SIP, or Session Initiation Protocol, was published as a proposed standard by the Internet Engineering Task Force (IETF) in 1999. SIP is a signaling protocol for multimedia session control. This includes signaling for voice, data, and video sessions between just two parties or for multi-party conferencing sessions.
SIP end-points negotiate the media parameters using Session Description Protocol (SDP). SDP allows the SIP terminal or application to negotiate media type (audio, video, or data), transport protocol (RTP), and media encoding method.
Unlike the switched circuit network, IP networks developed using SIP will easily support the concepts of presence, availability, and mobility since it is part of the SIP standard.
The IETF defines standard behavior for key SIP Server elements such as registration, redirect, and proxy servers. SIP registration servers allow users to register with the SIP network using a SIP address that is similar to a URL or Web site location. Once a user registers, the local SIP proxy server routes messages on behalf of the user to the proper destination address. The SIP redirect server is a special server that returns a new or changed address for the destination party that may have temporarily changed locations (from work to home for example). Each of these servers has a particular behavior defined in the IETF standard and maintains session state information in a standard manner, leading to interoperability.
SIP Advantages
SIP's main advantages over H.323 are its extensibility and flexibility for adding new features.
H.323 is a more complex architecture and uses binary encoding of message parameters. The simplicity of SIP makes it easier to develop and debug applications leading to lower product costs for equipment providers.
The SIP client-server and header format design is based on proven Internet standards such as Hyper Text Transfer Protocol (used on commercial web servers) and Simple Mail Transfer Protocol (used for internet email services). The client-server design means that each SIP request will result in a well-defined set of SIP responses. This modular design makes it easy to extend SIP to support new operations by simply adding a new request (or SIP Method) and its corresponding response message. Because SIP is text-based, it is easy to parse the various SIP commands.
The SIP Forum is an industry organization with members from the leading SIP technology companies. Its mission is to advance the adoption of products and services based on SIP.
Read more about SIP protocol design and SIP network elements.
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