"This document describes the usage of the Session Initiation Protocol (SIP) for subscriptions and notifications of presence. Presence is defined as the willingness and ability of a user to communicate with other users on the network. Historically@ presence has been limited to ""on-line"" and ""off-line"" indicators; the notion of presence here is broader. Subscriptions and notifications of presence are supported by defining an event package within the general SIP event notification framework. This protocol is also compliant with the Common Presence Profile (CPP) framework."
RFC 3856-2004 history
2004RFC 3856-2004 A Presence Event Package for the Session Initiation Protocol (SIP)