannot be reliably determined, it will return
   442 - Syntax Error in Command as the reply to the last action it can
   parse.

8.2.3 TransactionPending

   The receiver invokes the TransactionPending.  A TransactionPending
   indicates that the Transaction is actively being processed, but has
   not been completed.  It is used to prevent the sender from assuming
   the TransactionRequest was lost where the Transaction will take some
   time to complete.

     TransactionPending(TransactionID { } )

   The TransactionID parameter must be the same as that of the
   corresponding TransactionRequest.  A property of root
   (normalMGExecutionTime) is settable by the MGC to indicate the
   interval within which the MGC expects a response to any transaction
   from the MG.  Another property (normalMGCExecutionTime) is settable
   by the MGC to indicate the interval within which the MG should expect
   a response to any transaction from the MGC.  Senders may receive more
   than one TransactionPending for a command.  If a duplicate request is


Groves, et al.              Standards Track                    [Page 71]

RFC 3525                Gateway Control Protocol               June 2003


   received when pending, the responder may send a duplicate pending
   immediately, or continue waiting for its timer to trigger another
   TransactionPending.

8.3   Messages

   Multiple Transactions can be concatenated into a Message.  Messages
   have a header, which includes the identity of the sender.  The
   Message Identifier (MID) of a message is set to a provisioned name
   (e.g., domain address/domain name/device name) of the entity
   transmitting the message.  Domain name is a suggested default.  An
   H.248.1 entity (MG/MGC) must consistently use the same MID in all
   messages it originates for the duration of control association with
   the peer (MGC/MG).

   Every Message contains a Version Number identifying the version of
   the protocol the message conforms to.  Versions consist of one or two
   digits, beginning with version 1 for the present version of the
   protocol.

   The transactions in a message are treated independently.  There is no
   order implied; there is no application or protocol acknowledgement of
   a message.  A message is essentially a transport mechanism.  For
   example, message X containing transaction requests A, B, and C may be
   responded to with message Y containing replies to A and C and message
   Z containing the reply to B.  Likewise, message L containing request
   D and message M containing request E may be responded to with message
   N containing replies to both D and E.

9  Transport

   The transport mechanism for the protocol should allow the reliable
   transport of transactions between a MGC and MG.  The transport shall
   remain independent of what particular commands are being sent and
   shall be applicable to all application states.  There are several
   transports defined for the protocol, which are defined in Annexes to
   this RFC and other Recommendations of the H.248
   sub-series.  Additional Transports may be defined as additional

   Recommendations of the H.248 sub-series.  For transport of the
   protocol over IP, MGCs shall implement both TCP and UDP/ALF, a MG
   shall implement TCP or UDP/ALF or both.

   The MG is provisioned with a name or address (such as DNS name or IP
   address) of a primary and zero or more secondary MGCs (see 7.2.8)
   that is the address the MG uses to send messages to the MGC.  If TCP
   or UDP is used as the protocol transport and the port to which the
   initial ServiceChange request is to be sent is not otherwise known,


Groves, et al.              Standards Track                    [Page 72]

RFC 3525                Gateway Control Protocol               June 2003


   that request should be sent to the default port number for the
   protocol.  This port number is 2944 for text-encoded operation or
   2945 for binary-encoded operation, for either UDP or TCP.  The MGC
   receives the message containing the ServiceChange request from the MG
   and can determine the MG's address from it.  As described in 7.2.8,
   either the MG or the MGC may supply an address in the
   ServiceChangeAddress parameter to which subsequent transaction
   requests must be addressed, but responses (including the response to
   the initial ServiceChange request) must always be sent back to the
   address which was the source of the corresponding request.  For
   example, in IP networks, this is the source address in the IP header
   and the source port number in the TCP/UDP/SCTP header.

9.1   Ordering of Commands

   This RFC does not mandate that the underlying transport protocol
   guarantees the sequencing of transactions sent to an entity.  This
   property tends to maximize the timeliness of actions, but it has a
   few drawbacks.  For example:

   -  Notify commands may be delayed and arrive at the MGC after the
      transmission of a new command changing the EventsDescriptor.

   -  If a new command is transmitted before a previous one is
      acknowledged, there is no guarantee that prior command will be
      executed before the new one.

   Media Gateway Controllers that want to guarantee consistent operation
   of the Media Gateway may use the following rules.  These rules are
   with respect to commands that are in different transactions.
   Commands that are in the same transaction are executed in order (see
   clause 8).

   1) When a Media Gateway handles several Terminations, commands
      pertaining to the different Terminations may be sent in parallel,
      for example following a model where each Termination (or group of
      Terminations) is controlled by its own process or its own thread.

   2) On a Termination, there should normally be at most one outstanding
      command (Add or Modify or Move), unless the outstanding commands
      are in the same transaction.  However, a Subtract command may be
      issued at any time.  In consequence, a Media Gateway may sometimes
      receive a Modify command that applies to a previously subtracted
      Termination.  Such commands should be ignored, and an error code
      should be returned.


Groves, et al.              Standards Track                    [Page 73]

RFC 3525                Gateway Control Protocol               June 2003


   3) For transports that do not guarantee in-sequence delivery of
      messages (i.e., UDP), there should normally be on a given
      Termination at most one outstanding Notify command at any time.

   4) In some cases, an implicitly or explicitly wildcarded Subtract
      command that applies to a group of Terminations may step in front
      of a pending Add command.  The Media Gateway Controller should
      individually delete all Terminations for which an Add command was
      pending at the time of the global Subtract command.  Also, new Add
      commands for Terminations named by the wildcarding (or implied in
      a Multiplex descriptor) should not be sent until the wildcarded
      Subtract command is acknowledged.

   5) AuditValue and AuditCapability are not subject to any sequencing.

   6) ServiceChange shall always be the first command sent by a MG as
      defined by the restart procedure.  Any other command or response
      must be delivered after this ServiceChange command.

   These rules do not affect the command responder, which should always
   respond to commands.

9.2   Protection against Restart Avalanche

   In the event that a large number of Media Gateways are powered on
   simultaneously and they were to all initiate a ServiceChange
   transaction, the Media Gateway Controller would very likely be
   swamped, leading to message losses and network congestion during the
   critical period of service restoration.  In order to prevent such
   avalanches, the following behaviour is suggested:

   1) When a Media Gateway is powered on, it should initiate a restart
      timer to a random value, uniformly distributed between 0 and a
      maximum waiting delay (MWD).  Care should be taken to avoid
      synchronicity of the random number generation between multiple
      Media Gateways that would use the same algorithm.

   2) The Media Gateway should then wait for either the end of this
      timer or the detection of a local user activity, such as for
      example an off-hook transition on a residential Media Gateway.

   3) When the timer elapses, or when an activity is detected, the Media
      Gateway should initiate the restart procedure.

   The restart procedure simply requires the MG to guarantee that the
   first message that the Media Gateway Controller sees from this MG is
   a ServiceChange message informing the Media Gateway Controller about
   the restart.


Groves, et al.              Standards Track                    [Page 74]

RFC 3525                Gateway Control Protocol               June 2003


     NOTE - The value of MWD is a configuration parameter that depends
     on the type of the Media Gateway.  The following reasoning may be
     used to determine the value of this delay on residential gateways.

   Media Gateway Controllers are typically dimensioned to handle the
   peak hour traffic load, during which, in average, 10% of the lines
   will be busy, placing calls whose average duration is typically 3
   minutes.  The processing of a call typically involves 5 to 6 Media
   Gateway Controller transactions between each Media Gateway and the
   Media Gateway Controller.  This simple calculation shows that the
   Media Gateway Controller is expected to handle 5 to 6 transactions
   for each Termination, every 30 minutes on average, or, to put it
   otherwise, about one transaction per Termination every 5 to 6 minutes
   on average.  This suggests that a reasonable value of MWD for a
   residential gateway would be 10 to 12 minutes.  In the absence of
   explicit configuration, residential gateways should adopt a value of
   600 seconds for MWD.

   The same reasoning suggests that the value of MWD should be much
   shorter for trunking gateways or for business gateways, because they
   handle a large number of Terminations, and also because the usage
   rate of these Terminations is much higher than 10% during the peak
   busy hour, a typical value being 60%.  These Terminations, during the
   peak hour, are this expected to contribute about one transaction per
   minute to the Media Gateway Controller load.  A reasonable algorithm
   is to make the value of MWD per "trunk" Termination six times shorter
   than the MWD per residential gateway, and also inversely proportional
   to the number of Terminations that are being restarted.  For example
   MWD should be set to 2.5 seconds for a gateway that handles a T1
   line, or to 60 milliseconds for a gateway that handles a T3 line.

10 Security Considerations

   This clause covers security when using the protocol in an IP
   environment.

10.1  Protection of Protocol Connections

   A security mechanism is clearly needed to prevent unauthorized
   entities from using the protocol defined in this RFC for setting up
   unauthorized calls or interfering with authorized calls.  The
   security mechanism for the protocol when transported over IP networks
   is IPsec [RFC 2401 to RFC 2411].

   The AH header [RFC 2402] affords data origin authentication,
   connectionless integrity and optional anti-replay protection of
   messages passed between the MG and the MGC.  The ESP header [RFC
   2406] provides confidentiality of messages, if desired.  For


Groves, et al.              Standards Track                    [Page 75]

RFC 3525                Gateway Control Protocol               June 2003


   instance, the ESP encryption service should be requested if the
   session descriptions are used to carry session keys, as defined in
   SDP.

   Implementations of the protocol defined in this RFC employing the ESP
   header SHALL comply with section 5 of [RFC 2406], which defines a
   minimum set of algorithms for integrity checking and encryption.
   Similarly, implementations employing the AH header SHALL comply with
   section 5 of [RFC 2402], which defines a minimum set of algorithms
   for integrity checking using manual keys.

   Implementations SHOULD use IKE [RFC 2409] to permit more robust
   keying options.  Implementations employing IKE SHOULD support
   authentication with RSA signatures and RSA public key encryption.

10.2  Interim AH scheme

   Implementation of IPsec requires that the AH or ESP header be
   inserted immediately after the IP header.  This cannot be easily done
   at the application level.  Therefore, this presents a deployment
   problem for the protocol defined in this RFC where the underlying
   network implementation does not support IPsec.

   As an interim solution, an optional AH header is defined within the
   H.248.1 protocol header.  The header fields are exactly those of the
   SPI, SEQUENCE NUMBER and DATA fields as defined in [RFC 2402].  The
   semantics of the header fields are the same as the "transport mode"
   of [RFC 2402], except for the calculation of the Integrity Check
   Value (ICV).  In IPsec, the ICV is calculated over the entire IP
   packet including the IP header.  This prevents spoofing of the IP
   addresses.  To retain the same functionality, the ICV calculation
   should be performed across all the transactions (concatenated) in the
   message prepended by a synthesized IP header consisting of a 32-bit
   source IP address, a 32-bit destination address and a 16-bit UDP
   destination port encoded as 20 hex digits.  When the interim AH
   mechanism is employed when TCP is the transport Layer, the UDP Port
   above becomes the TCP port, and all other operations are the same.

   Implementations of the H.248.1 protocol SHALL implement IPsec where
   the underlying operating system and the transport network supports
   IPsec.  Implementations of the protocol using IPv4 SHALL implement
   the interim AH scheme.  However, this interim scheme SHALL NOT be
   used when the underlying network layer supports IPsec.  IPv6
   implementations are assumed to support IPsec and SHALL NOT use the
   interim AH scheme.


Groves, et al.              Standards Track                    [Page 76]

RFC 3525                Gateway Control Protocol               June 2003


   All implementations of the interim AH mechanism SHALL comply with
   section 5 of RFC 2402 which defines a minimum set of algorithms for
   integrity checking using manual keys.

   The interim AH interim scheme does not provide protection against
   eavesdropping, thus forbidding third parties from monitoring the
   connections set up by a given Termination.  Also, it does not provide
   protection against replay attacks.  These procedures do not
   necessarily protect against denial of service attacks by misbehaving
   MGs or misbehaving MGCs.  However, they will provide an
   identification of these misbehaving entities, which should then be
   deprived of their authorization through maintenance procedures.

10.3  Protection of Media Connections

   The protocol allows the MGC to provide MGs with "session keys" that
   can be used to encrypt the audio messages, protecting against
   eavesdropping.

   A specific problem of packet networks is "uncontrolled barge-in".
   This attack can be performed by directing media packets to the IP
   address and UDP port used by a connection.  If no protection is
   implemented, the packets must be decompressed and the signals must be
   played on the "line side".

   A basic protection against this attack is to only accept packets from
   known sources, checking for example that the IP source address and
   UDP source port match the values announced in the Remote descriptor.
   This has two inconveniences: it slows down connection establishment
   and it can be fooled by source spoofing:

   -  To enable the address-based protection, the MGC must obtain the
      remote session description of the egress MG and pass it to the
      ingress MG.  This requires at least one network round trip, and
      leaves us with a dilemma: either allow the call to proceed without
      waiting for the round trip to complete, and risk for example,
      "clipping" a remote announcement, or wait for the full round trip
      and settle for slower call-set up procedures.

   -  Source spoofing is only effective if the attacker can obtain valid
      pairs of source destination addresses and ports, for example by
      listening to a fraction of the traffic.  To fight source spoofing,
      one could try to control all access points to the network.  But
      this is in practice very hard to achieve.


Groves, et al.              Standards Track                    [Page 77]

RFC 3525                Gateway Control Protocol               June 2003


   An alternative to checking the source address is to encrypt and
   authenticate the packets, using a secret key that is conveyed during
   the call set-up procedure.  This will not slow down the call set-up,
   and provides strong protection against address spoofing.

11 MG-MGC Control Interface

   The control association between MG and MGC is initiated at MG cold
   start, and announced by a ServiceChange message, but can be changed
   by subsequent events, such as failures or manual service events.
   While the protocol does not have an explicit mechanism to support
   multiple MGCs controlling a physical MG, it has been designed to
   support the multiple logical MG (within a single physical MG) that
   can be associated with different MGCs.

11.1  Multiple Virtual MGs

   A physical Media Gateway may be partitioned into one or more Virtual
   MGs.  A virtual MG consists of a set of statically partitioned
   physical Terminations and/or sets of ephemeral Terminations.  A
   physical Termination is controlled by one MGC.  The model does not
   require that other resources be statically allocated, just
   Terminations.  The mechanism for allocating Terminations to virtual
   MGs is a management method outside the scope of the protocol.  Each
   of the virtual MGs appears to the MGC as a complete MG client.

   A physical MG may have only one network interface, which must be
   shared across virtual MGs.  In such a case, the packet/cell side
   Termination is shared.  It should be noted however, that in use, such
   interfaces require an ephemeral instance of the Termination to be
   created per flow, and thus sharing the Termination is
   straightforward.  This mechanism does lead to a complication, namely
   that the MG must always know which of its controlling MGCs should be
   notified if an event occurs on the interface.

   In normal operation, the Virtual MG will be instructed by the MGC to
   create network flows (if it is the originating side), or to expect
   flow requests (if it is the terminating side), and no confusion will
   arise.  However, if an unexpected event occurs, the Virtual MG must
   know what to do with respect to the physical resources it is
   controlling.

   If recovering from the event requires manipulation of a physical
   interface's state, only one MGC should do so.  These issues are
   resolved by allowing any of the MGCs to create EventsDescriptors to
   be notified of such events, but only one MGC can have read/write


Groves, et al.              Standards Track                    [Page 78]

RFC 3525                Gateway Control Protocol               June 2003


   access to the physical interface properties; all other MGCs have
   read-only access.  The management mechanism is used to designate
   which MGC has read/write capability, and is designated the Master
   MGC.

   Each virtual MG has its own Root Termination.  In most cases the
   values for the properties of the Root Termination are independently
   settable by each MGC.  Where there can only be one value, the
   parameter is read-only to all but the Master MGC.

   ServiceChange may only be applied to a Termination or set of
   Terminations partitioned to the Virtual MG or created (in the case of
   ephemeral Terminations) by that Virtual MG.

11.2  Cold start

   A MG is pre-provisioned by a management mechanism outside the scope
   of this protocol with a primary and (optionally) an ordered list of
   secondary MGCs.  Upon a cold start of the MG, it will issue a
   ServiceChange command with a "Restart" method, on the Root
   Termination to its primary MGC.  If the MGC accepts the MG, it sends
   a Transaction Reply not including a ServiceChangeMgcId parameter.  If
   the MGC does not accept the MG's registration, it sends a Transaction
   Reply, providing the address of an alternate MGC to be contacted by
   including a ServiceChangeMgcId parameter.

   If the MG receives a Transaction Reply that includes a
   ServiceChangeMgcId parameter, it sends a ServiceChange to the MGC
   specified in the ServiceChangeMgcId.  It continues this process until
   it gets a controlling MGC to accept its registration, or it fails to
   get a reply.  Upon failure to obtain a reply, either from the primary
   MGC, or a designated successor, the MG tries its pre-provisioned
   secondary MGCs, in order.  If the MG is unable to establish a control
   relationship with any MGC, it shall wait a random amount of time as
   described in 9.2 and then start contacting its primary, and if
   necessary, its secondary MGCs again.

   It is possible that the reply to a ServiceChange with Restart will be
   lost, and a command will be received by the MG prior to the receipt
   of the ServiceChange response.  The MG shall issue Error 505 -
   Command Received before a ServiceChange Reply has been received.

11.3  Negotiation of protocol version

   The first ServiceChange command from a MG shall contain the version
   number of the protocol supported by the MG in the
   ServiceChangeVersion parameter.  Upon receiving such a message, if
   the MGC supports only a lower version, then the MGC shall send a


Groves, et al.              Standards Track                    [Page 79]

RFC 3525                Gateway Control Protocol               June 2003


   ServiceChangeReply with the lower version and thereafter all the
   messages between MG and MGC shall conform to the lower version of the
   protocol.  If the MG is unable to comply and it has established a
   transport connection to the MGC, it should close that connection.  In
   any event, it should reject all subsequent requests from the MGC with
   error 406 - Version Not Supported.

   If the MGC supports a higher version than the MG but is able to
   support the lower version proposed by the MG, it shall send a
   ServiceChangeReply with the lower version and thereafter all the
   messages between MG and MGC shall conform to the lower version of the
   protocol.  If the MGC is unable to comply, it shall reject the
   association, with error 406 - Version Not Supported.

   Protocol version negotiation may also occur at "handoff" and
   "failover" ServiceChanges.

   When extending the protocol with new versions, the following rules
   should be followed:

   1) Existing protocol elements, i.e., procedures, parameters,
      descriptor, property, values, should not be changed unless a
      protocol error needs to be corrected or it becomes necessary to
      change the operation of the service that is being supported by the
      protocol.

   2) The semantics of a command, a parameter, a descriptor, a property,
      or a value should not be changed.

   3) Established rules for formatting and encoding messages and
      parameters should not be modified.

   4) When information elements are found to be obsolete they can be
      marked as not used.  However, the identifier for that information
      element will be marked as reserved.  In that way it can not be
      used in future versions.

11.4  Failure of a MG

   If a MG fails, but is capable of sending a message to the MGC, it
   sends a ServiceChange with an appropriate method (graceful or forced)
   and specifies the Root TerminationID.  When it returns to service, it
   sends a ServiceChange with a "Restart" method.

   Allowing the MGC to send duplicate messages to both MGs accommodates
   pairs of MGs that are capable of redundant failover of one of the
   MGs.  Only the Working MG shall accept or reject transactions.  Upon
   failover, the primary MG sends a ServiceChange command with a


Groves, et al.              Standards Track                    [Page 80]

RFC 3525                Gateway Control Protocol               June 2003


   "Failover" method and a "MG Impending Failure" reason.  The MGC then
   uses the secondary MG as the active MG.  When the error condition is
   repaired, the Working MG can send a "ServiceChange" with a "Restart"
   method.

     Note: Redundant failover MGs require a reliable transport, because
     the protocol provides no means for a secondary MG running ALF to
     acknowledge messages sent from the MGC.

11.5  Failure of an MGC

   If the MG detects a failure of its controlling MGC, it attempts to
   contact the next MGC on its pre-provisioned list.  It starts its
   attempts at the beginning (primary MGC), unless that was the MGC that
   failed, in which case it starts at its first secondary MGC.  It sends
   a ServiceChange message with a "Failover" method and a "MGC Impending
   Failure" reason.  If the MG is unable to establish a control
   relationship with any MGC, it shall wait a random amount of time as
   described in section 9.2 and then start again contacting its primary,
   and (if necessary) its secondary MGCs.  When contacting its
   previously controlling MGC, the MG sends the ServiceChange message
   with "Disconnected" method.

   In partial failure, or for manual maintenance reasons, an MGC may
   wish to direct its controlled MGs to use a different MGC.  To do so,
   it sends a ServiceChange method to the MG with a "HandOff" method,
   and its designated replacement in ServiceChangeMgcId.  If "HandOff"
   is supported, the MG shall send a ServiceChange message with a
   "Handoff" method and a "MGC directed change" reason to the designated
   MGC.  If it fails to get a reply from the designated MGC, the MG
   shall behave as if its MGC failed, and start contacting secondary
   MGCs as specified in the previous paragraph.  If the MG is unable to
   establish a control relationship with any MGC, it shall wait a random
   amount of time as described in 9.2 and then start contacting its
   primary, and if necessary, its secondary MGCs again.

   No recommendation is made on how the MGCs involved in the Handoff
   maintain state information; this is considered to be out of scope of
   this RFC.  The MGC and MG may take the following steps when Handoff
   occurs.  When the MGC initiates a HandOff, the handover should be
   transparent to Operations on the Media Gateway.  Transactions can be
   executed in any order, and could be in progress when the
   ServiceChange is executed.  Accordingly, commands in progress
   continue and replies to all commands from the original MGC must be
   sent to the transport address from which they were sent.  If the
   service relationship with the sending MGC has ended, the replies
   should be discarded.  The MG may receive outstanding transaction
   replies from the new MGC.  No new messages shall be sent to the new


Groves, et al.              Standards Track                    [Page 81]

RFC 3525                Gateway Control Protocol               June 2003


   MGC until the control association is established.  Repeated
   transaction requests shall be directed to the new MGC.  The MG shall
   maintain state on all Terminations and Contexts.

   It is possible that the MGC could be implemented in such a way that a
   failed MGC is replaced by a working MGC where the identity of the new
   MGC is the same as the failed one.  In such a case,
   ServiceChangeMgcId would be specified with the previous value and the
   MG shall behave as if the value was changed, and send a ServiceChange
   message, as above.

   Pairs of MGCs that are capable of redundant failover can notify the
   controlled MGs of the failover by the above mechanism.

12 Package definition

   The primary mechanism for extension is by means of Packages.
   Packages define additional Properties, Events, Signals and Statistics
   that may occur on Terminations.

   Packages defined by IETF will appear in separate RFCs.

   Packages defined by ITU-T may appear in the relevant Recommendations
   (e.g., as Recommendations of the H.248 sub-series).

   1) A public document or a standard forum document, which can be
      referenced as the document that describes the package following
      the guideline above, should be specified.

   2) The document shall specify the version of the Package that it
      describes.

   3) The document should be available on a public web server and should
      have a stable URL.  The site should provide a mechanism to provide
      comments and appropriate responses should be returned.

12.1  Guidelines for defining packages

   Packages define Properties, Events, Signals, and Statistics.

   Packages may also define new error codes according to the guidelines
   given in 13.2.  This is a matter of documentary convenience: the
   package documentation is submitted to IANA in support of the error
   code registration.  If a package is modified, it is unnecessary to
   provide IANA with a new document reference in support of the error
   code unless the description of the error code itself is modified.


Groves, et al.              Standards Track                    [Page 82]

RFC 3525                Gateway Control Protocol               June 2003


   Names of all such defined constructs shall consist of the PackageID
   (which uniquely identifies the package) and the ID of the item (which
   uniquely identifies the item in that package).  In the text encoding
   the two shall be separated by a forward slash ("/") character.
   Example: togen/playtone is the text encoding to refer to the play
   tone signal in the tone generation package.

   A Package will contain the following sections:

12.1.1   Package

   Overall description of the package, specifying:

      Package Name: only descriptive

      PackageID: is an identifier

      Description:

      Version:

         A new version of a package can only add additional Properties,
         Events, Signals, Statistics and new possible values for an
         existing parameter described in the original package.  No
         deletions or modifications shall be allowed.  A version is an
         integer in the range from 1 to 99.

      Designed to be extended only (Optional):

         This indicates that the package has been expressly designed to
         be extended by others, not to be directly referenced.  For
         example, the package may not have any function on its own or be
         nonsensical on its own.  The MG SHOULD NOT publish this
         PackageID when reporting packages.

      Extends (Optional): existing package Descriptor

         A package may extend an existing package.  The version of the
         original package must be specified.  When a package extends
         another package it shall only add additional Properties,
         Events, Signals, Statistics and new possible values for an
         existing parameter described in the original package.  An
         extended package shall not redefine or overload an identifier
         defined in the original package and packages it may have
         extended (multiple levels of extension).  Hence, if package B
         version 1 extends package A version 1, version 2 of B will not
         be able to extend the A version 2 if A version 2 defines a name
         already in B version 1.


Groves, et al.              Standards Track                    [Page 83]

RFC 3525                Gateway Control Protocol               June 2003


12.1.2   Properties

   Properties defined by the package, specifying:

      Property Name: only descriptive

      PropertyID: is an identifier

      Description:

      Type: One of:

         Boolean

         String: UTF-8 string

         Octet String: A number of octets.  See Annex A and Annex B.3
         for encoding

         Integer: 4 byte signed integer

         Double: 8 byte signed integer

         Character: unicode UTF-8 encoding of a single letter.  Could be
         more than one octet.

         Enumeration: one of a list of possible unique values (see 12.3)

         Sub-list: a list of several values from a list.  The type of
         sub-list SHALL also be specified.  The type shall be chosen
         from the types specified in this section (with the exception of
         sub-list).  For example, Type: sub-list of enumeration.  The
         encoding of sub-lists is specified in Annexes A and B.3.

      Possible values:

         A package MUST specify either a specific set of values or a
         description of how values are determined.  A package MUST also
         specify a default value or the default behaviour when the value
         is omitted from its descriptor.  For example, a package may
         specify that procedures related to the property are suspended
         when its value is omitted.  A default value (but not
   procedures)
         may be specified as provisionable.

      Defined in:

         Which H.248.1 descriptor the property is defined in.


Groves, et al.              Standards Track                    [Page 84]

RFC 3525                Gateway Control Protocol               June 2003


         LocalControl is for stream dependent properties.
         TerminationState is for stream independent properties.  These
         are expected to be the most common cases, but it is possible
         for properties to be defined in other descriptors.

      Characteristics: Read/Write or both, and (optionally), global:

         Indicates whether a property is read-only, or read-write, and
         if it is global.  If Global is omitted, the property is not
         global.  If a property is declared as global, the value of the
         property is shared by all Terminations realizing the package.

12.1.3   Events

   Events defined by the package, specifying:

      Event name: only descriptive

      EventID: is an identifier

      Description:

      EventsDescriptor Parameters:

         Parameters used by the MGC to configure the event, and found in
         the EventsDescriptor.  See 12.2.

      ObservedEventsDescriptor Parameters:

         Parameters returned to the MGC in Notify requests and in
         replies to command requests from the MGC that audit
         ObservedEventsDescriptor, and found in the
         ObservedEventsDescriptor.  See 12.2.

12.1.4   Signals

   Signals defined by the package, specifying:

      Signal Name: only descriptive

      SignalID: is an identifier.  SignalID is used in a
      SignalsDescriptor

      Description

      SignalType: one of:

         OO (On/Off)


Groves, et al.              Standards Track                    [Page 85]

RFC 3525                Gateway Control Protocol               June 2003


         TO (TimeOut)

         BR (Brief)

      NOTE - SignalType may be defined such that it is dependent on the
      value of one or more parameters.  The package MUST specify a
      default signal type.  If the default type is TO, the package MUST
      specify a default duration which may be provisioned.  A default
      duration is meaningless for BR.

      Duration: in hundredths of seconds

      Additional Parameters: see 12.2

12.1.5   Statistics

   Statistics defined by the package, specifying:

      Statistic name: only descriptive

      StatisticID: is an identifier

      StatisticID is used in a StatisticsDescriptor

      Description:

      Units: unit of measure, e.g., milliseconds, packets

12.1.6   Procedures

   Additional guidance on the use of the package.

