Menu
Home
People
Places
Arts
History
Plants & Animals
Science
Life & Culture
Technology
Reference.org
Session Initiation Protocol
open-in-new
<p>The Session Initiation Protocol (SIP) is a <a href="/facts/Signaling_protocol/839NyIvT">signaling protocol</a> used for initiating, maintaining, and terminating <a href="/facts/Communication_session/PRQuXhIL">communication sessions</a> that include voice, video and messaging applications. SIP is used in <a href="/facts/Internet_telephony/5QGBbXYg">Internet telephony</a>, in private IP telephone systems, as well as mobile phone calling over <a href="/facts/LTE_(telecommunication)/72QhwXHT">LTE</a> (<a href="/facts/VoLTE/Rp3slvPE">VoLTE</a>). </p><p>The protocol defines the specific format of messages exchanged and the sequence of communications for cooperation of the participants. SIP is a <a href="/facts/Text-based_protocol/uavpbuft">text-based protocol</a>, incorporating many elements of the <a href="/facts/Hypertext_Transfer_Protocol/TWtvf7JD">Hypertext Transfer Protocol</a> (HTTP) and the <a href="/facts/Simple_Mail_Transfer_Protocol/TeCRKDpJ">Simple Mail Transfer Protocol</a> (SMTP). A call established with SIP may consist of multiple <a href="/facts/Media_stream/4rPhSq0Y">media streams</a>, but no separate streams are required for applications, such as <a href="/facts/Text_messaging/AjCD61SD">text messaging</a>, that exchange data as payload in the SIP message. </p><p>SIP works in conjunction with several other protocols that specify and carry the session media. Most commonly, media type and parameter negotiation and media setup are performed with the <a href="/facts/Session_Description_Protocol/WMnnA5jb">Session Description Protocol</a> (SDP), which is carried as payload in SIP messages. SIP is designed to be independent of the underlying <a href="/facts/Transport_layer/UmIGUCeK">transport layer</a> protocol and can be used with the <a href="/facts/User_Datagram_Protocol/dxLU0D6q">User Datagram Protocol</a> (UDP), the <a href="/facts/Transmission_Control_Protocol/WW4sPrt9">Transmission Control Protocol</a> (TCP), and the <a href="/facts/Stream_Control_Transmission_Protocol/s8tmzUhz">Stream Control Transmission Protocol</a> (SCTP). For secure transmissions of SIP messages over insecure network links, the protocol may be encrypted with <a href="/facts/Transport_Layer_Security/pGy16TnA">Transport Layer Security</a> (TLS). For the transmission of media streams (voice, video) the SDP payload carried in SIP messages typically employs the <a href="/facts/Real-time_Transport_Protocol/YKQK1RK2">Real-time Transport Protocol</a> (RTP) or the <a href="/facts/Secure_Real-time_Transport_Protocol/LBXehUtz">Secure Real-time Transport Protocol</a> (SRTP). </p>