== 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


=== 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)

==== 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
|===