Re: [MMUSIC] Simultanous usage of a=rtcp and a=rtcp-mux

Paul Kyzivat <> Fri, 03 August 2012 02:41 UTC

Return-Path: <>
Received: from localhost (localhost []) by (Postfix) with ESMTP id 9009311E8072 for <>; Thu, 2 Aug 2012 19:41:07 -0700 (PDT)
X-Virus-Scanned: amavisd-new at
X-Spam-Flag: NO
X-Spam-Score: 0.221
X-Spam-Status: No, score=0.221 tagged_above=-999 required=5 tests=[AWL=-0.542, BAYES_00=-2.599, FH_RELAY_NODNS=1.451, HELO_MISMATCH_NET=0.611, J_CHICKENPOX_14=0.6, J_CHICKENPOX_15=0.6, RDNS_NONE=0.1]
Received: from ([]) by localhost ( []) (amavisd-new, port 10024) with ESMTP id fR1EkGQNLbNm for <>; Thu, 2 Aug 2012 19:41:07 -0700 (PDT)
Received: from ( [IPv6:2001:558:fe14:43:76:96:62:24]) by (Postfix) with ESMTP id 0128411E80A4 for <>; Thu, 2 Aug 2012 19:41:06 -0700 (PDT)
Received: from ([]) by with comcast id i2ex1j00B0QuhwU512h9nJ; Fri, 03 Aug 2012 02:41:09 +0000
Received: from ([IPv6:2001:df8:0:64:19d6:c06f:f140:d490]) by with comcast id i2gu1j00E3NDGqc3N2gvm0; Fri, 03 Aug 2012 02:40:55 +0000
Message-ID: <>
Date: Thu, 02 Aug 2012 19:41:04 -0700
From: Paul Kyzivat <>
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.7; rv:12.0) Gecko/20120428 Thunderbird/12.0.1
MIME-Version: 1.0
References: <>
In-Reply-To: <>
Content-Type: text/plain; charset="ISO-8859-1"; format="flowed"
Content-Transfer-Encoding: 7bit
Subject: Re: [MMUSIC] Simultanous usage of a=rtcp and a=rtcp-mux
X-Mailman-Version: 2.1.12
Precedence: list
List-Id: Multiparty Multimedia Session Control Working Group <>
List-Unsubscribe: <>, <>
List-Archive: <>
List-Post: <>
List-Help: <>
List-Subscribe: <>, <>
X-List-Received-Date: Fri, 03 Aug 2012 02:41:07 -0000

On 8/2/12 4:31 PM, Christer Holmberg wrote:
> Hi,
> One of the issues which was raised during the MMUSIC BUNDLE presentation, but which is not BUNDLE specfic, was the case when an offer contains both a=rtcp and a=rtcp-mux.
> A case where this can happen is ICE, which requires the inclusion of the a=rtcp attribute.
>        "If the agent is utilizing RTCP, it MUST encode the RTCP candidate using the a=rtcp attribute as defined in RFC 3605 [RFC3605]." (RFC 5245/Section 4.3)
> Example:
> m=audio 20000
> a=rtcp: 25000
> a=rtcp-mux
> Q: If the receiver supports both attributes, to which port would it send RTCP?
> As indicated during the meeting, in theory this could be solved by using a=rtcp: 20000 (ie same port as RTP), but people indicated that it is not allowed (eventhough not explicitly forbidden, afaik).
> As also indicated, another way to solve this could be by saying: "If the receiver supports a=rtcp-mux, use that, otherwise use a=rtcp". But, I have not found any text which would support such intepretion.

RFC 5761 says:

    If the answerer wishes to multiplex RTP and RTCP onto a single port,
    it MUST include a media-level "a=rtcp-mux" attribute in the answer.
    The RTP payload types used in the answer MUST conform to the rules in
    Section 4.

    If the answer does not contain an "a=rtcp-mux" attribute, the offerer
    MUST NOT multiplex RTP and RTCP packets on a single port.  Instead,
    it should send and receive RTCP on a port allocated according to the
    usual port-selection rules (either the port pair, or a signalled port
    if the "a=rtcp:" attribute [10] is also included).  This will occur
    when talking to a peer that does not understand the "a=rtcp-mux"

Seems pretty explicit to me. Also, I don't see how it could work 
otherwise and still obey the standard SDP backward compatibility rule 
for unknown attributes.