Session Initiation Protocol (SIP)

Session Initiation Protocol

The Session Initiation Protocol (SIP) is acommunications protocol for signaling and controlling multimedia communicationsessions. The most common applications of SIP are in Internet telephony forvoice and video calls, as well as instant messaging, over Internet Protocol(IP) networks.

The protocol defines the messages that aresent between endpoints, which govern establishment, termination and otheressential elements of a call. SIP can be used for creating, modifying andterminating sessions consisting of one or several media streams. SIP is anapplication layer protocol designed to be independent of the underlyingtransport layer. It is a text-based protocol, incorporating many elements ofthe Hypertext Transfer Protocol (HTTP) and the Simple Mail Transfer Protocol(SMTP).

SIP works in conjunction with several otherapplication layer protocols that identify and carry the session media. Media identificationand negotiation is achieved with the Session Description Protocol (SDP). Forthe transmission of media streams (voice, video) SIP typically employs theReal-time Transport Protocol (RTP) or Secure Real-time Transport Protocol(SRTP). For secure transmissions of SIP messages, the protocol may be encryptedwith Transport Layer Security (TLS).