[MMUSIC] Definition of a=rid in draft-ietf-mmusic-rid-09 (was WGLC on draft-ietf-mmusic-sdp-simulcast-07)

worley@ariadne.com (Dale R. Worley) Mon, 06 March 2017 19:50 UTC

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From: worley@ariadne.com (Dale R. Worley)
To: Paul Kyzivat <pkyzivat@alum.mit.edu>
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Subject: [MMUSIC] Definition of a=rid in draft-ietf-mmusic-rid-09 (was WGLC on draft-ietf-mmusic-sdp-simulcast-07)
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Paul Kyzivat <pkyzivat@alum.mit.edu> writes:
> On 3/3/17 10:56 AM, Dale R. Worley wrote:
>> Inaki Baz Castillo <ibc@aliax.net> writes:
>>>> but according to https://tools.ietf.org/html/draft-ietf-mmusic-rid-09
>>>> it seems that "direction" (send/recv) should be placed *before*
>>>> "pt=xx":
>>>> a=rid:<rid-id> <direction> [pt=<fmt-list>;]<restriction>=<value>...
>>> In fact, pt=xx seems to be yet another "param".
>> That purported BNF is really bizarre, since it generates the clearly
>> incorrect form:
> IIUC you are commenting on the ABNF in draft-ietf-mmusic-rid-08, not in 
> draft-ietf-mmusic-sdp-simulcast-07, right?
>>    a=rid:<rid-id> <direction> <restriction>=<value><restriction>=<value>
> I'm not seeing how the ABNF generates that.

What I'm commenting on is this line way back in the innermost quoted

>>>> a=rid:<rid-id> <direction> [pt=<fmt-list>;]<restriction>=<value>...

which was in turn quoted from draft-ietf-mmusic-rid-09.

In BNF-like notations, "..." means "repetition of the preceding thing",
which I took to mean repetition of "<restriction>=<value>".  Although
that's probably not the correct interpretation, and rather it means
repetition of "<value>".

>> A correct description is:
>>    a=rid:<rid-id> <direction> ( pt=<fmt-list> | <restriction>=<value> ) *( ; <restriction>=<value> )
> ISTM the ABNF in the draft is equivalent to what you have written. 
> (Though yours is clearer.)

What I don't see is why mmusic-rid-09 requires the pt restriction to be
in first position.  I'm not read into it, but it seems that there's no
semantic reason why it must be first, and it makes the grammar messier
to force it to be so.  Without that restriction, you could just say

>>    a=rid:<rid-id> <direction> <restriction>=<value> *( ; <restriction>=<value> )

and allow pt to be one of the <restriction>s.