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

Christer Holmberg <christer.holmberg@ericsson.com> Thu, 02 August 2012 23:32 UTC

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From: Christer Holmberg <christer.holmberg@ericsson.com>
To: "mmusic@ietf.org" <mmusic@ietf.org>
Date: Fri, 03 Aug 2012 01:31:59 +0200
Thread-Topic: Simultanous usage of a=rtcp and a=rtcp-mux
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Subject: [MMUSIC] Simultanous usage of a=rtcp and a=rtcp-mux
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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)


m=audio 20000
a=rtcp: 25000

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.