The Session Initiation Protocol (SIP) is a protocol at the application layer designed to set up, modify, and end multimedia sessions or conferences. These sessions involve data exchanges among participants in activities such as Internet telephony and video calls. SIP enables the formation of multicast conferences involving multiple participants, allowing new participants to join ongoing sessions and permitting the addition or removal of media in these sessions.
A key feature of SIP is its support for personal mobility, allowing users to keep a consistent external identifier irrespective of their location in the network. It also seamlessly integrates name mapping and redirection services to facilitate this mobility.
SIP encompasses several critical aspects of multimedia communication establishment and termination:
- User Location: Identifying the appropriate end system for communication.
- User Availability: Assessing the called party’s readiness to communicate.
- User Capabilities: Determining suitable media and media parameters.
- Session Setup: Initiating session parameters and signaling at both the caller and receiver’s end.
- Session Management: This includes transferring and ending sessions, altering session parameters, and activating services.
SIP functions as a modular component, not as an all-inclusive communications system. It integrates with other Internet protocols to create a comprehensive multimedia architecture, but its core functions are independent of these protocols.
The details and specifications of SIP are documented in RFC 3261.
The current list of official warnings (registered in the SIP Parameters IANA registry) can be found on Wikipedia.