[SIPForum-discussion] FW: [Sip] MUST 200 OK contain SDP? Thanks

[Yuantao Zhang]

Dear all

I am really confused by the following part in RFC_3261. Could you please
help us to explain it? Thank you very much.
"
13.2.1 Creating the Initial INVITE
.
.
.
      o  If the initial offer is in an INVITE, the answer MUST be in a
         reliable non-failure message from UAS back to UAC which is
         correlated to that INVITE.  For this specification, that is
         only the final 2xx response to that INVITE.  That same exact
         answer MAY also be placed in any provisional responses sent
         prior to the answer.  The UAC MUST treat the first session
         description it receives as the answer, and MUST ignore any
         session descriptions in subsequent responses to the initial
         INVITE.
.
.
."

Let's break it into 2 parts, in part 1, 
         "If the initial offer is in an INVITE, the answer MUST be in a
         reliable non-failure message from UAS back to UAC which is
         correlated to that INVITE.  For this specification, that is
         only the final 2xx response to that INVITE. "
Does that mean if "Invite" contains SDP offer, "200 OK" MUST contain SDP
answer.

In part 2,
         "That same exact answer MAY also be placed in any provisional
responses sent
         prior to the answer.  The UAC MUST treat the first session
         description it receives as the answer, and MUST ignore any
         session descriptions in subsequent responses to the initial
         INVITE."
Does that mean if "Invite" contains SDP offer, the answer SDP MAY be in
"180 ring" and not in "200 OK"? I think part 1 and part 2 are
inconsistent.

The attached sequence chart is the real problem we meet. Sx1 sends "bye"
to sx2 after the "200 OK" because there is no SDP answer in "200 OK".
But the producer of sx2 argues that the SDP answer is in "180 ring"
already. No need to repeat it in "200 OK". Could you please tell me who
is right? MUST 200 OK contain SDP? Thanks you very much.



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