Session Initiation Protocol (SIP)

This definition is part of our Essential Guide: The basics of SIP trunking explained

The Session Initiation Protocol (SIP) is an Internet Engineering Task Force (IETF) standard protocol for initiating an interactive user session that involves multimedia elements such as video, voice, chat, gaming, and virtual reality.

Like HTTP or SMTP, SIP works in the Application layer of the Open Systems Interconnection (OSI) communications model. The Application layer is the level responsible for ensuring that communication is possible. SIP can establish multimedia sessions or Internet telephony calls, and modify, or terminate them. The protocol can also invite participants to unicast or multicast sessions that do not necessarily involve the initiator. Because the SIP supports name mapping and redirection services, it makes it possible for users to initiate and receive communications and services from any location, and for networks to identify the users whereever they are.

SIP is a request-response protocol, dealing with 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. Once these are assured, SIP establishes call parameters at either end of the communication, and handles call transfer and termination.

The Session Initiation Protocol is specified in IETF Request for Comments [RFC] 2543.


Getting started with SIP
To explore how SIP is used in the enterprise, here are some additional resources:
SIP tutorial: This comprehensive tutorial details the ins and outs of SIP, the VoIP and UC industries' signalling protocol of choice.
SIP network security measures: Learn about specific security measures that can protect your oganization in the event of a SIP-based network attack.

This was first published in March 2008

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