Definition

Session Initiation Protocol (SIP)

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

Session Initiation Protocol (SIP) is a signaling protocol used for initiating, maintaining, modifying and terminating real-time sessions that involve video, voice, messaging and other communications applications and services between two or more endpoints on IP networks.

SIP was developed by the Internet Engineering Task Force (IETF) to address the evolving needs of IP-based communications. Native support for mobility, interoperability and multimedia were among the drivers behind SIP's development. SIP complements other communications protocols, such as Real-Time Transport Protocol (RTP) and Real-Time Streaming Protocols (RTSP), used in IP-based sessions.

SIP features

The SIP communications protocol determines five attributes when establishing and terminating multimedia sessions:

  • User location
  • User availability
  • User capabilities
  • Session setup
  • Session management

Different types of multimedia SIP sessions include internet telephony calls, video conferencing and other forms of unified communications. The protocol can be used to invite participants to unicast or multicast sessions that do not necessarily involve the initiator.

SIP itself does not provide communication services. Instead, the protocol's specification defines interoperable implementations of SIP features, called primitives, that can be used to facilitate different services. Primitives allow additional information to be embedded in a SIP message, such as linking a user's photo to directory information to enable media-rich caller ID.

Legacy PBX connected to SIP provider over the Internet

SIP also supports name mapping and redirection services, which are two key ways the protocol enables mobility. Users and endpoints are detected with a single identifier, known as a uniform resource identifier (URI), which is independent of their network location. URIs are alphanumeric, using a syntax that looks more like an email address than a phone number or IP address.

In addition to real-time services, SIP can be used for asynchronous event notifications, such as automatic callbacks, message-waiting indicators and buddy lists based on presence.

How does SIP work?

Like HTTP or SMTP, SIP works in the application layer of the Open Systems Interconnection (OSI) communications model.

SIP is a request-response protocol, receiving requests from clients and responses from servers. Requests can be sent through any transport protocol, such as UDP, SCTP or TCP.

Legacy PBX connected to SIP Provider via dedicated line

SIP determines the end system to be used for the session, the communication media and media parameters, and whether the called party agrees to engage in communication. Once these are assured, SIP establishes call parameters at either end of the communication, also handling call transfer and termination.

The protocol is supported by both IPv4 and IPv6.

This was last updated in March 2017

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