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SIP (Session Initiation Protocol)

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Note: Many topics at this site are reduced versions of the text in "The Encyclopedia of Networking and Telecommunications." Search results will not be as extensive as a search of the book's CD-ROM.

SIP is an Internet protocol that provides simple application layer signaling for setting up, maintaining, and terminating multimedia sessions such as voice calls, videoconferences, and even instant messaging sessions. SIP performs many of the functions of the ITU H.323 multimedia conferencing standard, which was largely specified by the telecoms. SIP provides a more-scalable, higher-performance, and more-efficient calling model. Because it is designed on the Internet model, it is inherently distributed and supports the development of telephony applications on Internet systems.

SIP is described in RFC 2543 (SIP: Session Initiation Protocol, March 1999). A related document is RFC 2327 (SDP: Session Description Protocol, April 1998). SDP is a protocol for describing multimedia session for the purposes of session announcement, session invitation, and other forms of multimedia session initiation.

SIP normally runs over UDP or TCP, but it can run over other protocols such as IP, ATM, or X.25. It requires only a datagram service and is independent of the packet layer. It can provide "out-of-band" call setup services in which the SIP exchanges take place over UDP or TCP, but actual data transmission takes place over the public telephone network.

SIP addresses are expressed as URLs. For example, a SIP URL might be sip:+12125551212@, where is the host serving as a gateway into the PSTN.

This topic continues in "The Encyclopedia of Networking and Telecommunications" with a discussion of the following:

  • Call setup, maintenance, and termination
  • SIP textual syntax
  • Request and response messages
  • SIP entities, including user agents, proxy agents, and redirect servers.
  • SIP extensions
  • Related Internet RFCs
  • SCTP (Stream Control Transmission Protocol)

See "Multimedia" and "Voice over IP (VoIP)" for more information about how SIP relates to multimedia conferencing and related protocols.

Copyright (c) 2001 Tom Sheldon and Big Sur Multimedia.
All rights reserved under Pan American and International copyright conventions.