Re: [MMUSIC] Proposal for what bundle should say about demux

Emil Ivov <emcho@jitsi.org> Fri, 24 May 2013 09:15 UTC

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Message-ID: <519F2F8B.5080004@jitsi.org>
Date: Fri, 24 May 2013 12:14:51 +0300
From: Emil Ivov <emcho@jitsi.org>
Organization: Jitsi
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To: Kevin Dempsey <kevindempsey70@gmail.com>
References: <C5E08FE080ACFD4DAE31E4BDBF944EB11350F3C8@xmb-aln-x02.cisco.com> <519E80E4.6010904@jitsi.org> <519E9131.2080400@alum.mit.edu> <519F1424.8020803@jitsi.org> <CAMvTgcdpz5dh2zxRGu_xu1QdqsdAapgT-zziT5GzyzyodQxYXA@mail.gmail.com> <519F2BD8.8060803@jitsi.org> <CAMvTgcfx1t0d-NEavTxdq=9FEH9Z-wjX2S+HcSimFUgzg5-58g@mail.gmail.com>
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Cc: mmusic <mmusic@ietf.org>, Paul Kyzivat <pkyzivat@alum.mit.edu>
Subject: Re: [MMUSIC] Proposal for what bundle should say about demux
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On 24.05.13, 12:02, Kevin Dempsey wrote:
> I didn't say anything about changing the PTs.

Sorry, I misunderstood.

> The answerer decides what
> it codecs it will use from the ones listed in the offer. If it doesn't
> support any other mechanism to allow demux then it must choose codecs
> that have PTs on only one m-line, or not accept the BUNDLE.

Matching packets to m= lines is the concern of the receiving party. So
is the mechanism that it uses. What you are suggesting is for the
answerer to basically say: "It seems to me that you won't be able to
demux these two payloads so I will not send them to you because I know
better".

In the same time the offerer may be planning on using a demux technique
that the answerer simply isn't aware of. For example (and this is really
just to make a point, so please don't assume I am suggesting something
like this in general) if 101 is mapped to both telephone-event and VP8,
demuxing can be easily made without any other knowledge of SSRC or
header extensions.

Cheers,
Emil


> 
> 
> On 24 May 2013 09:59, Emil Ivov <emcho@jitsi.org
> <mailto:emcho@jitsi.org>> wrote:
> 
>     On 24.05.13, 11:40, Kevin Dempsey wrote:
>     > Isn't the onus on the answerer to ensure the demux is possible,
>     since it
>     > is accepting the bundle. Therefore if it doesn't support signalling
>     > SSRCs or an offerred 'fancy-RFC m-line matching' method, it must only
>     > send PTs that are tied to a single m-line.
> 
>     PTs are defined by the offerer. The answerer cannot change them for the
>     offerer-bound direction (even if it could request different values for
>     the packets that it, the answerer, is going to receive).
> 
>     Emil
> 
> 
>     >
>     >
>     > On 24 May 2013 08:17, Emil Ivov <emcho@jitsi.org
>     <mailto:emcho@jitsi.org>
>     > <mailto:emcho@jitsi.org <mailto:emcho@jitsi.org>>> wrote:
>     >
>     >     On 24.05.13, 00:59, Paul Kyzivat wrote:
>     >     > IMO the "before the O/A completes" issues are separate from
>     the "after
>     >     > the O/A completes" issues. I'm still not convinced that the
>     >     "before the
>     >     > O/A completes" issues are important. (We have reliable 183
>     >     responses to
>     >     > deal with it.) But I'm ok with Cullen using PT to solve that
>     >     problem if
>     >     > he thinks he needs to.
>     >
>     >     I wasn't referring to "before the O/A completes" situations.
>     If your
>     >     peer does not support fancy-RFC m-line matching nor does it
>     provide
>     >     SSRCs, you have to be able to fall back to PT demuxing.
>     >
>     >     Therefore, all your offers have to accommodate for that
>     possibility
>     >     unless you know that the offer is going to someone who also
>     supports
>     >     other mechanisms.
>     >
>     >     Emil
>     >
>     >     --
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