== Configuring OsmoSGSN

Contrary to other network elements (like OsmoBSC, OsmoNITB), the
OsmoSGSN has a relatively simple configuration.

On the one hand, this is primary because the PCU configuration happens
from the BSC side.

On the other hand, it is because the Gb interface does not need an
explicit configuration of all each PCU connecting to the SGSN.  The
administrator only has to ensure that the NS and BSSGP layer identities
(NSEI, NSVCI, BVCI) are unique for each PCU connecting to the SGSN.

=== Configuring the Gp interface

The Gp interface is the GTP-C and GTP-U based interface between the SGSN
and the GGSNs.  It is implemented via UDP on well-known source and
destination ports.

When a MS requests establishment of a PDP context, it specifies the APN
(Access Point Name) to which the context shall be established.  This APN
determines which GGSN shall be used, and that in turn determines which
external IP network the MS will be connected to.

There are two modes in which GGSNs can be configured:

. static GGSN/APN configuration
. dynamic GGSN/APN configuration

==== Static GGSN/APN configuration

In this mode, there is a static list of GGSNs and APNs configured in
OsmoSGSN via the VTY / config file.

This is a non-standard method outside of the 3GPP specifications for the
SGSN, and is typically only used in private/small GPRS networks without
any access to a GRX.

.Example: Static GGSN/APN configuration (single catch-all GGSN)
----
OsmoSGSN(config-sgsn)# gtp local-ip 172.0.0.1 <1>
OsmoSGSN(config-sgsn)# ggsn 0 remote-ip 127.0.0.2 <2>
OsmoSGSN(config-sgsn)# ggsn 0 gtp-version 1 <3>
OsmoSGSN(config-sgsn)# apn * ggsn 0 <4>
----
<1> Configure the local IP address at the SGSN used for Gp/GTP
<2> Specify the remote IP address of the GGSN (for GGSN 0)
<3> Specify the GTP protocol version used for GGSN 0
<4> Route all APN names to GGSN 0


==== Dynamic GGSN/APN configuration

In this mode, the SGSN will use a DNS-based method to perform the lookup
from the APN (as specified by the MS) towards the GGSN IP address.

This is the official method as per the 3GPP specifications for the SGSN,
and what is used on GRX.

.Example: Dynamic GGSN/APN configuration
----
OsmoSGSN(config-sgsn)# gtp local-ip 192.168.0.11 <1>
OsmoSGSN(config-sgsn)# ggsn dynamic <2>
OsmoSGSN(config-sgsn)# grx-dns-add 1.2.3.4 <3>
----
<1> Configure the local IP address at the SGSN used for Gp/GTP
<2> Enable the dynamic GGSN resolving mode
<3> Specify the IP address of a DNS server for APN resolution

[[auth-pol]]
=== Authorization Policy

The authorization policy controls by which rules a subscriber is accepted or
rejected. The possible options range from accepting just all subscribers without
further checking, to a fine grained access-control, handled by an external HLR.

accept-all:: All subscribers that attempt to attach to the GPRS network are
accepted without further checking. This option is intended to be used for
testing in a controlled environment only. A wide-open network may attract
subscribers from foreign networks and disrupt their service. It is highly
recommended to pick one of the options below.

remote:: This option allows to connect OsmoSGSN to an external HLR via the
GSUP protocol. This will be the preferred option in larger networks.

acl-only:: If no external HLR is available, the network operator has the
option to control the access using an access control list. The access control
list contains the IMSI numbers of the allowed subscribers. This method offers
fine grained access control and is ideal for small networks and lab test
environments.

closed:: This policy mode softens the strict *acl-only* only mode by also
implicitly accepting home network subscribers. The decision is made by the MCC
and MNC part of the IMSI number. The combination of MCC and MNC fully identifies
a subscribers home network, also known as a Home Network Identity (HNI, i.e.
MCC and MNC found at the start of the IMSI, e.g. MCC 901 and MNC 700 with
IMSI 901700000003080).

NOTE: The policy mode *closed* must not be confused with the equally named
policy that is defined for osmo-nitb!


.Example: Assign or change authorization policy:
----
OsmoSGSN> enable
OsmoSGSN# configure terminal
OsmoSGSN(config)# sgsn
OsmoSGSN(config-sgsn)# auth-policy acl-only <1>
OsmoSGSN(config-sgsn)# write <2>
Configuration saved to sgsn.cfg
OsmoSGSN(config-sgsn)# end
OsmoSGSN# disable
OsmoSGSN>
----
<1> 'acl-only' is selected as authorization policy
<2> Saves current changes to cofiguration to make this policy
persistent

.Example: Access control list:
----
sgsn
 auth-policy acl-only <1>
 imsi-acl add 001010000000003
 imsi-acl add 001010000000002
 imsi-acl add 001010000000001
 imsi-acl add 901700000000068 <2>
----
<1> Set the authorization policy
<2> Add as many subscribers as required

=== Subscriber Configuration

As opposed to OsmoNITB, OsmoSGSN does not feature a built-in HLR.

It can thus operate only in the following two modes:

. Accessing an external HLR (or HLR gateway) via the GSUP protocol
. Accepting subscribers based on internal ACL (access control list),
  see also <<auth-pol>>

==== Accessing an external HLR via GSUP

The non-standard GSUP protocol was created to provide OsmoSGSN with
access to an external HLR while avoiding the complexities of the
TCAP/MAP protocol stack commonly used by HLRs.

A custom HLR could either directly implement GSUP, or an external gateway
can be used to convert GSUP to the respective MAP operations.