12.2  Guidelines to defining Parameters to Events and Signals

   Parameter Name: only descriptive

   ParameterID: is an identifier.  The textual ParameterID of parameters
   to Events and Signals shall not start with "EPA" and "SPA",
   respectively.  The textual ParameterID shall also not be "ST",
   "Stream", "SY", "SignalType", "DR", "Duration", "NC",
   "NotifyCompletion", "KA", "Keepactive", "EB", "Embed", "DM" or
   "DigitMap".

   Type: One of:

      Boolean

      String: UTF-8 octet string


Groves, et al.              Standards Track                    [Page 86]

RFC 3525                Gateway Control Protocol               June 2003


      Octet String: A number of octets.  See Annex A and Annex B.3 for
      encoding

      Integer: 4-octet signed integer

      Double: 8-octet signed integer

      Character: unicode UTF-8 encoding of a single letter.  Could be
      more than one octet.

      Enumeration: one of a list of possible unique values (see 12.3)

      Sub-list: a list of several values from a list (not supported for
      statistics).  The type of sub-list SHALL also be specified.  The
      type shall be chosen from the types specified in this section
      (with the exception of sub-list).  For example, Type: sub-list of
      enumeration.  The encoding of sub-lists is specified in Annexes A
      and B.3.

   Possible values:

      A package MUST specify either a specific set of values or a
      description of how values are determined.  A package MUST also
      specify a default value or the default behavior when the value is
      omitted from its descriptor.  For example, a package may specify
      that procedures related to the parameter are suspended when it
      value is omitted.  A default value (but not procedures) may be
      specified as provisionable.

   Description:

12.3  Lists

   Possible values for parameters include enumerations.  Enumerations
   may be defined in a list.  It is recommended that the list be IANA
   registered so that packages that extend the list can be defined
   without concern for conflicting names.

12.4  Identifiers

   Identifiers in text encoding shall be strings of up to 64 characters,
   containing no spaces, starting with an alphabetic character and
   consisting of alphanumeric characters and/or digits, and possibly
   including the special character underscore ("_").


Groves, et al.              Standards Track                    [Page 87]

RFC 3525                Gateway Control Protocol               June 2003


   Identifiers in binary encoding are 2 octets long.

   Both text and binary values shall be specified for each identifier,
   including identifiers used as values in enumerated types.

12.5  Package registration

   A package can be registered with IANA for interoperability reasons.
   See clause 13 for IANA Considerations.

13 IANA Considerations

13.1  Packages

   The following considerations SHALL be met to register a package with
   IANA:

   1) A unique string name, unique serial number and version number is
      registered for each package.  The string name is used with text
      encoding.  The serial number shall be used with binary encoding.
      Serial Numbers 0x8000 to 0xFFFF are reserved for private use.
      Serial number 0 is reserved.

   2) A contact name, email and postal addresses for that contact shall
      be specified.  The contact information shall be updated by the
      defining organization as necessary.

   3) A reference to a document that describes the package, which should
      be public:

      The document shall specify the version of the Package that it
      describes.

      If the document is public, it should be located on a public web
      server and should have a stable URL.  The site should provide a
      mechanism to provide comments and appropriate responses should be
      returned.

   4) Packages registered by other than recognized standards bodies
      shall have a minimum package name length of 8 characters.

   5) All other package names are first come-first served if all other
      conditions are met.


Groves, et al.              Standards Track                    [Page 88]

RFC 3525                Gateway Control Protocol               June 2003


13.2  Error codes

   The following considerations SHALL be met to register an error code
   with IANA:

   1) An error number and a one-line (80-character maximum) string is
      registered for each error.

   2) A complete description of the conditions under which the error is
      detected shall be included in a publicly available document.  The
      description shall be sufficiently clear to differentiate the error
      from all other existing error codes.

   3) The document should be available on a public web server and should
      have a stable URL.

   4) Error numbers registered by recognized standards bodies shall have
      3- or 4-character error numbers.

   5) Error numbers registered by all other organizations or individuals
      shall have 4-character error numbers.

   6) An error number shall not be redefined nor modified except by the
      organization or individual that originally defined it, or their
      successors or assigns.

13.3  ServiceChange reasons

   The following considerations SHALL be met to register service change
   reason with IANA:

   1) A one-phrase, 80-character maximum, unique reason code is
      registered for each reason.

   2) A complete description of the conditions under which the reason is
      used is detected shall be included in a publicly available
      document.  The description shall be sufficiently clear to
      differentiate the reason from all other existing reasons.

   3) The document should be available on a public web server and should
      have a stable URL.


Groves, et al.              Standards Track                    [Page 89]

RFC 3525                Gateway Control Protocol               June 2003


ANNEX A - Binary encoding of the protocol

   This annex specifies the syntax of messages using the notation
   defined in Recommendation X.680; Information technology - Abstract
   Syntax Notation One (ASN.1): Specification of basic notation.
   Messages shall be encoded for transmission by applying the basic
   encoding rules specified in Recommendation X.690, Information
   Technology - ASN.1 Encoding Rules: Specification of Basic Encoding
   Rules (BER), Canonical Encoding Rules (CER) and Distinguished
   Encoding Rules.

A.1   Coding of wildcards

   The use of wildcards ALL and CHOOSE is allowed in the protocol.  This
   allows a MGC to partially specify Termination IDs and to let the MG
   choose from the values that conform to the partial specification.
   Termination IDs may encode a hierarchy of names.  This hierarchy is
   provisioned.  For instance, a TerminationID may consist of a trunk
   group, a trunk within the group and a circuit.  Wildcarding must be
   possible at all levels.  The following paragraphs explain how this is
   achieved.

   The ASN.1 description uses octet strings of up to 8 octets in length
   for Termination IDs.  This means that Termination IDs consist of at
   most 64 bits.  A fully specified Termination ID may be preceded by a
   sequence of wildcarding fields.  A wildcarding field is one octet in
   length.  Bit 7 (the most significant bit) of this octet specifies
   what type of wildcarding is invoked: if the bit value equals 1, then
   the ALL wildcard is used; if the bit value if 0, then the CHOOSE
   wildcard is used.  Bit 6 of the wildcarding field specifies whether
   the wildcarding pertains to one level in the hierarchical naming
   scheme (bit value 0) or to the level of the hierarchy specified in
   the wildcarding field plus all lower levels (bit value 1).  Bits 0
   through 5 of the wildcarding field specify the bit position in the
   Termination ID at which the wildcarding starts.

   We illustrate this scheme with some examples.  In these examples, the
   most significant bit in a string of bits appears on the left hand
   side.

   Assume that Termination IDs are three octets long and that each octet
   represents a level in a hierarchical naming scheme.  A valid
   Termination ID is:

      00000001 00011110 01010101.


Groves, et al.              Standards Track                    [Page 90]

RFC 3525                Gateway Control Protocol               June 2003


   Addressing ALL names with prefix 00000001 00011110 is done as
   follows:

      wildcarding field: 10000111

      Termination ID: 00000001 00011110 xxxxxxxx.

   The values of the bits labeled "x" is irrelevant and shall be ignored
   by the receiver.

   Indicating to the receiver that it must choose a name with 00011110
   as the second octet is done as follows:

      wildcarding fields: 00010111 followed by 00000111

      Termination ID: xxxxxxxx 00011110 xxxxxxxx.

   The first wildcard field indicates a CHOOSE wildcard for the level in
   the naming hierarchy starting at bit 23, the highest level in our
   assumed naming scheme.  The second wildcard field indicates a CHOOSE
   wildcard for the level in the naming hierarchy starting at bit 7, the
   lowest level in our assumed naming scheme.

   Finally, a CHOOSE-wildcarded name with the highest level of the name
   equal to 00000001 is specified as follows:

      wildcard field: 01001111

      Termination ID: 0000001 xxxxxxxx xxxxxxxx .

   Bit value 1 at bit position 6 of the first octet of the wildcard
   field indicates that the wildcarding pertains to the specified level
   in the naming hierarchy and all lower levels.

   Context IDs may also be wildcarded.  In the case of Context IDs,
   however, specifying partial names is not allowed.  Context ID 0x0
   SHALL be used to indicate the NULL Context, Context ID 0xFFFFFFFE
   SHALL be used to indicate a CHOOSE wildcard, and Context ID
   0xFFFFFFFF SHALL be used to indicate an ALL wildcard.

   TerminationID 0xFFFFFFFFFFFFFFFF SHALL be used to indicate the ROOT
   Termination.


Groves, et al.              Standards Track                    [Page 91]

RFC 3525                Gateway Control Protocol               June 2003


A.2   ASN.1 syntax specification

   This subclause contains the ASN.1 specification of the H.248.1
   protocol syntax.

     NOTE 1 - In case a transport mechanism is used that employs
     application level framing, the definition of Transaction below
     changes.  Refer to the annex or to the Recommendation of the H.248
     sub-series defining the transport mechanism for the definition that
     applies in that case.

     NOTE 2 - The ASN.1 specification below contains a clause defining
     TerminationIDList as a sequence of TerminationIDs.  The length of
     this sequence SHALL be one, except possibly when used in
     contextAuditResult.

     NOTE 3 - This syntax specification does not enforce all
     restrictions on element inclusions and values.  Some additional
     restrictions are stated in comments and other restrictions appear
     in the text of this RFC.  These additional restrictions
     are part of the protocol even though not enforced by this
     specification.

     NOTE 4 - The ASN.1 module in this Annex uses octet string types to
     encode values for property parameter, signal parameter and event
     parameter values and statistics.  The actual types of these values
     vary and are specified in Annex C or the relevant package
     definition.

   A value is first BER-encoded based on its type using the table below.
   The result of this BER-encoding is then encoded as an ASN.1 octet
   string, "double wrapping" the value.  The format specified in Annex C
   or the package relates to BER encoding according to the following
   table:

   Type Specified in Package   ASN.1 BER Type

   String                      IA5String or UTF8String (Note 4)

   Integer (4 Octet)           INTEGER

   Double (8 octet signed int) INTEGER (Note 3)

   Character (UTF-8, Note 1)   IA5String

   Enumeration                 ENUMERATED

   Boolean                     BOOLEAN


Groves, et al.              Standards Track                    [Page 92]

RFC 3525                Gateway Control Protocol               June 2003


   Unsigned Integer (Note 2)   INTEGER  (Note 3)

   Octet (String)              OCTET STRING

     Note 1: Can be more than one byte

     Note 2: Unsigned integer is referenced in Annex C

     Note 3: The BER encoding of INTEGER does not imply the use of 4
     bytes.

     Note 4: String should be encoded as IA5String when the contents
     are all ASCII characters, but as UTF8String if it contains any
     Non-ASCII characters.

   See ITU-T Rec.  X.690, 8.7, for the definition of the encoding of an
   octet string value.

   MEDIA-GATEWAY-CONTROL DEFINITIONS AUTOMATIC TAGS::=
   BEGIN

   MegacoMessage ::= SEQUENCE
   {
      authHeader     AuthenticationHeader OPTIONAL,
      mess           Message
   }

   AuthenticationHeader ::= SEQUENCE
   {
      secParmIndex   SecurityParmIndex,
      seqNum         SequenceNum,
      ad             AuthData
   }

   SecurityParmIndex ::= OCTET STRING(SIZE(4))

   SequenceNum       ::= OCTET STRING(SIZE(4))

   AuthData          ::= OCTET STRING (SIZE (12..32))

   Message ::= SEQUENCE
   {
      version           INTEGER(0..99),
      -- The version of the protocol defined here is equal to 1.
      mId               MId,  -- Name/address of message originator
      messageBody       CHOICE
      {
         messageError      ErrorDescriptor,


Groves, et al.              Standards Track                    [Page 93]

RFC 3525                Gateway Control Protocol               June 2003


         transactions      SEQUENCE OF Transaction
      },
      ...
   }

   MId ::= CHOICE
   {
      ip4Address           IP4Address,
      ip6Address           IP6Address,
      domainName           DomainName,
      deviceName           PathName,
      mtpAddress           OCTET STRING(SIZE(2..4)),
      -- Addressing structure of mtpAddress:
      --     25 - 15           0
      --        |  PC        | NI |
      --      24 - 14 bits    2 bits
      -- Note: 14 bits are defined for international use.
      -- Two national options exist where the point code is 16 or 24
      -- bits.
      -- To octet align the mtpAddress, the MSBs shall be encoded as 0s.
             ...
   }

   DomainName ::= SEQUENCE
   {
      name        IA5String,
      -- The name starts with an alphanumeric digit followed by a
      -- sequence of alphanumeric digits, hyphens and dots.  No two
      -- dots shall occur consecutively.
      portNumber     INTEGER(0..65535) OPTIONAL
   }

   IP4Address ::= SEQUENCE
   {
      address        OCTET STRING (SIZE(4)),
      portNumber     INTEGER(0..65535) OPTIONAL
   }

   IP6Address ::= SEQUENCE
   {
      address        OCTET STRING (SIZE(16)),
      portNumber     INTEGER(0..65535) OPTIONAL
   }

   PathName ::= IA5String(SIZE (1..64))
   -- See A.3

   Transaction ::= CHOICE


Groves, et al.              Standards Track                    [Page 94]

RFC 3525                Gateway Control Protocol               June 2003


   {
      transactionRequest   TransactionRequest,
      transactionPending   TransactionPending,
      transactionReply     TransactionReply,
      transactionResponseAck  TransactionResponseAck,
          -- use of response acks is dependent on underlying transport
      ...
   }

   TransactionId ::= INTEGER(0..4294967295)  -- 32-bit unsigned integer

   TransactionRequest ::= SEQUENCE
   {
      transactionId        TransactionId,
      actions              SEQUENCE OF ActionRequest,
      ...
   }

   TransactionPending ::= SEQUENCE
   {
      transactionId        TransactionId,
      ...
   }

   TransactionReply ::= SEQUENCE
   {
      transactionId        TransactionId,
      immAckRequired       NULL OPTIONAL,
      transactionResult    CHOICE
      {
           transactionError   ErrorDescriptor,
           actionReplies      SEQUENCE OF ActionReply
      },
      ...
   }

   TransactionResponseAck ::= SEQUENCE OF TransactionAck
   TransactionAck ::= SEQUENCE
   {
      firstAck       TransactionId,
      lastAck        TransactionId OPTIONAL
   }

   ErrorDescriptor ::= SEQUENCE
   {
      errorCode      ErrorCode,
      errorText      ErrorText OPTIONAL
   }


Groves, et al.              Standards Track                    [Page 95]

RFC 3525                Gateway Control Protocol               June 2003


   ErrorCode ::= INTEGER(0..65535)
   -- See clause 13 for IANA Considerations with respect to error codes

   ErrorText ::= IA5String

   ContextID ::= INTEGER(0..4294967295)

   -- Context   NULL Value: 0
   -- Context CHOOSE Value: 4294967294 (0xFFFFFFFE)
   -- Context    ALL Value: 4294967295 (0xFFFFFFFF)


   ActionRequest ::= SEQUENCE
   {
      contextId         ContextID,
      contextRequest       ContextRequest OPTIONAL,
      contextAttrAuditReq  ContextAttrAuditRequest OPTIONAL,
      commandRequests   SEQUENCE OF CommandRequest
   }

   ActionReply ::= SEQUENCE
   {
      contextId         ContextID,
      errorDescriptor   ErrorDescriptor OPTIONAL,
      contextReply      ContextRequest OPTIONAL,
      commandReply      SEQUENCE OF CommandReply
   }

   ContextRequest ::= SEQUENCE
   {
      priority       INTEGER(0..15) OPTIONAL,
      emergency      BOOLEAN OPTIONAL,
      topologyReq    SEQUENCE OF TopologyRequest OPTIONAL,
      ...
   }

   ContextAttrAuditRequest ::= SEQUENCE
   {
      topology    NULL OPTIONAL,
      emergency   NULL OPTIONAL,
      priority    NULL OPTIONAL,
      ...
   }

   CommandRequest ::= SEQUENCE
   {
      command           Command,


Groves, et al.              Standards Track                    [Page 96]

RFC 3525                Gateway Control Protocol               June 2003


      optional          NULL OPTIONAL,
      wildcardReturn    NULL OPTIONAL,
      ...
   }

   Command ::= CHOICE
   {
      addReq               AmmRequest,
      moveReq              AmmRequest,
      modReq               AmmRequest,
      -- Add, Move, Modify requests have the same parameters
      subtractReq          SubtractRequest,
      auditCapRequest      AuditRequest,
      auditValueRequest    AuditRequest,
      notifyReq            NotifyRequest,
      serviceChangeReq     ServiceChangeRequest,
      ...
   }

   CommandReply ::= CHOICE
   {
      addReply                AmmsReply,
      moveReply               AmmsReply,
      modReply                AmmsReply,
      subtractReply           AmmsReply,
      -- Add, Move, Modify, Subtract replies have the same parameters
      auditCapReply           AuditReply,
      auditValueReply         AuditReply,
      notifyReply             NotifyReply,
      serviceChangeReply      ServiceChangeReply,
      ...
   }

   TopologyRequest ::= SEQUENCE
   {
      terminationFrom         TerminationID,
      terminationTo           TerminationID,
      topologyDirection       ENUMERATED
      {
         bothway(0),
         isolate(1),
         oneway(2)
      },
      ...
   }

   AmmRequest ::= SEQUENCE
   {


Groves, et al.              Standards Track                    [Page 97]

RFC 3525                Gateway Control Protocol               June 2003


      terminationID        TerminationIDList,
      descriptors          SEQUENCE OF AmmDescriptor,
      -- At most one descriptor of each type (see AmmDescriptor)
      -- allowed in the sequence.
      ...
   }

   AmmDescriptor ::= CHOICE
   {
      mediaDescriptor         MediaDescriptor,
      modemDescriptor         ModemDescriptor,
      muxDescriptor           MuxDescriptor,
      eventsDescriptor        EventsDescriptor,
      eventBufferDescriptor   EventBufferDescriptor,
      signalsDescriptor       SignalsDescriptor,
      digitMapDescriptor      DigitMapDescriptor,
      auditDescriptor         AuditDescriptor,
      ...
   }

   AmmsReply ::= SEQUENCE
   {
      terminationID        TerminationIDList,
      terminationAudit     TerminationAudit OPTIONAL,
      ...
   }

   SubtractRequest ::= SEQUENCE
   {
      terminationID        TerminationIDList,
      auditDescriptor      AuditDescriptor OPTIONAL,
      ...
   }

   AuditRequest ::= SEQUENCE
   {
      terminationID        TerminationID,
      auditDescriptor      AuditDescriptor,
      ...
   }

   AuditReply ::= CHOICE
   {
      contextAuditResult   TerminationIDList,
      error                ErrorDescriptor,
      auditResult          AuditResult,
      ...
   }


Groves, et al.              Standards Track                    [Page 98]

RFC 3525                Gateway Control Protocol               June 2003


   AuditResult ::= SEQUENCE
   {

      terminationID           TerminationID,
      terminationAuditResult  TerminationAudit
   }

   TerminationAudit ::= SEQUENCE OF AuditReturnParameter

   AuditReturnParameter ::= CHOICE
   {
      errorDescriptor         ErrorDescriptor,
      mediaDescriptor         MediaDescriptor,
      modemDescriptor         ModemDescriptor,
      muxDescriptor           MuxDescriptor,
      eventsDescriptor        EventsDescriptor,
      eventBufferDescriptor   EventBufferDescriptor,
      signalsDescriptor       SignalsDescriptor,
      digitMapDescriptor      DigitMapDescriptor,
      observedEventsDescriptor   ObservedEventsDescriptor,
      statisticsDescriptor    StatisticsDescriptor,
      packagesDescriptor      PackagesDescriptor,
      emptyDescriptors        AuditDescriptor,
      ...
   }

   AuditDescriptor ::= SEQUENCE
   {
      auditToken  BIT STRING
         {
            muxToken(0), modemToken(1), mediaToken(2),
            eventsToken(3), signalsToken(4),
            digitMapToken(5), statsToken(6),
            observedEventsToken(7),
            packagesToken(8), eventBufferToken(9)
         } OPTIONAL,
      ...
   }

   NotifyRequest ::= SEQUENCE
   {
      terminationID              TerminationIDList,
      observedEventsDescriptor   ObservedEventsDescriptor,
      errorDescriptor            ErrorDescriptor OPTIONAL,
      ...
   }


Groves, et al.              Standards Track                    [Page 99]

RFC 3525                Gateway Control Protocol               June 2003


   NotifyReply ::= SEQUENCE
   {
      terminationID           TerminationIDList,
      errorDescriptor         ErrorDescriptor OPTIONAL,
      ...
   }

   ObservedEventsDescriptor ::= SEQUENCE
   {
      requestId               RequestID,
      observedEventLst        SEQUENCE OF ObservedEvent
   }

   ObservedEvent ::= SEQUENCE
   {
      eventName            EventName,
      streamID             StreamID OPTIONAL,
      eventParList         SEQUENCE OF EventParameter,
      timeNotation         TimeNotation OPTIONAL,
      ...
   }

   EventName ::= PkgdName

   EventParameter ::= SEQUENCE
   {
      eventParameterName      Name,
      value                   Value,
   -- For use of extraInfo see the comment related to PropertyParm
      extraInfo CHOICE
      {
         relation Relation,
         range    BOOLEAN,
         sublist  BOOLEAN
      } OPTIONAL,
      ...
   }

   ServiceChangeRequest ::= SEQUENCE
   {
      terminationID           TerminationIDList,
      serviceChangeParms      ServiceChangeParm,
      ...
   }

   ServiceChangeReply ::= SEQUENCE
   {
      terminationID           TerminationIDList,


Groves, et al.              Standards Track                   [Page 100]

RFC 3525                Gateway Control Protocol               June 2003


      serviceChangeResult     ServiceChangeResult,
      ...
   }

   -- For ServiceChangeResult, no parameters are mandatory.  Hence the
   -- distinction between ServiceChangeParm and ServiceChangeResParm.

   ServiceChangeResult ::= CHOICE
   {
      errorDescriptor            ErrorDescriptor,
      serviceChangeResParms      ServiceChangeResParm
   }

   WildcardField ::= OCTET STRING(SIZE(1))

   TerminationID ::= SEQUENCE
   {
      wildcard SEQUENCE OF WildcardField,
      id    OCTET STRING(SIZE(1..8)),
      ...
   }
   -- See A.1 for explanation of wildcarding mechanism.
   -- Termination ID 0xFFFFFFFFFFFFFFFF indicates the ROOT Termination.

   TerminationIDList ::= SEQUENCE OF TerminationID

   MediaDescriptor ::= SEQUENCE
   {

      termStateDescr TerminationStateDescriptor OPTIONAL,
      streams     CHOICE
      {
         oneStream      StreamParms,
         multiStream    SEQUENCE OF StreamDescriptor
      } OPTIONAL,
      ...
   }

   StreamDescriptor ::= SEQUENCE
   {
      streamID          StreamID,
      streamParms       StreamParms
   }

   StreamParms ::= SEQUENCE
   {
      localControlDescriptor     LocalControlDescriptor OPTIONAL,
      localDescriptor            LocalRemoteDescriptor OPTIONAL,


Groves, et al.              Standards Track                   [Page 101]

RFC 3525                Gateway Control Protocol               June 2003


      remoteDescriptor           LocalRemoteDescriptor OPTIONAL,
      ...
   }

   LocalControlDescriptor ::= SEQUENCE
   {

      streamMode        StreamMode OPTIONAL,
      reserveValue      BOOLEAN OPTIONAL,
      reserveGroup      BOOLEAN OPTIONAL,
      propertyParms     SEQUENCE OF PropertyParm,
      ...
   }

   StreamMode ::= ENUMERATED
   {
      sendOnly(0),
      recvOnly(1),
      sendRecv(2),
      inactive(3),
      loopBack(4),
         ...
   }

   -- In PropertyParm, value is a SEQUENCE OF octet string.  When sent
   -- by an MGC the interpretation is as follows:
   -- empty sequence means CHOOSE
   -- one element sequence specifies value
   -- If the sublist field is not selected, a longer sequence means
   -- "choose one of the values" (i.e., value1 OR value2 OR ...)
   -- If the sublist field is selected,
   -- a sequence with more than one element encodes the value of a
   -- list-valued property (i.e., value1 AND value2 AND ...).
   -- The relation field may only be selected if the value sequence
   -- has length 1.  It indicates that the MG has to choose a value
   -- for the property.  E.g., x > 3 (using the greaterThan
   -- value for relation) instructs the MG to choose any value larger
   -- than 3 for property x.
   -- The range field may only be selected if the value sequence
   -- has length 2.  It indicates that the MG has to choose a value
   -- in the range between the first octet in the value sequence and
   -- the trailing octet in the value sequence, including the
   -- boundary values.
   -- When sent by the MG, only responses to an AuditCapability request
   -- may contain multiple values, a range, or a relation field.

   PropertyParm ::= SEQUENCE
   {


Groves, et al.              Standards Track                   [Page 102]

RFC 3525                Gateway Control Protocol               June 2003


      name        PkgdName,
      value       SEQUENCE OF OCTET STRING,
      extraInfo   CHOICE
      {
         relation    Relation,
         range       BOOLEAN,
         sublist     BOOLEAN
      } OPTIONAL,
      ...
   }

   Name ::= OCTET STRING(SIZE(2))

   PkgdName ::= OCTET STRING(SIZE(4))
   -- represents Package Name (2 octets) plus Property, Event,
   -- Signal Names or Statistics ID. (2 octets)
   -- To wildcard a package use 0xFFFF for first two octets, choose
   -- is not allowed.  To reference native property tag specified in
   -- Annex C, use 0x0000 as first two octets.
   -- To wildcard a Property, Event, Signal, or Statistics ID, use
   -- 0xFFFF for last two octets, choose is not allowed.
   -- Wildcarding of Package Name is permitted only if Property,
   -- Event, Signal, or Statistics ID are
   -- also wildcarded.

