[MMUSIC] Sending a=rtcp-mux-only w/o a=rtcp-mux

Eric Rescorla <ekr@rtfm.com> Fri, 03 February 2017 17:39 UTC

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From: Eric Rescorla <ekr@rtfm.com>
Date: Fri, 3 Feb 2017 09:38:39 -0800
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Subject: [MMUSIC] Sending a=rtcp-mux-only w/o a=rtcp-mux
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I have been reading the mux-exclusive document and I'm not sure it says
quite what we want. Specifically, S 4.2 says:

   When an offerer sends the initial offer, if the offerer wants to
   indicate exclusive RTP/RTCP multiplexing for RTP-based media, the
   offerer MUST associate an SDP 'rtcp-mux-only' attribute with the
   associated SDP media description ("m=" line).

   In addition, if the offerer associates an SDP 'rtcp-mux-only'
   attribute with an SDP media description ("m=" line), the offerer MAY
   also associate an SDP 'rtcp-mux' attribute with the same SDP media
   description ("m=" line), following the procedures in [RFC5761].

As I understand this text, the offerer may say the following things:

 1. No a=rtcp-mux: No muxing.
 2. a=rtcp-mux: I am offering RTCP mux
 3. a=rtcp-mux-only + a=rtcp-mux: I will only do RTCP mux
 4. a=rtcp-mux-only: I will only do RTCP mux (same as #3).

I don't think the last of these is sensible. No current implementation
will know what to do with a=rtcp-mux-only w/o a=rtcp-mux, so this will
result in interop failures. Thus the MAY in the second graf needs to be
a MUST.

-Ekr