Re: [MMUSIC] FW: Issue #27: Allow RTP attributes in non-media m= sections

Paul Kyzivat <pkyzivat@alum.mit.edu> Fri, 17 February 2017 16:51 UTC

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To: Taylor Brandstetter <deadbeef@google.com>
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From: Paul Kyzivat <pkyzivat@alum.mit.edu>
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Date: Fri, 17 Feb 2017 11:51:25 -0500
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Cc: IETF MMUSIC WG <mmusic@ietf.org>, Christer Holmberg <christer.holmberg@ericsson.com>
Subject: Re: [MMUSIC] FW: Issue #27: Allow RTP attributes in non-media m= sections
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On 2/16/17 9:27 PM, Taylor Brandstetter wrote:
>     So, to make this work I think it will be necessary to ammend the
>     definitions of the particular attributes to specify this usage. And
>     then future new attributes that pertain to RTP would also need to
>     address this.
>
>
> sdp-mux-attributes already amends the definitions of these attributes,
> such that a TRANSPORT attribute in m= section "A" can be used for media
> described by m= section "B". So, I don't see why it couldn't go a step
> further, and explicitly allow TRANSPORT and IDENTICAL category
> attributes to appear in m= sections with proto values not normally used
> with those attributes.

I will reserve judgement until I see specific text.

	Thanks,
	Paul

> On Thu, Feb 16, 2017 at 3:50 PM, Paul Kyzivat <pkyzivat@alum.mit.edu
> <mailto:pkyzivat@alum.mit.edu>> wrote:
>
>     On 2/16/17 12:10 PM, Christer Holmberg wrote:
>
>         Hi Paul,
>
>         I assume you have an opinion on this :)
>
>         The suggestion is to allow RTP-specific parameters (SDP rtcp-mux
>         attributes etc) in non-RTP m= lines (e.g., data channel).
>
>
>     This is a nasty issue.
>
>     The problem with allowing this is: what do these parameters *mean*
>     when so attached? And where do I look to find out?
>
>     I'm not certain if they are currently permitted or not. AFAIK there
>     is no *general* mechanism for specifying with which proto values a
>     particular attribute may be used. I haven't studied the definitions
>     of the "RTP-related" attributes to see if they make a specific
>     statement about this. My guess is that they don't, but that they
>     only define the meaning in the context of an RTP session.
>
>     If that is so, perhaps the rule that unknown attributes are to be
>     ignored should apply to those attributes when used with a non-RTP
>     media section. But if that rule were to apply, then we would expect
>     that with O/A the rules for how these attributes in an offer affect
>     what goes in the answer would not apply. I guess that won't be
>     sufficient here.
>
>     So, to make this work I think it will be necessary to ammend the
>     definitions of the particular attributes to specify this usage. And
>     then future new attributes that pertain to RTP would also need to
>     address this.
>
>     IMO this is a can of worms. So my opinion is that these should *not*
>     be used with non-RTP m-lines, with or without bundle.
>
>     Note that data channel is a special case. While we have agreed not
>     to consider it for now, it is possible, in principle, to run RTP
>     over a data channel. If that were defined then these attributes
>     would also be needed. But then they would not be used as media-level
>     attributes. Instead, they would be dcsa attributes.
>
>             Thanks,
>             Paul
>
>         *From:*mmusic [mailto:mmusic-bounces@ietf.org
>         <mailto:mmusic-bounces@ietf.org>] *On Behalf Of *Christer
>         Holmberg
>         *Sent:* 16 February 2017 19:07
>         *To:* Eric Rescorla <ekr@rtfm.com <mailto:ekr@rtfm.com>>; mmusic
>         WG <mmusic@ietf.org <mailto:mmusic@ietf.org>>
>         *Subject:* Re: [MMUSIC] Issue #27: Allow RTP attributes in
>         non-media m=
>         sections
>
>
>
>         Hi,
>
>         * *
>
>         *>*See:
>
>             https://github.com/cdh4u/draft-sdp-bundle/issues/27
>             <https://github.com/cdh4u/draft-sdp-bundle/issues/27>
>
>
>             https://github.com/rtcweb-wg/jsep/issues/528
>             <https://github.com/rtcweb-wg/jsep/issues/528>
>
>
>
>
>             The basic issue is that it's possible to have a situation
>             where you
>
>         have both
>
>             media and data m= sections but the BUNDLE tag is associated
>             with the data
>
>
>             m= section and now you need to put the TRANSPORT and IDENTICAL
>
>
>             attributes somewhere. The JSEP editors discussed this and
>             came to the
>
>
>             conclusion that it should go with the BUNDLE tag (i.e., in
>             the data m=
>
>         section)
>
>             and that BUNDLE should forbid this, but it requires a change
>             to BUNDLE.
>
>
>
>
>         Did you mean to say that BUNDLE should NOT forbid this?
>
>
>
>         Based on your GitHub discussion, my understanding is that you
>         want to
>         allow to include RTP-specific parameters (‘rtcp-mux’, ‘rtcp’,
>         ‘rtcp-mux-only’ attributes etc) in the data m= section.
>
>
>
>         To repeat what I said on GitHub:
>
>
>
>         This has been discussed in the past, and the outcome has been to now
>         allow RTP-specific parameters in non-RTP m= sections.
>
>
>
>         A solution would be to simply change the bundle tag when the RTP m=
>         sections are added.
>
>
>
>         …OR, we change the mux category for the RTP-specific parameters.
>         But,
>         that of course means they have to be added to every RTP m= section.
>
>
>
>         Regards,
>
>
>
>         Christer
>
>
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