   Relation ::= ENUMERATED
   {
      greaterThan(0),
      smallerThan(1),
      unequalTo(2),
      ...
   }

   LocalRemoteDescriptor ::= SEQUENCE
   {
      propGrps SEQUENCE OF PropertyGroup,
      ...
   }

   PropertyGroup ::= SEQUENCE OF PropertyParm

   TerminationStateDescriptor ::= SEQUENCE
   {
      propertyParms        SEQUENCE OF PropertyParm,
      eventBufferControl   EventBufferControl OPTIONAL,
      serviceState         ServiceState OPTIONAL,
      ...
   }


Groves, et al.              Standards Track                   [Page 103]

RFC 3525                Gateway Control Protocol               June 2003


   EventBufferControl ::= ENUMERATED
   {
      off(0),
      lockStep(1),
      ...
   }

   ServiceState ::= ENUMERATED

   {
      test(0),
      outOfSvc(1),
      inSvc(2),
         ...
   }

   MuxDescriptor   ::= SEQUENCE
   {
      muxType           MuxType,
      termList          SEQUENCE OF TerminationID,
      nonStandardData   NonStandardData OPTIONAL,
      ...
   }

   MuxType ::= ENUMERATED
   {
      h221(0),
      h223(1),
      h226(2),
      v76(3),
      ...
   }

   StreamID ::= INTEGER(0..65535)   -- 16-bit unsigned integer

   EventsDescriptor ::= SEQUENCE
   {
      requestID      RequestID OPTIONAL,
                  -- RequestID must be present if eventList
                  -- is non empty
      eventList      SEQUENCE OF RequestedEvent,
      ...
   }

   RequestedEvent ::= SEQUENCE
   {
      pkgdName       PkgdName,


Groves, et al.              Standards Track                   [Page 104]

RFC 3525                Gateway Control Protocol               June 2003


      streamID       StreamID OPTIONAL,
      eventAction    RequestedActions OPTIONAL,
      evParList      SEQUENCE OF EventParameter,
      ...
   }

   RequestedActions ::= SEQUENCE
   {
      keepActive        BOOLEAN OPTIONAL,
      eventDM           EventDM OPTIONAL,
      secondEvent          SecondEventsDescriptor OPTIONAL,
      signalsDescriptor    SignalsDescriptor OPTIONAL,
      ...
   }

   EventDM ::= CHOICE
   {  digitMapName   DigitMapName,
      digitMapValue  DigitMapValue
   }

   SecondEventsDescriptor ::= SEQUENCE
   {
      requestID         RequestID OPTIONAL,
      eventList         SEQUENCE OF SecondRequestedEvent,
      ...
   }

   SecondRequestedEvent ::= SEQUENCE
   {
      pkgdName          PkgdName,
      streamID          StreamID OPTIONAL,
      eventAction       SecondRequestedActions OPTIONAL,
      evParList         SEQUENCE OF EventParameter,
      ...
   }

   SecondRequestedActions ::= SEQUENCE
   {
      keepActive           BOOLEAN OPTIONAL,
      eventDM              EventDM OPTIONAL,
      signalsDescriptor    SignalsDescriptor OPTIONAL,
      ...
   }

   EventBufferDescriptor ::= SEQUENCE OF EventSpec

   EventSpec ::= SEQUENCE
   {


Groves, et al.              Standards Track                   [Page 105]

RFC 3525                Gateway Control Protocol               June 2003


      eventName      EventName,
      streamID       StreamID OPTIONAL,
      eventParList   SEQUENCE OF EventParameter,
      ...
   }

   SignalsDescriptor ::= SEQUENCE OF SignalRequest

   SignalRequest ::=CHOICE
   {
      signal         Signal,
      seqSigList     SeqSigList,
      ...
   }

   SeqSigList ::= SEQUENCE
   {
      id                INTEGER(0..65535),
      signalList        SEQUENCE OF Signal
   }

   Signal ::= SEQUENCE
   {
      signalName        SignalName,
      streamID          StreamID OPTIONAL,
      sigType           SignalType OPTIONAL,
      duration          INTEGER (0..65535) OPTIONAL,
      notifyCompletion  NotifyCompletion OPTIONAL,
      keepActive        BOOLEAN OPTIONAL,
      sigParList        SEQUENCE OF SigParameter,
      ...
   }

   SignalType ::= ENUMERATED
   {
      brief(0),
      onOff(1),
      timeOut(2),
      ...
   }

   SignalName ::= PkgdName

   NotifyCompletion ::= BIT STRING
   {
      onTimeOut(0), onInterruptByEvent(1),
      onInterruptByNewSignalDescr(2), otherReason(3)
   }


Groves, et al.              Standards Track                   [Page 106]

RFC 3525                Gateway Control Protocol               June 2003


   SigParameter ::= SEQUENCE
   {
      sigParameterName     Name,
      value                Value,
      -- For use of extraInfo see the comment related to PropertyParm
      extraInfo CHOICE
      {
         relation Relation,
         range    BOOLEAN,
         sublist  BOOLEAN

      } OPTIONAL,
      ...
   }

   -- For an AuditCapReply with all events, the RequestID SHALL be ALL.
   -- ALL is represented by 0xffffffff.

   RequestID ::= INTEGER(0..4294967295)   -- 32-bit unsigned integer

   ModemDescriptor ::= SEQUENCE
   {
      mtl               SEQUENCE OF ModemType,
      mpl               SEQUENCE OF PropertyParm,
      nonStandardData   NonStandardData OPTIONAL
   }

   ModemType ::= ENUMERATED
   {
      v18(0),
      v22(1),
      v22bis(2),
      v32(3),
      v32bis(4),
      v34(5),
      v90(6),
      v91(7),
      synchISDN(8),
      ...
   }

   DigitMapDescriptor ::= SEQUENCE
   {
      digitMapName   DigitMapName   OPTIONAL,
      digitMapValue  DigitMapValue  OPTIONAL
   }


Groves, et al.              Standards Track                   [Page 107]

RFC 3525                Gateway Control Protocol               June 2003


   DigitMapName ::= Name

   DigitMapValue ::= SEQUENCE
   {
      startTimer     INTEGER(0..99) OPTIONAL,
      shortTimer     INTEGER(0..99) OPTIONAL,
      longTimer      INTEGER(0..99) OPTIONAL,
      digitMapBody      IA5String,
   -- Units are seconds for start, short and long timers, and
   -- hundreds of milliseconds for duration timer.  Thus start,
   -- short, and long range from 1 to 99 seconds and duration
   -- from 100 ms to 9.9 s
      -- See A.3 for explanation of digit map syntax
      ...
   }

   ServiceChangeParm ::= SEQUENCE
   {
      serviceChangeMethod     ServiceChangeMethod,
      serviceChangeAddress    ServiceChangeAddress OPTIONAL,
      serviceChangeVersion    INTEGER(0..99) OPTIONAL,
      serviceChangeProfile    ServiceChangeProfile OPTIONAL,
      serviceChangeReason     Value,
   -- A serviceChangeReason consists of a numeric reason code
   -- and an optional text description.
   -- The serviceChangeReason SHALL be a string consisting of
   -- a decimal reason code, optionally followed by a single
   -- space character and a textual description string.
   -- This string is first BER-encoded as an IA5String.
   -- The result of this BER-encoding is then encoded as
   -- an ASN.1 OCTET STRING type, "double wrapping" the
   -- value as was done for package elements.
      serviceChangeDelay      INTEGER(0..4294967295) OPTIONAL,
                                        -- 32-bit unsigned integer
      serviceChangeMgcId      MId OPTIONAL,
      timeStamp               TimeNotation OPTIONAL,
      nonStandardData         NonStandardData OPTIONAL,
      ...
   }

   ServiceChangeAddress ::= CHOICE
   {
      portNumber        INTEGER(0..65535),    -- TCP/UDP port number
      ip4Address        IP4Address,
      ip6Address        IP6Address,
      domainName        DomainName,
      deviceName        PathName,
      mtpAddress        OCTET STRING(SIZE(2..4)),


Groves, et al.              Standards Track                   [Page 108]

RFC 3525                Gateway Control Protocol               June 2003


      ...
   }

   ServiceChangeResParm ::= SEQUENCE
   {
      serviceChangeMgcId      MId OPTIONAL,
      serviceChangeAddress    ServiceChangeAddress OPTIONAL,
      serviceChangeVersion    INTEGER(0..99) OPTIONAL,
      serviceChangeProfile    ServiceChangeProfile OPTIONAL,
      timestamp               TimeNotation OPTIONAL,
      ...
   }

   ServiceChangeMethod ::= ENUMERATED

   {
      failover(0),
      forced(1),
      graceful(2),
      restart(3),
      disconnected(4),
      handOff(5),
      ...
   }

   ServiceChangeProfile ::= SEQUENCE
   {
      profileName    IA5String(SIZE (1..67))
      -- 64 characters for name, 1 for "/", 2 for version to match ABNF
   }

   PackagesDescriptor ::= SEQUENCE OF PackagesItem

   PackagesItem ::= SEQUENCE
   {
      packageName       Name,
      packageVersion    INTEGER(0..99),
      ...
   }

   StatisticsDescriptor ::= SEQUENCE OF StatisticsParameter

   StatisticsParameter ::= SEQUENCE
   {
      statName       PkgdName,
      statValue      Value OPTIONAL
   }


Groves, et al.              Standards Track                   [Page 109]

RFC 3525                Gateway Control Protocol               June 2003


   NonStandardData ::= SEQUENCE
   {
      nonStandardIdentifier   NonStandardIdentifier,
      data                    OCTET STRING
   }

   NonStandardIdentifier ::= CHOICE
   {
      object            OBJECT IDENTIFIER,
      h221NonStandard   H221NonStandard,
      experimental      IA5String(SIZE(8)),
          -- first two characters should be "X-" or "X+"
      ...
   }

   H221NonStandard ::= SEQUENCE
   {  t35CountryCode1   INTEGER(0..255),
      t35CountryCode2   INTEGER(0..255),      -- country, as per T.35
      t35Extension      INTEGER(0..255),      -- assigned nationally
      manufacturerCode     INTEGER(0..65535), -- assigned nationally
      ...
   }

   TimeNotation ::= SEQUENCE
   {
      date     IA5String(SIZE(8)),  -- yyyymmdd format
      time     IA5String(SIZE(8))   -- hhmmssss format
      -- per ISO 8601:1988
   }

   Value ::= SEQUENCE OF OCTET STRING

   END


Groves, et al.              Standards Track                   [Page 110]

RFC 3525                Gateway Control Protocol               June 2003


A.3   Digit maps and path names

   From a syntactic viewpoint, digit maps are strings with syntactic
   restrictions imposed upon them.  The syntax of valid digit maps is
   specified in ABNF [RFC 2234].  The syntax for digit maps presented in
   this subclause is for illustrative purposes only.  The definition of
   digitMap in Annex B takes precedence in the case of differences
   between the two.

     digitMap = (digitString / LWSP "(" LWSP digitStringList LWSP ")"
               LWSP)

     digitStringList = digitString *( LWSP "|" LWSP digitString )
     digitString = 1*(digitStringElement)
     digitStringElement = digitPosition [DOT]
     digitPosition = digitMapLetter / digitMapRange
     digitMapRange = ("x" / (LWSP "[" LWSP digitLetter LWSP "]" LWSP))
     digitLetter = *((DIGIT "-" DIGIT) /digitMapLetter)
     digitMapLetter = DIGIT           ;digits 0-9
             / %x41-4B / %x61-6B    ;a-k and A-K
             / "L"/ "S"       ;Inter-event timers
                                 ;(long, short)
             / "Z"            ;Long duration event
     DOT = %x2E ; "."
     LWSP = *(WSP / COMMENT / EOL)
     WSP = SP / HTAB
     COMMENT = ";" *(SafeChar / RestChar / WSP) EOL
     EOL = (CR [LF]) / LF
     SP = %x20
     HTAB = %x09
     CR = %x0D
     LF = %x0A
     SafeChar = DIGIT / ALPHA / "+" / "-" / "&" / "!" / "_" / "/" /
         "'" / "?" / "@" / "^" / "`" / "~" / "*" / "$" / "\" /
         "(" / ")" / "%" / "."
     RestChar = ";" / "[" / "]" / "{" / "}" / ":" / "," / "#" /
         "<" / ">" / "=" / %x22
     DIGIT = %x30-39       ; digits 0 through 9
     ALPHA = %x41-5A / %x61-7A; A-Z, a-z

   A path name is also a string with syntactic restrictions imposed upon
   it.  The ABNF production defining it is copied from Annex B.

     ; Total length of pathNAME must not exceed 64 chars.
     pathNAME = ["*"] NAME *("/" / "*"/ ALPHA / DIGIT /"_" / "$" )
                            ["@" pathDomainName ]


Groves, et al.              Standards Track                   [Page 111]

RFC 3525                Gateway Control Protocol               June 2003


     ; ABNF allows two or more consecutive "." although it is
     ; meaningless in a path domain name.
     pathDomainName       = (ALPHA / DIGIT / "*" )
                            *63(ALPHA / DIGIT / "-"
     NAME = ALPHA *63(ALPHA / DIGIT / "_" )


Groves, et al.              Standards Track                   [Page 112]

RFC 3525                Gateway Control Protocol               June 2003


ANNEX B - Text encoding of the protocol

B.1   Coding of wildcards

   In a text encoding of the protocol, while TerminationIDs are
   arbitrary, by judicious choice of names, the wildcard character, "*"
   may be made more useful.  When the wildcard character is encountered,
   it will "match" all TerminationIDs having the same previous and
   following characters (if appropriate).  For example, if there were
   TerminationIDs of R13/3/1, R13/3/2 and R13/3/3, the TerminationID
   R13/3/* would match all of them.  There are some circumstances where
   ALL Terminations must be referred to.  The TerminationID "*"
   suffices, and is referred to as ALL.  The CHOOSE TerminationID "$"
   may be used to signal to the MG that it has to create an ephemeral
   Termination or select an idle physical Termination.

B.2   ABNF specification

   The protocol syntax is presented in ABNF according to RFC 2234.

      Note 1 - This syntax specification does not enforce all
      restrictions on element inclusions and values.  Some additional
      restrictions are stated in comments and other restrictions appear
      in the text of this RFC.  These additional restrictions are part
      of the protocol even though not enforced by this specification.

      Note 2 - The syntax is context-dependent.  For example, "Add" can
      be the AddToken or a NAME depending on the context in which it
      occurs.

   Everything in the ABNF and text encoding is case insensitive.  This
   includes TerminationIDs, digitmap Ids etc.  SDP is case sensitive as
   per RFC 2327.

   ; NOTE -- The ABNF in this section uses the VALUE construct (or lists
   ; of VALUE constructs) to encode various package element values
   ; (properties, signal parameters, etc.).  The types of these values
   ; vary and are specified the relevant package definition.  Several
   ; such types are described in section 12.2.
   ;
   ; The ABNF specification for VALUE allows a quotedString form or a
   ; collection of SafeChars.  The encoding of package element values
   ; into ABNF VALUES is specified below.  If a type's encoding allows
   ; characters other than SafeChars, the quotedString form MUST be used
   ; for all values of that type, even for specific values that consist
   ; only of SafeChars.
   ;


Groves, et al.              Standards Track                   [Page 113]

RFC 3525                Gateway Control Protocol               June 2003


   ; String:  A string MUST use the quotedString form of VALUE and can
   ; contain anything allowable in the quotedString form.
   ;
   ; Integer, Double, and Unsigned Integer:  Decimal values can be
   ; encoded using characters 0-9.  Hexadecimal values must be prefixed
   ; with '0x' and can use characters 0-9,a-f,A-F.  An octal format is
   ; not supported.  Negative integers start with '-' and MUST be
   ; Decimal.  The SafeChar form of VALUE MUST be used.
   ;
   ; Character:  A UTF-8 encoding of a single letter surrounded by
   ; double quotes.
   ;
   ; Enumeration:  An enumeration MUST use the SafeChar form of VALUE
   ; and can contain anything allowable in the SafeChar form.
   ;
   ; Boolean:  Boolean values are encoded as "on" and "off" and are
   ; case insensitive.  The SafeChar form of VALUE MUST be used.
   ;
   ; Future types:  Any defined types MUST fit within
   ; the ABNF specification of VALUE.  Specifically, if a type's
   ; encoding allows characters other than SafeChars, the quotedString
   ; form MUST be used for all values of that type, even for specific
   ; values that consist only of SafeChars.
   ;
   ; Note that there is no way to use the double quote character within
   ; a value.
   ;
   ; Note that SDP disallows whitespace at the beginning of a line,
   ; Megaco ABNF allows whitespace before the beginning of the SDP in
   ; the Local/Remote descriptor.  Parsers should accept whitespace
   ; between the LBRKT following the Local/Remote token and the
   ; beginning of the SDP.

   megacoMessage        = LWSP [authenticationHeader SEP ] message

   authenticationHeader = AuthToken EQUAL SecurityParmIndex COLON
                          SequenceNum COLON AuthData

   SecurityParmIndex    = "0x" 8(HEXDIG)
   SequenceNum          = "0x" 8(HEXDIG)
   AuthData             = "0x" 24*64(HEXDIG)

   message            = MegacopToken SLASH Version SEP mId SEP
   messageBody
   ; The version of the protocol defined here is equal to 1.

   messageBody          = ( errorDescriptor / transactionList )


Groves, et al.              Standards Track                   [Page 114]

RFC 3525                Gateway Control Protocol               June 2003


   transactionList      = 1*( transactionRequest / transactionReply /
                          transactionPending / transactionResponseAck )
   ;Use of response acks is dependent on underlying transport


   transactionPending   = PendingToken EQUAL TransactionID LBRKT
   RBRKT

   transactionResponseAck = ResponseAckToken LBRKT transactionAck
                  *(COMMA transactionAck) RBRKT
   transactionAck = transactionID / (transactionID "-" transactionID)

   transactionRequest   = TransToken EQUAL TransactionID LBRKT
                          actionRequest *(COMMA actionRequest) RBRKT

   actionRequest        = CtxToken EQUAL ContextID LBRKT ((
                          contextRequest [COMMA  commandRequestList])
                          / commandRequestList) RBRKT

   contextRequest    = ((contextProperties [COMMA contextAudit])
               / contextAudit)

   contextProperties    = contextProperty *(COMMA contextProperty)

   ; at-most-once
   contextProperty    = (topologyDescriptor / priority / EmergencyToken)

   contextAudit   = ContextAuditToken LBRKT contextAuditProperties
                         *(COMMA contextAuditProperties) RBRKT

   ; at-most-once
   contextAuditProperties = ( TopologyToken / EmergencyToken /
                              PriorityToken )

   ; "O-" indicates an optional command
   ; "W-" indicates a wildcarded response to a command
   commandRequestList = ["O-"] ["W-"] commandRequest
                        *(COMMA ["O-"] ["W-"]commandRequest)

   commandRequest      = ( ammRequest / subtractRequest / auditRequest /
                           notifyRequest / serviceChangeRequest)

   transactionReply     = ReplyToken EQUAL TransactionID LBRKT
                     [ ImmAckRequiredToken COMMA]
                   ( errorDescriptor / actionReplyList ) RBRKT

   actionReplyList      = actionReply *(COMMA actionReply )


Groves, et al.              Standards Track                   [Page 115]

RFC 3525                Gateway Control Protocol               June 2003


   actionReply          = CtxToken EQUAL ContextID LBRKT
                     ( errorDescriptor / commandReply ) /
            (commandReply COMMA errorDescriptor) ) RBRKT

   commandReply      = (( contextProperties [COMMA commandReplyList] ) /
                           commandReplyList )


   commandReplyList     = commandReplys *(COMMA commandReplys )

   commandReplys        = (serviceChangeReply / auditReply / ammsReply /
                           notifyReply )

   ;Add Move and Modify have the same request parameters
   ammRequest           = (AddToken / MoveToken / ModifyToken ) EQUAL
                          TerminationID [LBRKT ammParameter *(COMMA
                          ammParameter) RBRKT]

   ;at-most-once
   ammParameter         = (mediaDescriptor / modemDescriptor /
                           muxDescriptor / eventsDescriptor /
                           signalsDescriptor / digitMapDescriptor /
                           eventBufferDescriptor / auditDescriptor)

   ammsReply            = (AddToken / MoveToken / ModifyToken /
                           SubtractToken ) EQUAL TerminationID [ LBRKT
                           terminationAudit RBRKT ]

   subtractRequest      =  SubtractToken EQUAL TerminationID
                           [ LBRKT auditDescriptor RBRKT]

   auditRequest         =  (AuditValueToken / AuditCapToken ) EQUAL
                           TerminationID LBRKT auditDescriptor RBRKT

   auditReply           = (AuditValueToken / AuditCapToken )
                          ( contextTerminationAudit  / auditOther)

   auditOther           = EQUAL TerminationID [LBRKT
                          terminationAudit RBRKT]

   terminationAudit = auditReturnParameter *(COMMA auditReturnParameter)

   contextTerminationAudit = EQUAL CtxToken ( terminationIDList /
                          LBRKT errorDescriptor RBRKT )

   auditReturnParameter = (mediaDescriptor / modemDescriptor /
                           muxDescriptor / eventsDescriptor /
                           signalsDescriptor / digitMapDescriptor /


Groves, et al.              Standards Track                   [Page 116]

RFC 3525                Gateway Control Protocol               June 2003


                      observedEventsDescriptor / eventBufferDescriptor /
                           statisticsDescriptor / packagesDescriptor /
                            errorDescriptor / auditItem)

   auditDescriptor      = AuditToken LBRKT [ auditItem
                          *(COMMA auditItem) ] RBRKT

   notifyRequest        = NotifyToken EQUAL TerminationID
                          LBRKT ( observedEventsDescriptor
                                [ COMMA errorDescriptor ] ) RBRKT

   notifyReply          = NotifyToken EQUAL TerminationID
                          [ LBRKT errorDescriptor RBRKT ]

   serviceChangeRequest = ServiceChangeToken EQUAL TerminationID
                          LBRKT serviceChangeDescriptor RBRKT

   serviceChangeReply   = ServiceChangeToken EQUAL TerminationID
                          [LBRKT (errorDescriptor /
                          serviceChangeReplyDescriptor) RBRKT]

   errorDescriptor   = ErrorToken EQUAL ErrorCode
                       LBRKT [quotedString] RBRKT

   ErrorCode            = 1*4(DIGIT) ; could be extended

   TransactionID        = UINT32

   mId                  = (( domainAddress / domainName )
                          [":" portNumber]) / mtpAddress / deviceName

   ; ABNF allows two or more consecutive "." although it is meaningless
   ; in a domain name.
   domainName           = "<" (ALPHA / DIGIT) *63(ALPHA / DIGIT / "-" /
                          ".") ">"
   deviceName           = pathNAME

   ;The values 0x0, 0xFFFFFFFE and 0xFFFFFFFF are reserved.
   ContextID            = (UINT32 / "*" / "-" / "$")

   domainAddress        = "[" (IPv4address / IPv6address) "]"
   ;RFC2373 contains the definition of IP6Addresses.
   IPv6address          = hexpart [ ":" IPv4address ]
   IPv4address          = V4hex DOT V4hex DOT V4hex DOT V4hex
   V4hex                = 1*3(DIGIT) ; "0".."255"
   ; this production, while occurring in RFC2373, is not referenced
   ; IPv6prefix           = hexpart SLASH 1*2DIGIT
   hexpart           = hexseq "::" [ hexseq ] / "::" [ hexseq ] / hexseq


Groves, et al.              Standards Track                   [Page 117]

RFC 3525                Gateway Control Protocol               June 2003


   hexseq               = hex4 *( ":" hex4)
   hex4                 = 1*4HEXDIG

   portNumber           = UINT16

   ; Addressing structure of mtpAddress:
   ; 25 - 15            0
   ;    |  PC        | NI |
   ;    24 - 14 bits    2 bits
   ; Note: 14 bits are defined for international use.
   ; Two national options exist where the point code is 16 or 24 bits.
   ; To octet align the mtpAddress the MSBs shall be encoded as 0s.
   ; An octet shall be represented by 2 hex digits.
   mtpAddress           = MTPToken LBRKT 4*8 (HEXDIG) RBRKT

   terminationIDList  = LBRKT TerminationID *(COMMA TerminationID) RBRKT

   ; Total length of pathNAME must not exceed 64 chars.
   pathNAME      = ["*"] NAME *("/" / "*"/ ALPHA / DIGIT /"_" / "$" )
                          ["@" pathDomainName ]

   ; ABNF allows two or more consecutive "." although it is meaningless
   ; in a path domain name.
   pathDomainName       = (ALPHA / DIGIT / "*" )
                          *63(ALPHA / DIGIT / "-" / "*" / ".")

   TerminationID        = "ROOT" / pathNAME / "$" / "*"

   mediaDescriptor = MediaToken LBRKT mediaParm *(COMMA mediaParm) RBRKT

   ; at-most one terminationStateDescriptor
   ; and either streamParm(s) or streamDescriptor(s) but not both
   mediaParm            = (streamParm / streamDescriptor /
                           terminationStateDescriptor)

   ; at-most-once per item
   streamParm           = ( localDescriptor / remoteDescriptor /
                           localControlDescriptor )

   streamDescriptor     = StreamToken EQUAL StreamID LBRKT streamParm
                          *(COMMA streamParm) RBRKT

   localControlDescriptor = LocalControlToken LBRKT localParm
                            *(COMMA localParm) RBRKT

   ; at-most-once per item except for propertyParm
   localParm = ( streamMode / propertyParm / reservedValueMode
                  / reservedGroupMode )


Groves, et al.              Standards Track                   [Page 118]

RFC 3525                Gateway Control Protocol               June 2003


   reservedValueMode    = ReservedValueToken EQUAL ( "ON" / "OFF" )
   reservedGroupMode    = ReservedGroupToken EQUAL ( "ON" / "OFF" )

   streamMode           = ModeToken EQUAL streamModes

   streamModes     = (SendonlyToken / RecvonlyToken / SendrecvToken /
                          InactiveToken / LoopbackToken )

   propertyParm         = pkgdName parmValue
   parmValue            = (EQUAL alternativeValue/ INEQUAL VALUE)
   alternativeValue     = ( VALUE
                  / LSBRKT VALUE *(COMMA VALUE) RSBRKT
                   ; sublist (i.e., A AND B AND ...)
                  / LBRKT VALUE *(COMMA VALUE) RBRKT
                   ; alternatives (i.e., A OR B OR ...)
                  /  LSBRKT VALUE COLON VALUE RSBRKT )
                   ; range

   INEQUAL              = LWSP (">" / "<" / "#" ) LWSP
   LSBRKT               = LWSP "[" LWSP
   RSBRKT               = LWSP "]" LWSP

   ; Note - The octet zero is not among the permitted characters in
   ; octet string.  As the current definition is limited to SDP, and a
   ; zero octet would not be a legal character in SDP, this is not a
   ; concern.

   localDescriptor      = LocalToken LBRKT octetString RBRKT

   remoteDescriptor     = RemoteToken LBRKT octetString RBRKT

   eventBufferDescriptor= EventBufferToken [ LBRKT eventSpec
                          *( COMMA eventSpec) RBRKT ]

   eventSpec      = pkgdName [ LBRKT eventSpecParameter
                *(COMMA eventSpecParameter) RBRKT ]
   eventSpecParameter   = (eventStream / eventOther)

   eventBufferControl     = BufferToken EQUAL ( "OFF" / LockStepToken )

   terminationStateDescriptor = TerminationStateToken LBRKT
              terminationStateParm *( COMMA terminationStateParm ) RBRKT

   ; at-most-once per item except for propertyParm
   terminationStateParm = (propertyParm / serviceStates /
                           eventBufferControl )


Groves, et al.              Standards Track                   [Page 119]

RFC 3525                Gateway Control Protocol               June 2003


   serviceStates        = ServiceStatesToken EQUAL ( TestToken /
                          OutOfSvcToken / InSvcToken )

   muxDescriptor        = MuxToken EQUAL MuxType  terminationIDList

   MuxType              = ( H221Token / H223Token / H226Token / V76Token
                           / extensionParameter )

   StreamID             = UINT16
   pkgdName     = (PackageName SLASH ItemID) ;specific item
                / (PackageName SLASH "*") ;all items in package
                / ("*" SLASH "*") ; all items supported by the MG
   PackageName          = NAME
   ItemID               = NAME

   eventsDescriptor     = EventsToken [ EQUAL RequestID LBRKT
                        requestedEvent *( COMMA requestedEvent ) RBRKT ]

   requestedEvent       = pkgdName [ LBRKT eventParameter
                          *( COMMA eventParameter ) RBRKT ]

   ; at-most-once each of KeepActiveToken , eventDM and eventStream
   ;at most one of either embedWithSig or embedNoSig but not both
   ;KeepActiveToken and embedWithSig must not both be present
   eventParameter       = ( embedWithSig / embedNoSig / KeepActiveToken
                            /eventDM / eventStream / eventOther )

   embedWithSig         = EmbedToken LBRKT signalsDescriptor
                            [COMMA embedFirst ] RBRKT
   embedNoSig        = EmbedToken LBRKT embedFirst RBRKT

   ; at-most-once of each
   embedFirst      = EventsToken [ EQUAL RequestID LBRKT
              secondRequestedEvent *(COMMA secondRequestedEvent) RBRKT ]

   secondRequestedEvent = pkgdName [ LBRKT secondEventParameter
                          *( COMMA secondEventParameter ) RBRKT ]

   ; at-most-once each of embedSig , KeepActiveToken, eventDM or
   ; eventStream
   ; KeepActiveToken and embedSig must not both be present
   secondEventParameter = ( embedSig / KeepActiveToken / eventDM /
                            eventStream / eventOther )

   embedSig  = EmbedToken LBRKT signalsDescriptor RBRKT

   eventStream          = StreamToken EQUAL StreamID


Groves, et al.              Standards Track                   [Page 120]

