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

[Eric Rescorla <[e-mail]>]

On Tue, Feb 7, 2017 at 9:40 AM, Christer Holmberg <
[e-mail]> wrote:

> 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.
> >
> > What does the other side do with that?
>
> Well, it knows that it doesn't have to include a=rtcp-mux the next time it
> wants to do mux-only.
>
> Obviously, as you suggested in your original e-mail, if we wouldn't allow
> a=rtcp-mux-only without a=rtcp-mux (alt #4) in an offer to begin with, it
> doesn't matter.
>

Yeah, I don't think this is a plausible option.

At this point it would be great to hear from anyone who thinks that we
should allow
a=rtcp-mux-only without a=rtcp-mux....

-Ekr


>
> > Is there any precedent for this in SDP?
>
> Not anything I can think of.
>
> Regards,
>
> Christer
>
>
> From: Eric Rescorla <[e-mail]>
> Date: Monday 6 February 2017 at 16:32
> To: Christer Holmberg <[e-mail]>
> Cc: "[e-mail]" <[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]> 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]> 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
>
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