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SIP Makes Your Presence Known: Page 4 of 8

RFC 3261 SIP: Session Initiation Protocol

RFC 3856: A Presence Event Package for the Session Initiation Protocol

RFC 3859: Common Profile for Presence

PUA (presence user agent) software, such as Microsoft Communicator 2005 and Microsoft Windows Messenger manipulates presence information. Presence information changes when a user logs on or off the SIP endpoint, or when the user deploys the PUA to make these changes himself. That's akin to a user setting his or her IM client to offline mode to avoid being interrupted.

A user can have multiple devices, such as a PDA, laptop or cell phone, distributing his or her presence information. For each device, a PUA pushes presence information to PAs collocated with SIP proxy servers or included with standalone PSs.