[SIPForum-discussion] Call Gets Answered During UPDATE?

[Dale R. Worley]

Heather Mayhan <heather.mayhan at ericsson.com> writes:
> I have been trying to muddle through the question of, say you have a
> call IN PROGRESS, but not yet answered.

The specifications are in RFC 3311, with clarifications in RFC 6337.
The crucial thing is that if an INVITE contains an offer, and the caller
receives a PRACK containing an answer, the offer/answer transaction is
finished, and the later arrival of an answer in a 200 for the INVITE has
no effect on the media session.

> SIP UPDATE method is being used several times during the call set up,
> and being sent end-to-end.
>
> What would the expected behavior be if the 200 OK indicating call
> ANSWER is received in between a SIP UPDATE and the corresponding 200
> OK for that UPDATE?
>
> Would the 200 OK indicating call ANSWER prevail, or should the UPDATE
> be acknowledged before sending on 200 OK for ANSWER?

The central rule is that for the caller to send an UPDATE, the callee
must have *already* sent an answer to the offer in the INVITE.  So the
offer/answer exchange from the INVITE has already been completed.  The
UPDATE starts a *new* offer/answer exchange.

Dale