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Re: [MMUSIC] New Draft: SDP Grouping Identifiers

Christer Holmberg <christer.holmberg@ericsson.com> Tue, 03 December 2013 18:47 UTC

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From: Christer Holmberg <christer.holmberg@ericsson.com>
To: Paul Kyzivat <pkyzivat@alum.mit.edu>, "mmusic@ietf.org" <mmusic@ietf.org>
Thread-Topic: [MMUSIC] New Draft: SDP Grouping Identifiers
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Date: Tue, 03 Dec 2013 18:47:26 +0000
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Subject: Re: [MMUSIC] New Draft: SDP Grouping Identifiers
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Hi,

I share Paul's concern regarding mandating a specific order.

There are parsers that do not necessarily keep the order when parsing an SDP body, why e.g. something that "enter" a SIP proxy is not necessarily what "comes out".

Regards,

Christer
________________________________________
From: mmusic [mmusic-bounces@ietf.org] on behalf of Paul Kyzivat [pkyzivat@alum.mit.edu]
Sent: Tuesday, 03 December 2013 7:19 PM
To: mmusic@ietf.org
Subject: Re: [MMUSIC] New Draft: SDP Grouping Identifiers

On 12/3/13 9:11 AM, Adam Roach wrote:
> Based on the conversations we had during the partial offer/partial
> answer draft discussion in Vancouver, Martin and I put together a
> separate document that formally defines a group identifier for RFC 5888
> SDP groups:
>
> http://tools.ietf.org/html/draft-roach-mmusic-groupid-00
>
> We've made this a separate document because the utility of identifying
> groups extends beyond its (upcoming) use in the partial offer/partial
> answer draft.
>
> The document is mercifully short. Feedback is solicited.

This is yet another example of the contortions that we are now going
through because of the inadequacy of SDP syntax. :-(

I find the approach troubling because of strict ordering requirement on
attributes. AFAIK this is the first time time I have seen any ordering
requirements on attributes other than the session level/media level
distinction.

Clearly there is a need to bind a group-id to a particular group line,
so there must be some ordering requirement. I see no reason that the two
lines must be adjacent, but once the need for some attribute ordering
has been accepted I guess there is no reason not to accept this stronger
requirement.

Since group-ids are required to be unique within the session, why is it
necessary to include the group semantic in the in-group attribute. IOW,
why not:
   a=in-group abc
rather than
   a=in-group LS:abc


It would be "cleaner" (to the extent that anything is clean in SDP) to
just define a new alternative grouping syntax that isn't backward
compatible. It could be as simple as using an a=in-group attribute (this
time including the semantic tag) as the only definer of groups.
Is there any great need to preserve backward compatibility here?

        Thanks,
        Paul


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