What is SIP?

According to Wikipedia,

Session Initiation Protocol (SIP) is a protocol developed by the IETF MMUSIC Working Group and proposed standard for initiating, modifying, and terminating an interactive user session that involves multimedia elements such as video, voice, instant messaging, online games, and virtual reality. In November 2000, SIP was accepted as a 3GPP signaling protocol and permanent element of the IMS architecture . It is one of the leading signalling protocols for Voice over IP, along with H.323.


According to Henning Schulzrinne,

SIP provides the necessary protocol mechanisms so that end systems and proxy servers can provide services:

• call forwarding, including
  • the equivalent of 700-, 800- and 900- type calls;
  • call-forwarding no answer;
  • call-forwarding busy;
  • call-forwarding unconditional;
  • other address-translation services;
• callee and calling ``number'' delivery, where numbers can be any (preferably unique) naming scheme;
• personal mobility, i.e., the ability to reach a called party under a single, location-independent address even when the user changes terminals;
• terminal-type negotiation and selection: a caller can be given a choice how to reach the party, e.g., via Internet telephony, mobile phone, an answering service, etc.;
• terminal capability negotiation;
• caller and callee authentication;
• blind and supervised call transfer;
• invitations to multicast conferences.