Subject: [MMUSIC] Offer/answer model: counter-offer possibility

From: [email-masked]
Date: Fri, 15 Mar 2002 10:08:04 +0100

Hello,

I think that in the past there was some debate around a model
Offer/Counter-Offer/Answer, but I didn't find the results, and the
Offer/Answer draft says nothing about it.
My question is : How can this work when using SIP ?
See the example below that shows what I'm looking for.

User A                                 User B
                  SDP(offer)
      ----------------------------------->
           SDP(counter-offer)
     <-----------------------------------
           SDP(final-answer)
     ----------------------------------->

In SIP, this could be done in one of the two following ways:

1) Using PRACK for the final answer

User A                                 User B
             INVITE[SDP(offer)]
      ----------------------------------->
         183[SDP(counter-offer)]
     <-----------------------------------
    PRACK[SDP(final-answer)]
     ----------------------------------->

2) Using re-INVITE:

User A                                 User B
             INVITE[SDP(offer)]
      ----------------------------------->
         200[SDP(answer)]
     <-----------------------------------
                        ACK
     ----------------------------------->
re-INVITE[SDP(counter-offer)]
    <-----------------------------------
         200[SDP(final-answer)]
   ------------------------------------>
                        ACK
   <-----------------------------------

The first solution is preferred, since it is the shortest one; in the
second solution the number of messages is multiplied by two.
Comments ?

By the way, two small typos in the Offer Answer draft:

Section 5, bottom of page 4: according RFC2327, the "s=" line conveys the
subject of the session, and not the subject of the media stream.

Section 10, second SDP on page 20: the first SDP in the same page shows
that the first video component was already dropped (port number equal to
zero); therefore, in the Alice answer (second SDP on page 20), this
"m=video" line should have also the port number equal to zero.

Best regards
Juan Carlos

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---

From: Jonathan Rosenberg <[email-masked]>
Date: Sat, 16 Mar 2002 15:30:39 -0500

> Offer/Answer draft says nothing about it.

The result was that there was nothing you could do with a counter-offer
model that you couldnt do with a sequence of offer/answers. Since a
counter-offer model, through the usage of another protocol option, would
seriously complicate things, including 3pcc, we elected not to do it.
That is why it is not mentioned in the document.

> subject of the session, and not the subject of the media stream.

Correct. We may be able to fix it during authors 48 hours (the
specification has already been approved by IESG, so there will be no
further versions).

> "m=video" line should have also the port number equal to zero.

Correct. Same as above; we may be able to fix. Otherwise, we will
document through an errata site.

Thanks,
Jonathan R.

-- 
Jonathan D. Rosenberg, Ph.D.            72 Eagle Rock Avenue
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