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

Paul Kyzivat <pkyzivat@alum.mit.edu> Fri, 03 March 2017 19:07 UTC

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From: Paul Kyzivat <pkyzivat@alum.mit.edu>
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Date: Fri, 3 Mar 2017 14:07:40 -0500
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Subject: Re: [MMUSIC] WGLC on draft-ietf-mmusic-sdp-simulcast-07
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Dale,

On 3/3/17 10:56 AM, Dale R. Worley wrote:
> 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:

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.

> 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.)

Meanwhile, the ABNF of draft-ietf-mmusic-sdp-simulcast-07 does some 
small problems:

1) it allows either one or two 'sc-str-list's. I presume this is so you 
can have both send and recv lists, but it also allows two send lists or 
two recv lists.

2) it references rid-identifier from draft-ietf-mmusic-rid-08, but that 
isn't defined there. I guess it means to reference rid-id.

3) this syntax defines the syntax of the entire attribute, including 
"a=" and the separation between attribute name and value. We are trying 
to get away from that as part of the cleanup in rfc4566bis. (Because 
people keep getting it wrong, so that it doesn't match with the generic 
syntax of an attribute.)

I suggest a better syntax for this would be:

    sc-value     = sc-send [SP sc-recv] / sc-recv [SP sc-send]
    sc-send      = "send" sc-str-list
    sc-recv      = "recv" sc-str-list
    sc-str-list  = SP sc-alt-list *( ";" sc-alt-list )
    sc-alt-list  = sc-id *( "," sc-id )
    sc-id-paused = "~"
    sc-id        = [sc-id-paused] rid-id
    ; SP defined in [RFC5234]
    ; rid-id defined in [I-D.ietf-mmusic-rid]

	Thanks,
	Paul