Towards a real SIGTRAN/SS7 stack in libosmo-sigtran

In the good old days ever since the late 1980ies - and a surprising
amount even still today - telecom signaling traffic is still carried
over circuit-switched SS7 with its TDM lines as physical layer, and not
an IP/Ethernet based transport.

When Holger first created OsmoBSC, the BSC-only version of OpenBSC some
7-8 years ago, he needed to implement a minimal subset of SCCP wrapped
in TCP called SCCP Lite. This was due to the simple fact that the MSC
to which it should operate implemented this non-standard protocol
stacking that was developed + deployed before the IETF SIGTRAN WG
specified M3UA or SUA came around. But even after those were specified
in 2004, the 3GPP didn't specify how to carry A over IP in a standard
way until the end of 2008, when a first ).

So in order to proceed to implement the required
SCCP-over-M3UA-over-SCTP stacking, I originally thought well, take
Holgers old SCCP code, remove it from the IPA multiplex below, stack it
on top of a new M3UA codebase that is copied partially from SUA.

However, this falls short of the goals in several ways:

• The application shouldn't care whether it runs on top of SUA or SCCP,
  it should use a unified interface towards the SCCP Provider.
  OsmoHNBGW and the SUA code already introduce such an interface baed on
  the SCCP-User-SAP implemented using .




• Migrate the existing SUA code on top of that, maintaining the existing
  osmo_prim based
  to makes sure the bulk of the code has to deal only with one message
  format (parsed SUA).




• Introduce a
  and add ASPTM/ASPSM support to SUA (was missing so far) * Implement




• Implement M3UA using the xUA ASP and AS FSMs as well as the general
  xUA message encoder/decoder,
  offering the MTP SAP toward SCCP

And then finally stack all those bits on top of each other, rendering a
fairly clean and modern implementation that can be used with the IuCS of
the virtually unmodified OsmmoHNBGW, OsmoCSCN and OsmoSGSN for testing.

Next steps in the direction of the AoIP are:

• Implementation of the MTP-SAP based on the IPA transport




• Binding the new SCCP code on top of that




• Converting OsmoBSC code base to use the SCCP-User-SAP for its
  signaling connection

From that point onwards, OsmoBSC doesn't care anymore whether it
transports the BSSAP/BSSMAP messages of the A interface over
SCCP/IPA/TCP/IP (SCCPlite) SCCP/M3UA/SCTP/IP (3GPP AoIP), or even
something like SUA/SCTP/IP.

However, the 3GPP AoIP specs (unlike SCCPlite) actually modify the
BSSAP/BSSMAP payload. Rather than using Circuit Identifier Codes and
then mapping the CICs to UDP ports based on some secret conventions,
they actually encapsulate the IP address and UDP port information for
the RTP streams. This is of course the cleaner and more flexible
approach, but it means we'll have to do some further changes inside the
actual BSC code to accommodate this.