[MMUSIC] Query: payload type collision with offer/answer

Harald Alvestrand <harald@alvestrand.no> Wed, 09 January 2013 19:25 UTC

Return-Path: <harald@alvestrand.no>
X-Original-To: mmusic@ietfa.amsl.com
Delivered-To: mmusic@ietfa.amsl.com
Received: from localhost (localhost [127.0.0.1]) by ietfa.amsl.com (Postfix) with ESMTP id E0CA821F882D for <mmusic@ietfa.amsl.com>; Wed, 9 Jan 2013 11:25:07 -0800 (PST)
X-Virus-Scanned: amavisd-new at amsl.com
X-Spam-Flag: NO
X-Spam-Score: -110.299
X-Spam-Level:
X-Spam-Status: No, score=-110.299 tagged_above=-999 required=5 tests=[AWL=0.300, BAYES_00=-2.599, RCVD_IN_DNSWL_HI=-8, USER_IN_WHITELIST=-100]
Received: from mail.ietf.org ([64.170.98.30]) by localhost (ietfa.amsl.com [127.0.0.1]) (amavisd-new, port 10024) with ESMTP id owdkw-QGSO9i for <mmusic@ietfa.amsl.com>; Wed, 9 Jan 2013 11:25:07 -0800 (PST)
Received: from eikenes.alvestrand.no (eikenes.alvestrand.no [158.38.152.233]) by ietfa.amsl.com (Postfix) with ESMTP id 36A1721F87CB for <mmusic@ietf.org>; Wed, 9 Jan 2013 11:25:07 -0800 (PST)
Received: from localhost (localhost [127.0.0.1]) by eikenes.alvestrand.no (Postfix) with ESMTP id AAB8E39E2A2 for <mmusic@ietf.org>; Wed, 9 Jan 2013 20:25:05 +0100 (CET)
X-Virus-Scanned: Debian amavisd-new at eikenes.alvestrand.no
Received: from eikenes.alvestrand.no ([127.0.0.1]) by localhost (eikenes.alvestrand.no [127.0.0.1]) (amavisd-new, port 10024) with ESMTP id novqCfemOneT for <mmusic@ietf.org>; Wed, 9 Jan 2013 20:25:04 +0100 (CET)
Received: from hta-dell.lul.corp.google.com (unknown [IPv6:2620:0:1043:1:be30:5bff:fede:bcdc]) by eikenes.alvestrand.no (Postfix) with ESMTPSA id 8559B39E241 for <mmusic@ietf.org>; Wed, 9 Jan 2013 20:25:04 +0100 (CET)
Message-ID: <50EDC40F.3010501@alvestrand.no>
Date: Wed, 09 Jan 2013 20:25:03 +0100
From: Harald Alvestrand <harald@alvestrand.no>
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/17.0 Thunderbird/17.0
MIME-Version: 1.0
To: mmusic@ietf.org
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 7bit
Subject: [MMUSIC] Query: payload type collision with offer/answer
X-BeenThere: mmusic@ietf.org
X-Mailman-Version: 2.1.12
Precedence: list
List-Id: Multiparty Multimedia Session Control Working Group <mmusic.ietf.org>
List-Unsubscribe: <https://www.ietf.org/mailman/options/mmusic>, <mailto:mmusic-request@ietf.org?subject=unsubscribe>
List-Archive: <http://www.ietf.org/mail-archive/web/mmusic>
List-Post: <mailto:mmusic@ietf.org>
List-Help: <mailto:mmusic-request@ietf.org?subject=help>
List-Subscribe: <https://www.ietf.org/mailman/listinfo/mmusic>, <mailto:mmusic-request@ietf.org?subject=subscribe>
X-List-Received-Date: Wed, 09 Jan 2013 19:25:08 -0000

Hello,
I have encountered an issue, and I'm not sure it's an issue or not. SDP 
wisdom is sought.

Suppose two entities exchange the following offer/answer:

offer from X:

m=video 97
a=rtpmap:97 foo

answer from Y:

m=video 98
a=rtpmap:98 foo

It's clear from RFC 3264 that now X must send codec foo with payload 
type 98, and Y must send codec foo with payload type 97. That's not the 
question.

But suppose these are the offer and answer:

offer from X:

m=video 97 98
a=rtpmap:97 foo
a=rtpmap:98 bar

answer from Y:

m=video 97 98
a=rtpmap:97 bar
a=rtpmap:98 foo

That is, the payload types collide.

It would be possible to write code so that X sends foo with 98 and bar 
with 97, while Y sends foo with 97 and bar with 98. But is it conformant 
with the specs to do so?

And if this has a clear answer - which paragraph of which RFC makes this 
clear?
(Yes, this discussion is triggered from a real world problem.)

                    Harald