SCTP (Stream Control Transmission Protocol)

SCTP is a new reliable transport protocol that operates on top of a connectionless packet network such as IP, and operates at the same layer as TCP. It establishes a connection between two endpoints, called an association, for transmission of user messages. To establish an association between SCTP endpoints, one endpoint provides the other with a list of its transport addresses (one or more IP addresses in combination with an SCTP port). These transport addresses identify the addresses that will send and receive SCTP packets. SCTP was developed to eliminate deficiencies in TCP and offers acknowledged, error-free, non-duplicated user data transport.

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 end point to another) to ensure in-sequence delivery between associated end points. 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 does enforce head-of-line blocking, the SIGTRAN Working Group recommends SCTP rather than TCP for the transmission of signaling messages over IP networks.

Security

SCTP provides certain transport-related security features, such as resistance against blind "denial of service" attacks, masquerades, or improper monopolization of services.

SIGTRAN protocols do not define new security mechanisms, as the currently available security protocols provide the necessary mechanisms for secure transmission of SS7 messages over IP networks.

Deviations

The following sections summarize the most important deviations from the IETF RFCs that Oracle has made. Refer to the protocol compliance matrices for details (see Oracle Communications Internal References). Contact your Sales Representative for access to the information contained in these documents.

SCTP Multiple Streams

There are several architectural issues regarding the use of multiple streams as described in the SCTP protocol. These issues include:

• Synchronization between data streams
• Synchronization from control stream to data streams
• Load-sharing implementation based on Signaling Link Selection (SLS) across streams, either within a connection or across all of the connections in an Application Server

  Since the underlying SS7 network is connectionless, a stringent requirement for mis-sequenced messages has been set because it is often easier to recover from the loss of a message by a time-out than from one message delivered out-of-sequence. The Message Transfer Part (MTP) is able to maintain a high probability of message sequencing. This is ensured by the MTP user, which generates a value for a Signaling Link Selection (SLS) field as a parameter for each message. As the message is routed through the network, wherever there is a choice to be made between alternate routes, the link selection is made based on the SLS value in the message.

• Connection behavior when a stream becomes congested

A lack of consensus on the IETF SIGTRAN mailing list regarding these issues resulted in supporting a maximum of two streams: one control stream and one data stream.

SCTP Timer

Based on experiences in the field, Oracle has deviated from some RFC-recommended timer settings, especially those related to retransmission, to better accommodate signaling networks.

The default mode for the retransmission timer (RMODE) is linear, whereas the RFC-recommended timer setting is exponential. Oracle makes both settings available through configuring an association to use either the Linear (LIN) or the exponential (RFC) method. For more information about both modes and the timer settings, see SCTP Timers.