Support forwarding RIM messages over GTPCv1 EUTRAN<->GERAN

MMEs connect over Gn interface using GTPCv1 towards the SGSN in order to
exchange RIM PDUs by using "RAN Information Relay" GTPCv1 message type.
For more info, see 3GPP TS 29.060 sec 7.5.14.1 "RAN Information Relay"

In order to support it, this commit does the following:

* Uses new libgtp APIs to rx and tx RAN Information Relay messages. The
  same "gsn" object is reused, ie. the local GTPCv1 socket address used
  for exchanging messages against GGSN is reused.
* Adds a new "sgsn_mme_ctx" struct holding information about MMEs
  allowed by the SGSN, each one containing information about the GTP
  address it uses, the in/out routing based on TAI requests, etc. The
  set of MMEs and their config can be set up using new VTY node introduced
  in this commit.
* The RIM related code in SGSN is refactored to allow forwarding from
  and to several types of addresses/interfaces.

Depends: osmo-ggsn.git Change-Id Iea3eb032ccd4aed5187baca7f7719349d76039d4
Depends: libosmocore.git Change-Id I534db7d8bc5ceb19a2a6866f07d5f5c70e456c5c
Related: SYS#5314
Change-Id: I396450b8d8b66595dab8ff7bf41cbf964bb40d93

1 files changed, 54 insertions, 1 deletions

diff --git a/doc/manuals/chapters/configuration.adoc b/doc/manuals/chapters/configuration.adoc
index 7d3072ec0..c31761c5f 100644
--- a/doc/manuals/chapters/configuration.adoc
+++ b/doc/manuals/chapters/configuration.adoc
@@ -11,7 +11,8 @@ explicit configuration of 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
+[[gp-if-ggsn]]
+=== Configuring the Gp interface (towards GGSN)
 
 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
@@ -67,6 +68,58 @@ OsmoSGSN(config-sgsn)# grx-dns-add 1.2.3.4 <3>
 <2> Enable the dynamic GGSN resolving mode
 <3> Specify the IP address of a DNS server for APN resolution
 
+[[gp-if-mme]]
+=== Configuring the Gp interface (towards MME)
+
+The Gp interface also contains the GTP-C v1 based interface between the SGSN
+and the MMEs. This interface between SGSN and MMEs is used to transfer _RAN
+Information Relay_ GTP-C messages between them, which are used as containers to
+allow PCUs under the SGSN and eNodeBs under MMEs to  exchange cell information
+(RIM).
+
+In the SGSN, this interface re-uses the same socket local configuration as per
+the GGSN connections (see _gtp local-ip_ VTY command in <<gp-if-ggsn>>).
+
+Similarly as with GGSNs, (again see <<gp-if-ggsn>>), selection of destination
+peers for the _RAN Information Relay_ message can be configured statically or
+dynamically over GRX.
+
+
+==== Static MME/TAI configuration
+
+In this mode, there is a static list of MMEs and TAIs configured in
+OsmoSGSN via the VTY / config file. One MME in the list can be configured as the
+_default route_, where all unspecified TAIs are routed too.
+
+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 MME/TAI configuration (single catch-all GGSN)
+----
+sgsn
+...
+ gtp local-ip 192.168.0.10 <1>
+ mme test-mme0 <2>
+  gtp remote-ip 192.168.0.20 <3>
+  gtp ran-info-relay 262 42 3 <4>
+  gtp ran-info-relay 262 42 4
+ mme test-mme1 <5>
+  gtp remote-ip 192.168.0.30
+  gtp ran-info-relay default  <6>
+----
+<1> Configure the local IP address at the SGSN used for Gp/GTP
+<2> Configure an MME named "test-mme0"
+<3> Specify the remote IP address of the MME (for MME "test-mme0")
+<4> Route specified TAIs towards this MME
+<5> Configure an MME named "test-mme1"
+<6> Route all TAIs with an unspecified MME towards MM "test-mme1"
+
+==== Dynamic MME/TAI configuration
+
+Dynamic MME/TAI peer look up over GRX is not yet supported by OsmoSGSN.
+
+
 [[auth-pol]]
 === Authorization Policy