RFC 3525                Gateway Control Protocol               June 2003


   eventOther           = eventParameterName parmValue

   eventParameterName   = NAME

   eventDM              = DigitMapToken EQUAL(( digitMapName ) /
                          (LBRKT digitMapValue RBRKT ))

   signalsDescriptor    = SignalsToken LBRKT [ signalParm
                          *(COMMA signalParm)] RBRKT

   signalParm           = signalList / signalRequest

   signalRequest        = signalName [ LBRKT sigParameter
                          *(COMMA sigParameter) RBRKT ]

   signalList           = SignalListToken EQUAL signalListId LBRKT
                          signalListParm *(COMMA signalListParm) RBRKT

   signalListId         = UINT16

   ;exactly once signalType, at most once duration and every signal
   ;parameter
   signalListParm       = signalRequest

   signalName           = pkgdName
   ;at-most-once sigStream, at-most-once sigSignalType,
   ;at-most-once sigDuration, every signalParameterName at most once
   sigParameter = sigStream / sigSignalType / sigDuration / sigOther
               / notifyCompletion / KeepActiveToken
   sigStream            = StreamToken EQUAL StreamID
   sigOther             = sigParameterName parmValue
   sigParameterName     = NAME
   sigSignalType        = SignalTypeToken EQUAL signalType
   signalType           = (OnOffToken / TimeOutToken / BriefToken)
   sigDuration          = DurationToken EQUAL UINT16
   notifyCompletion     = NotifyCompletionToken EQUAL (LBRKT
            notificationReason *(COMMA notificationReason) RBRKT)

   notificationReason   = ( TimeOutToken / InterruptByEventToken
                        / InterruptByNewSignalsDescrToken
                        / OtherReasonToken )
   observedEventsDescriptor = ObservedEventsToken EQUAL RequestID
                      LBRKT observedEvent *(COMMA observedEvent) RBRKT

   ;time per event, because it might be buffered
   observedEvent        = [ TimeStamp LWSP COLON] LWSP
                          pkgdName [ LBRKT observedEventParameter
                          *(COMMA observedEventParameter) RBRKT ]


Groves, et al.              Standards Track                   [Page 121]

RFC 3525                Gateway Control Protocol               June 2003


   ;at-most-once eventStream, every eventParameterName at most once
   observedEventParameter = eventStream / eventOther

   ; For an AuditCapReply with all events, the RequestID should be ALL.
   RequestID            = ( UINT32 / "*" )

   modemDescriptor      = ModemToken (( EQUAL modemType) /
                      (LSBRKT modemType *(COMMA modemType) RSBRKT))
                     [ LBRKT propertyParm *(COMMA propertyParm) RBRKT ]


   ; at-most-once except for extensionParameter
   modemType            = (V32bisToken / V22bisToken / V18Token /
                           V22Token / V32Token / V34Token / V90Token /
                         V91Token / SynchISDNToken / extensionParameter)

   digitMapDescriptor  = DigitMapToken EQUAL
                        ( ( LBRKT digitMapValue RBRKT ) /
                        (digitMapName [ LBRKT digitMapValue RBRKT ]) )
   digitMapName        = NAME
   digitMapValue       = ["T" COLON Timer COMMA] ["S" COLON Timer COMMA]
                         ["L" COLON Timer COMMA] digitMap
   Timer               = 1*2DIGIT
   ; Units are seconds for T, S, and L timers, and hundreds of
   ; milliseconds for Z timer.  Thus T, S, and L range from 1 to 99
   ; seconds and Z from 100 ms to 9.9 s
   digitMap = (digitString /
               LWSP "(" LWSP digitStringList LWSP ")" LWSP)
   digitStringList   = digitString *( LWSP "|" LWSP digitString )
   digitString       = 1*(digitStringElement)
   digitStringElement = digitPosition [DOT]
   digitPosition     = digitMapLetter / digitMapRange
   digitMapRange     = ("x" / (LWSP "[" LWSP digitLetter LWSP "]" LWSP))
   digitLetter       = *((DIGIT "-" DIGIT ) / digitMapLetter)
   digitMapLetter    = DIGIT   ;Basic event symbols
               / %x41-4B / %x61-6B ; a-k, A-K
               / "L" / "S"   ;Inter-event timers (long, short)
               / "Z"         ;Long duration modifier

   ;at-most-once, and DigitMapToken and PackagesToken are not allowed
   ;in AuditCapabilities command
   auditItem            = ( MuxToken / ModemToken / MediaToken /
                           SignalsToken / EventBufferToken /
                           DigitMapToken / StatsToken / EventsToken /
                           ObservedEventsToken / PackagesToken )


Groves, et al.              Standards Track                   [Page 122]

RFC 3525                Gateway Control Protocol               June 2003


   serviceChangeDescriptor = ServicesToken LBRKT serviceChangeParm
                            *(COMMA serviceChangeParm) RBRKT

   ; each parameter at-most-once
   ; at most one of either serviceChangeAddress or serviceChangeMgcId
   ; but not both
   ; serviceChangeMethod and serviceChangeReason are REQUIRED
   serviceChangeParm    = (serviceChangeMethod / serviceChangeReason /
                          serviceChangeDelay / serviceChangeAddress /
                          serviceChangeProfile / extension / TimeStamp /
                          serviceChangeMgcId / serviceChangeVersion )

   serviceChangeReplyDescriptor = ServicesToken LBRKT
                        servChgReplyParm *(COMMA servChgReplyParm) RBRKT

   ; at-most-once.  Version is REQUIRED on first ServiceChange response
   ; at most one of either serviceChangeAddress or serviceChangeMgcId
   ; but not both
   servChgReplyParm     = (serviceChangeAddress / serviceChangeMgcId /
                          serviceChangeProfile / serviceChangeVersion /
                          TimeStamp)
   serviceChangeMethod  = MethodToken EQUAL (FailoverToken /
                          ForcedToken / GracefulToken / RestartToken /
                          DisconnectedToken / HandOffToken /
                          extensionParameter)
   ; A serviceChangeReason consists of a numeric reason code
   ; and an optional text description.
   ; A serviceChangeReason MUST be encoded using the quotedString
   ; form of VALUE.
   ; The quotedString SHALL contain a decimal reason code,
   ; optionally followed by a single space character and a
   ; textual description string.


   serviceChangeReason  = ReasonToken  EQUAL VALUE
   serviceChangeDelay   = DelayToken   EQUAL UINT32
   serviceChangeAddress = ServiceChangeAddressToken EQUAL ( mId /
                          portNumber )
   serviceChangeMgcId   = MgcIdToken   EQUAL mId
   serviceChangeProfile = ProfileToken EQUAL NAME SLASH Version
   serviceChangeVersion = VersionToken EQUAL Version
   extension            = extensionParameter parmValue

   packagesDescriptor   = PackagesToken LBRKT packagesItem
                          *(COMMA packagesItem) RBRKT

   Version              = 1*2(DIGIT)
   packagesItem         = NAME "-" UINT16


Groves, et al.              Standards Track                   [Page 123]

RFC 3525                Gateway Control Protocol               June 2003


   TimeStamp            = Date "T" Time ; per ISO 8601:1988
   ; Date = yyyymmdd
   Date                 = 8(DIGIT)
   ; Time = hhmmssss
   Time                 = 8(DIGIT)
   statisticsDescriptor = StatsToken LBRKT statisticsParameter
                         *(COMMA statisticsParameter ) RBRKT

   ;at-most-once per item
   statisticsParameter  = pkgdName [EQUAL VALUE]

   topologyDescriptor   = TopologyToken LBRKT topologyTriple
                          *(COMMA topologyTriple) RBRKT
   topologyTriple       = terminationA COMMA
                          terminationB COMMA topologyDirection
   terminationA         = TerminationID
   terminationB         = TerminationID
   topologyDirection    = BothwayToken / IsolateToken / OnewayToken

   priority             = PriorityToken EQUAL UINT16

   extensionParameter   = "X"  ("-" / "+") 1*6(ALPHA / DIGIT)

   ; octetString is used to describe SDP defined in RFC2327.
   ; Caution should be taken if CRLF in RFC2327 is used.
   ; To be safe, use EOL in this ABNF.
   ; Whenever "}" appears in SDP, it is escaped by "\", e.g., "\}"
   octetString          = *(nonEscapeChar)
   nonEscapeChar        = ( "\}" / %x01-7C / %x7E-FF )
   ; Note - The double-quote character is not allowed in quotedString.
   quotedString         = DQUOTE *(SafeChar / RestChar/ WSP) DQUOTE

   UINT16               = 1*5(DIGIT)  ; %x0-FFFF
   UINT32               = 1*10(DIGIT) ; %x0-FFFFFFFF

   NAME                 = ALPHA *63(ALPHA / DIGIT / "_" )
   VALUE                = quotedString / 1*(SafeChar)
   SafeChar             = DIGIT / ALPHA / "+" / "-" / "&" /
                          "!" / "_" / "/" / "\'" / "?" / "@" /
                          "^" / "`" / "~" / "*" / "$" / "\" /
                          "(" / ")" / "%" / "|" / "."

   EQUAL                = LWSP %x3D LWSP ; "="
   COLON                = %x3A           ; ":"
   LBRKT                = LWSP %x7B LWSP ; "{"
   RBRKT                = LWSP %x7D LWSP ; "}"
   COMMA                = LWSP %x2C LWSP ; ","


Groves, et al.              Standards Track                   [Page 124]

RFC 3525                Gateway Control Protocol               June 2003


   DOT                  = %x2E           ; "."
   SLASH                = %x2F           ; "/"
   ALPHA                = %x41-5A / %x61-7A ; A-Z / a-z
   DIGIT                = %x30-39         ; 0-9
   DQUOTE               = %x22            ; " (Double Quote)
   HEXDIG               = ( DIGIT / "A" / "B" / "C" / "D" / "E" / "F" )
   SP                   = %x20        ; space
   HTAB                 = %x09        ; horizontal tab
   CR                   = %x0D        ; Carriage return
   LF                   = %x0A        ; linefeed
   LWSP                 = *( WSP / COMMENT / EOL )
   EOL                  = (CR [LF] / LF )
   WSP                  = SP / HTAB ; white space
   SEP                  = ( WSP / EOL / COMMENT) LWSP
   COMMENT              = ";" *(SafeChar/ RestChar / WSP / %x22) EOL
   RestChar            = ";" / "[" / "]" / "{" / "}" / ":" / "," / "#" /
                          "<" / ">" / "="

   ; New Tokens added to sigParameter must take the format of SPA*
   ; * may be of any form i.e., SPAM
   ; New Tokens added to eventParameter must take the form of EPA*
   ; * may be of any form i.e., EPAD

   AddToken                   = ("Add"                   / "A")
   AuditToken                 = ("Audit"                 / "AT")
   AuditCapToken              = ("AuditCapability"       / "AC")
   AuditValueToken            = ("AuditValue"            / "AV")
   AuthToken                  = ("Authentication"        / "AU")
   BothwayToken               = ("Bothway"               / "BW")
   BriefToken                 = ("Brief"                 / "BR")
   BufferToken                = ("Buffer"                / "BF")
   CtxToken                   = ("Context"               / "C")
   ContextAuditToken       = ("ContextAudit"    / "CA")
   DigitMapToken              = ("DigitMap"              / "DM")
   DisconnectedToken          = ("Disconnected"          / "DC")
   DelayToken                 = ("Delay"                 / "DL")
   DurationToken              = ("Duration"              / "DR")
   EmbedToken                 = ("Embed"                 / "EM")
   EmergencyToken             = ("Emergency"             / "EG")
   ErrorToken                 = ("Error"                 / "ER")
   EventBufferToken           = ("EventBuffer"           / "EB")
   EventsToken                = ("Events"                / "E")
   FailoverToken              = ("Failover"              / "FL")
   ForcedToken                = ("Forced"                / "FO")
   GracefulToken              = ("Graceful"              / "GR")
   H221Token                  = ("H221" )
   H223Token                  = ("H223" )
   H226Token                  = ("H226" )


Groves, et al.              Standards Track                   [Page 125]

RFC 3525                Gateway Control Protocol               June 2003


   HandOffToken               = ("HandOff"               / "HO")
   ImmAckRequiredToken        = ("ImmAckRequired"        / "IA")
   InactiveToken              = ("Inactive"              / "IN")
   IsolateToken               = ("Isolate"               / "IS")
   InSvcToken                 = ("InService"             / "IV")
   InterruptByEventToken      = ("IntByEvent"            / "IBE")
   InterruptByNewSignalsDescrToken
                              = ("IntBySigDescr"         / "IBS")
   KeepActiveToken            = ("KeepActive"            / "KA")
   LocalToken                 = ("Local"                 / "L")
   LocalControlToken          = ("LocalControl"          / "O")
   LockStepToken              = ("LockStep"              / "SP")
   LoopbackToken              = ("Loopback"              / "LB")
   MediaToken                 = ("Media"                 / "M")
   MegacopToken               = ("MEGACO"                / "!")
   MethodToken                = ("Method"                / "MT")
   MgcIdToken                 = ("MgcIdToTry"            / "MG")
   ModeToken                  = ("Mode"                  / "MO")
   ModifyToken                = ("Modify"                / "MF")
   ModemToken                 = ("Modem"                 / "MD")
   MoveToken                  = ("Move"                  / "MV")
   MTPToken                   = ("MTP")
   MuxToken                   = ("Mux"                   / "MX")
   NotifyToken                = ("Notify"                / "N")
   NotifyCompletionToken      = ("NotifyCompletion"      / "NC")
   ObservedEventsToken        = ("ObservedEvents"        / "OE")
   OnewayToken                = ("Oneway"                / "OW")
   OnOffToken                 = ("OnOff"                 / "OO")
   OtherReasonToken           = ("OtherReason"           / "OR")
   OutOfSvcToken              = ("OutOfService"          / "OS")
   PackagesToken              = ("Packages"              / "PG")
   PendingToken               = ("Pending"               / "PN")
   PriorityToken              = ("Priority"              / "PR")
   ProfileToken               = ("Profile"               / "PF")
   ReasonToken                = ("Reason"                / "RE")
   RecvonlyToken              = ("ReceiveOnly"           / "RC")
   ReplyToken                 = ("Reply"                 / "P")
   RestartToken               = ("Restart"               / "RS")
   RemoteToken                = ("Remote"                / "R")
   ReservedGroupToken         = ("ReservedGroup"         / "RG")
   ReservedValueToken         = ("ReservedValue"         / "RV")
   SendonlyToken              = ("SendOnly"              / "SO")
   SendrecvToken              = ("SendReceive"           / "SR")
   ServicesToken              = ("Services"              / "SV")
   ServiceStatesToken         = ("ServiceStates"         / "SI")
   ServiceChangeToken         = ("ServiceChange"         / "SC")
   ServiceChangeAddressToken  = ("ServiceChangeAddress"  / "AD")
   SignalListToken            = ("SignalList"            / "SL")


Groves, et al.              Standards Track                   [Page 126]

RFC 3525                Gateway Control Protocol               June 2003


   SignalsToken               = ("Signals"               / "SG")
   SignalTypeToken            = ("SignalType"            / "SY")
   StatsToken                 = ("Statistics"            / "SA")
   StreamToken                = ("Stream"                / "ST")
   SubtractToken              = ("Subtract"              / "S")
   SynchISDNToken             = ("SynchISDN"             / "SN")
   TerminationStateToken      = ("TerminationState"      / "TS")
   TestToken                  = ("Test"                  / "TE")
   TimeOutToken               = ("TimeOut"               / "TO")
   TopologyToken              = ("Topology"              / "TP")
   TransToken                 = ("Transaction"           / "T")
   ResponseAckToken           = ("TransactionResponseAck" / "K")
   V18Token                   = ("V18")
   V22Token                   = ("V22")
   V22bisToken                = ("V22b")
   V32Token                   = ("V32")
   V32bisToken                = ("V32b")
   V34Token                   = ("V34")
   V76Token                   = ("V76")
   V90Token                   = ("V90")
   V91Token                   = ("V91")
   VersionToken               = ("Version"               / "V")

B.3   Hexadecimal octet coding

   Hexadecimal octet coding is a means for representing a string of
   octets as a string of hexadecimal digits, with two digits
   representing each octet.  This octet encoding should be used when
   encoding octet strings in the text version of the protocol.  For each
   octet, the 8-bit sequence is encoded as two hexadecimal digits.  Bit
   0 is the first transmitted; bit 7 is the last.  Bits 7-4 are encoded
   as the first hexadecimal digit, with Bit 7 as MSB and Bit 4 as LSB.
   Bits 3-0 are encoded as the second hexadecimal digit, with Bit 3 as
   MSB and Bit 0 as LSB. Examples:

        Octet bit pattern                   Hexadecimal coding
        00011011                            D8
        11100100                            27
        10000011 10100010 11001000 00001001 C1451390

B.4   Hexadecimal octet sequence

   A hexadecimal octet sequence is an even number of hexadecimal digits,
   terminated by a <CR> character.


Groves, et al.              Standards Track                   [Page 127]

RFC 3525                Gateway Control Protocol               June 2003


ANNEX C - Tags for media stream properties

   Parameters for Local, Remote and LocalControl descriptors are
   specified as tag-value pairs if binary encoding is used for the
   protocol.  This annex contains the property names (PropertyID), the
   tags (Property tag), type of the property (Type) and the values
   (Value).  Values presented in the Value field when the field contains
   references shall be regarded as "information".  The reference
   contains the normative values.  If a value field does not contain a
   reference, then the values in that field can be considered as
   "normative".

   Tags are given as hexadecimal numbers in this annex.  When setting
   the value of a property, a MGC may underspecify the value according
   to one of the mechanisms specified in 7.1.1.

   It is optional to support the properties in this Annex or any of its
   sub-sections.  For example, only three properties from C.3 and only
   five properties from C.8 might be implemented.

   For type "enumeration" the value is represented by the value in
   brackets, e.g., Send(0), Receive(1).  Annex C properties with the
   types "N bits" or "M Octets" should be treated as octet strings when
   encoding the protocol.  Properties with "N bit integer" shall be
   treated as an integers.  "String" shall be treated as an IA5String
   when encoding the protocol.

   When a type is smaller than one octet, the value shall be stored in
   the low-order bits of an octet string of size 1.

C.1   General media attributes

   PropertyID    Property Type          Value
                 tag

   Media         1001     Enumeration   Audio(0), Video(1), Data(2)

   Transmission  1002     Enumeration   Send(0), Receive(1),
   mode                                 Send&Receive(2)

   Number of     1003     Unsigned      0-255
   Channels               integer

   Sampling      1004     Unsigned      0-2^32
   rate                   integer

   Bitrate       1005     Integer       (0..4294967295)NOTE - Units of
                                        100 bit/s.


Groves, et al.              Standards Track                   [Page 128]

RFC 3525                Gateway Control Protocol               June 2003


   ACodec        1006     Octet string  Audio Codec Type:
                                        Ref.: ITU-T Q.765
                                        Non-ITU-T codecs are defined
                                        with the appropriate standards
                                        organization under a defined
                                        Organizational Identifier.

   Samplepp      1007     Unsigned      Maximum samples or frames per
                          integer       packet: 0..65535

   Silencesupp   1008     Boolean       Silence Suppression: True/False

   Encrypttype   1009     Octet string  Ref.: ITU-T H.245

   Encryptkey    100A     Octet string  Encryption key
                          size          Ref.: ITU-T H.235
                          (0..65535)

   Echocanc      100B                    Not Used.  See H.248.1 E.13 for
                                        an example of possible Echo
                                        Control properties.

   Gain          100C     Unsigned      Gain in dB: 0..65535
                          integer

   Jitterbuff    100D     Unsigned      Jitter buffer size in ms:
                          integer       0..65535

   PropDelay     100E     Unsigned      Propagation Delay: 0..65535
                          integer       Maximum propagation delay in
                                        milliseconds for the bearer
                                        connection between two media
                                        gateways.  The maximum delay
                                        will be dependent on the bearer
                                        technology.

   RTPpayload    100F     Integer       Payload type in RTP Profile for
                                        Audio and Video Conferences
                                        with Minimal Control
                                        Ref.: RFC 1890


Groves, et al.              Standards Track                   [Page 129]

RFC 3525                Gateway Control Protocol               June 2003


C.2   Mux properties

   PropertyID Property tag Type         Value

   H222       2001         Octet string H222LogicalChannelParameters
                                         Ref.: ITU-T H.245

   H223       2002         Octet string H223LogicalChannelParameters
                                         Ref.: ITU-T H.245

   V76        2003         Octet string V76LogicalChannelParameters
                                         Ref.: ITU-T H.245

   H2250      2004         Octet string H2250LogicalChannelParameters
                                         Ref.: ITU-T H.245

C.3   General bearer properties

   PropertyID Property   Type        Value
              tag

   Mediatx    3001       Enumeration Media Transport TypeTDM
                                      Circuit(0), ATM(1), FR(2),
                                      Ipv4(3), Ipv6(4), ...

   BIR        3002       4 octets    Value depends on transport
                                      technology

   NSAP       3003       1-20 octets See NSAP.
                                      Ref.: Annex A/X.213

C.4   General ATM properties

   PropertyID Property Type              Value
              tag

   AESA       4001     20 octets         ATM End System Address

   VPVC       4002     4 octets: VPCI    VPCI/VCI
                        in first two
                        least             Ref.: ITU-T Q.2931
                        significant
                        octets, VCI in
                        second two
                        octets


Groves, et al.              Standards Track                   [Page 130]

RFC 3525                Gateway Control Protocol               June 2003


   SC         4003     Enumeration       Service Category: CBR(0),
                                          nrt-VBR1(1), nrt  VBR2(2),
                                          nrt-VBR3(3), rt-VBR1(4),
                                          rt  VBR2(5), rt-VBR3(6),
                                          UBR1(7), UBR2(8), ABR(9).
                                          Ref.: ATM Forum UNI 4.0

   BCOB       4004     5-bit integer     Broadband Bearer Class
                                          Ref.: ITU-T Q.2961.2

   BBTC       4005     7-bit integer     Broadband Transfer Capability
                                          Ref.: ITU-T Q.2961.1

   ATC        4006     Enumeration       I.371 ATM Traffic
                                          CapabilityDBR(0), SBR1(1),
                                          SBR2(2), SBR3(3), ABT/IT(4),
                                          ABT/DT(5), ABR(6)
                                          Ref.: ITU-T I.371

   STC        4007     2 bits            Susceptibility to clipping:
                                          Bits
                                          2 1
                                          ---
                                          0 0     not susceptible to
                                                  clipping
                                          0 1     susceptible to
                                                  clipping
                                          Ref.: ITU-T Q.2931

   UPCC       4008     2 bits            User Plane Connection
                                          configuration:
                                          Bits
                                          2 1
                                          ---
                                          0 0     point-to-point
                                          0 1     point-to-multipoint
                                          Ref.: ITU-T Q.2931

   PCR0       4009     24-bit integer    Peak Cell Rate (For CLP = 0)
                                          Ref.: ITU-T Q.2931

   SCR0       400A     24-bit integer    Sustainable Cell Rate (For
                                          CLP = 0)
                                          Ref.: ITU-T Q.2961.1

   MBS0       400B     24-bit integer    Maximum Burst Size (For CLP =
                                          0)
                                          Ref.: ITU-T Q.2961.1


Groves, et al.              Standards Track                   [Page 131]

RFC 3525                Gateway Control Protocol               June 2003


   PCR1       400C     24-bit integer    Peak Cell Rate (For CLP = 0 +
                                          1)
                                          Ref.: ITU-T Q.2931

   SCR1       400D     24-bit integer    Sustainable Cell Rate (For
                                          CLP = 0 + 1)
                                          Ref.: ITU-T Q.2961.1

   MBS1       400E     24-bit integer    Maximum Burst Size (For CLP =
                                          0 + 1)
                                          Ref.: ITU-T Q.2961.1

   BEI        400F     Boolean           Best Effort Indicator
                                          Value 1 indicates that BEI is
                                          to be included in the ATM
                                          signaling; value 0 indicates
                                          that BEI is not to be
                                          included in the ATM
                                          signaling.
                                          Ref.: ATM Forum UNI 4.0

   TI         4010     Boolean           Tagging Indicator
                                          Value 0 indicates that
                                          tagging is not allowed; value
                                          1 indicates that tagging is
                                          requested.
                                          Ref.: ITU-T Q.2961.1

   FD         4011     Boolean           Frame Discard
                                          Value 0 indicates that no
                                          frame discard is allowed;
                                          value 1 indicates that frame
                                          discard is allowed.
                                          Ref.: ATM Forum UNI 4.0

   A2PCDV     4012     24-bit integer    Acceptable 2-point CDV
                                          Ref.: ITU-T Q.2965.2

   C2PCDV     4013     24-bit integer    Cumulative 2-point CDV
                                          Ref.: ITU-T Q.2965.2

   APPCDV     4014     24-bit integer    Acceptable P-P CDV
                                          Ref.: ATM Forum UNI 4.0

   CPPCDV     4015     24-bit integer    Cumulative P-P CDV
                                          Ref.: ATM Forum UNI 4.0


Groves, et al.              Standards Track                   [Page 132]

RFC 3525                Gateway Control Protocol               June 2003


   ACLR       4016     8-bit integer     Acceptable Cell Loss Ratio
                                          Ref.: ITU-T Q.2965.2, ATM
                                          Forum UNI 4.0

   MEETD      4017     16-bit integer    Maximum End-to-end transit
                                          delay
                                          Ref.: ITU-T Q.2965.2, ATM
                                          Forum UNI 4.0

   CEETD      4018     16-bit integer    Cumulative End-to-end transit
                                          delay
                                          Ref.: ITU-T Q.2965.2, ATM
                                          Forum UNI 4.0

   QosClass   4019     Integer 0-5          QoS Class

                                            QoS Class     Meaning

                                            0             Default QoS
                                                           associated
                                                           with the ATC
                                                           as defined
                                                           in ITU-T
                                                           Q.2961.2

                                            1             Stringent

                                            2             Tolerant

                                            3             Bi-level

                                            4             Unbounded

                                            5             Stringent
                                                           Bi-level
                                          Ref.: ITU-T Q.2965.1

   AALtype    401A     1 octet           AAL Type
                                          Bits
                                          8 7 6 5 4 3 2 1
                                          ---------------
                                          0 0 0 0 0 0 0 0   AAL for
                                                            voice
                                          0 0 0 0 0 0 0 1   AAL type 1
                                          0 0 0 0 0 0 1 0   AAL type 2
                                          0 0 0 0 0 0 1 1   AAL type
                                                            3/4
                                          0 0 0 0 0 1 0 1   AAL type 5


Groves, et al.              Standards Track                   [Page 133]

RFC 3525                Gateway Control Protocol               June 2003


                                          0 0 0 1 0 0 0 0   user-
                                                            defined AAL
                                          Ref.: ITU-T Q.2931

C.5   Frame Relay

   PropertyID         Property    Type          Value
                      tag

   DLCI               5001        Unsigned      Data link connection
                                  integer       id

   CID                5002        Unsigned      sub-channel id
                                  integer

   SID/Noiselevel     5003        Unsigned      silence insertion
                                  integer       descriptor

   Primary Payload    5004        Unsigned      Primary Payload Type
   type                           integer       Covers FAX and codecs

C.6   IP

   PropertyID Property tag Type                Value

   IPv4       6001         32 bits Ipv4Address Ipv4Address
                                                Ref.: IETF RFC 791

   IPv6       6002         128 bits            IPv6 Address
                                                Ref.: IETF RFC 2460

   Port       6003         Unsigned integer    0..65535

   Porttype   6004         Enumerated          TCP(0), UDP(1), SCTP(2)


C.7   ATM AAL2

   PropertyID Property Type                   Value
              tag

   AESA       7001     20 octets              AAL2 service endpoint
                                                address as defined in
                                                the referenced
                                                Recommendation.
                                                ESEANSEA
                                                Ref.: ITU-T Q.2630.1


Groves, et al.              Standards Track                   [Page 134]

RFC 3525                Gateway Control Protocol               June 2003


   BIR        See C.3  4 octets               Served user generated
                                                reference as defined in
                                                the referenced
                                                Recommendation.
                                                SUGR
                                                Ref.: ITU-T Q.2630.1

   ALC        7002     12 octets              AAL2 link
                                                characteristics as
                                                defined in the
                                                referenced
                                                Recommendation.
                                                Maximum/Average CPS-SDU
                                                bit rate;
                                                Maximum/Average CPS-SDU
                                                size
                                                Ref.: ITU-T Q.2630.1

   SSCS       7003     I.366.2: Audio (8      Service specific
                        octets); Multirate (3  convergence sublayer
                        octets), or I.366.1:   information as defined
                        SAR-assured (14        in:
                        octets);SAR-unassured  - ITU-T Q.2630.1,and
                        (7 octets).            used in:
                                                - ITU-T I.366.2:
                                                Audio/Multirate;
                                                - ITU-T I.366.1: SAR-
                                                assured/unassured.
                                                Ref.: ITU-T Q.2630.1,
                                                I.366.1 and I.366.2

   SUT        7004     1..254 octets          Served user transport
                                                parameter as defined in
                                                the referenced
                                                Recommendation.
                                                Ref.: ITU-T Q.2630.1

   TCI        7005     Boolean                Test connection
                                                indicator as defined in
                                                the referenced
                                                Recommendation.
                                                Ref.: ITU-T Q.2630.1

   Timer_CU   7006     32-bit integer         Timer-CU
                                                Milliseconds to hold
                                                partially filled cell
                                                before sending.


