Re: [MMUSIC] Open Issues: draft-ietf-mmusic-sdp-simulcast-03

Martin Thomson <martin.thomson@gmail.com> Fri, 23 October 2015 17:22 UTC

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Date: Fri, 23 Oct 2015 10:22:16 -0700
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From: Martin Thomson <martin.thomson@gmail.com>
To: Paul Kyzivat <pkyzivat@alum.mit.edu>
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Subject: Re: [MMUSIC] Open Issues: draft-ietf-mmusic-sdp-simulcast-03
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On 23 October 2015 at 09:49, Paul Kyzivat <pkyzivat@alum.mit.edu> wrote:
>       simulcast-value = sc-send-opt [SP sc-recv-opt]
>                       / sc-recv-opt

I like this, but I think that it can be improved further.

One annoying feature of your grammar is that this is valid:

a=simulcast:send rid=1 paused=2 recv pt=3

This makes it much harder to parse than ideal because you can't just
split on spaces, you have to process the paused as if it might also be
"recv".

It also permits rid and pt on the same line.

How about:

simulcast-value = simulcast-pt / simulcast-rid
simulcast-pt = ( simulcast-pt-send [ SP simulcast-pt-recv ] )
             / simulcast-pt-recv
simulcast-rid = ( simulcast-rid-send [ SP simulcast-rid-recv ] )
             / simulcast-rid-recv

simulcast-pt-send = "send" SP simulcast-pt-args
simulcast-pt-recv = "recv" SP simulcast-pt-args
simulcast-pt-args = simulcast-pt-list *(";" simulcast-pt-list)
simulcast-pt-list = payload-type *("," payload-type)

simulcast-rid-send = "send" SP simulcast-rid-args
simulcast-rid-recv = "recv" SP simulcast-rid-args
simulcast-rid-args = simulcast-rid-list *(";" simulcast-rid-list)
simulcast-rid-list = simulcast-rid-id *("," simulcast-rid-id)
simulcast-rid-id = [simulcast-rid-paused] rid-identifier
simulcast-rid-paused = "!"

Which would have paused streams prefixed with a character (I chose "!"
because it is the first character that is permitted by the SDP token
grammar but not the rid-identifier grammar, feel free to paint that
bikeshed any colour you like).

This:
      a=simulcast: send rid=1,2,3 recv rid=4 paused=3
Becomes:
      a=simulcast:send rid=1,2,3 recv rid=4,!4

And this is now invalid:
      a=simulcast:send pt=1 recv=rid=2