The primitives/operations of GSUP are modelled to have a 1:1
correspondence to their MAP counterparts.  However, the encoding is much
simplified by use of a binary TLV encoding similar to Layer 3 of
GSM/GPRS.

GSUP performs a challenge-response authentication protocol called OAP,
which uses the standard MILEAGE algorithm for mutual authentication
between OsmoSGSN and the HLR/HLR-GW.

[[sgsn-ex-gsup]]
.Example: Using an external HLR via GSUP
----
OsmoSGSN(config-sgsn)# gsup remote-ip 2.3.4.5 <1>
OsmoSGSN(config-sgsn)# gsup remote-port 10000 <2>
OsmoSGSN(config-sgsn)# gsup oap-k 000102030405060708090a0b0c0d0e0f <3>
OsmoSGSN(config-sgsn)# gsup oap-opc 101112131415161718191a1b1c1d1e1f <4>
----
<1> Configure the IP address of the (remote) HLR or HLR-GW
<2> Configure the TCP port of the (remote) HLR or HLR-GW
<3> Specify the OAP shared key
<4> Specify the OAP shared OPC


=== CDR configuration

OsmoSGSN can write a text log file containing CDR (call data records),
which are commonly used for accounting/billing purpose.

.Example: CDR configuration
----
OsmoSGSN(config-sgsn)# cdr filename /var/log/osmosgsn.cdr
OsmoSGSN(config-sgsn)# cdr interval 600 <1>
----
<1> Periodically log existing PDP contexts every 600 seconds (10 min)

The CDR file is a simple CSV file including a header line naming the
individual fields of each CSV line.

[[sgsn-cdr]]
.Description of CSV fields in OsmoSGSN CDR file
[options="header",cols="15%,85%"]
|===
|Field Name|Description
|timestamp|Timestamp in YYYYMMDDhhmmssXXX where XXX are milli-seconds
|imsi|IMSI causing this CDR
|imei|IMEI causing this CDR
|msisdn|MSISDN causing this CDR (if known)
|cell_id|Cell ID in which the MS was registered last
|lac|Location Area Code in which the MS was registered last
|hlr|HLR of the subscriber
|event|Possible events are explained below in <<sgsn-cdr-event>>
|pdp|
|pdp_duration|duration of the PDP context so far
|ggsn_addr|GGSN related to the PDP context
|sgsn_addr|SGSN related to the PDP context
|apni|APN identifier of the PDP context
|eua_addr|IP address allocated to the PDP context
|vol_in|Number of bytes in MO direction
|vol_out|Number of bytes in MT direction
|charging_id|Related charging ID
|===

[[sgsn-cdr-event]]
.Description of OsmoSGSN CDR Events
[options="header",cols="15%,85%"]
|===
|Event|Description
|attach|GMM ATTACH COMPLETE about to be sent to MS
|update|GMM ROUTING AREA UPDATE COMPLETE about to be sent to MS
|detach|GMM DETACH REQUEST received from MS
|free|Release of the MM context memory
|pdp-act|GTP CREATE PDP CONTEXT CONFIRM received from GGSN
|pdp-deact|GTP DELETE PDP CONTEXT CONFIRM received from GGSN
|pdp-terminate|Forced PDP context termination during MM context release
|pdp-free|Release of the PDP context memory
|===


=== User traffic compression

In order to save optimize GPRS bandwith, OsmoSGSN implements header and data
compression schemes. The compression will reduce the packet length in order
to save radio bandwith.

==== Header compression

On TCP/IP connections, each packet is prepended with a fairly long TCP/IP
header. The header contains a lot of static information that never changes
throughout the connection. (source and destination address, port numbers etc.)
OsmoSGSN implements a TCP/IP header compression scheme called RFC1144, also
known as SLHC. This type of header compression removes the TCP/IP header
entirely and replaces it with a shorter version, that only contains the
information that is absolutely necessary to identify and check the packet.
The receiving part then restores the original header and forwards it to higher
layers.

*compression rfc1144 passive*::
TCP/IP header compression has to be actively requested by the modem. The
network will not promote compression by itself. This is the recommended mode
of operation.

*compression rfc1144 active slots <1-256>*::
TCP/IP header compression is actively promoted by the network. Modems may still
actively request different compression parameters or reject the offered
compression parameters entirely. The number of slots is the maximum number
of packet headers per subscriber that can be stored in the codebook.

.Example: Accept compression if requested:
----
sgsn
 compression rfc1144 passive
----

.Example: Actively promote compression:
----
sgsn
 compression rfc1144 active slots 8
----

NOTE: The usage of TCP/IP options may disturb the RFC1144 header compression
scheme. TCP/IP options may render RFC1144 ineffective if variable data is
encoded into the option section of the TCP/IP packet. (e.g. TCP option 8,
Timestamp)


==== Data compression

Data compression works on the raw packet data, including the header part of the
packet. If enabled, header compression is applied before first data compression
is applied. OsmoSGSN implements the V.42bis data compression scheme.

*compression rfc1144 passive*::
V42bis data compression has to be actively requested by the modem. The network
will not promote compression by itself. This is the recommended mode of
operation.

*compression v42bis active direction (ms|sgsn|both) codewords <512-65535> strlen <6-250>*::
V42bis data compression is actively promoted by the network. Modems may still
actively request different compression parameters or reject the offered
compression parameters entirely. The direction configures which sides are
allowed to send compressed packets. For most cases, compressing 'both'
directions will be the preferred option. The following to parameters configure
the codebook size by the maxium number ('codewords') and size ('strlen') of
entries.

.Example: Accept compression if requested:
----
sgsn
 compression v42bis passive
----

.Example: Actively promote compression:
----
sgsn
 compression v42bis active direction both codewords 512 strlen 20
----