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Re: [MMUSIC] WGLC on draft-ietf-mmusic-sdp-simulcast-07

worley@ariadne.com (Dale R. Worley) Fri, 03 March 2017 15:57 UTC

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From: worley@ariadne.com
To: Iñaki Baz Castillo <ibc@aliax.net>
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Cc: fandreas@cisco.com, mmusic@ietf.org, draft-ietf-mmusic-sdp-simulcast@ietf.org
Subject: Re: [MMUSIC] WGLC on draft-ietf-mmusic-sdp-simulcast-07
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Iñaki 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:

   a=rid:<rid-id> <direction> <restriction>=<value><restriction>=<value>

A correct description is:

   a=rid:<rid-id> <direction> ( pt=<fmt-list> | <restriction>=<value> ) *( ; <restriction>=<value> )

The problem is that BNF doesn't allow you to directly specify what you
want to say:  (1) pt is like a parameter, but if it's present, it must
be first, and (2) there must be at least one pt or restriction.  And
there's no simple construction for "a list of one or more X's separated
by Y's".

Dale

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