Session Initiation Protocol (SIP)

Session Initiation Protocol (SIP) is an application-layer control protocol that can establish, modify, and terminate multimedia sessions (conferences). A session is considered as an exchange of data between an association of participants, such as Internet telephony calls and video telephony. SIP is able to support multicast conferences with more than two participants. Participants can be invited to already existing sessions. Media can be added to (and removed from) an existing session.

SIP supports personal mobility - users can maintain a single externally visible identifier regardless of their network location. SIP transparently supports name mapping and redirection services, to support this mobility.

SIP supports five facets of establishing and terminating multimedia communications:

• User location: determination of the end system to be used for communication;
• User availability: determination of the willingness of the called party to engage in communications;
• User capabilities: determination of the media and media parameters to be used;
• Session setup: "ringing", establishment of session parameters at both called and calling party;
• Session management: including transfer and termination of sessions, modifying session parameters, and invoking services.

SIP is not a vertically integrated communications system. SIP is rather a component that can be used with other Internet protocols to build a complete multimedia architecture. However, the basic functionality and operation of SIP does not depend on any of these protocols.

SIP is described in RFC 3261.