The main purpose behind RFC 3261, Session Initiation Protocol (SIP), is to to set up, modify and tear down multi-media communications sessions on the internet.
An important feature of SIP is its addressing model:
To: E. Schroedinger <sip:schro@quantum-heaven.com>
From: Werner Heisenberg <sip:heisi@uncertainty-zone.com>
This addressing model is opaque for internet routing and direct access to user end-points. (If they were just e-mail addresses, we sould know how to address the SMTP servers.) Somehow SIP addresses need to be associated with actual end-point addresses on the network. IP addresses of users need to be obtained. That's where the rest of SIP comes in. Once you have these features, we can start talking about the use of SIP in VoIP, IP mobility, IMS and more.
[Names for addresses are inspired by examples in "SIP: Understanding the session initiation protocol" by Alan B. Johnson]
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