The Session Initiation Protocol (SIP) is an Internet Engineering Task Force standard protocol for initiating an interactive user session that could involve multiple elements such as video, voice, and data.

Like other Internet protocols such as HTTP or SMTP, SIP works in the application layer of the Open Systems Interconnection communications model. This layer is responsible for ensuring that communication is possible. SIP can initiate multimedia sessions or Internet telephony calls, and modify or terminate them. Because SIP supports name mapping and redirection, it enables users to initiate and receive data flow from any location, and for networks to identify the users where ever they are.

SIP is a request-response protocol. That is, SIP manages requests from clients and responses from servers. Participants are identified by SIP URLs. Requests can be sent through any transport protocol, such as UDP, SCTP, or TCP. SIP determines the end system to be used for the session, the communication media and media parameters, and the called party's desire to engage in the communication. SIP then establishes call parameters at either end of the communication, and handles call transfer and termination.

SIP presents an enormous opportunity to redefine how networks communicate. SIP has proved to be such an outstanding and flexible protocol that vendors and service providers are spending significant resources to best leverage this market opportunity. Some of the primary drivers for SIP include:

     # Interoperability of IP devices
     # Easy deployment of multimedia applications through seamless voice, video, and data integration
     # Strong presence and instant messaging capabilities
     # Tight device/application integration through SIP support in Windows XP and AOL Instant Messenger
     # 3G Mobile device support