Re: [MMUSIC] The simulcast colon

Paul Kyzivat <pkyzivat@alum.mit.edu> Sat, 24 October 2015 23:19 UTC

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From: Paul Kyzivat <pkyzivat@alum.mit.edu>
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Subject: Re: [MMUSIC] The simulcast colon
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Christer, Bo,

Haven't the two of you seen the conversation between me and Martin? That 
thread starts at:
https://mailarchive.ietf.org/arch/msg/mmusic/7bUkyhJAN2IRjm51DyvrPhS_yPI

That first message gives a list of issues that we have been trying to 
address. Here is a restatement of that list:

- the "colon issue"
- syntax allows two sends, or two recvs, but those are semantically
   forbidden
- the syntax allows paused with PTs, but that is semantically forbidden.
- syntax allows pt values that aren't numeric
- use of WSP (SP or HTAB). Contrary to typical SDP usage.
- the conventions for attribute syntax, introduced in rfc4566bis,
   call for only the syntax for the attribute *value* is given, not for
   the "a=" or the attribute name, or the ":".

We have been negotiating a new syntax proposal. We have been converging, 
but not completely converged.

There are a few esthetic loose ends in that proposed syntax. Based on 
that discussion, below is a new proposal for you. It doesn't have some 
of the extensibility of the syntax in -03 because I couldn't see how to 
do it in a meaningful way. I don't think it is a great loss here.

This may perhaps need further tweaking based on the ongoing discussion 
of pt vs rid.

The following would replace the beginning of section 6.1, or maybe be a 
new section/subsection. It also ought to be cross referenced from IANA 
considerations as a request to add it to the iana sdp attribute registry.

	Thanks,
	Paul

6.1 Simulcast attribute

    Name: simulcast

    Value: simulcast-value

    Usage Level: media

    Charset Dependent: no

    Syntax:

       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 = "~" ; OR (bikeshed color) "/" or "!"

       ; SP defined in [RFC5234]
       ; payload-type, defined in [draft-ietf-mmusic-rfc4566bis-15]
       ; rid-identifier defined in [I-D.pthatcher-mmusic-rid]

    Examples:

       a=simulcast: send pt=97;98 recv pt=97
       a=simulcast: recv pt=97
       a=simulcast: send rid=1,~2,3 recv rid=~A,B

On 10/24/15 1:46 PM, Christer Holmberg wrote:
> Hi,
>
> A few comments on the ABNF:
>
> *Q1*: The colon MUST be there. As Bo pointed out, it’s part of the core
> syntax for attributes.
>
> In order to now have both the colon and the space, the ABNF can be
> modified e.g. in the following way:
>
> *sc-attr     = "a=simulcast:" sc-str-list  [WSP sc-str-list] [WSP
> sc-pause-list]*
>
> *Q2*: I don’t think SDP allows usage of whitespace (WSP).
>
> RFC 4566 & draft-4566bis say:
>
>                “Whitespace MUST NOT be used on either side of the "=" sign.
>
> In addition, as far as I know, SDP is very strict with spaces, which is
> the reason they are explicitly stated in the SDP ABNF.
>
>                “In general, <value> is either a number of fields
> delimited by a *single*
>
> *              space character*or a free format string…”
>
> Regards,
>
> Christer
>
> *From:*mmusic [mailto:mmusic-bounces@ietf.org] *On Behalf Of *Bo Burman
> *Sent:* 24 October 2015 20:21
> *To:* Mo Zanaty (mzanaty) <mzanaty@cisco.com>om>; Peter Thatcher
> <pthatcher@google.com>
> *Cc:* mmusic@ietf.org
> *Subject:* Re: [MMUSIC] The simulcast colon
>
> I note that 4566bis ABNF for “attribute” says:
>
>     ; sub-rules of 'a='
>
>     attribute =           (att-field ":" att-value) / att-field
>
>     att-field =           token
>
>     att-value =           byte-string
>
> The colon does not seem to be a “should” to me, but rather a requirement
> for any SDP attribute that is not a boolean.
>
> Is there really any slack in how this can be implemented by SDP parsers?
>
> To comply with 4566bis ABNF and still keep a single separator, you
> should rather remove the space.
>
> /Bo
>
> *From:*mmusic [mailto:mmusic-bounces@ietf.org] *On Behalf Of *Mo Zanaty
> (mzanaty)
> *Sent:* den 22 oktober 2015 15:48
> *To:* Peter Thatcher
> *Cc:* mmusic@ietf.org <mailto:mmusic@ietf.org>
> *Subject:* Re: [MMUSIC] The simulcast colon
>
> The colon was added after others pointed out that attributes with
> further parameters should have a colon after the attribute. I don't know
> how many parsers actually care about that rule, but it seems safer to
> stick with the convention even if it bothers a few human eyes.
>
> Mo
>
>
> On Oct 22, 2015, at 12:54 AM, Peter Thatcher <pthatcher@google.com
> <mailto:pthatcher@google.com>> wrote:
>
> +1 to removing the colon
>
> https://en.wikiquote.org/wiki/Consistency
>
> On Wed, Oct 21, 2015 at 1:05 PM, Martin Thomson
> <martin.thomson@gmail.com <mailto:martin.thomson@gmail.com>> wrote:
>
> Is annoying me.  Do we really need two separators (i.e., the colon and
> the space)?
>
> BTW,  the examples in the draft have c= and m= lines in the wrong
> order, though I'm sure that I'm not the first to note this.
>
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