Osmocom Libraries » libosmo-sccp + libosmo-sigtran

Reading through RFC4165 and taking some random notes:

• two SCTP streams per direction (one for Link Status, another for User Data)
• M2PA retains classic notion of ss7 links + linksets
  • 1:1 mapping beteween SCTP association and SS7 link
    • same SLC for same link (==association) on both ends
• empty user data messages used to acknowledge receipt in some situations

Message format / protocol elements¶

• 24-bit FSN / BSN sequence numbers in each header
• only one message class with two message types
  • User Data
    • contains MTP3 message with PRI/SIO/SIF fields
  • Link Status
    • Alignment / Proving Normal / Proving Emergency / Ready / Processor Outage / Processor Recovered / Busy / Busy Ended / OOS
    • not all are sent on Stream0, but some on Stream1
• local processor outage
  • MTP3 from peer must be buffered
• remote processor outage
  • MTP3 messages might also need buffering
• flow control / congestion timers
  • criteria how to determine congestion onset is "implementation dependent"
• signaling of SCTP association failures up to MTP3 / take link out of service
• change-over procedure in case of link failure
  • message retrieval: obtain all not-yet-transmitted and all sent-but-not-yet-acknowledged messages from the old link
  • MTP3 XCO / XCA messages instead of COO/COA due to longer FSM/BSN range

FSMs¶

• MTP2 IAC (Initial Alignment Control) as per Q.703 Figure 4 / Figure 9
• MTP2 LSC (Link State Control) as per Q.703 Figure 3 / Figure 8
• MTP3 SLTC (Signalling Link Test Control) as per Q.707
• MTP3 HMDT (Message Distribution) as per Q.704