Session Initiation Protocol (SIP)

The Session Initiation Protocol (SIP) is the IETF standard for the establishment of multimedia sessions. These sessions might be used for audio, video, instant messaging, or other real-time data communication sessions.

Building a complete system that utilizes the SIP protocol requires the implementer to read the base SIP specification and any number of protocol documents that are targeted for a particular application. For example, the use of SIP for Instant Messaging is defined separately from the SIP specification and the use of SIP within Voice over IP requires the implementer to be familiar with SDP, which is an entirely separate RFC. This "modular" design is considered a strength of the SIP protocol.

The scope of SIP is relatively broad, including the establishment of virtually any kind of "session" between two parties. SIP is also entirely independent of the underlying transport, though TCP and UDP are used almost exclusively.

SIP was initially published as an Internet Draft by the IETF in 1996, with the first RFC in 1999. As of this writing, the most recent SIP specification is published in RFC 3261.

Resources: SIP Information Site, SIP Forum

See Also: Voice over IP (VoIP), H.323, H.248 (or Megaco), Media Gateway Control Protocol (MGCP), Instant Messaging (IM)