Groves, et al.              Standards Track                   [Page 135]

RFC 3525                Gateway Control Protocol               June 2003


   MaxCPSSDU  7007     8-bit integer          Maximum Common Part
                                                Sublayer Service Data
                                                Unit
                                                Ref.: ITU-T Q.2630.1

   CID        7008     8 bits                 subchannel id: 0-255
                                                Ref.: ITU-T I.363.2
C.8   ATM AAL1

   PropertyID Property   Type        Value
              tag

   BIR        See table  4-29 octets GIT (Generic Identifier
              in C.3                 Transport)
                                      Ref.: ITU-T Q.2941.1

   AAL1ST     8001       1 octet     AAL1 Subtype
                                      Bits
                                      8 7 6 5 4 3 2 1
                                      ---------------
                                      0 0 0 0 0 0 0 0     null
                                      0 0 0 0 0 0 0 1     voiceband
                                      signal transport on 64 kbit/s
                                      0 0 0 0 0 0 1 0     circuit
                                      transport
                                      0 0 0 0 0 1 0 0     high-quality
                                      audio signal transport
                                      0 0 0 0 0 1 0 1     video signal
                                      transport
                                      Ref.: ITU-T Q.2931

   CBRR       8002       1 octet     CBR Rate
                                      Bits
                                      8 7 6 5 4 3 2 1
                                      ---------------
                                      0 0 0 0 0 0 0 1       64 kbit/s
                                      0 0 0 0 0 1 0 0     1544 kbit/s
                                      0 0 0 0 0 1 0 1     6312 kbit/s
                                      0 0 0 0 0 1 1 0   32 064 kbit/s
                                      0 0 0 0 0 1 1 1   44 736 kbit/s
                                      0 0 0 0 1 0 0 0   97 728 kbit/s
                                      0 0 0 1 0 0 0 0     2048 kbit/s
                                      0 0 0 1 0 0 0 1     8448 kbit/s
                                      0 0 0 1 0 0 1 0   34 368 kbit/s
                                      0 0 0 1 0 0 1 1  139 264 kbit/s
                                      0 1 0 0 0 0 0 0   n x 64 kbit/s
                                      0 1 0 0 0 0 0 1    n x 8 kbit/s
                                      Ref.: ITU-T Q.2931


Groves, et al.              Standards Track                   [Page 136]

RFC 3525                Gateway Control Protocol               June 2003


   MULT       See table              Multiplier, or n x 64k/8k/300
              in C.9                 Ref.: ITU-T Q.2931

   SCRI       8003       1 octet     Source Clock Frequency Recovery
                                      Method
                                      Bits
                                      8 7 6 5 4 3 2 1
                                      ---------------
                                      0 0 0 0 0 0 0 0     null
                                      0 0 0 0 0 0 0 1     SRTS
                                      0 0 0 0 0 0 1 0     ACM
                                      Ref.: ITU-T Q.2931

   ECM        8004       1 octet     Error Correction Method
                                      Bits
                                      8 7 6 5 4 3 2 1
                                      ---------------
                                      0 0 0 0 0 0 0 0     null
                                      0 0 0 0 0 0 0 1     FEC - Loss
                                      0 0 0 0 0 0 1 0     FEC - Delay
                                      Ref.: ITU-T Q.2931

   SDTB       8005       16-bit      Structured Data Transfer
                         integer     Blocksize
                                      Block size of SDT CBR service
                                      Ref.: ITU-T I.363.1

   PFCI       8006       8-bit       Partially filled cells identifier
                         integer     1-47
                                      Ref.: ITU-T I.363.1

C.9   Bearer capabilities

   The table entries referencing Recommendation Q.931 refer to the
   encoding in the bearer capability information element of Q.931, not
   to the low layer information element.

   PropertyID    Tag    Type      Value

   TMR           9001   1 octet   Transmission Medium
                                  Requirement (Q.763)
                                  Bits
                                  87654321
                                  --------
                                  00000000  speech
                                  00000001  spare
                                  00000010  64 kbit/s
                                  unrestricted


Groves, et al.              Standards Track                   [Page 137]

RFC 3525                Gateway Control Protocol               June 2003


                                  00000011  3.1 kHz audio
                                  00000100  reserved for
                                  alternate speech (service
                                  2)/64 kbit/s unrestricted
                                  (service 1)
                                  00000101  reserved for
                                  alternate 64 kbit/s
                                  unrestricted (service
                                  1)/speech (service 2)
                                  00000110  64 kbit/s preferred

                                  The assigned codepoints
                                  listed below are all for
                                  unrestricted service.
                                  00000111  2 x 64 kbit/s
                                  00001000  384 kbit/s
                                  00001001  1536 kbit/s
                                  00001010  1920 kbit/s
                                  00001011
                                   through
                                  00001111  spare
                                  00010000
                                   through
                                  00101010:
                                     3 x 64 kbit/s through
                                    29 x 64 kbit/s
                                  except
                                  00010011  spare
                                  00100101  spare

                                  00101011
                                   through
                                  11111111  spare
                                  Ref.: ITU-T Q.763

   TMRSR         9002   1 octet   Transmission Medium
                                  Requirement Subrate
                                  0     unspecified
                                  1      8 kbit/s
                                  2     16 kbit/s
                                  3     32 kbit/s

   Contcheck     9003   Boolean   Continuity Check
                                  0     continuity check not
                                  required on this circuit
                                  1     continuity check
                                  required on this circuit
                                  Ref.: ITU-T Q.763


Groves, et al.              Standards Track                   [Page 138]

RFC 3525                Gateway Control Protocol               June 2003


   ITC           9004   5 bits    Information Transfer
                                  Capability
                                  Bits
                                  5 4 3 2 1
                                  ---------
                                  0 0 0 0 0     Speech
                                  0 1 0 0 0     Unrestricted
                                  digital information
                                  0 1 0 0 1     Restricted
                                  digital information
                                  1 0 0 0 0     3.1 kHz audio
                                  1 0 0 0 1     Unrestricted
                                  digital information with
                                  tones/announcements
                                  1 1 0 0 0     Video
                                  All other values are
                                  reserved.
                                  Ref.: ITU-T Q.763

   TransMode     9005   2 bits    Transfer Mode
                                  Bits
                                  2 1
                                  ---
                                  0 0     Circuit mode
                                  1 0     Packet mode
                                  Ref.: ITU-T Q.931

   TransRate     9006   5 bits    Transfer Rate
                                  Bits
                                  5 4 3 2 1
                                  ---------
                                  0 0 0 0 0     This code shall
                                  be used for packet mode calls
                                  1 0 0 0 0      64 kbit/s
                                  1 0 0 0 1  2 x 64 kbit/s
                                  1 0 0 1 1     384 kbit/s
                                  1 0 1 0 1    1536 kbit/s
                                  1 0 1 1 1    1920 kbit/s
                                  1 1 0 0 0  Multirate (64
                                  kbit/s base rate)
                                  Ref.: ITU-T Q.931

   MULT          9007   7 bits    Rate Multiplier
                                  Any value from 2 to n
                                  (maximum number of B-
                                  channels)
                                  Ref.: ITU-T Q.931


Groves, et al.              Standards Track                   [Page 139]

RFC 3525                Gateway Control Protocol               June 2003


   layer1prot    9008   5 bits    User Information Layer 1
                                  Protocol
                                  Bits
                                  5 4 3 2 1
                                  ---------
                                  0 0 0 0 1     ITU-T
                                  standardized rate adaption
                                  V.110 and X.30.
                                  0 0 0 1 0     Recommendation
                                  G.711 m-law
                                  0 0 0 1 1     Recommendation
                                  G.711 A-law
                                  0 0 1 0 0     Recommendation
                                  G.721 32 kbit/s ADPCM and
                                  Recommendation I.460
                                  0 0 1 0 1     Recommendations
                                  H.221 and H.242
                                  0 0 1 1 0     Recommendations
                                  H.223 and H.245
                                  0 0 1 1 1     Non-ITU-T
                                  standardized rate adaption.
                                  0 1 0 0 0     ITU-T
                                  standardized rate adaption
                                  V.120.
                                  0 1 0 0 1     ITU-T
                                  standardized rate adaption
                                  X.31 HDLC flag stuffing
                                  All other values are
                                  reserved.
                                  Ref.: ITU Recommendation
                                  Q.931

   syncasync     9009   Boolean   Synchronous/Asynchronous
                                  0     Synchronous data
                                  1     Asynchronous data
                                  Ref.: ITU-T Q.931

   negotiation   900A   Boolean   Negotiation
                                  0     In-band negotiation
                                  possible
                                  1     In-band negotiation not
                                  possible
                                  Ref.: ITU-T Q.931

   Userrate      900B   5 bits    User Rate
                                  Bits
                                  5 4 3 2 1


Groves, et al.              Standards Track                   [Page 140]

RFC 3525                Gateway Control Protocol               June 2003


                                  ---------
                                  0 0 0 0 0     Rate is
                                  indicated by E-bits specified
                                  in Recommendation I.460 or
                                  may be negotiated in-band
                                  0 0 0 0 1     0.6 kbit/s
                                  Recommendations V.6 and X.1
                                  0 0 0 1 0     1.2 kbit/s
                                  Recommendation V.6
                                  0 0 0 1 1     2.4 kbit/s
                                  Recommendations V.6 and X.1
                                  0 0 1 0 0     3.6 kbit/s
                                  Recommendation V.6
                                  0 0 1 0 1     4.8 kbit/s
                                  Recommendations V.6 and X.1
                                  0 0 1 1 0     7.2 kbit/s
                                  Recommendation V.6
                                  0 0 1 1 1     8 kbit/s
                                  Recommendation I.460
                                  0 1 0 0 0     9.6 kbit/s
                                  Recommendations V.6 and X.1
                                  0 1 0 0 1     14.4 kbit/s
                                  Recommendation V.6
                                  0 1 0 1 0     16 kbit/s
                                  Recommendation I.460
                                  0 1 0 1 1     19.2 kbit/s
                                  Recommendation V.6
                                  0 1 1 0 0     32 kbit/s
                                  Recommendation I.460
                                  0 1 1 0 1     38.4 kbit/s
                                  Recommendation V.110
                                  0 1 1 1 0     48 kbit/s
                                  Recommendations V.6 and X.1
                                  0 1 1 1 1     56 kbit/s
                                  Recommendation V.6
                                  1 0 0 1 0     57.6 kbit/s
                                  Recommendation V.14 extended
                                  1 0 0 1 1     28.8 kbit/s
                                  Recommendation V.110
                                  1 0 1 0 0     24 kbit/s
                                  Recommendation V.110
                                  1 0 1 0 1     0.1345 kbit/s
                                  Recommendation X.1
                                  1 0 1 1 0     0.100 kbit/s
                                  Recommendation X.1
                                  1 0 1 1 1     0.075/1.2
                                  kbit/s Recommendations V.6
                                  and X.1


Groves, et al.              Standards Track                   [Page 141]

RFC 3525                Gateway Control Protocol               June 2003


                                  1 1 0 0 0     1.2/0.075
                                  kbit/s Recommendations V.6
                                  and X.1
                                  1 1 0 0 1     0.050 kbit/s
                                  Recommendations V.6 and X.1
                                  1 1 0 1 0     0.075 kbit/s
                                  Recommendations V.6 and X.1
                                  1 1 0 1 1     0.110 kbit/s
                                  Recommendations V.6 and X.1
                                  1 1 1 0 0     0.150 kbit/s
                                  Recommendations V.6 and X.1
                                  1 1 1 0 1     0.200 kbit/s
                                  Recommendations V.6 and X.1
                                  1 1 1 1 0     0.300 kbit/s
                                  Recommendations V.6 and X.1
                                  1 1 1 1 1     12 kbit/s
                                  Recommendation V.6
                                  All other values are
                                  reserved.
                                  Ref.: ITU-T Q.931
   INTRATE       900C   2 bits    Intermediate Rate
                                  Bits
                                  2 1
                                  ---
                                  0 0     Not used
                                  0 1     8 kbit/s
                                  1 0     16 kbit/s
                                  1 1     32 kbit/s
                                  Ref.: ITU-T Q.931

   nictx         900D   Boolean   Network Independent Clock
                                  (NIC) on transmission
                                  0     Not required to send
                                  data with network independent
                                  clock
                                  1     Required to send data
                                  with network independent
                                  clock
                                  Ref.: ITU-T Q.931

   nicrx         900E   Boolean   Network independent clock
                                  (NIC) on reception
                                  0     Cannot accept data with
                                  network independent clock
                                  (i.e., sender does not support
                                  this optional procedure)
                                  1     Can accept data with
                                  network independent clock


Groves, et al.              Standards Track                   [Page 142]

RFC 3525                Gateway Control Protocol               June 2003


                                  (i.e., sender does support
                                  this optional procedure)
                                  Ref.: ITU-T Q.931

   flowconttx    900F   Boolean   Flow Control on transmission
                                  (Tx)
                                  0     Not required to send
                                  data with flow control
                                  mechanism
                                  1     Required to send data
                                  with flow control mechanism
                                  Ref.: ITU-T Q.931

   flowcontrx    9010   Boolean   Flow control on reception
                                  (Rx)
                                  0     Cannot accept data with
                                  flow control mechanism (i.e.,
                                  sender does not support this
                                  optional procedure)
                                  1     Can accept data with
                                  flow control mechanism (i.e.,
                                  sender does support this
                                  optional procedure)
                                  Ref.: ITU-T Q.931

   rateadapthdr  9011   Boolean   Rate adaption header/no
                                  header
                                  0     Rate adaption header
                                  not included
                                  1     Rate adaption header
                                  included
                                  Ref.: ITU-T Q.931

   multiframe    9012   Boolean   Multiple frame establishment
                                  support in data link
                                  0     Multiple frame
                                  establishment not supported.
                                  Only UI frames allowed
                                  1     Multiple frame
                                  establishment supported
                                  Ref.: ITU-T Q.931

   OPMODE        9013   Boolean   Mode of operation
                                  0     Bit transparent mode of
                                  operation
                                  1     Protocol sensitive mode
                                  of operation
                                  Ref.: ITU-T Q.931


Groves, et al.              Standards Track                   [Page 143]

RFC 3525                Gateway Control Protocol               June 2003


   llidnegot     9014   Boolean   Logical link identifier
                                  negotiation
                                  0     Default, LLI = 256 only
                                  1     Full protocol
                                  negotiation
                                  Ref.: ITU-T Q.931

   assign        9015   Boolean   Assignor/assignee
                                  0     Message originator is
                                  "default assignee"
                                  1     Message originator is
                                  "assignor only"
                                  Ref.: ITU-T Q.931

   inbandneg     9016   Boolean   In-band/out-band negotiation
                                  0     Negotiation is done
                                  with USER INFORMATION
                                  messages on a temporary
                                  signalling connection
                                  1     Negotiation is done in-
                                  band using logical link zero
                                  Ref.: ITU-T Q.931

   stopbits      9017   2 bits    Number of stop bits
                                  Bits
                                  2 1
                                  ---
                                  0 0     Not used
                                  0 1     1 bit
                                  1 0     1.5 bits
                                  1 1     2 bits
                                  Ref.: ITU-T Q.931

   databits      9018   2 bits    Number of data bits excluding
                                  parity bit if present
                                  Bits
                                  2 1
                                  ---
                                  0 0     Not used
                                  0 1     5 bits
                                  1 0     7 bits
                                  1 1     8 bits
                                  Ref.: ITU-T Q.931

   parity        9019   3 bits    Parity information
                                  Bits
                                  3 2 1


Groves, et al.              Standards Track                   [Page 144]

RFC 3525                Gateway Control Protocol               June 2003


                                  ------
                                  0 0 0     Odd
                                  0 1 0     Even
                                  0 1 1     None
                                  1 0 0     Forced to 0
                                  1 0 1     Forced to 1
                                  All other values are
                                  reserved.
                                  Ref.: ITU-T Q.931

   duplexmode    901A   Boolean   Mode duplex
                                  0     Half duplex
                                  1     Full duplex
                                  Ref.: ITU-T Q.931

   modem         901B   6 bits    Modem Type
                                  Bits
                                  6 5 4 3 2 1
                                  -----------
                                  0 0 0 0 0 0 through
                                  0 0 0 1 0 1   National use
                                  0 1 0 0 0 1   Rec.  V.21
                                  0 1 0 0 1 0     Rec.  V.22
                                  0 1 0 0 1 1     Rec.  V.22 bis
                                  0 1 0 1 0 0     Rec.  V.23
                                  0 1 0 1 0 1     Rec.  V.26
                                  0 1 1 0 0 1     Rec.  V.26 bis
                                  0 1 0 1 1 1     Rec.  V.26 ter
                                  0 1 1 0 0 0     Rec.  V.27
                                  0 1 1 0 0 1     Rec.  V.27 bis
                                  0 1 1 0 1 0     Rec.  V.27 ter
                                  0 1 1 0 1 1     Rec.  V.29
                                  0 1 1 1 0 1     Rec.  V.32
                                  0 1 1 1 1 0     Rec.  V.34
                                  1 0 0 0 0 0  through
                                  1 0 1 1 1 1    National use
                                  1 1 0 0 0 0  through
                                  1 1 1 1 1 1    User specified
                                  Ref.: ITU-T Q.931

   layer2prot    901C   5 bits    User information layer 2
                                  protocol
                                  Bits
                                  5 4 3 2 1
                                  ---------
                                  0 0 0 1 0    Rec.  Q.921/I.441
                                  0 0 1 1 0    Rec.  X.25, link
                                  layer


Groves, et al.              Standards Track                   [Page 145]

RFC 3525                Gateway Control Protocol               June 2003


                                  0 1 1 0 0    LAN logical link
                                  control (ISO/IEC 8802  2)
                                  All other values are
                                  reserved.
                                  Ref.: ITU-T Q.931

   layer3prot    901D   5 bits    User information layer 3
                                  protocol
                                  Bits
                                  5 4 3 2 1
                                  ---------
                                  0 0 0 1 0     ITU-T Q.931
                                  0 0 1 1 0     ITU-T X.25,
                                  packet layer
                                  0 1 0 1 1     ISO/IEC TR 9577
                                  (Protocol identification in
                                  the network layer)
                                  All other values are
                                  reserved.
                                  Ref.: ITU-T Q.931

   addlayer3prot 901E   Octet     Additional User Information
                                  layer 3 protocol
                                  Bits        Bits
                                  4 3 2 1     4 3 2 1
                                  -------     -------
                                  1 1 0 0     1 1 0 0
                                  Internet Protocol (RFC 791)
                                  (ISO/IEC TR 9577)
                                  1 1 0 0     1 1 1 1
                                  Point-to-point Protocol (RFC
                                  1661)
                                  Ref.: ITU-T Q.931

   DialledN      901F   30        Dialled Number
                        octets

   DiallingN     9020   30        Dialling Number
                        octets

   ECHOCI        9021             Not Used.  See H.248.1 E.13
                                  for an example of possible
                                  Echo Control properties.

   NCI           9022   1 octet   Nature of Connection
                                  Indicators
                                  Bits
                                  2 1     Satellite Indicator


Groves, et al.              Standards Track                   [Page 146]

RFC 3525                Gateway Control Protocol               June 2003


                                  ---
                                  0 0     no satellite circuit
                                  in the connection
                                  0 1     one satellite circuit
                                  in the connection
                                  1 0     two satellite
                                  circuits in the connection
                                  1 1     spare

                                  Bits
                                  4 3     Continuity check
                                  ---     indicator
                                  0 0     continuity check not
                                  required
                                  0 1     continuity check
                                  required on this circuit
                                  1 0     continuity check
                                  performed on a previous
                                  circuit
                                  1 1     spare

                                  Bit
                                  5     Echo control device
                                  -     indicator
                                  0     outgoing echo control
                                  device not included
                                  1     outgoing echo control
                                  device included

                                  Bits
                                  8 7 6     Spare
                                  Ref.: ITU-T Q.763

   USI           9023   Octet     User Service Information
                        string    Ref.: ITU-T Q.763 Clause 3.57

C.10  AAL5 properties

   PropertyID Property    Type       Value
              tag

   FMSDU      A001        32-bit     Forward Maximum CPCS-SDU Size:
                          integer    Maximum CPCS-SDU size sent in the
                                     direction from the calling user to
                                     the called user.
                                     Ref.: ITU-T Q.2931


Groves, et al.              Standards Track                   [Page 147]

RFC 3525                Gateway Control Protocol               June 2003


   BMSDU      A002        32-bit     Backwards Maximum CPCS-SDU Size:
                          integer    Maximum CPCS-SDU size sent in the
                                     direction from the called user to
                                     the calling user.
                                     Ref.: ITU-T Q.2931

   SSCS       See table   See table  See table in C.7
              in C.7      in C.7     Additional values:
                                     VPI/VCI

C.11  SDP equivalents

   PropertyID Property    Type   Value
              tag

   SDP_V      B001        String Protocol Version
                                  Ref.: RFC 2327

   SDP_O      B002        String Owner/creator and session ID
                                  Ref.: RFC 2327

   SDP_S      B003        String Session name
                                  Ref.: RFC 2327

   SDP_I      B004        String Session identifier
                                  Ref.: RFC 2327

   SDP_U      B005        String URI of descriptor
                                  Ref.: RFC 2327

   SDC_E      B006        String email address
                                  Ref.: RFC 2327

   SDP_P      B007        String phone number
                                  Ref.: RFC 2327

   SDP_C      B008        String Connection information
                                  Ref.: RFC 2327

   SDP_B      B009        String Bandwidth Information
                                  Ref.: RFC 2327

   SDP_Z      B00A        String Time zone adjustment
                                  Ref.: RFC 2327

   SDP_K      B00B        String Encryption Key
                                  Ref.: RFC 2327


Groves, et al.              Standards Track                   [Page 148]

RFC 3525                Gateway Control Protocol               June 2003


   SDP_A      B00C        String Zero or more session attributes
                                  Ref.: RFC 2327

   SDP_T      B00D        String Active Session Time
                                  Ref.: RFC 2327

   SDP_R      B00E        String Zero or more repeat times
                                  Reference: RFC 2327

   SDP_M      B00F        String Media type, port, transport and format
                                  Ref.: RFC 2327

C.12  H.245

   PropertyID Property   Type     Value
              tag

   OLC        C001       Octet    The value of H.245
                                   OpenLogicalChannel structure.
                         string   Ref.: ITU-T H.245

   OLCack     C002       Octet    The value of H.245
                         string   OpenLogicalChannelAck structure.
                                   Ref.: ITU-T H.245

   OLCcnf     C003       Octet    The value of H.245
                         string   OpenLogicalChannelConfirm structure.
                                   Ref.: ITU-T H.245

   OLCrej     C004       Octet    The value of H.245
                         string   OpenLogicalChannelReject structure.
                                   Ref.: ITU-T H.245

   CLC        C005       Octet    The value of H.245
                         string   CloseLogicalChannel structure.
                                   Ref.: ITU-T H.245

   CLCack     C006       Octet    The value of H.245
                         string   CloseLogicalChannelAck structure.
                                   Ref.: ITU-T H.245


Groves, et al.              Standards Track                   [Page 149]

RFC 3525                Gateway Control Protocol               June 2003


ANNEX D - Transport over IP

D.1   Transport over IP/UDP using Application Level Framing (ALF)

   Protocol messages defined in this RFC may be transmitted over UDP.
   When no port is provided by the peer (see 7.2.8), commands should be
   sent to the default port number: 2944 for text-encoded operation, or
   2945 for binary-encoded operation.  Responses must be sent to the
   address and port from which the corresponding commands were sent.

   ALF is a set of techniques that allows an application, as opposed to
   a stack, to affect how messages are sent to the other side.  A
   typical ALF technique is to allow an application to change the order
   of messages sent when there is a queue after it has queued them.
   There is no formal specification for ALF.  The procedures in Annex
   D.1 contain a minimum suggested set of ALF behaviours

   Implementors using IP/UDP with ALF should be aware of the
   restrictions of the MTU on the maximum message size.

D.1.1 Providing At-Most-Once functionality

   Messages, being carried over UDP, may be subject to losses.  In the
   absence of a timely response, commands are repeated.  Most commands
   are not idempotent.  The state of the MG would become unpredictable
   if, for example, Add commands were executed several times.  The
   transmission procedures shall thus provide an "At-Most-Once"
   functionality.

   Peer protocol entities are expected to keep in memory a list of the
   responses that they sent to recent transactions and a list of the
   transactions that are currently outstanding.  The transaction
   identifier of each incoming message is compared to the transaction
   identifiers of the recent responses sent to the same MId.  If a match
   is found, the entity does not execute the transaction, but simply
   repeats the response.  If no match is found, the message will be
   compared to the list of currently outstanding transactions.  If a
   match is found in that list, indicating a duplicate transaction, the
   entity does not execute the transaction (see D.1.4 for procedures on
   sending TransactionPending).

   The procedure uses a long timer value, noted LONG-TIMER in the
   following.  The timer should be set larger than the maximum duration
   of a transaction, which should take into account the maximum number


Groves, et al.              Standards Track                   [Page 150]

RFC 3525                Gateway Control Protocol               June 2003


   of repetitions, the maximum value of the repetition timer and the
   maximum propagation delay of a packet in the network.  A suggested
   value is 30 seconds.

   The copy of the responses may be destroyed either LONG-TIMER seconds
   after the response is issued, or when the entity receives a
   confirmation that the response has been received, through the
   "Response Acknowledgement parameter".  For transactions that are
   acknowledged through this parameter, the entity shall keep a copy of
   the transaction-id for LONG-TIMER seconds after the response is
   issued, in order to detect and ignore duplicate copies of the
   transaction request that could be produced by the network.

D.1.2 Transaction identifiers and three-way handshake

D.1.2.1  Transaction identifiers

   Transaction identifiers are 32-bit integer numbers.  A Media Gateway
   Controller may decide to use a specific number space for each of the
   MGs that they manage, or to use the same number space for all MGs
   that belong to some arbitrary group.  MGCs may decide to share the
   load of managing a large MG between several independent processes.
   These processes will share the same transaction number space.  There
   are multiple possible implementations of this sharing, such as having
   a centralized allocation of transaction identifiers, or
   pre-allocating non-overlapping ranges of identifiers to different
   processes.  The implementations shall guarantee that unique
   transaction identifiers are allocated to all transactions that
   originate from a logical MGC (identical mId).  MGs can simply detect
   duplicate transactions by looking at the transaction identifier and
   mId only.

D.1.2.2  Three-way handshake

   The TransactionResponse Acknowledgement parameter can be found in any
   message.  It carries a set of "confirmed transaction-id ranges".
   Entities may choose to delete the copies of the responses to
   transactions whose id is included in "confirmed transaction-id
   ranges" received in the transaction response messages.  They should
   silently discard further commands when the transaction-id falls
   within these ranges.

   The "confirmed transaction-id ranges" values shall not be used if
   more than LONG-TIMER seconds have elapsed since the MG issued its
   last response to that MGC, or when a MG resumes operation.  In this
   situation, transactions should be accepted and processed, without any
   test on the transaction-id.


Groves, et al.              Standards Track                   [Page 151]

RFC 3525                Gateway Control Protocol               June 2003


   Messages that carry the "Transaction Response Acknowledgement"
   parameter may be transmitted in any order.  The entity shall retain
   the "confirmed transaction-id ranges" received for LONG-TIMER
   seconds.

   In the binary encoding, if only the firstAck is present in a response
   acknowledgement (see A.2), only one transaction is acknowledged.  If
   both firstAck and lastAck are present, then the range of transactions
   from firstAck to lastAck is acknowledged.  In the text encoding, a
   horizontal dash is used to indicate a range of transactions being
   acknowledged (see B.2).

