[Sip] Offer by the callee

[Juan-Carlos.Rojas@alcatel.fr]

Hello,

I'm trying to connect the offer/answer, update and manyfolks drafts for the
following scenario:
- Alice sends an INVITE to Bob with "offer-1"
- Bob wants to make an "offer-2" to Alice, before the completion of the
session establishment
- Bob would like to ask Alice to start resource reservation only based on
the acceptation of "offer-2", and not before (in fact Bob knows that it is
useless to start resource reservation based on offer-1, because he knows he
will send immediatly a new offer-2,  but he is obliged to wait for Prack
before)

The Update draft defines the response 155 to allow the answerer (the callee
in our case) to request "Mr. the calling, please *send me* a new offer",
but there is no way for the same callee to announce "Mr. the calling, *I
want to send you* a new offer right now".
So, one possible way to achieve that is that the callee sends an answer to
the original offer but disabling all the media (in order to avoid to start
resource reservation), and next proposing a new offer, as described in the
call flow below (assume for the example that they are using end-to-end
resource reservation, but please don't generate a new debate on this
topic):

   Alice
Bob
                                      INVITE (offer-1)
       ----------------------------------------------------------------->
      183 (answer-to-offer-1, all port numbers to zero)
      <-----------------------------------------------------------------
                                         PRACK
       ----------------------------------------------------------------->
                                      200 PRACK
      <-----------------------------------------------------------------
                 UPDATE (offer-2, status-type=e2e)
      <-----------------------------------------------------------------
                  200 UPDATE (answer-to-offer-2)
       ----------------------------------------------------------------->

   <========== Resource Reservation ==========>

           UPDATE (current-status = desired-status)
       ----------------------------------------------------------------->
                                      200 UPDATE
      <-----------------------------------------------------------------
                                        200 INVITE
      <-----------------------------------------------------------------
                                              ACK
       ----------------------------------------------------------------->

Is this scenario compliant with the drafts Offer/Answer, Update and
Manyfolks ?

Thank you for your answer
Best regards
Juan Carlos



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