Initially only the traditional switch-based telephone system was the main medium for transmitting messages. However with the advent of the Internet, the need was felt to fabricate a system, which connects people over the IP based network. Different communities put forward different solutions but the solution presented by IETF was finally accepted as the most general one. However the development of SIP in IETF was not a one-step process.
Initial Internet drafts were produced in the form of -
Session Invitation Protocol (SIP) – M.Handley, E.Schooler
Simple Conference Invitation Protocol (SCIP) – H.Schulzrinne
SIP was originally intended to create a mechanism for inviting people to large-scale multipoint conferences on the Internet Multicast Backbone (Mbone). At this stage, IP telephony didn't really exist. The first draft was known as "draft-ietf-mmusic-sip-00”. It included only one request type, which was a call setup request. (Wondering what music is doing in SIP??? Well, it is an acronym for Multiparty Multimedia Session Control. IETF people are not that music crazy after all.)
A newer version “draft-ietf-mmusic-sip-01” was proposed as a modification to SIP-0. Still it was yet to take the shape of SIP as we know it now.
The IETF published the draft called "draft-ietf-mmusic-sip-12". It contained the six requests that SIP has today.
SIP published RFC 2543 as a standard.
It was modified further to generate the more modern version of RFC 3261.
Let's leave the history to get older and concentrate on perhaps the most important part of this tutorial. Let's know about the functions of SIP.