D.1.3 Computing retransmission timers

   It is the responsibility of the requesting entity to provide suitable
   timeouts for all outstanding transactions, and to retry transactions
   when timeouts have been exceeded.  Furthermore, when repeated
   transactions fail to be acknowledged, it is the responsibility of the
   requesting entity to seek redundant services and/or clear existing or
   pending connections.

   The specification purposely avoids specifying any value for the
   retransmission timers.  These values are typically network dependent.
   The retransmission timers should normally estimate the timer value by
   measuring the time spent between the sending of a command and the
   return of a response.  Implementations SHALL ensure that the
   algorithm used to calculate retransmission timing performs an
   exponentially increasing backoff of the retransmission timeout for
   each retransmission or repetition after the first one.

     NOTE - One possibility is to use the algorithm implemented in
     TCP-IP, which uses two variables:

   -  The average acknowledgement delay (AAD), estimated through an
      exponentially smoothed average of the observed delays.

   -  The average deviation (ADEV), estimated through an exponentially
      smoothed average of the absolute value of the difference between
      the observed delay and the current average.  The retransmission
      timer, in TCP, is set to the sum of the average delay plus N times
      the average deviation.  The maximum value of the timer should
      however be bounded for the protocol defined in this
      RFC, in order to guarantee that no repeated packet
      would be received by the gateways after LONG-TIMER seconds.  A
      suggested maximum value is 4 seconds.


Groves, et al.              Standards Track                   [Page 152]

RFC 3525                Gateway Control Protocol               June 2003


   After any retransmission, the entity SHOULD do the following:

   -  It should double the estimated value of the average delay, AAD.

   -  It should compute a random value, uniformly distributed between
      0.5 AAD and AAD.

   -  It should set the retransmission timer to the sum of that random
      value and N times the average deviation.

   This procedure has two effects.  Because it includes an exponentially
   increasing component, it will automatically slow down the stream of
   messages in case of congestion.  Because it includes a random
   component, it will break the potential synchronization between
   notifications triggered by the same external event.

D.1.4 Provisional responses

   Executing some transactions may require a long time.  Long execution
   times may interact with the timer-based retransmission procedure.
   This may result either in an inordinate number of retransmissions, or
   in timer values that become too long to be efficient.  Entities that
   can predict that a transaction will require a long execution time may
   send a provisional response, "Transaction Pending".  They SHOULD send
   this response if they receive a repetition of a transaction that is
   still being executed.

   Entities that receive a Transaction Pending shall switch to a
   different repetition timer for repeating requests.  The root
   Termination has a property (ProvisionalResponseTimerValue), which can
   be set to the requested maximum number of milliseconds between
   receipt of a command and transmission of the TransactionPending
   response.  Upon receipt of a final response following receipt of
   provisional responses, an immediate confirmation shall be sent, and
   normal repetition timers shall be used thereafter.  An entity that
   sends a provisional response, SHALL include the immAckRequired field
   in the ensuing final response, indicating that an immediate
   confirmation is expected.  Receipt of a Transaction Pending after
   receipt of a reply shall be ignored.

D.1.5 Repeating Requests, Responses and Acknowledgements

   The protocol is organized as a set of transactions, each of which is
   composed of a request and a response, commonly referred to as an
   acknowledgement.  The protocol messages, being carried over UDP, may
   be subject to losses.  In the absence of a timely response,
   transactions are repeated.  Entities are expected to keep in memory a


Groves, et al.              Standards Track                   [Page 153]

RFC 3525                Gateway Control Protocol               June 2003


   list of the responses that they sent to recent transactions, i.e., a
   list of all the responses they sent over the last LONG-TIMER seconds,
   and a list of the transactions that are currently being executed.

   The repetition mechanism is used to guard against three types of
   possible errors:

   -  transmission errors, when for example a packet is lost due to
      noise on a line or congestion in a queue;

   -  component failure, when for example an interface to a entity
      becomes unavailable;

   -  entity failure, when for example an entire entity becomes
      unavailable.

   The entities should be able to derive from the past history an
   estimate of the packet loss rate due to transmission errors.  In a
   properly configured system, this loss rate should be kept very low,
   typically less than 1%.  If a Media Gateway Controller or a Media
   Gateway has to repeat a message more than a few times, it is very
   legitimate to assume that something else than a transmission error is
   occurring.   For example, given a loss rate of 1%, the probability
   that five consecutive transmission attempts fail is 1 in 100 billion,
   an event that should occur less than once every 10 days for a Media
   Gateway Controller that processes 1000 transactions per second.
   (Indeed, the number of repetition that is considered excessive should
   be a function of the prevailing packet loss rate.)  We should note
   that the "suspicion threshold", which we will call "Max1", is
   normally lower than the "disconnection threshold", which should be
   set to a larger value.

   A classic retransmission algorithm would simply count the number of
   successive repetitions, and conclude that the association is broken
   after retransmitting the packet an excessive number of times
   (typically between 7 and 11 times.)  In order to account for the
   possibility of an undetected or in  progress "failover", we modify
   the classic algorithm so that if the Media Gateway receives a valid
   ServiceChange message announcing a failover, it will start
   transmitting outstanding commands to that new MGC.  Responses to
   commands are still transmitted to the source address of the command.

   In order to automatically adapt to network load, this RFC specifies
   exponentially increasing timers.  If the initial timer is set to 200
   milliseconds, the loss of a fifth retransmission will be detected
   after about 6 seconds.  This is probably an acceptable waiting delay
   to detect a failover.  The repetitions should continue after that
   delay not only in order to perhaps overcome a transient connectivity


Groves, et al.              Standards Track                   [Page 154]

RFC 3525                Gateway Control Protocol               June 2003


   problem, but also in order to allow some more time for the execution
   of a failover  (waiting a total delay of 30 seconds is probably
   acceptable).

   It is, however, important that the maximum delay of retransmissions
   be bounded.  Prior to any retransmission, it is checked that the time
   elapsed since the sending of the initial datagram is no greater than
   T-MAX.  If more than T-MAX time has elapsed, the MG concludes that
   the MGC has failed, and it begins its recovery process as described
   in section 11.5.  If the MG retries to connect to the current MGC it
   shall use a ServiceChange with ServiceChangeMethod set to
   Disconnected so that the new MGC will be aware that the MG lost one
   or more transactions.  The value T-MAX is related to the LONG-TIMER
   value: the LONG-TIMER value is obtained by adding to T  MAX the
   maximum propagation delay in the network.

D.2   Using TCP

   Protocol messages as defined in this RFC may be transmitted over TCP.
   When no port is specified by the other side (see 7.2.8), the commands
   should be sent to the default port.  The defined protocol has
   messages as the unit of transfer, while TCP is a stream-oriented
   protocol.  TPKT, according to RFC 1006, SHALL be used to delineate
   messages within the TCP stream.

   In a transaction-oriented protocol, there are still ways for
   transaction requests or responses to be lost.  As such, it is
   recommended that entities using TCP transport implement application
   level timers for each request and each response, similar to those
   specified for application level framing over UDP.

D.2.1 Providing the At-Most-Once functionality

   Messages, being carried over TCP, are not subject to transport
   losses, but loss of a transaction request or its reply may
   nonetheless be noted in real implementations.  In the absence of a
   timely response, commands are repeated.  Most commands are not
   idempotent.  The state of the MG would become unpredictable if, for
   example, Add commands were executed several times.

   To guard against such losses, it is recommended that entities follow
   the procedures in D.1.1.

D.2.2 Transaction identifiers and three-way handshake

   For the same reasons, it is possible that transaction replies may be
   lost even with a reliable delivery protocol such as TCP.  It is
   recommended that entities follow the procedures in D.1.2.2.


Groves, et al.              Standards Track                   [Page 155]

RFC 3525                Gateway Control Protocol               June 2003


D.2.3 Computing retransmission timers

   With reliable delivery, the incidence of loss of a transaction
   request or reply is expected to be very low.  Therefore, only simple
   timer mechanisms are required.  Exponential back-off algorithms
   should not be necessary, although they could be employed where, as in
   an MGC, the code to do so is already required, since MGCs must
   implement ALF/UDP as well as TCP.

D.2.4 Provisional responses

   As with UDP, executing some transactions may require a long time.
   Entities that can predict that a transaction will require a long
   execution time may send a provisional response, "Transaction
   Pending".  They should send this response if they receive a
   repetition of a transaction that is still being executed.

   Entities that receive a Transaction Pending shall switch to a longer
   repetition timer for that transaction.

   Entities shall retain Transactions and replies until they are
   confirmed.  The basic procedure of D.1.4 should be followed, but
   simple timer values should be sufficient.  There is no need to send
   an immediate confirmation upon receipt of a final response.

D.2.5 Ordering of commands

   TCP provides ordered delivery of transactions.  No special procedures
   are required.  It should be noted that ALF/UDP allows sending entity
   to modify its behaviour under congestion, and in particular, could
   reorder transactions when congestion is encountered.  TCP could not
   achieve the same results.


Groves, et al.              Standards Track                   [Page 156]

RFC 3525                Gateway Control Protocol               June 2003


ANNEX E - Basic packages

   This annex contains definitions of some packages for use with
   Recommendation H.248.1.

E.1   Generic

   PackageID: g (0x0001)
   Version: 1
   Extends: None

   Description:
      Generic package for commonly encountered items.

E.1.1 Properties

   None.

E.1.2 Events

   Cause

      EventID: cause (0x0001)
      Generic error event

      EventsDescriptor parameters:  None

      ObservedEvents Descriptor Parameters:

         General Cause
         ParameterID: Generalcause (0x0001)

            This parameter groups the failures into six groups, which
            the MGC may act upon.

            Type: enumeration

            Possible values:
                     "NR" Normal Release (0x0001)
                     "UR" Unavailable Resources (0x0002)
                     "FT" Failure, Temporary (0x0003)
                     "FP" Failure, Permanent (0x0004)
                     "IW" Interworking Error (0x0005)
                     "UN" Unsupported (0x0006)

         Failure Cause
         ParameterID: Failurecause (0x0002)


Groves, et al.              Standards Track                   [Page 157]

RFC 3525                Gateway Control Protocol               June 2003


            Possible values:  OCTET STRING

            Description: The Failure Cause is the value generated by the
            Released equipment, i.e., a released network connection.
            The concerned value is defined in the appropriate bearer
            control protocol.

   Signal Completion

      EventID: sc (0x0002)

      Indicates the termination of a signal for which the
      notifyCompletion parameter was set to enable reporting of a
      completion event.  For further procedural description, see 7.1.1,
      7.1.17 and 7.2.7.

      EventsDescriptor parameters:  None

      ObservedEvents Descriptor parameters:

         Signal Identity
         ParameterID: SigID (0x0001)

            This parameter identifies the signal which has terminated.
            For a signal that is contained in a signal list, the signal
            list identity parameter should also be returned indicating
            the appropriate list.

            Type: Binary: octet (string), Text: string

            Possible values: a signal which has terminated.  A signal
            shall be identified using the pkgdName syntax without
            wildcarding.

         Termination Method
         ParameterID: Meth (0x0002)

            Indicates the means by which the signal terminated.

            Type: enumeration

            Possible values:
               "TO" (0x0001) Signal timed out or otherwise completed on
               its own
               "EV" (0x0002) Interrupted by event
               "SD" (0x0003) Halted by new Signals descriptor
               "NC" (0x0004) Not completed, other cause


Groves, et al.              Standards Track                   [Page 158]

RFC 3525                Gateway Control Protocol               June 2003


         Signal List ID
         ParameterID:  SLID (0x0003)

            Indicates to which signal list a signal belongs.  The
            SignalList ID is only returned in cases where the signal
            resides in a signal list.

            Type: integer

            Possible values: any integer

E.1.3 Signals

   None.

E.1.4 Statistics

   None.

E.2   Base Root Package

   PackageID: root (0x0002)
   Version: 1
   Extends: None

   Description:
      This package defines Gateway wide properties.

E.2.1 Properties

   MaxNrOfContexts
   PropertyID: maxNumberOfContexts (0x0001)

      The value of this property gives the maximum number of contexts
      that can exist at any time.  The NULL context is not included in
      this number.

      Type: double

      Possible values: 1 and up

      Defined in: TerminationState

      Characteristics: read only

   MaxTerminationsPerContext
   PropertyID: maxTerminationsPerContext (0x0002)


Groves, et al.              Standards Track                   [Page 159]

RFC 3525                Gateway Control Protocol               June 2003


      The maximum number of allowed terminations in a context, see 6.1

      Type: integer

      Possible values: any integer

      Defined in: TerminationState

      Characteristics: read only

   normalMGExecutionTime
   PropertyId: normalMGExecutionTime (0x0003)

      Settable by the MGC to indicate the interval within which the MGC
      expects a response to any transaction from the MG (exclusive of
      network delay)

      Type: integer

      Possible values: any integer, represents milliseconds

      Defined in: TerminationState

      Characteristics: read / write

   normalMGCExecutionTime
   PropertyId: normalMGCExecutionTime (0x0004)

      Settable by the MGC to indicate the interval within which the MG
      should expects a response to any transaction from the MGC
      (exclusive of network delay)

      Type: integer

      Possible values: any integer, represents milliseconds

      Defined in: TerminationState

      Characteristics: read / write

   MGProvisionalResponseTimerValue
   PropertyId: MGProvisionalResponseTimerValue (0x0005)

      Indicates the time within which the MGC should expect a Pending
      Response from the MG if a Transaction cannot be completed.

      Initially set to normalMGExecutionTime plus network delay, but may
      be lowered.


Groves, et al.              Standards Track                   [Page 160]

RFC 3525                Gateway Control Protocol               June 2003


      Type: Integer

      Possible Values: any integer, represents milliseconds

      Defined in: TerminationState

      Characteristics: read / write

   MGCProvisionalResponseTimerValue
   PropertyId: MGCProvisionalResponseTimerValue (0x0006)

      Indicates the time within which the MG should expect a Pending
      Response from the MGC if a Transaction cannot be completed.
      Initially set to normalMGCExecutionTime plus network delay, but
      may be lowered.

      Type: Integer

      Possible Values: any integer, represents milliseconds

      Defined in: TerminationState

      Characteristics: read / write

E.2.2 Events

   None.

E.2.3 Signals

   None.

E.2.4 Statistics

   None.

E.2.5 Procedures

   None.

E.3   Tone Generator Package

   PackageID: tonegen (0x0003)
   Version: 1
   Extends: None


Groves, et al.              Standards Track                   [Page 161]

RFC 3525                Gateway Control Protocol               June 2003


   Description:

      This package defines signals to generate audio tones.  This
      package does not specify parameter values.  It is intended to be
      extendable.  Generally, tones are defined as an individual signal
      with a parameter, ind, representing "interdigit" time delay, and a
      tone id to be used with playtones.  A tone id should be kept
      consistent with any tone generation for the same tone.  MGs are
      expected to be provisioned with the characteristics of appropriate
      tones for the country in which the MG is located.

   Designed to be extended only.

E.3.1 Properties

   None.

E.3.2 Events

   None.

E.3.3 Signals

   Play tone
   SignalID: pt (0x0001)

      Plays audio tone over an audio channel

      Signal Type: Brief

      Duration: Provisioned

      Additional parameters:

         Tone id list
         ParameterID: tl (0x0001)

            Type: list of tone ids

            List of tones to be played in sequence.  The list SHALL
            contain one or more tone ids.

         Inter signal duration
         ParameterID: ind (0x0002)

            Type: integer

            Timeout between two consecutive tones in milliseconds


Groves, et al.              Standards Track                   [Page 162]

RFC 3525                Gateway Control Protocol               June 2003


   No tone ids are specified in this package.  Packages that extend this
   package can add possible values for tone id as well as adding
   individual tone signals.

E.3.4 Statistics

   None.

E.3.5 Procedures

   None.

E.4   Tone Detection Package

   PackageID: tonedet (0x0004)
   Version: 1
   Extends: None

   This Package defines events for audio tone detection.  Tones are
   selected by name (tone id).  MGs are expected to be provisioned with
   the characteristics of appropriate tones for the country in which the
   MG is located.

   Designed to be extended only:
      This package does not specify parameter values.  It is intended to
      be extendable.

E.4.1 Properties

   None.

E.4.2 Events

   Start tone detected
   EventID: std, 0x0001

      Detects the start of a tone.  The characteristics of positive tone
      detection are implementation dependent.

      EventsDescriptor parameters:

         Tone id list
         ParameterID: tl (0x0001)

            Type: list of tone ids


Groves, et al.              Standards Track                   [Page 163]

RFC 3525                Gateway Control Protocol               June 2003


            Possible values: The only tone id defined in this package is
            "wild card" which is "*" in text encoding and 0x0000 in
            binary.  Extensions to this package would add possible
            values for tone id.  If tl is "wild card", any tone id is
            detected.

         ObservedEventsDescriptor parameters:

         Tone id
         ParameterID: tid (0x0003)

            Type: enumeration

            Possible values: "wildcard" as defined above is the only
            value defined in this package.  Extensions to this package
            would add additional possible values for tone id.

         End tone detected
         EventID: etd, 0x0002

         Detects the end of a tone.

         EventDescriptor parameters:

            Tone id list
            ParameterID: tl (0x0001)

               Type: enumeration or list of enumerated types

               Possible values: No possible values are specified in this
               package.  Extensions to this package would add possible
               values for tone id.

         ObservedEventsDescriptor parameters:

            Tone id
            ParameterID: tid (0x0003)

               Type: enumeration

               Possible values: "wildcard" as defined above is the only
               value defined in this package.  Extensions to this
               package would add possible values for tone id.

            Duration
            ParameterId: dur (0x0002)

               Type: integer, in milliseconds


Groves, et al.              Standards Track                   [Page 164]

RFC 3525                Gateway Control Protocol               June 2003


               This parameter contains the duration of the tone from
               first detection until it stopped.

   Long tone detected
   EventID: ltd, 0x0003

      Detects that a tone has been playing for at least a certain amount
      of time.

      EventDescriptor parameters:

         Tone id list
         ParameterID: tl (0x0001)

            Type: enumeration or list

            Possible values: "wildcard" as defined above is the only
            value defined in this package.  Extensions to this package
            would add possible values for tone id.

         Duration
         ParameterID: dur (0x0002)

            Type: integer, duration to test against

            Possible values: any legal integer, expressed in
            milliseconds

      ObservedEventsDescriptor parameters:

         Tone id
         ParameterID: tid (0x0003)

            Type: Enumeration

            Possible values: No possible values are specified in this
            package.  Extensions to this package would add possible
            values for tone id.

E.4.3 Signals

   None.

E.4.4 Statistics

   None.


Groves, et al.              Standards Track                   [Page 165]

RFC 3525                Gateway Control Protocol               June 2003


E.4.5 Procedures

   None.

E.5   Basic DTMF Generator Package

   PackageID: dg (0x0005)
   Version: 1
   Extends: tonegen version 1

   This package defines the basic DTMF tones as signals and extends the
   allowed values of parameter tl of playtone in tonegen.

E.5.1 Properties

   None.

E.5.2 Events

   None.

E.5.3 Signals

   DTMF character 0
   SignalID: d0 (0x0010)

      Generate DTMF 0 tone.  The physical characteristic of DTMF 0 is
      defined in the gateway.

      Signal Type: Brief

      Duration: Provisioned

      Additional parameters:

         None.

   Additional values:

      d0 (0x0010) is defined as a tone id for playtone

   The other DTMF characters are specified in exactly the same way.  A
   table with all signal names and signal IDs is included.  Note that
   each DTMF character is defined as both a signal and a tone id, thus
   extending the basic tone generation package.  Also note that DTMF
   SignalIds are different from the names used in a digit map.


Groves, et al.              Standards Track                   [Page 166]

RFC 3525                Gateway Control Protocol               June 2003


                     Signal name     Signal ID/Tone id

                    DTMF character 0    d0 (0x0010)
                    DTMF character 1    d1 (0x0011)
                    DTMF character 2    d2 (0x0012)
                    DTMF character 3    d3 (0x0013)
                    DTMF character 4    d4 (0x0014)
                    DTMF character 5    d5 (0x0015)
                    DTMF character 6    d6 (0x0016)
                    DTMF character 7    d7 (0x0017)
                    DTMF character 8    d8 (0x0018)
                    DTMF character 9    d9 (0x0019)
                    DTMF character *    ds (0x0020)
                    DTMF character #    do (0x0021)
                    DTMF character A    da (0x001a)
                    DTMF character B    db (0x001b)
                    DTMF character C    dc (0x001c)
                    DTMF character D    dd (0x001d)

E.5.4 Statistics

   None.

E.5.5 Procedures

   None.

E.6   DTMF detection Package

   PackageID: dd (0x0006)
   Version: 1
   Extends: tonedet version 1

   This package defines the basic DTMF tones detection.  This Package
   extends the possible values of tone id in the "start tone detected"
   "end tone detected" and "long tone detected" events.

   Additional tone id values are all tone ids described in package dg
   (basic DTMF generator package).

   The following table maps DTMF events to digit map symbols as
   described in 7.1.14.

                           DTMF Event Symbol

                           d0   "0"
                           d1   "1"
                           d2   "2"


Groves, et al.              Standards Track                   [Page 167]

RFC 3525                Gateway Control Protocol               June 2003


                           d3   "3"
                           d4   "4"
                           d5   "5"
                           d6   "6"
                           d7   "7"
                           d8   "8"
                           d9   "9"
                           da   "A" or "a"
                           db   "B" or "b"
                           dc   "C" or "c"
                           dd   "D" or "d"
                           ds   "E" or "e"
                           do   "F" or "f"

E.6.1 Properties

   None.

E.6.2 Events

   DTMF digits

      EventIds are defined with the same names as the SignalIds defined
      in the table found in E.5.3.

   DigitMap Completion Event
   EventID: ce, 0x0004

      Generated when a digit map completes as described in 7.1.14.

      EventsDescriptor parameters: None.

      ObservedEventsDescriptor parameters:

         DigitString
         ParameterID: ds (0x0001)

            Type: string of digit map symbols (possibly empty) returned
            as a quotedString

            Possible values: a sequence of the characters "0" through
            "9", "A" through "F", and the long duration modifier "Z".

            Description: the portion of the current dial string as
            described in 7.1.14 which matched part or all of an
            alternative event sequence specified in the digit map.


Groves, et al.              Standards Track                   [Page 168]

RFC 3525                Gateway Control Protocol               June 2003


         Termination Method
         ParameterID: Meth (0x0003)

            Type: enumeration

            Possible values:

               "UM" (0x0001) Unambiguous match

               "PM" (0x0002) Partial match, completion by timer expiry
               or unmatched event

               "FM" (0x0003) Full match, completion by timer expiry or
               unmatched event

            Description: indicates the reason for generation of the
            event.  See the procedures in 7.1.14.

E.6.3 Signals

   None.

E.6.4 Statistics

   None.

E.6.5 Procedures

   Digit map processing is activated only if an events descriptor is
   activated that contains a digit map completion event as defined in
   Section E.6.2 and that digit map completion event contains an eventDM
   field in the requested actions as defined in Section 7.1.9.  Other
   parameters such as KeepActive or embedded events of signals
   descriptors may also be present in the events descriptor and do not
   affect the activation of digit map processing.

E.7   Call Progress Tones Generator Package

   PackageID: cg, 0x0007
   Version: 1
   Extends: tonegen version 1

   This package defines the basic call progress tones as signals and
   extends the allowed values of the tl parameter of playtone in
   tonegen.


Groves, et al.              Standards Track                   [Page 169]

RFC 3525                Gateway Control Protocol               June 2003


E.7.1 Properties

   None.

E.7.2 Events

   None.

E.7.3 Signals

   Dial Tone
   SignalID: dt (0x0030)

      Generate dial tone.  The physical characteristic of dial tone is
      available in the gateway.

      Signal Type: TimeOut

      Duration: Provisioned

      Additional parameters:

         None.

   Additional values:

      dt (0x0030) is defined as a tone id for playtone

   The other tones of this package are defined in exactly the same way.
   A table with all signal names and signal IDs is included.  Note that
   each tone is defined as both a signal and a tone id, thus extending
   the basic tone generation package.

     Signal Name                 Signal ID/tone id

     Dial Tone                   dt (0x0030)
     Ringing Tone                rt (0x0031)
     Busy Tone                   bt (0x0032)
     Congestion Tone             ct (0x0033)
     Special Information Tone    sit(0x0034)
     Warning Tone                wt (0x0035)
     Payphone Recognition Tone   prt (0x0036)
     Call Waiting Tone           cw (0x0037)
     Caller Waiting Tone         cr (0x0038)

E.7.4 Statistics

   None.


Groves, et al.              Standards Track                   [Page 170]

RFC 3525                Gateway Control Protocol               June 2003


E.7.5 Procedures

      NOTE - The required set of tone ids corresponds to those defined
      in Recommendation E.180/Q.35.  See Recommendation E.180/Q.35 for
      definition of the meanings of these tones.


E.8   Call Progress Tones Detection Package

   PackageID: cd (0x0008)
   Version: 1
   Extends: tonedet version 1

   This package defines the basic call progress detection tones.  This
   package extends the possible values of tone id in the "start tone
   detected", "end tone detected" and "long tone detected" events.

   Additional values

      toneID values are defined for start tone detected, end tone
      detected and long tone detected with the same values as those in
      package cg (call progress tones generation package).

   The required set of tone ids corresponds to Recommendation
   E.180/Q.35.  See Recommendation E.180/Q.35 for definition of the
   meanings of these tones.

E.8.1 Properties

   None.

E.8.2 Events

   Events are defined as in the call progress tones generator package
   (cg) for the tones listed in the table of E.7.3.

E.8.3 Signals

   None.

E.8.4 Statistics

   None.

E.8.5 Procedures

   None.


Groves, et al.              Standards Track                   [Page 171]

RFC 3525                Gateway Control Protocol               June 2003


E.9   Analog Line Supervision Package

   PackageID: al, 0x0009
   Version: 1
   Extends: None

   This package defines events and signals for an analog line.

   E.9.1 Properties

   None.

E.9.2 Events

   onhook
   EventID: on (0x0004)

      Detects handset going on hook.  Whenever an events descriptor is
      activated that requests monitoring for an on-hook event and the
      line is already on-hook, then the MG shall behave according to the
      setting of the "strict" parameter.

      EventDescriptor parameters:

         Strict Transition
         ParameterID: strict (0x0001)

            Type: enumeration

            Possible values: "exact" (0x00), "state" (0x01), "failWrong"
            (0x02)

              "exact" means that only an actual hook state transition to
              on-hook is to be recognized;

              "state" means that the event is to be recognized either if
              the hook state transition is detected or if the hook state
              is already on-hook;

              "failWrong" means that if the hook state is already
              on-hook, the command fails and an error is reported.

      ObservedEventsDescriptor parameters:

         Initial State
         ParameterID: init (0x0002)

            Type: Boolean


Groves, et al.              Standards Track                   [Page 172]

RFC 3525                Gateway Control Protocol               June 2003


            Possible values:

               "True" means that the event was reported because the line
               was already on-hook when the events descriptor containing
               this event was activated;

               "False" means that the event represents an actual state
               transition to on-hook.

   offhook
   EventID: of (0x0005)

      Detects handset going off hook.  Whenever an events descriptor is
      activated that requests monitoring for an off-hook event and the
      line is already off-hook, then the MG shall behave according to
      the setting of the "strict" parameter.

      EventDescriptor parameters:

         Strict Transition
         ParameterID: strict (0x0001)

            Type: enumeration

            Possible values: "exact" (0x00), "state" (0x01), "failWrong"
            (0x02)

               "exact" means that only an actual hook state transition
               to off-hook is to be recognized;

               "state" means that the event is to be recognized either
               if the hook state transition is detected or if the hook
               state is already off-hook;

               "failWrong" means that if the hook state is already off-
               hook, the command fails and an error is reported.

      ObservedEventsDescriptor parameters

         Initial State
         ParameterID: init (0x0002)

            Type: Boolean


Groves, et al.              Standards Track                   [Page 173]

RFC 3525                Gateway Control Protocol               June 2003


            Possible values:

               "True" means that the event was reported because the line
               was already off-hook when the events descriptor
               containing this event was activated;

               "False" means that the event represents an actual state
               transition to off-hook.

   flashhook
   EventID: fl, 0x0006

      Detects handset flash.  A flash occurs when an onhook is followed
      by an offhook between a minimum and maximum duration.

