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

Paul Kyzivat <> Fri, 23 October 2015 16:49 UTC

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Subject: Re: [MMUSIC] Open Issues: draft-ietf-mmusic-sdp-simulcast-03
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On 10/22/15 4:52 PM, Adam Roach wrote:

> BIKESHED TIME! I think Martin is correct that the "space-colon"
> delimiter is unaesthetic; I'll go further and point out that it appears
> to be unprecedented (I could be missing something), which will make it
> unnecessary parsing exception as compared to other attributes. Mo's
> point that we want a colon is well-taken, but that doesn't justify the
> space. It seems that the current formulation was selected to make the
> ABNF easy to write. To that end, I've put together a formulation for the
> "just a colon" format.
> sc-attr     = "a=simulcast:" sc-str-list [WSP sc-str-list] [WSP sc-pause-list]
> Is there any reason we can't make the format consistent with all the
> other "a=foo:bar" syntaxes out there, as described by this rule?

Yes please. And please follow the format for attribute definition 
established in rfc4566bis.

Also, I have some other issues:
- syntax allows two sends, or two recvs, but those are semantically 
- 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.

Below is a proposal for revised syntax that follows the spirit, but 
aligns better with other attributes and 4566bis. It doesn't have some of 
the extensibility of the prior syntax 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.


6.1 Simulcast attribute

    Name: simulcast

    Value: simulcast-value

    Usage Level: media

    Charset Dependent: no


       simulcast-value = sc-send-opt [SP sc-recv-opt]
                       / sc-recv-opt
       sc-send-opt = "send" SP sc-sendrecv-args
       sc-recv-opt = "recv" SP sc-sendrecv-args

       sc-sendrecv-args = sc-pt-args / sc-rid-args
       sc-pt-args = "pt=" sc-pt-list *(";" sc-pt-list)
       sc-rid-args = "rid=" sc-rid-list *(";" sc-rid-list)
                     [SP sc-pause-opt]

       sc-pt-list = payload-type *("," payload-type)
       sc-rid-list = rid-identifier *("," rid-identifier)
       sc-pause-opt = "paused=" sc-rid-list

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


       a=simulcast: send pt=97;98 recv pt=97
       a=simulcast: send rid=1,2,3 recv rid=4 paused=3