The Session Initiation Protocol (SIP) UPDATE Method
Abstract
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.
Phone System 