[SIPForum-discussion] Offer-Answer Model

[Retesh]

Hi
In your scenario, I am assuming that 183 is non-reliable. In that
case, UPDATE MUST not be sent with a new offer as offer-answer is not
complete. UAS needs to send the same sdp as in 200 ok.
Check the following extract from RFC 3311

      o  If the UPDATE is being sent before completion of the initial
         INVITE transaction, and the initial INVITE contained an offer,
         the UPDATE can contain an offer if the callee generated an
         answer in a reliable provisional response, and the caller has
         received answers to any other offers it sent in either PRACK or
         UPDATE, and has generated answers for any offers it received in
         an UPDATE from the callee.


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With Regards
Retesh Chadha