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

[Christer Holmberg <[e-mail]>]

Hi,

We had a long discussion about this, with many different opinions, and it would take some time to go through the archive and check everything. But, one opinion was that it IS useful to send the attribute, as it indicates support of the mechanism.

Regards,

Christer

From: Eric Rescorla <[e-mail]<mailto:[e-mail]>>
Date: Monday 6 February 2017 at 16:32
To: Christer Holmberg <[e-mail]<mailto:[e-mail]>>
Cc: "[e-mail]<mailto:[e-mail]>" <[e-mail]<mailto:[e-mail]>>
Subject: Re: [MMUSIC] Sending a=rtcp-mux-only w/o a=rtcp-mux



On Mon, Feb 6, 2017 at 5:57 AM, Christer Holmberg <[e-mail]<mailto:[e-mail]>> wrote:
Hi,

>>> Following up to myself, I don't think it's sensible for answers to
>>>contain a=rtcp-mux-only, because either you accepted mux, in which case
>>>all is good, or you rejected it, in which case it was rejected.
>>
>> While I agree that a=rtcp-mux would be enough in the Answer as far as
>>indicating mux is concerned, including a=rtcp-mux-only in the Answer
>>does indicate that the Answerer supports the mux-exclusive mechanism.
>
> I don't see how that's really that useful

But what harm does it cause?

I don't think that's the standard here. We should only send indicators in SDP when they
do something useful.

-Ekr


Regards,

Christer




On Fri, Feb 3, 2017 at 9:38 AM, Eric Rescorla
<[e-mail]<mailto:[e-mail]>> wrote:

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