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

[mjhorse]

Hello
 According to my knowledge, 200 OK doesn't need to contain SDP every time.
 In ordinary situation, where I mean the UAs are not implemented with PRACK extension, 200 OK must take SDP body serving as a offer to prior SDP body containd in corresponding INVITE, or as a answer if there is no SDP in corresponding INVITE.
 However, if PRACK extension is implemented, there are some cases in which PRACK can take on the same media negotiation function as 200 OK does which described in the 1st paragraph. So, in such cases, if there is no need for a further media negotiation, 200 OK doesn't have to carry SDP body.
  
 Correct me if I am wrong.
  
 ------------------
  by mj

  
  
 ------------------ Original ------------------
  From:  "Yuantao Zhang"<yuantao.zhang at ericsson.com>;
 Date:  Wed, May 21, 2008 04:03 PM
 To:  "discussion"<discussion at sipforum.org>; 
 
 Subject:  [SIPForum-discussion] FW: [Sip] MUST 200 OK contain SDP? Thanks

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