Re: [AVTCORE] [MMUSIC] RTP/RTCP-mux: Attribute in answer if not included in offer?

Christer Holmberg <> Wed, 22 June 2016 17:30 UTC

Return-Path: <>
Received: from localhost (localhost []) by (Postfix) with ESMTP id 5D76812D9B7; Wed, 22 Jun 2016 10:30:22 -0700 (PDT)
X-Virus-Scanned: amavisd-new at
X-Spam-Flag: NO
X-Spam-Score: -4.22
X-Spam-Status: No, score=-4.22 tagged_above=-999 required=5 tests=[BAYES_00=-1.9, HTML_MESSAGE=0.001, RCVD_IN_DNSWL_MED=-2.3, RCVD_IN_MSPIKE_H3=-0.01, RCVD_IN_MSPIKE_WL=-0.01, SPF_PASS=-0.001] autolearn=ham autolearn_force=no
Received: from ([]) by localhost ( []) (amavisd-new, port 10024) with ESMTP id 9cF2H9y-LfMT; Wed, 22 Jun 2016 10:30:17 -0700 (PDT)
Received: from ( []) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by (Postfix) with ESMTPS id A0FC312D991; Wed, 22 Jun 2016 10:30:16 -0700 (PDT)
X-AuditID: c1b4fb30-f79486d0000069d0-63-576acb25618f
Received: from (Unknown_Domain []) by (Symantec Mail Security) with SMTP id F4.82.27088.52BCA675; Wed, 22 Jun 2016 19:30:14 +0200 (CEST)
Received: from ([]) by ([]) with mapi id 14.03.0294.000; Wed, 22 Jun 2016 19:29:36 +0200
From: Christer Holmberg <>
To: Roni Even <>, 'Suhas Nandakumar' <>
Thread-Topic: [MMUSIC] [AVTCORE] RTP/RTCP-mux: Attribute in answer if not included in offer?
Date: Wed, 22 Jun 2016 17:29:36 +0000
Message-ID: <>
References: <> <>, <04d601d1ba7e$189ec900$49dc5b00$> <> <13f501d1cc9b$c8409d50$58c1d7f0$>
In-Reply-To: <13f501d1cc9b$c8409d50$58c1d7f0$>
Accept-Language: en-US
Content-Language: en-US
x-originating-ip: []
Content-Type: multipart/alternative; boundary="_000_7594FB04B1934943A5C02806D1A2204B38100A24ESESSMB209erics_"
MIME-Version: 1.0
X-Brightmail-Tracker: H4sIAAAAAAAAA+NgFtrMIsWRmVeSWpSXmKPExsUyM2K7k67a6axwg5+3mS1e9qxkt1j+8gSj xdTlj1ks/rYzW+yc28HswOox7f59No+ds+6yeyxZ8pMpgDmKyyYlNSezLLVI3y6BK2PTzi2M BdNfM1acPN3B2sC48hJjFyMnh4SAicStTY+hbDGJC/fWs3UxcnEICRxhlFh5YzozhLOEUWLJ jJ8sXYwcHGwCFhLd/7RBTBGBIIl9DQIgvcwCixklfn4zAgkLC8RK7HxgDBIWEYiTOLX7DQuE 7SfRfnspO4jNIqAqsbb7HlicV8BXYtOTF6wgtpDAJiaJebMUQWxOoEX/5h4EO40R6LTvp9Yw QawSl7j1ZD4TxMkCEkv2nGeGsEUlXj7+xwphK0ksuv0Zqj5fYuLzr1C7BCVOznzCMoFRdBaS UbOQlM1CUgYRN5A4t30jO4StLbFs4WtmCFtfYvvtmazI4gsY2VcxihanFiflphsZ6aUWZSYX F+fn6eWllmxiBEblwS2/DXYwvnzueIhRgINRiYf3wY7McCHWxLLiytxDjBIczEoivAHHssKF eFMSK6tSi/Lji0pzUosPMUpzsCiJ8/q/VAwXEkhPLEnNTk0tSC2CyTJxcEo1MG48s1ejWJvz 4lKu+BqzpWpP7x2Pe63iWDnF7pO9o5e2hvTRb8oiL9P2JWQ1xd2um7libs23Au/+HX78X64t vHFvnmrnyjPLHbPzZn5uu8505cIn/rTjAot2r1l479jetUK+NxV8Nkv3d89+tOTm7H2vO0z+ u357XrR551+7qFdbZ9+02uWQ2flSiaU4I9FQi7moOBEATlb1HMYCAAA=
Archived-At: <>
Cc: Magnus Westerlund <>, "" <>, 'Colin Perkins' <>, "" <>
Subject: Re: [AVTCORE] [MMUSIC] RTP/RTCP-mux: Attribute in answer if not included in offer?
X-Mailman-Version: 2.1.17
Precedence: list
List-Id: Audio/Video Transport Core Maintenance <>
List-Unsubscribe: <>, <>
List-Archive: <>
List-Post: <>
List-Help: <>
List-Subscribe: <>, <>
X-List-Received-Date: Wed, 22 Jun 2016 17:30:22 -0000


I don’t think one should use a=rtcp to request RTP/RTCP multiplex.



