Re: [MMUSIC] Why do we need rtcp-mux-exclusive?

Eric Rescorla <ekr@rtfm.com> Fri, 04 March 2016 11:58 UTC

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From: Eric Rescorla <ekr@rtfm.com>
Date: Fri, 04 Mar 2016 03:57:21 -0800
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To: Christer Holmberg <christer.holmberg@ericsson.com>
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Subject: Re: [MMUSIC] Why do we need rtcp-mux-exclusive?
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On Fri, Mar 4, 2016 at 12:24 AM, Christer Holmberg <
christer.holmberg@ericsson.com> wrote:

> Hi,
>
> >I've been reading this document and I'm finding myself confused about
> >why we need it.
> >
> >As I understand it, the current state of play is that mux-only
> >endpoints will send offers with both a=rtcp-mux and one set of
> >candidates for each m= line (and with trickle, perhaps no candidates
> >in the initial offer).  If that offer is processed by a non-mux
> >endpoint, that endpoint will reject rtcp-mux and then eventually you
> >end up with a failure to negotiate transport connections because there
> >will never be RTCP-candidates.
> >
> >What this draft seems to offer is that a way for the mux-only endpoint
> >to indicate that it is mux-only, so that non-mux endpoints can instead
> >fail the connection immediately (section 4.3). So you get a hard
> >failure instead of a transport failure. Is that correct so far?
>
> Correct.
>
> >If so, here's my problem: because unknown a= lines are ignored, old
> >endpoints won't know about rtcp-mux-exclusive and so they will just
> >ignore it. Thus, this attribute is only useful for *new* endpoints
> >which don't want to do mux (but do know about this attribute). I
> >suppose that endpoints like that might exist (maybe gateways) but I'm
> >skeptical that it's worth extra effort to have a hard fail here.
>
> Unfortunately this is related to a general problem with SDP -you can't
> mandate support of features (for SIP, one can always define an option tag
> and place it in a Require header field).
>
> But, we agreed that we want to have an explicit indicator,


My point is that I think that that agreement was wrong and we should not do
so.


and then we can only hope people will implement it (by mandating support of
> it etc).
>

But again, why would you?

-Ekr