Sipping on a latte

One of the interesting aspects of the Session Initiation Protocol (SIP) is that is uses the “offer/answer” model to negotiate how a session is to be established… and to also update that session while it is in progress. A basic example would be the exchange of Session Description Protocol (SDP) information during an initial SIP INVITE to determine what audio codecs to use.

As SIP uses the offer/answer model in a number of ways, some folks very helpfully put together this Internet Draft:

http://tools.ietf.org/html/draft-ietf-sipping-sip-offeranswer

The abstract is:

The Session Initiation Protocol (SIP) utilizes the offer/answer model to establish and update multimedia sessions using the Session Description Protocol (SDP). The description of the offer/answer model in SIP is dispersed across multiple RFCs. This document summarizes all the current usages of the offer/answer model in SIP communication. Also, this document tries to incorporate the results of the discussions on the controversial issues to avoid repeating the same discussions later.

If you are seeking to understand how SIP works down at a protocol layer, this document is a useful guide to an important part of SIP.

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