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Understanding the SIP Protocol
November 2009 245
Numerous protocols carry various forms of real-time multimedia
session data such as voice, video, or text messages. The Session
Initiation Protocol (SIP) works in concert with these protocols by
enabling internet endpoints (called user agents) to discover one
another and to agree on a characterization of a session they would
like to share.
For locating prospective session participants, and for other
functions, SIP enables the creation of an infrastructure of network
hosts (called proxy servers) to which user agents can send
registrations, invitations to sessions, and other requests. SIP is an
agile, general-purpose tool for creating, modifying, and terminating
sessions that works independently of underlying transport protocols,
regardless of the type of session being established.
Overview of SIP Functionality
SIP is an application-layer control protocol that can establish,
modify, and terminate multimedia sessions (conferences) such as
internet telephony calls. SIP can also invite participants to already
existing sessions, such as multicast conferences. Media is added to
(and removed from) an existing session. SIP transparently supports
name mapping and redirection services, supporting personal
mobility - users can maintain a single externally visible identifier
regardless of their network location.
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.