IETF RFC 3311
The Session Initiation Protocol (SIP) UPDATE Method
|Publication Date:||1 September 2002|
This specification defines the new UPDATE method for the Session Initiation Protocol (SIP). UPDATE allows a client to update parameters of a session (such as the set of media streams and their codecs) but has no impact on the state of a dialog. In that sense, it is like a re-INVITE, but unlike re-INVITE, it can be sent before the initial INVITE has been completed. This makes it very useful for updating session parameters within early dialogs.