SCTP: Stream Control Transmission Protocol

To reliably transport SS7 messages over IP networks, the Internet Engineering Task force sigtran working group devised the Stream Control Transmission Protocol (SCTP). SCTP allows the reliable transfer of signaling messages between signaling endpoints in an IP network.

To establish an association between SCTP endpoints, one endpoint provides the other endpoint with a list of its transport addresses (multiple IP addresses in combination with an SCTP port). These transport addresses identify the addresses which will send and receive SCTP packets.

IP signaling traffic is usually composed of many independent message sequences between many different signaling endpoints. SCTP allows signaling messages to be independently ordered within multiple streams (unidirectional logical channels established from one SCTP endpoint to another) to ensure in-sequence delivery between associated endpoints. By transferring independent message sequences in separate SCTP streams, it is less likely that the retransmission of a lost message will affect the timely delivery of other messages in unrelated sequences (called head-of-line blocking). Because TCP/IP does enforce head-of-line blocking, the sigtran Working Group recommends SCTP rather than TCP/IP for the transmission of signaling messages over IP networks.

There are three types of messages in SS7:

  • Message Signal Units (MSUs)
  • Link Status Signal Units (LSSUs)
  • Fill-In Signal Units (FISUs)

MSUs originate at a higher level than MTP Level 2 and are destined for a peer at another node. LSSUs allow peer MTP Level 2 layers to exchange link status information. FISUs are sent when no other signal units are waiting to be sent across the synchronous link. This purpose is preserved by the heartbeat messages in SCTP. FISUs also carry acknowledgment of messages, a function also assumed by SCTP.

In summary, SCTP provides:

  • acknowledged error-free non-duplicated transfer of signaling information  
  • in-sequence delivery of messages within multiple streams, with an option for order-of-arrival delivery of individual messages
  • optional bundling of multiple messages into a single SCTP packet  
  • data fragmentation as required  
  • network-level fault tolerance through support of multi-homing at either or both ends of an association
  • appropriate congestion avoidance behavior and resistance to flooding (denial-of-service) and masquerade attacks

To meet stringent SS7 signaling reliability and performance requirements for carrier-grade networks, VoIP network operators ensure that there is no single point of failure in the end-to-end network architecture between an SS7 node and a media gateway controller. To achieve carrier-grade reliability in IP networks, links in a linkset are typically distributed amongst multiple signaling gateways, media gateway controllers are distributed over multiple CPU hosts, and redundant IP network paths are provisioned to ensure survivability of SCTP associations between SCTP endpoints.

 

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