http://www.rfc-editor.org/rfc/rfc4479.txt
RFC 4479: A Data Model for Presence, J. Rosenberg2006-07-01T23:00:00-00:00This document defines the underlying presence data model used by Session Initiation Protocol (SIP) for Instant Messaging and Presence Leveraging Extensions (SIMPLE) presence agents. The data model provides guidance on how to map various communications systems into presence documents in a consistent fashion. [STANDARDS-TRACK]