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SCTP (Stream Control Transmission Protocol)

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SCTP is a protocol for transporting PSTN signaling messages over IP networks. In addition, the protocol designers state that SCTP is capable of broader applications, including transport of broadcast and streaming data, without the need for TCP. SCTP is a connection-oriented protocol like TCP that provides many of the reliability features of TCP such as acknowledgments, fragmentation, and sequencing. But SCTP eliminates much of the overhead inherent in TCP that can cause delays. It also provides additional features that optimize it for signal transport.

PSTN signaling is important in IP environments when IP telephony clients wish to establish call connections with PSTN telephone users. SCTP provides SS7 (Signaling System 7) functionality over IP networks and, therefore, supports voice call setup over the Internet. In the PSTN, SS7 is an out-of-band signaling system used to set up voice calls. SCTP is used to transport SS7 messages across IP networks and the Internet in situations in which an IP telephony user wants to call a traditional PSTN telephone (or vice versa). These calls are actually set up across signaling and media gateways that translate between SS7 and Internet signaling.

SCTP's special feature is the ability to deliver SS7 messages with little loss and delay. This is critical in order to interface with SS7 since SS7 does not tolerate high delays. In particular, SCTP attempts to reduce delays caused by "head-of-line blocking," which is a queuing problem that can cause delays while some messages wait for other messages to be serviced. TCP causes this problem because it enforces a strict order of delivery. SCTP overcomes the problem by assuming that low delay is more important than order of delivery.

SCTP is described in RFC 2960 (Stream Control Transmission Protocol, October 2000). It was developed by the IETF Signaling Transport (sigtran) Working Group, which is working on related protocols.

Copyright (c) 2001 Tom Sheldon and Big Sur Multimedia.
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