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

From: "Yuantao Zhang" <[email-masked]>
Date: Thu, 22 May 2008 16:44:18 +0800

Dear Sir or Madam

I am an R&D engineer in Ericsson and 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
.
.
.
        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? Thank you very much.

Sx1                              xs2
   ====1   invite(SDP)    ====> 
  <====2   100 trying     ====
  <====3   180 ringing(SDP)==== 
   ====4   prack          ====> 
  <====5   200 ack        ====> 
  <====6   200 OK (no SDP)====> 

Best Regards
Yuantao Zhang

I&V Engineer 
Broadband Networks R&D Center
Ericsson (China) Company Ltd.	
Ericsson Tower 		Phone :+86 10 8476 7240
No. 5 Lize East Street		Internal:11240
Chaoyang District		Fax     :+86 10 8476 7738
Beijing 100102		Mobile :+86 13911606213
P.R. China
http://www.ericsson.com	     e-mail :[email-masked]

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---

From: "孙永光" <[email-masked]>
Date: Wed, 28 May 2008 08:34:15 +0800

Hi Yuantao
   I'm SunYongGuang, my msn is [email-masked]

it is ok,

        "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.
"the answer MUST be in a reliable non-failure message from UAS back to UAC
which is
  correlated to that INVITE" in your scenario  , the 180 with 100rel  is
reliable non-failure message and it includes SDP, the offer/answer is
completed, so the 200 response may include SDP or not, please refer RFC3262

---

From: Paul Kyzivat <[email-masked]>
Date: Tue, 27 May 2008 23:10:22 -0400

Take a look at

http://www.ietf.org/internet-drafts/draft-ietf-sipping-sip-offeranswer-08.txt

It should answer your questions.

	Thanks,
	Paul

孙永光 wrote:
> <mailto:[email-masked]>
>  
> it is ok,
> 
> "the answer MUST be in a reliable non-failure message from UAS back to 
> UAC which is
> 2008/5/22, Yuantao Zhang <[email-masked] 
> <mailto:[email-masked]>>:
> 
>     Dear Sir or Madam
> 
>     <mailto:[email-masked]>
> 
>     Use [email-masked]
>     <mailto:[email-masked]> for questions on current sip
>     Use [email-masked] <mailto:[email-masked]> for new developments
>     on the application of sip
> 
> 
> 
> ------------------------------------------------------------------------
>