What is SIP?

Session Initiation Protocol (SIP), a signaling protocol, is used for establishing a session in an IP network — from a simple two-way telephone call to a multi-media conference call session with many participants. The IP telephony industry has recently adopted SIP, an RFC standard (RFC 3261) from the Internet Engineering Task Force (IETF), as the protocol of choice for signaling because of its ability to facilitate Internet applications by working with other protocols. It is not the be-all and end-all of protocols — it was designed to be a facilitation mechanism, not an all-inclusive solution. Its flexibility is what makes it so powerful, and an all-inclusive approach does not offer this level of flexibility.

Essentially, SIP establishes, manipulates and tears down sessions, and its main purpose is to help session originators deliver invitations to potential session participants wherever they may be. It uses URLs to address participants and SDP to convey session information and it’s easy to combine SIP with other applications, like Web browsers and messaging. The bottom line is that it’s a modular approach to maximizing IP telephony protocols. SIP can find and invite call invitees wherever they are. It facilitates multi-media calls with many participants who may join and leave at will.