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

Paul Kyzivat <pkyzivat@alum.mit.edu> Fri, 23 October 2015 18:00 UTC

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To: Martin Thomson <martin.thomson@gmail.com>
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From: Paul Kyzivat <pkyzivat@alum.mit.edu>
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Date: Fri, 23 Oct 2015 14:00:49 -0400
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Subject: Re: [MMUSIC] Open Issues: draft-ietf-mmusic-sdp-simulcast-03
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Martin,

Yeah, what you propose below is better.

But I'm not loving "!" - it is hard to distinguish from "1". Of course 
this isn't a problem for parsers, only for us humans reading examples 
and traces. I would prefer some other character, perhaps "-", "~" or "/".

	Thanks,
	Paul

On 10/23/15 1:22 PM, Martin Thomson wrote:
> 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
>