Consider a B2BUA application that connects party-A with party-B. Now,
in the mid-way, it puts party-B on hold or sends BYE and negotiates
the already established session of A with new party C. There are many
such applications which does this, like network initiated
call-transfer, announcements etc.
Now my query is that how party-A is notified by standard SIP mechanism
that he is now connected to a new party-C?

RFC 4235 (An INVITE-Initiated Dialog Event Package for the Session
Initiation Protocol) can be used here for this purpose. Apart from
this, is there any straight-forward way like some header in re-INVITE
towards party-A which would indicate the the peer has changed and
revel the identity of party-C? This re-INVITE would normally be sent
by B2BUA for session negotiation between A and C.
One way would be to change the "From" header's uri of this re-INVITE
to reveal identity of party C. This should be perfectly valid under
RFC 3261 but no-body cares of the re-INVITE's From header for
determining peer's identity. They all look for this in new-INVITE.

Another close contending header is "Refer-To" header but it has to be
used in REFER only.

Any suggestions/criticism?

Thanks,
~Vikram
(http://www.veraznetworks.com)
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