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What is SIP (Session Initiation Protocol)?

The definition of session initiation protocol is a communications protocol used for controlling communications sessions.  Over the past decade, SIP has become a standard protocol in the telecommunications industry for calls over the data network. A typical SIP “session” might include two or more parties engaged in a voice over IP (VoIP) call. SIP sessions could also include video calls, as well as for instant messaging over IP networks.

How SIP works

SIP works by defining messages sent between endpoints, which govern the beginning, the end and other parts of a call. SIP is frequently used to create, modify and end sessions consisting of one or multiple media streams. As an application layer protocol, SIP is designed to be independent of the underlying transport layer. SIP is a text-based protocol that incorporates several elements of the Hypertext Transfer Protocol (HTTP).

SIP can be configured to work with several other application layer protocols that identify and carry the session media. Session Description Protocol (SDP) can be used to achieve media identification. While SIP often employs real-time transport protocol (RTP) or Secure Real-time Transport Protocol (SRTP) for for transmitting media streams such as voice and video. For secure transmissions of SIP messages, protocols may be encrypted with Transport Layer Security (TLS) in order to transmit secure SIP messages.