From: Roni Even []
Sent: 22 June 2016 18:36
To: Christer Holmberg <>om>; 'Suhas Nandakumar' <>
Cc: Magnus Westerlund <>om>; 'Colin Perkins' <>rg>;;
Subject: RE: [MMUSIC] [AVTCORE] RTP/RTCP-mux: Attribute in answer if not included in offer?

Hi Christer,
I am not sure I understand you answer. I was wondering if the answerer can ask to receive RTP/RTCP multiplex even if the offer did not include the rtp/rtcp-mux attribute by providing in an
A=rtcp the port of for the RTP stream?

From: Christer Holmberg []
Sent: Monday, May 30, 2016 5:26 PM
To: Roni Even; 'Suhas Nandakumar'
Cc: Magnus Westerlund; 'Colin Perkins';<>;<>
Subject: RE: [MMUSIC] [AVTCORE] RTP/RTCP-mux: Attribute in answer if not included in offer?


In an offer a=rtcp is used to provide a fallback port, in case the answerer does not support RTP/RTCP-mux (but does support a=rtcp).



Sent from my Windows Phone
From: Roni Even<>
Sent: ‎30/‎05/‎2016 17:19
To: 'Suhas Nandakumar'<>; Christer Holmberg<>
Cc: Magnus Westerlund<>; 'Colin Perkins'<>;<>;<>
Subject: Re: [MMUSIC] [AVTCORE] RTP/RTCP-mux: Attribute in answer if not included in offer?
Agree with Suhas but have a question
What if the offer has an a=rtcp with a port different from the RTP one, and the response include a=rtcp using the same port as the RTP port? Is it allowed, at least in RFC3605 did not see any discussion about this


From: mmusic [] On Behalf Of Suhas Nandakumar
Sent: Friday, May 27, 2016 4:26 AM
To: Christer Holmberg
Cc: Magnus Westerlund; Colin Perkins (<>);<>;<>
Subject: Re: [MMUSIC] [AVTCORE] RTP/RTCP-mux: Attribute in answer if not included in offer?

Going along O/A negotiation procedures, i would lean towards your former conclusion that the offer needs to include the rtcp-mux ability.
If the offer doesn't include a=rtcp-mux and the answerer wants to mux , then it needs a new O/A from the answerer's perspectives.

If we make a=rtcp-mux a declarative SDP it might solve the other use-case ..


On Thu, May 26, 2016 at 1:01 PM, Christer Holmberg <<>> wrote:
(Sorry for the cross-posting)


There has been a discussion in 3GPP regarding RFC 5761 (RTP/RTCP-mux).

Section 5.1.1 says:

5.1.1.  SDP Signalling

   When the Session Description Protocol (SDP) [8] is used to negotiate
   RTP sessions following the offer/answer model [9], the "a=rtcp-mux"
   attribute (see Section 8) indicates the desire to multiplex RTP and
   RTCP onto a single port.  The initial SDP offer MUST include this
   attribute at the media level to request multiplexing of RTP and RTCP
   on a single port.  For example:

       o=csp 1153134164 1153134164 IN IP6 2001:DB8::211:24ff:fea3:7a2e
       c=IN IP6 2001:DB8::211:24ff:fea3:7a2e
       t=1153134164 1153137764
       m=audio 49170 RTP/AVP 97
       a=rtpmap:97 iLBC/8000

   This offer denotes a unicast voice-over-IP session using the RTP/AVP
   profile with iLBC coding.  The answerer is requested to send both RTP
   and RTCP to port 49170 on IPv6 address 2001:DB8::211:24ff:fea3:7a2e.

   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"

Q: The question is whether it is allowed to include a=rtcp-mux in an answer if the associated offer did NOT contain a=rtcp-mux.

The second last paragraph in the text above does say that, if the answer wishes to do RTP/RTCP-mux, it includes a=rtcp-mux in the answer.

BUT, in my view that is it based on the first paragraph, which assumes a=rtcp-mux is also included in the offer.

Or, am I wrong? Is it allowed to include a=rtcp-mux in the answer, even if it’s not included in the offer? If so, how will the offerer then indicate whether mux is used or not.



Audio/Video Transport Core Maintenance<>