      EventDescriptor parameters:

         Minimum duration
         ParameterID: mindur (0x0004)

            Type: integer in milliseconds

            Default value is provisioned.

         Maximum duration
         ParameterID: maxdur (0x0005)

            Type: integer in milliseconds

            Default value is provisioned.

      ObservedEventsDescriptor parameters:

         None

E.9.3 Signals

   ring
   SignalID: ri, 0x0002

      Applies ringing on the line

      Signal Type: TimeOut

      Duration: Provisioned


Groves, et al.              Standards Track                   [Page 174]

RFC 3525                Gateway Control Protocol               June 2003


      Additional parameters:

         Cadence
         ParameterID: cad (0x0006)

            Type: list of integers representing durations of alternating
            on and off segments, constituting a complete ringing cycle
            starting with an on.  Units in milliseconds

            Default is fixed or provisioned.  Restricted function MGs
            may ignore cadence values they are incapable of generating.

         Frequency
         ParameterID: freq (0x0007)

            Type: integer in Hz

            Default is fixed or provisioned.  Restricted function MGs
            may ignore frequency values they are incapable of
            generating.

E.9.4 Statistics

   None.

E.9.5 Procedures

   If the MGC sets an EventsDescriptor containing a hook state
   transition event (on-hook or off-hook) with the "strict" (0x0001)
   parameter set to "failWrong", and the hook state is already what the
   transition implies, the execution of the command containing that
   EventsDescriptor fails.  The MG SHALL include error code 540
   "Unexpected initial hook state" in its reponse.

E.9.6 Error code

   This package defines a new error code:

      540 - Unexpected initial hook state

   The procedure for use of this code is given in E.9.5.

E.10  Basic Continuity Package

   PackageID: ct (0x000a)
   Version: 1
   Extends: None


Groves, et al.              Standards Track                   [Page 175]

RFC 3525                Gateway Control Protocol               June 2003


   This package defines events and signals for continuity test.  The
   continuity test includes provision of either a loopback or
   transceiver functionality.

E.10.1   Properties

   None.

E.10.2   Events

   Completion
   EventID: cmp, 0x0005

      This event detects test completion of continuity test.

      EventDescriptor parameters

         None.

      ObservedEventsDescriptor parameters

         Result
         ParameterID: res (0x0008)

            Type: enumeration

            Possible values: success (0x0001), failure (0x0000)

E.10.3   Signals

   Continuity test
   SignalID: ct (0x0003)

      Initiates sending of continuity test tone on the termination to
      which it is applied.

      Signal Type: TimeOut

      Default value is provisioned

      Additional parameters:

         None.

   Respond
   SignalID: rsp (0x0004)


Groves, et al.              Standards Track                   [Page 176]

RFC 3525                Gateway Control Protocol               June 2003


      The signal is used to respond to a continuity test.  See E.10.5
      for further explanation.

      Signal Type: On/Off

      Default duration is provisioned

      Additional parameters:

         None.

E.10.4   Statistics

   None.

E.10.5   Procedures

   When a MGC wants to initiate a continuity test, it sends a command to
   the MG containing:

   -  a signals descriptor with the ct signal; and

   -  an events descriptor containing the cmp event.

   Upon reception of a command containing the ct signal and cmp event,
   the MG initiates the continuity test tone for the specified
   Termination.  If the return tone is detected and any other required
   conditions are satisfied before the signal times out, the cmp event
   shall be generated with the value of the result parameter equal to
   success.  In all other cases, the cmp event shall be generated with
   the value of the result parameter equal to failure.

   When a MGC wants the MG to respond to a continuity test, it sends a
   command to the MG containing a signals descriptor with the rsp
   signal.  Upon reception of a command with the rsp signal, the MG
   either applies a loopback or (for 2-wire circuits) awaits reception
   of a continuity test tone.  In the loopback case, any incoming
   information shall be reflected back as outgoing information.  In the
   2-wire case, any time the appropriate test tone is received, the
   appropriate response tone should be sent.  The MGC determines when to
   remove the rsp signal.

   When a continuity test is performed on a Termination, no echo devices
   or codecs shall be active on that Termination.

   Performing voice path assurance as part of continuity testing is
   provisioned by bilateral agreement between network operators.


Groves, et al.              Standards Track                   [Page 177]

RFC 3525                Gateway Control Protocol               June 2003


      (Informative Note) Example tones and test procedure details are
      given in Q.724 sections 7 and 8, Q.764 section 2.1.8 and Q.1902.4.

E.11  Network Package

   PackageID: nt (0x000b)
   Version: 1
   Extends: None

   This package defines properties of network terminations independent
   of network type.

E.11.1   Properties

   Maximum Jitter Buffer
   PropertyID: jit (0x0007)

      This property puts a maximum size on the jitter buffer.

      Type: integer in milliseconds

      Possible values: This property is specified in milliseconds.

      Defined in: LocalControlDescriptor

      Characteristics: read/write

E.11.2   Events

   network failure
   EventID: netfail, 0x0005

      The termination generates this event upon detection of a failure
      due to external or internal network reasons.

      EventDescriptor parameters

         None.

      ObservedEventsDescriptor parameters

         cause
         ParameterID: cs (0x0001)

            Type: string

            Possible values: any text string


Groves, et al.              Standards Track                   [Page 178]

RFC 3525                Gateway Control Protocol               June 2003


            This parameter may be included with the failure event to
            provide diagnostic information on the reason of failure.

   quality alert
   EventID: qualert, 0x0006

      This property allows the MG to indicate a loss of quality of the
      network connection.  The MG may do this by measuring packet loss,
      interarrival jitter, propagation delay and then indicating this
      using a percentage of quality loss.

      EventDescriptor parameters

         Threshold
         ParameterId: th (0x0001)

            Type: integer

            Possible values: 0 to 99

            Description: threshold for percent of quality loss measured,
            calculated based on a provisioned method, that could take
            into consideration packet loss, jitter, and delay for
            example.  Event is triggered when calculation exceeds the
            threshold.

      ObservedEventsDescriptor parameters

         Threshold
         ParameterId: th (0x0001)

            Type: integer

            Possible values: 0 to 99

            Description: percent of quality loss measured, calculated
            based on a provisioned method, that could take into
            consideration packet loss, jitter, and delay for example.

E.11.3   Signals

   None.


Groves, et al.              Standards Track                   [Page 179]

RFC 3525                Gateway Control Protocol               June 2003


E.11.4   Statistics

   Duration
   StatisticsID: dur (0x0001)

      Description: provides duration of time the termination has been in
      the Context.

      Type: double, in milliseconds

   Octets Sent
   StatisticID: os (0x0002)

      Type: double

      Possible values: any 64-bit integer

   Octets Received
   StatisticID: or (0x0003)

      Type: double

      Possible values: any 64-bit integer

E.11.5   Procedures

   None.

E.12  RTP Package

   PackageID: rtp (0x000c)
   Version: 1
   Extends: Network Package version 1

   This package is used to support packet-based multimedia data transfer
   by means of the Real-time Transport Protocol (RTP) [RFC 1889].

E.12.1   Properties

   None.

E.12.2   Events

   Payload Transition
   EventID: pltrans, 0x0001

      This event detects and notifies when there is a transition of the
      RTP payload format from one format to another.


Groves, et al.              Standards Track                   [Page 180]

RFC 3525                Gateway Control Protocol               June 2003


      EventDescriptor parameters

         None.

      ObservedEventsDescriptor parameters

         ParameterName: rtppayload
         ParameterID: rtppltype, 0x01

         Type: list of enumerated types.

         Possible values: The encoding method shall be specified by
         using one or several valid encoding names, as defined in the
         RTP AV Profile or registered with IANA.

E.12.3   Signals

   None.

E.12.4   Statistics

   Packets Sent
   StatisticID: ps (0x0004)

      Type: double

      Possible values: any 64-bit integer

   Packets Received
   StatisticID: pr (0x0005)

      Type: double

      Possible values: any 64-bit integer

   Packet Loss
   StatisticID: pl (0x0006)

      Describes the current rate of packet loss on an RTP stream, as
      defined in IETF RFC 1889.  Packet loss is expressed as percentage
      value: number of packets lost in the interval between two
      reception reports, divided by the number of packets expected
      during that interval.

      Type: double

      Possible values: a 32-bit whole number and a 32-bit fraction.


Groves, et al.              Standards Track                   [Page 181]

RFC 3525                Gateway Control Protocol               June 2003


   Jitter
   StatisticID: jit (0x0007)

      Requests the current value of the interarrival jitter on an RTP
      stream as defined in IETF RFC 1889.  Jitter measures the variation
      in interarrival time for RTP data packets.

   Delay
   StatisticID:delay (0x0008)

      Requests the current value of packet propagation delay expressed
      in timestamp units.  Same as average latency.

E.12.5   Procedures

   None.

E.13  TDM Circuit Package

      PackageID: tdmc (0x000d)
      Version: 1
      Extends: Network Package version 1

      This package may be used by any termination that supports gain and
      echo control.  It was originally intended for use on TDM circuits
      but may be more widely used.


      New versions or extensions of this package should take non-TDM use
      into account.

E.13.1   Properties

      Echo Cancellation
      PropertyID: ec (0x0008)

         Type: boolean

         Possible values:

            "on" (when the echo cancellation is requested) and

            "off" (when it is turned off.)

            The default is provisioned.

         Defined in: LocalControlDescriptor


Groves, et al.              Standards Track                   [Page 182]

RFC 3525                Gateway Control Protocol               June 2003


         Characteristics: read/write

      Gain Control
      PropertyID: gain (0x000a)

         Gain control, or usage of of signal level adaptation and
         noise level reduction is used to adapt the level of the signal.
         However, it is necessary, for example for modem calls, to turn
         off this function.

         Type: integer

         Possible values:

         The gain control parameter may either be specified as
         "automatic" (0xffffffff), or as an explicit number of decibels
         of gain (any other integer value).  The default is provisioned
         in the MG.

      Defined in: LocalControlDescriptor

      Characteristics: read/write

E.13.2   Events

   None.

E.13.3   Signals

   None.

E.13.4   Statistics

   None.

E.13.5   Procedures

   None.


Groves, et al.              Standards Track                   [Page 183]

RFC 3525                Gateway Control Protocol               June 2003


APPENDIX I  EXAMPLE CALL FLOWS (INFORMATIVE)

   All H.248.1 implementors must read the normative part of this RFC
   carefully before implementing from it.  The examples in this appendix
   should not be used as stand-alone explanations of how to create
   protocol messages.

   The examples in this appendix use SDP for encoding of the Local and
   and Remote stream descriptors. SDP is defined in RFC 2327. If there
   is is any discrepancy between the SDP in the examples, and RFC 2327,
   the the RFC should be consulted for correctness. Audio profiles used
   are are those defined in IETF RFC 1890, and others registered with
   IANA.  For example, G.711 A-law is called PCMA in SDP, and is
   assigned profile 0. G.723.1 is called G723 and is profile 4; H.263 is
   called H263 and is profile 34. See also
   http://www.iana.org/assignments/rtp-parameters.

A.1   Residential Gateway to Residential Gateway Call

   This example scenario illustrates the use of the elements of the
   protocol to set up a Residential Gateway to Residential Gateway call
   over an IP-based network.  For simplicity, this example assumes that
   both Residential Gateways involved in the call are controlled by the
   same Media Gateway Controller.

A.1.1 Programming Residential GW Analog Line Terminations for Idle
   Behavior

   The following illustrates the API invocations from the Media Gateway
   Controller and Media Gateways to get the Terminations in this
   scenario programmed for idle behavior.  Both the originating and
   terminating Media Gateways have idle AnalogLine Terminations
   programmed to look for call initiation events (i.e., -offhook) by
   using the Modify Command with the appropriate parameters.  The null
   Context is used to indicate that the Terminations are not yet
   involved in a Context.  The ROOT termination is used to indicate the
   entire MG instead of a termination within the MG.

   In this example, MG1 has the IP address 124.124.124.222, MG2 is
   125.125.125.111, and the MGC is 123.123.123.4. The default Megaco
   port is 55555 for all three.

   1. An MG registers with an MGC using the ServiceChange command:

   MG1 to MGC:

   MEGACO/1 [124.124.124.222] Transaction = 9998 {
       Context = - {


Groves, et al.              Standards Track                   [Page 184]

RFC 3525                Gateway Control Protocol               June 2003


           ServiceChange = ROOT {Services {
               Method=Restart,
               ServiceChangeAddress=55555, Profile=ResGW/1}
           }
       } }

   2. The MGC sends a reply:

   MGC to MG1:

   MEGACO/1 [123.123.123.4]:55555 Reply = 9998 {
      Context = - {ServiceChange = ROOT {
        Services {ServiceChangeAddress=55555, Profile=ResGW/1} } } }

   3. The MGC programs a Termination in the NULL context.  The
   terminationId is A4444, the streamId is 1, the requestId in the
   Events descriptor is 2222.  The mId is the identifier of the sender
   of this message, in this case, it is the IP address and port
   [123.123.123.4]:55555.  Mode for this stream is set to SendReceive.
   "al" is the analog line supervision package.  Local and Remote are
   assumed to be provisioned.

   MGC to MG1:

   MEGACO/1 [123.123.123.4]:55555 Transaction = 9999 {
       Context = - {
           Modify = A4444 {
               Media { Stream = 1 {
                        LocalControl {
                            Mode = SendReceive,
                            tdmc/gain=2,  ; in dB,
                            tdmc/ec=on
                        },

                    }
               },
               Events = 2222 {al/of(strict=state)}
           }
       } }


   The dialplan script could have been loaded into the MG previously.
   Its function would be to wait for the OffHook, turn on dialtone and
   start collecting DTMF digits.  However in this example, we use the
   digit map, which is put into place after the offhook is detected
   (step 5 below).


Groves, et al.              Standards Track                   [Page 185]

RFC 3525                Gateway Control Protocol               June 2003


   Note that the embedded EventsDescriptor could have been used to
   combine steps 3 and 4 with steps 8 and 9, eliminating steps 6 and 7.

   4. The MG1 accepts the Modify with this reply:

   MG1 to MGC:

   MEGACO/1 [124.124.124.222]:55555

   Reply = 9999 {
      Context = - {Modify = A4444} }

   5. A similar exchange happens between MG2 and the MGC, resulting in
   an idle Termination called A5555.

A.1.2 Collecting Originator Digits and Initiating Termination

   The following builds upon the previously shown conditions.  It
   illustrates the transactions from the Media Gateway Controller and
   originating Media Gateway (MG1) to get the originating Termination
   (A4444) through the stages of digit collection required to initiate a
   connection to the terminating Media Gateway (MG2).

   6. MG1 detects an offhook event from User 1 and reports it to the
   Media Gateway Controller via the Notify Command.

   MG1 to MGC:

   MEGACO/1 [124.124.124.222]:55555 Transaction = 10000 {
      Context = - {
          Notify = A4444 {ObservedEvents =2222 {
            19990729T22000000:al/of(init=false)}}
      } }

   7. And the Notify is acknowledged.

   MGC to MG1:

   MEGACO/1 [123.123.123.4]:55555 Reply = 10000 {

       Context = - {Notify = A4444} }


Groves, et al.              Standards Track                   [Page 186]

RFC 3525                Gateway Control Protocol               June 2003


   8. The MGC Modifies the termination to play dial tone, to look for
   digits according to Dialplan0 and to look for the on-hook event now.

   MGC to MG1:

   MEGACO/1 [123.123.123.4]:55555 Transaction = 10001 {
       Context = - {
           Modify = A4444 {
               Events = 2223 {
                   al/on(strict=state), dd/ce {DigitMap=Dialplan0}
               },
               Signals {cg/dt},
               DigitMap= Dialplan0{ (0| 00|[1-
   7]xxx|8xxxxxxx|Fxxxxxxx|Exx|91xxxxxxxxxx|9011x.)}
           }
       } }

   9. And the Modify is acknowledged.

   MG1 to MGC:

   MEGACO/1 [124.124.124.222]:55555 Reply = 10001 {
       Context = - {Modify = A4444} }

   10.   Next, digits are accumulated by MG1 as they are dialed by User
   1.  Dialtone is stopped upon detection of the first digit.  When an
   appropriate match is made of collected digits against the currently
   programmed Dialplan for A4444, another Notify is sent to the Media
   Gateway Controller.

   MG1 to MGC:

   MEGACO/1 [124.124.124.222]:55555 Transaction = 10002 {
      Context = - {
          Notify = A4444 {ObservedEvents =2223 {
            19990729T22010001:dd/ce{ds="916135551212",Meth=UM}}}
      } }

   11.   And the Notify is acknowledged.

   MGC to MG1:

   MEGACO/1 [123.123.123.4]:55555 Reply = 10002 {
       Context = - {Notify = A4444} }


   12.   The controller then analyses the digits and determines that a
   connection needs to be made from MG1 to MG2.  Both the TDM


Groves, et al.              Standards Track                   [Page 187]

RFC 3525                Gateway Control Protocol               June 2003


   termination A4444, and an RTP termination are added to a new context
   in MG1.  Mode is ReceiveOnly since Remote descriptor values are not
   yet specified.  Preferred codecs are in the MGC's preferred order of
   choice.

   MGC to MG1:

   MEGACO/1 [123.123.123.4]:55555 Transaction = 10003 {
       Context = $ {
          Add = A4444,
          Add = $ {
              Media {
                Stream = 1 {
                     LocalControl {
                         Mode = ReceiveOnly,

                         nt/jit=40 ; in ms
                     },
                     Local { v=0 c=IN IP4 $ m=audio $ RTP/AVP 4
   a=ptime:30 v=0 c=IN IP4 $ m=audio $ RTP/AVP 0
                     }
                }
             }
          }
       } }


      NOTE - The MGC states its preferred parameter values as a series
      of SDP blocks in  Local.  The MG fills in the Local Descriptor in
      the Reply.

   13.   MG1 acknowledges the new Termination and fills in the Local IP
   address and UDP port.  It also makes a choice for the codec based on
   the MGC preferences in Local.  MG1 sets the RTP port to 2222.

   MG1 -> MGC:

   MEGACO/1 [124.124.124.222]:55555 Reply = 10003 {
      Context = 2000 {
         Add = A4444,
         Add=A4445{
            Media {
                Stream = 1 {
                    Local { v=0 o=- 2890844526 2890842807 IN IP4
   124.124.124.222 s=- t= 0 0 c=IN IP4 124.124.124.222 m=audio 2222
   RTP/AVP 4 a=ptime:30 a=recvonly
                    } ; RTP profile for G.723.1 is 4
                }


Groves, et al.              Standards Track                   [Page 188]

RFC 3525                Gateway Control Protocol               June 2003


            }
         }
      } }

   14.   The MGC will now associate A5555 with a new Context on MG2, and
   establish an RTP Stream (i.e., A5556 will be assigned), SendReceive
   connection through to the originating user, User 1.  The MGC also
   sets ring on A5555.

   MGC to MG2:

   MEGACO/1 [123.123.123.4]:55555 Transaction = 50003 {
       Context = $ {
          Add = A5555  { Media {
               Stream = 1 {
                    LocalControl {Mode = SendReceive} }},
         Events=1234{al/of(strict=state)},
               Signals {al/ri}

               },
          Add  = $ {Media {
               Stream = 1 {
                    LocalControl {
                       Mode = SendReceive,
                       nt/jit=40 ; in ms
                    },
                    Local { v=0 c=IN IP4 $ m=audio $ RTP/AVP 4
   a=ptime:30
                    },
                    Remote { v=0 c=IN IP4 124.124.124.222 m=audio 2222
   RTP/AVP 4 a=ptime:30
                    } ; RTP profile for G.723.1 is 4
                }
             }
         }
      } }

   15.   This is acknowledged.  The stream port number is different from
   the control port number.  In this case it is 1111 (in the SDP).

   MG2 to MGC:

   MEGACO/1 [125.125.125.111]:55555 Reply = 50003 {
      Context = 5000 {
      Add = A5555,
         Add = A5556{
            Media {
               Stream = 1 {


Groves, et al.              Standards Track                   [Page 189]

RFC 3525                Gateway Control Protocol               June 2003


                   Local { v=0 o=- 7736844526 7736842807 IN IP4
   125.125.125.111 s=- t= 0 0 c=IN IP4 125.125.125.111 m=audio 1111
   RTP/AVP 4 }
               } ; RTP profile for G723.1 is 4
            }
          }

      } }

   16.   The above IPAddr and UDPport need to be given to MG1 now.

   MGC to MG1:

   MEGACO/1 [123.123.123.4]:55555 Transaction = 10005 {
     Context = 2000 {
       Modify = A4444 {
         Signals {cg/rt}
       },
       Modify = A4445 {
          Media {
               Stream = 1 {
                   Remote { v=0 o=- 7736844526 7736842807 IN IP4
   125.125.125.111 s=- t= 0 0 c=IN IP4 125.125.125.111 m=audio 1111
   RTP/AVP 4
                   }
               } ; RTP profile for G723.1 is 4
           }
       }
     } }


   MG1 to MGC:

   MEGACO/1 [124.124.124.222]:55555 Reply = 10005 {
      Context = 2000 {Modify = A4444, Modify = A4445} }

   17.   The two gateways are now connected and User 1 hears the
   RingBack.  The MG2 now waits until User2 picks up the receiver and
   then the two-way call is established.


Groves, et al.              Standards Track                   [Page 190]

RFC 3525                Gateway Control Protocol               June 2003


   From MG2 to MGC:

   MEGACO/1 [125.125.125.111]:55555 Transaction = 50005 {
      Context = 5000 {

          Notify = A5555 {ObservedEvents =1234 {
            19990729T22020002:al/of(init=false)}}
      } }

   From MGC to MG2:

   MEGACO/1 [123.123.123.4]:55555 Reply = 50005 {
       Context = - {Notify = A5555} }

   From MGC to MG2:

   MEGACO/1 [123.123.123.4]:55555 Transaction = 50006 {
      Context = 5000 {
         Modify = A5555 {
            Events = 1235 {al/on(strict=state)},
            Signals { } ; to turn off ringing
         }
      } }

   From MG2 to MGC:

   MEGACO/1 [125.125.125.111]:55555 Reply = 50006 {
    Context = 5000 {Modify = A4445} }

   18.   Change mode on MG1 to SendReceive, and stop the ringback.

   MGC to MG1:

   MEGACO/1 [123.123.123.4]:55555 Transaction = 10006 {
      Context = 2000 {
         Modify = A4445 {
            Media {
               Stream = 1 {
                  LocalControl {
                     Mode=SendReceive

                  }
               }
            }
         },
         Modify = A4444 {
            Signals { }
         }


Groves, et al.              Standards Track                   [Page 191]

RFC 3525                Gateway Control Protocol               June 2003


      } }

   from MG1 to MGC:

   MEGACO/1 [124.124.124.222]:55555 Reply = 10006 {
      Context = 2000 {Modify = A4445, Modify = A4444}}

   19.   The MGC decides to Audit the RTP termination on MG2.

   MGC -> MG2:

   MEGACO/1 [123.123.123.4]:55555 Transaction = 50007 {
      Context = - {AuditValue = A5556{
         Audit{Media, DigitMap, Events, Signals, Packages, Statistics }}
      } }

   20.   The MG2 replies.

   MG2 -> MGC:

   MEGACO/1 [125.125.125.111]:55555 Reply = 50007 {
      Context = - { AuditValue = A5556 {
             Media {
                TerminationState { ServiceStates = InService,
            Buffer = OFF },
          Stream = 1 {
                    LocalControl { Mode = SendReceive,
                       nt/jit=40 },
                    Local { v=0 o=- 7736844526 7736842807 IN IP4
   125.125.125.111 s=- t= 0 0 c=IN IP4 125.125.125.111 m=audio 1111
   RTP/AVP  4 a=ptime:30
                   },
                    Remote { v=0 o=- 2890844526 2890842807 IN IP4
   124.124.124.222 s=- t= 0 0 c=IN IP4 124.124.124.222 m=audio 2222
   RTP/AVP  4 a=ptime:30
                    } } },
              Events,
           Signals,
           DigitMap,
          Packages {nt-1, rtp-1},
             Statistics { rtp/ps=1200,  ; packets sent
                          nt/os=62300, ; octets sent
                          rtp/pr=700, ; packets received
                          nt/or=45100, ; octets received
                          rtp/pl=0.2,  ; % packet loss
                          rtp/jit=20,
                          rtp/delay=40 } ; avg latency
          }


Groves, et al.              Standards Track                   [Page 192]

RFC 3525                Gateway Control Protocol               June 2003


       } }

   21.   When the MGC receives an onhook signal from one of the MGs, it
   brings down the call.  In this example, the user at MG2 hangs up
   first.

   From MG2 to MGC:

   MEGACO/1 [125.125.125.111]:55555 Transaction = 50008 {
      Context = 5000 {
          Notify = A5555 {ObservedEvents =1235 {
             19990729T24020002:al/on(init=false)}
          }
      } }

   From MGC to MG2:

   MEGACO/1 [123.123.123.4]:55555 Reply = 50008 {

       Context = - {Notify = A5555} }

   22.   The MGC now sends both MGs a Subtract to take down the call.
   Only the subtracts to MG2 are shown here.  Each termination has its
   own set of statistics that it gathers.  An MGC may not need to
   request both to be returned.  A5555 is a physical termination, and
   A5556 is an RTP termination.

   From MGC to MG2:

   MEGACO/1 [123.123.123.4]:55555 Transaction = 50009 {
      Context = 5000 {
         Subtract = A5555 {Audit{Statistics}},
         Subtract = A5556 {Audit{Statistics}}
      } }

   From MG2 to MGC:

   MEGACO/1 [125.125.125.111]:55555 Reply = 50009 {
      Context = 5000 {
        Subtract = A5555 {
             Statistics {
                nt/os=45123, ; Octets Sent
                nt/dur=40 ; in seconds
                }
          },
          Subtract = A5556 {
             Statistics {
                rtp/ps=1245, ; packets sent


Groves, et al.              Standards Track                   [Page 193]

RFC 3525                Gateway Control Protocol               June 2003


                nt/os=62345, ; octets sent
                rtp/pr=780, ; packets received
                nt/or=45123, ; octets received
                rtp/pl=10, ;  % packets lost
                rtp/jit=27,
                rtp/delay=48 ; average latency
             }
          }
      } }

   23.   The MGC now sets up both MG1 and MG2 to be ready to detect the
   next off-hook event.  See step 1.  Note that this could be the
   default state of a termination in the null context, and if this were
   the case, no message need be sent from the MGC to the MG.  Once a
   termination returns to the null context, it goes back to the default
   termination values for that termination.


Groves, et al.              Standards Track                   [Page 194]

RFC 3525                Gateway Control Protocol               June 2003


APPENDIX II  Changes From RFC 3015

   In the following table, "source" indicates when the change was first
   approved.  It has the following values:

   IG1100: H.248 Implementor's Guide approved in November, 2000 (as TD
   Plen-39, Christian Groves, editor).

   IG0601: H.248 Implementor's Guide approved in June, 2001 (as  TD
   Plen-15, Christian Groves, editor).

   IGDUB: Draft H.248 Implementor's Guide approved at the Q.3
   Rapporteur's meeting held near Dublin, October 2001 (as TD-28, Terry
   Anderson, editor).

   GEN0202: added at the Geneva meeting, February 2002, which consented
   to H.248 v1 Amendment 1 (as TD Plen-36r1, Marcello Pantaleo, editor).

   ITUPOST: added in post-Geneva editing by the ITU-T.

   TTPOST: added in post-approval editing by the Megaco Chair, Tom
   Taylor, who assembled this document for submission.

   Section    Source                       Change

   1          ITUPOST   Reference changed from H.248 to H.248.1.

   2.1        ITUPOST   Reference added for error codes, changed from
                        H.248 Annex L to H.248.8 (2002).

   2.1        IG1100    Corrected Q.765 reference to Q.765.5.

   2.1        GEN0202   Added reference to X.690.

   2.2        GEN0202   Added reference to H.226.

   2.2         IGDUB    Added informative references to Q.724, Q.764,
                        and Q.1902.4.

   4          IG0601    Added expansion of ALF.

   5          TTPOST    Gave priority to IETF conventions (added at
                        start of document).


Groves, et al.              Standards Track                   [Page 195]

RFC 3525                Gateway Control Protocol               June 2003


   6.1.1      IG0601    Added text regarding use of wildcards for
                        context identifiers.  (This information
                        already appeared in section 8.1.2.  The IG
                        change subsequently disappeared.)

   6.1.1      IG1100    Added ranking of priority values.

   6.2         IGDUB    Deleted definition of signals.

   6.2        GEN0202   Expanded text and diagrams describing
                        multiplexing terminations.

   6.2        TTPOST    Added asterisks to multiplexing diagrams to
                        indicate centre of context.  Added Figure 6a
                        showing cascading of multiplexes.

   6.2.2      IG0601    Added text indicating that ALL does not
                        include ROOT.

   6.2.3      IG1100    Added text clarifying what must be supported
                        to claim support of a package.

   6.2.3      IG1100    Added text indicating what packages a peer can
                        indicate support for, when some of them are
                        extensions of others.

   6.2.4      IG0601    Added text on ability of provisioning to
                        override default values, and need for MGC to
                        audit to learn the provisioned defaults.

   6.2.4      IG0601    Added text indicating effect of omitting
                        specific properties from Descriptors in
                        commands modifying a termination.
                        Contradicted original text saying that omitted
                        properties retain their prior values (still
                        true for entirely-omitted Descriptors).

   6.2.4      GEN0202   Modified above text to restrict it to
                        read/write properties, allow for default
                        behaviour in place of default values if so
                        specified in the property definition.

   6.2.4       IGDUB    Trimmed definition of signals Descriptor in
                        table and inserted cross-reference to section
                        7.1.11.

   6.2.4      IG1100    Added Topology and Error Descriptors to table.


Groves, et al.              Standards Track                   [Page 196]

RFC 3525                Gateway Control Protocol               June 2003


   6.2.5       IGDUB    Specified error code to return if ROOT used
                        inappropriately.

   7.1.1      IG1100    Added qualification to explanation of effect
                        of missing Audit Descriptor, excepting
                        Subtract.

   7.1.3      GEN0202   Changed "inputs" to "bearers" to be consistent
                        with terminology in 6.2.

   7.1.4      IG0601    Small change to make clear that more than one
                        of Local, Remote, and LocalControl can be
                        included in the default streamId.

   7.1.7      IG0601    Default value for Mode specified to be
                        Inactive.

   7.1.7      GEN0202   Added text requiring processing of media in
                        any of the reserved formats, where more than
                        one has been reserved in a given stream.

   7.1.8       IGDUB    Added restriction to at most one m= line per
                        session description.

   7.1.9      IG0601    Text added to omit request identifier if the
                        EventsDescriptor is empty.  Further text added
                        at end to indicate the effects of an empty
                        EventsDescriptor and an empty
                        EventBufferDescriptor.

   7.1.9      IG0601    Fixed typo for destination of a Notify.

   7.1.9      IG1100    Added note to say event remains active after
                        it has been notified, so long as it is still
                        present in the active Events Descriptor.

   7.1.11      IGDUB    Added definition of signals.

   7.1.11     GEN0202   Modified definition to include example of more
                        complex signal, and added role of signal in
                        media preparation for future signals.

   7.1.11      IGDUB    The timeout completion reason was broadened to
                        include other circumstances where the signal
                        completed on its own.  Text added to indicate
                        that if default signal type changed to TO,
                        duration parameter must be provided.


Groves, et al.              Standards Track                   [Page 197]

RFC 3525                Gateway Control Protocol               June 2003


   7.1.11     GEN0202   Removed reference to BR signal being "so
                        short" it will stop on its own.  Added text
                        indicating that if the type of a signal is
                        changed to TO, the Duration parameter must be
                        supplied.

   7.1.11     IG1100    Deleted text discussing type of Signals List.

   7.1.12     GEN0202   Improved wording of introductory paragraph and
                        added text making content of returned
                        Descriptor clear.

   7.1.14.2   GEN0202   Added text indicating that when the start
                        timer is set to 0, initial digit timing is
                        disabled and the MG waits indefinitely for
                        digits.

   7.1.14.2   GEN0202   Added text pointing out that default digit
                        timer values should be provisioned, but can be
                        overridden in the digit map.

   7.1.14.3   GEN0202   Changed result of long-short digit timer
                        conflict from undefined to long.

   7.1.14.6   IG1100    Clarified that the digit map is provided by
                        the eventDM parameter, which must be present.

   7.1.14.7   GEN0202   Added text clarifying that events covered by
                        the digit map completion event have no side-
                        effects unless separately enabled.

   7.1.14.8   IG0601    Added requirement that the event specification
                        include the eventDM parameter.

   7.1.17      IGDUB    Added text to indicate timestamp is optional
                        and to include observed event parameters in
                        reported content.

   7.1.17     GEN0202   Deleted provision that time is expressed in
                        UTC (since intention was to use format, not
                        time zone).

   7.1.18      IGDUB    Added text indicating error to return if
                        topology option not supported.


Groves, et al.              Standards Track                   [Page 198]

RFC 3525                Gateway Control Protocol               June 2003


   7.1.18     IG1100    Added text clarifying effect of not mentioning
              TTPOST    a termination in a topology Descriptor, and
                        default topology for a new termination.  (This
                        text got lost between the Dublin meeting and
                        the production of H.248 Amendment 1 out of the
                        Geneva 02/02 meeting.  It has been added back
                        to the present document.)

   7.1.19     IG1100    New section to describe Error Descriptor.
              GEN0202   Slightly edited in Geneva 02/02 meeting.
              ITUPOST   Reference for error code documentation updated
                        to H.248.8.

   7.1.19     IG0601    Added paragraph giving guidance on level at
                        which errors should be reported.

   7.2        IG1100    Noted possibility of Error Descriptor in reply
                        to any command.

   7.2.1      IG1100    Added EventBufferDescriptor as Add parameter.

   7.2.1      IG1100    Removed restriction on use of CHOOSE wildcard.

   7.2.2      IG1100    Added EventBufferDescriptor as Modify
                        parameter.

   7.2.2      GEN0202   Added text on side-effects of Modify of a
                        multiplexing termination.

   7.2.3      IG1100    Added prohibition against subtracting from the
                        NULL context.

   7.2.3      GEN0202   Added text on side-effects of Subtract of a
                        multiplexing termination.

   7.2.3       IGDUB    Added text clarifying effect of empty
                        AuditDescriptor in Subtract.

   7.2.4      IG1100    Added EventBufferDescriptor as Move parameter.

   7.2.4      GEN0202   Removed misleading statement that Move acts as
                        subtract from original context.

   7.2.4      IG1100    Clarified effect of Move on properties of the
                        moved termination.

   7.2.4      GEN0202   Added text on side-effects of Move of a
                        multiplexing termination.


Groves, et al.              Standards Track                   [Page 199]

RFC 3525                Gateway Control Protocol               June 2003


   7.2.5      IG1100    Added examples showing W- wildcard usage.

   7.2.5      IG1100    Noted that returning a list of all contextIDs
                        requires that they be returned one per
                        ActionReply.

   7.2.5      IG1100    Added table entry (ALL, specific) to determine
                        context in which termination currently
                        resides.

   7.2.6      GEN0202   Added table similar to that in 7.2.5.

   7.2.7      IG0601    Added TerminationID to API.

   7.2.7       IGDUB    Indicated timestamp was optional in Notify, to
                        accord with syntax.

   7.2.7      IG1100    Noted possibility of sending Error Descriptor
                        in Notify.

   7.2.8      IG0601    Added text to description of Forced method to
                        indicate that Forced on ROOT indicates a cold
                        restart (all context state lost).

   7.2.8       IGDUB    Amplified explanation of Disconnected method
                        to emphasize return to the previously
                        controlling MGC.

   7.2.8      IG0601    Added text for MG use of Failover method when
                        it detects MGC failure.

   7.2.8      IG1100    Added notes discouraging use of
                        ServiceChangeAddress and warning that it could
                        be either a full address or just a port
                        number.

   7.2.8      IG0601    Added text indicating that timestamp does not
                        necessarily represent absolute time, only
                        local clock reading.

   7.2.8       IGDUB    Corrected "gateway" to "MGC" in discussion of
                        returned ServiceChangeMgcId parameter.

   7.3        IG0601    Removed error code documentation to Annex L
              ITUPOST   (now H.248.8).

   8          IG1100    Added requirement that an Action be non-empty.


Groves, et al.              Standards Track                   [Page 200]

RFC 3525                Gateway Control Protocol               June 2003


   8          GEN0202   Added context properties and context property
                        audit requests to commands as potential
                        contents of actions.

   8.1.2      GEN0202   Added prohibition on using partial contextIDs
                        with ALL wildcards.

   8.2.2      IG1100    Added text clarifying when in transaction
                        processing the requested actions have been
                        completed and a reply can be sent.

   8.2.2      IG1100    Added ALL as allowed contextID in
                        TransactionReply.

   8.2.2      GEN0202   Provided general reference to section 7.1.19
                        for generation of error Descriptors.

   8.2.2      IG0601    Corrected Actions to Commands when discussing
                        partially-understood action.

   8.3        IG0601    Added text specifying that the same MId value
                        must be used by a given entity throughout the
                        life of a control association.

   8.3        IG0601    Added text expanding on independence of
                        transactions from messages.

   9          ITUPOST   Indicated that additional transports may be
                        defined in separate Recommendations as well as
                        annexes to the primary specification.

   9          IG0601    Gave specific example of "request source
                        address" for IP.

   9.1        IG1100    Deleted restriction to one outstanding Notify
                        command on a termination at one time, since
                        this is transport-specific.

   9.1        IG0601    Restored restriction, but noted that it
                        applied only to transport not guaranteeing
                        ordered delivery.

   10.2       IG1100    Corrected length of synthesized address field
                        from 10 to 20 hex digits and indicated that
                        calculation should be over entire message, not
                        just one transaction.


Groves, et al.              Standards Track                   [Page 201]

RFC 3525                Gateway Control Protocol               June 2003


   11.2       IG1100    Corrected text in first two paragraphs
                        describing use of ServiceChangeMgcId
                        parameter.

   11.2       IG1100    Corrected "Transaction Accept" to "Transaction
                        Reply".

   11.4       IG0601    Noted that support of redundant MGs requires
              GEN0202   use of a reliable transport and support in the
                        MGC.  Added more explanation in Geneva.

   11.5       IG0601    Added text clarifying procedure if MG unable
                        to establish a control relationship with any
                        of its eligible MGCs.

   11.5        IGDUB    Added text indicating that when trying to
                        reestablish contact with the previously
                        controlling MGC the MG uses the Disconnected
                        method.

   11.5       IG1100    Clarified handoff procedure.

   11.5       GEN0202   Changed text on replies to transactions in
                        progress during handoff.  Replies now
                        discarded when the service relationship with
                        the old MGC has ended, rather than sent to the
                        new MGC.  The new MGC could still send replies
                        to requests sent to the old MGC.

   12.1.1     GEN0202   Added optional package designation as
                        "designed to be extended only".

   12.1.1     IG1100    Made prohibition on overloading of identifiers
                        in extended packages transitive through all
                        ancestors of the extended package.

   12.1.2      IGDUB    Clarified the set of types allowed for
                        properties.

   12.1.2     GEN0202   Added requirement to specify the base type of
                        a sub-list.

   12.1.2     GEN0202   Provided requirements for content of the
                        "Possible Values" template item, including
                        specification of default values or behaviour.


Groves, et al.              Standards Track                   [Page 202]

RFC 3525                Gateway Control Protocol               June 2003


   12.1.4     GEN0202   Added requirement to specify the default
                        signal type, and specify a default duration
                        for TO signals.  Also noted that duration is
                        meaningless for BR, and that the signal type
                        might be dependent on the values of other
                        signal parameters.

   12.2       GEN0202   Fixed section title (covers only event and
                        signal parameters, not properties or
                        statistics).

   12.2       IG1100    Reserved SPA and EPA prefixes, so they are not
                        to be used for signal and event parameter
                        tokens.

   12.2       IG0601    Expanded list of reserved prefixes.

   12.2        IGDUB    Clarified the set of types allowed for signal
                        and event parameters.

   12.2       GEN0202   Added requirement to specify the base type of
                        a sub-list.

   12.2       GEN0202   Provided requirements for content of the
                        "Possible Values" template item, including
                        specification of default values or behaviour.

   12.4        IGDUB    Corrected to indicate identifiers must start
                        with alphabetic rather than alphanumeric
                        character.

   13.1       IG0601    Changed private range of binary package
                        identifiers to convenient hex values.

   A          GEN0202   Removed versions from X.680 and X.690
                        references.

   A.2         IGDUB    Added note warning that the syntax alone does
                        not provide a complete description of the
                        constraints, but must be supplemented by a
                        reading of the text and comments.

   A.2        IG0601    Added description of double wrapping of
                        parameters declared as OCTET STRING.


Groves, et al.              Standards Track                   [Page 203]

RFC 3525                Gateway Control Protocol               June 2003


   A.2        GEN0202   Some editing of double wrapping description to
                        use ASN.1, BER in their proper places.  Added
                        possibility of encoding strings as UTF8String,
                        but only if they contain non-ASCII characters.

   A.2         IGDUB    Added line in table on double wrapping of true
                        octet strings.

   A.2        IG1100    Corrected and expanded comments describing
                        mtpAddress form of MId.  Fixed maximum length
                        of mtpAddress both here and in
                        ServiceChangeAddress.

   A.2        IG0601   Inserted missing lines in IP4Address
                        production.

   A.2        IG0601    Modified TransactionResponseAck to allow
                        acknowledgement of multiple ranges of
                        transactionIds.

   A.2        IG0601    Corrected numerical value of CHOOSE as a
                        context identifier.

   A.2         IGDUB    Added missing extension marker in
                        TopologyRequest.

   A.2        IG1100    AuditReply and AuditResult modified to bring
                        binary functionality into line with text
                        functionality.

   A.2        IG0601    Removed OPTIONAL tag from terminationID in
                        NotifyReply.

   A.2        IG0601    Added extraInfo substructure to EventParameter
                        and SigParameter.

   A.2        IG0601    Modified MediaDescriptor to make it optional
                        to specify a stream.

   A.2        IG0601    Added OPTIONAL tags to reserveValue and
                        reserveGroup.

   A.2         IGDUB    Added to comments for pkgdName to indicate
                        applicability to event names, signal names,
                        and statisticIds as well as property.


Groves, et al.              Standards Track                   [Page 204]

RFC 3525                Gateway Control Protocol               June 2003


   A.2        IG0601    RequestID made optional in EventsDescriptor
                        and SecondEventsDescriptor and comment added
                        saying it must be present if events are
                        present.

   A.2        IG1100    Added OPTIONAL tags on RequestActions and
                        SecondRequestedActions keepActive BOOLEANs.

   A.2        IG1100    Added comment to indicate requestID value to
                        use in an AuditCapReply.

   A.2        GEN0202   Added comment to DigitMapValue indicating time
                        units for timers.

   A.2        IG0601    Added comment indicating coding of Value for
              GEN0202   ServiceChangeReason.  Cleaned up in Geneva to
                        use ASN.1 and BER in their proper places.

   A.2        IG0601    Inserted missing extension marker in
                        ServiceChangeParm production.

   A.2        IG0601    Aligned definition of mtpAddress in
                        ServiceChangeAddress with that in MId.

   A.2        IG0601    Added timestamp to ServiceChangeResParm.

   A.2         IGDUB    Changed type of profileName in
                        ServiceChangeProfile to IA5String.

   A.2        IG0601    Made returned value optional in
                        statisticsParameter, to support
                        auditCapability result.

   A.2        GEN0202   Added reference to ISO 8601:1988 for
                        TimeNotation.

   A.2        IG1100    Value production modified to support the
                        sublist parameter type.

   A.3        IG1100    Corrected ABNF for digitStringlisT, replacing
                        "/" with "|".

   A.3        IG1100    Added parentheses to digitMapRange production.

   A.3        IG1100    Replaced more abbreviated syntax for pathName
                        with fuller definition and constraints copied
                        from B.2.


Groves, et al.              Standards Track                   [Page 205]

RFC 3525                Gateway Control Protocol               June 2003


   B.2         IGDUB    Added note warning that the syntax alone does
                        not provide a complete description of the
                        constraints, but must be supplemented by a
                        reading of the text and comments.

   B.2        IG0601    Added note warning that the interpretation of
                        symbols is context-dependent.

   B.2        IG1100    Added comment to indicate case insensitivity
                        of protocol (excepting SDP) and ABNF.

   B.2        IG0601    Expanded upon and capitalized this comment.

   B.2        IG0601    Lengthy note added on the coding of the VALUE
                        construct.

   B.2         IGDUB    Deleted sentence in note suggesting that
                        packages could add new types for properties,
                        parameters, or statistics.

   B.2        IG0601    Added note indicating that parsers should
                        allow for white space preceding the first line
                        of SDP in Local or Remote.

   B.2         IGDUB    Added comments identifying the O- and W- tags.

   B.2        IG1100    Moved wildcard tag up from individual commands
                        to commandRequestList.

   B.2        GEN0202   Added additional error case to actionReply.

   B.2        IG0601    Modified syntax of auditOther to allow return
                        of terminationID only.

   B.2         IGDUB    Corrected upper limit for V4hex.

   B.2        IG1100    Corrected and expanded comments describing
                        mtpAddress form of MId.

   B.2        IG0601    Modified comment to mediaParm to make
                        streamParms and StreamDescriptor mutually
                        exclusive.

   B.2        GEN0202   Modified comment further to indicate at most
                        one instance of terminationStateDescriptor.

   B.2        GEN0202   Expanded comment for streamParm to indicate
                        the restriction on repetition is per item.


Groves, et al.              Standards Track                   [Page 206]

RFC 3525                Gateway Control Protocol               June 2003


   B.2        IG0601    Modified "at most once" comments to localParm,
                        terminationStateParm, and modemType, to allow
                        multiple instances of propertyParm in the
                        first two cases and extensionParameter in the
                        last one.

   B.2        IG0601    Added note before description of Local and
                        Remote, pointing out that the octet value x00
                        is not allowed in octetString.

   B.2        IG0601    Syntax for eventsDescriptor, embedFirst, and
                        eventBufferDescriptor modified to make
                        contents beyond token optional.

   B.2         IGDUB    Replaced "event" by "item" in comment to
                        pkgdName because pkgdName applies to
                        properties, signals, and statistics as well.

   B.2        IG0601    Corrected placement of EQUAL in eventDM
                        production.

   B.2        IG1100    Added comment and syntax to indicate requestID
                        value to use in an AuditCapReply.

   B.2        IG1100    Corrected Modem Descriptor to allow package
                        items as properties.

   B.2        IG0601    Comment to modemType changed to allow multiple
                        instances of extensionParameter.

   B.2        GEN0202   Comment added to indicate units for Timer.

   B.2        IG1100    Added parentheses to digitMapRange production.

   B.2        IG1100    Added comment to serviceChangeParm,
                        restricting each parameter to one appearance.

   B.2        IG0601    Added comments making serviceChangeMgcId and
                        serviceChangeAddress mutually exclusive in
                        ServiceChangeParm and servChgReplyParm.

   B.2         IGDUB    Added comment to serviceChangeParm indicating
                        that ServiceChangeMethod and
                        ServiceChangeReason are required.

   B.2        IG0601    Added Timestamp to servChgReplyParm.


Groves, et al.              Standards Track                   [Page 207]

RFC 3525                Gateway Control Protocol               June 2003


   B.2        IG0601    Added comment indicating coding of Value for
                        ServiceChangeReason.

   B.2        IG0601    Modified ServiceChangeAddress to use MId
                        definition for full address.

   B.2        IG1100    Made returned value optional in
                        statisticsParameter, to support
                        auditCapability result.

   B.2        IG1100    Changed topologyDescriptor to allow multiple
                        triples.

   B.2        IG0601    Added comment forbidding use of a double quote
                        within a quotedString value.

   B.2        IG1100    Reserved prefixes for new tokens added to
                        signalParameter and eventParameter, to avoid
                        collision with package names.

   B.2        IG1100    EmbedToken and EmergencyToken changed to
                        remove clash with EventBufferToken.

   B.3        IG1100    New section describing hexadecimal octet
                        encoding.

   B.4        IG1100    New section describing hex octet sequence.

   C          IG1100    Added permission to use Annex C properties in
                        LocalControl as well as in Local and Remote.

   C          IG0601    Added text making support of all properties of
                        Annex C optional.

   C           IGDUB    Added directions to reconcile tabulated
                        formats with allowed types for properties.

   C.1        IG1100    Corrected Q.765 reference to Q.765.5 for
                        ACodec.

   C.1        IG1100    Deprecated Echocanc codepoint in favour of
                        package-defined property.

   C.4        ITUPOST   Updated references from Q.2961 to Q.2961.1.

   C.4         IGDUB    Added details on format of VPVC.

   C.9        IG1100    Renamed USI to layer1prot.


Groves, et al.              Standards Track                   [Page 208]

RFC 3525                Gateway Control Protocol               June 2003


   C.9        IG1100    Deprecated ECHOCI codepoint in favour of
                        package-defined property.

   C.9        IG1100    Added new USI property.

   C.11       IG1100    Added m= line tag.

   D.1        IG0601    Added explanation of ALF.

   D.1.5       IGDUB    Expanded text indicating that when trying to
                        reestablish contact with the previously
                        controlling MGC the MG uses the Disconnected
                        method.

   E.1.2      GEN0202   Added missing EventsDescriptor parameters
                        lines.

   E.1.2      GEN0202   For the Signal Completion event:
                        - corrected the description of how it is
                        enabled
                        - heavily edited the description of the Signal
                        Identity observed event parameter and added a
                        type.

   E.1.2       IGDUB    The timeout completion reason for the Signal
                        Completion event was broadened to include
                        other circumstances where the signal completed
                        on its own.

   E.1.2      IG1100    Added signal list ID observed event parameter
                        to the Signal Completion event.

   E.2.1      IG0601    Added missing read only, read-write
                        specifications.

   E.2.1      IG0601    Split ProvisionalResponseTimer properties into
                        one for MG, one for MGC.

   E.3        GEN0202   Added "Designed to be extended only" to
                        tonegen package description.

   E.4        GEN0202   Added "Designed to be extended only" to
                        tonedet package description.

   E.4.2      GEN0202   Added type for tone ID observed parameter for
                        Long Tone Detected event.


Groves, et al.              Standards Track                   [Page 209]

RFC 3525                Gateway Control Protocol               June 2003


   E.6.2      IG1100    Corrected binary identifier for digit map
                        completion event to avoid clash with base
                        package.

   E.6.2      IG1100    Removed procedural text.

   E.6.5      IG1100    Added procedural text indicating where to find
                        the applicable digit map and indicating the
                        error to return if the parameter is missing.

   E.6.5      IG0601    Further modified procedural text.

   E.7.3      IG1100    Corrected text identifier for payphone
                        recognition tone to avoid clash with base
                        package.

   E.10.5      IGDUB    Provided informative references for tones and
                        procedures for continuity check.

   E.13       GEN0202   Added note that TDM package could also apply
                        to other transports.

   E.13.1     IG1100    Changed default for echo cancellation from
                        "on" to provisioned.

   E.13.1     IG0601    Corrected type for gain property.

   Appendix   TTPOST    Included a number of corrections which were
      I                 not picked up in H.248.1 Amendment 1 but which
                        do appear in H.248.1 v2.

Intellectual Property Rights

   The ITU draws attention to the possibility that the practice or
   implementation of this RFC may involve the use of a claimed
   Intellectual Property Right.  The ITU takes no position concerning
   the evidence, validity or applicability of claimed Intellectual
   Property Rights, whether asserted by ITU members or others outside of
   the Recommendation development process.

   As of the date of approval of this RFC, the ITU had received notice
   of intellectual property, protected by patents, which may be required
   to implement this RFC.  However, implementors are cautioned that this
   may not represent the latest information and are therefore strongly
   urged to consult the TSB patent database.


Groves, et al.              Standards Track                   [Page 210]

RFC 3525                Gateway Control Protocol               June 2003


   The IETF has also received notice of intellectual property claims
   relating to Megaco/H.248.1.  Please consult the IETF IPR
   announcements at http://www.ietf.org/ipr.html.

Acknowledgments

   Megaco/H.248.1 is the result of hard work by many people in both the
   IETF and in ITU-T Study Group 16.  This section records those who
   played a prominent role in ITU-T meetings, on the Megaco list, or
   both.

   Megaco/H.248 owes a large initial debt to the MGCP protocol (RFC
   2705), and thus to its authors, Mauricio Arango, Andrew Dugan, Ike
   Elliott, Christian Huitema, and Scott Pickett.  Flemming Andreasen
   does not appear on this list of authors, but was a major contributor
   to the development of both MGCP and Megaco/H.248.1.  RFC 3435 has an
   extensive acknowledgement of many other people who worked on media
   gateway control before Megaco got started.

   The authors of the first Megaco RFCs (2805, then 3015) were Fernando
   Cuervo, Nancy Greene, Abdallah Rayhan, Christian Huitema, Brian
   Rosen, and John Segers.  Christian Groves conceived and was editor of
   Annex C.  The people most active on the Megaco list in the period
   leading up to the completion of RFC 2885 were Brian Rosen, Tom
   Taylor, Nancy Greene, Christian Huitema, Matt Holdrege, Chip Sharp,
   John Segers, Michael Thomas, Henry Sinnreich, and Paul Sijben.  The
   people who sacrificed sleep and meals to complete the massive amount
   of work required in the decisive Study Group 16 meeting of February,
   2000, were Michael Brown, Ranga Dendi, Larry Forni, Glen Freundlich,
   Christian Groves, Alf Heidemark, Steve Magnell, Selvam Rengasami,
   Rich Rubin, Klaus Sambor, John Segers, Chip Sharp, Tom Taylor, and
   Stephen Terrill.

   The most active people on the Megaco list in the period since the
   February 2000 have been Tom Taylor, Brian Rosen, Christian Groves,
   Madhu Babu Brahmanapally, Troy Cauble, Terry Anderson, Chuong Nguyen,
   and Kevin Boyle, but many other people have been regular
   contributors.  Brian Rosen did tremendous service in putting together
   the Megaco interoperability tests.  On the Study Group 16 side, the
   editorial team for the final revised document in February, 2002
   included Christian Groves, Marcello Pantaleo, Terry Anderson, Peter
   Leis, Kevin Boyle, and Tom Taylor.

   Tom Taylor as Megaco Chair managed the day to day operation of the
   Megaco list, with Brian Rosen taking an equal share of the burden for
   most of the last three years.  Glen Freundlich as the Study Group 16
   Rapporteur ran the ITU-T meetings and ensured that all of the work at
   hand was completed.  Without Glen's determination the Megaco/H.248


Groves, et al.              Standards Track                   [Page 211]

RFC 3525                Gateway Control Protocol               June 2003


   standard would have taken at least half a year longer to produce.
   Christian Groves filled in ably as Rapporteur when Glen could no
   longer take part.

Authors' Addresses

   Terry L. Anderson
   24 Hill St
   Bernardsville, NJ 07924
   USA

   EMail: tlatla@verizon.net


   Christian Groves
   Ericsson AsiaPacificLab Australia
   37/360 Elizabeth St
   Melbourne, Victoria 3000
   Australia

   EMail: Christian.Groves@ericsson.com.au


   Marcello Pantaleo
   Ericsson Eurolab Deuschland
   Ericsson Allee 1
   52134 Herzogenrath, Germany

   EMail: Marcello.Pantaleo@eed.ericsson.se


   Tom Taylor
   Nortel Networks
   1852 Lorraine Ave,
   Ottawa, Ontario
   Canada K1H 6Z8

   Phone: +1 613 736 0961
   EMail: taylor@nortelnetworks.com


Groves, et al.              Standards Track                   [Page 212]

RFC 3525                Gateway Control Protocol               June 2003


Full Copyright Statement

   Copyright (C) The Internet Society (2003).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.


Groves, et al.              Standards Track                   [Page 213]