Re: [MMUSIC] T140: Usage of "-"fmt value in dcsa encapsulated fmtp attribute

Gunnar Hellström <gunnar.hellstrom@omnitor.se> Fri, 13 December 2019 22:01 UTC

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From: Gunnar Hellström <gunnar.hellstrom@omnitor.se>
To: Christer Holmberg <christer.holmberg@ericsson.com>, Flemming Andreasen <fandreas@cisco.com>, Christer Holmberg <christer.holmberg=40ericsson.com@dmarc.ietf.org>, "mmusic (mmusic@ietf.org)" <mmusic@ietf.org>, "mmusic-chairs@ietf.org" <mmusic-chairs@ietf.org>
Thread-Topic: [MMUSIC] T140: Usage of "-"fmt value in dcsa encapsulated fmtp attribute
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Date: Fri, 13 Dec 2019 22:01:07 +0000
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Subject: Re: [MMUSIC] T140: Usage of "-"fmt value in dcsa encapsulated fmtp attribute
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Hi,

Den 2019-12-13 kl. 21:30, skrev Christer Holmberg:

Hi,

...



The question is whether it in such definition is allowed to "override" the 4566bis statement saying that the format
must be one of the formats specified for the media. As far as SDP is concerned, the only format is 'webrtc-datachannel'.



Again, I personally think we should keep "-" (I don't like to omit information elements), and add whatever text needed to
indicate that the 4566bis text does not apply to sub-protocol specific fmtp attributes.



Syntactical consistency and strict adherence to the wording in 4566bis would argue for "webrtc-datachannel",
whereas conceptual consistency would argue more for "t140". I'm not in favor of "-", since it doesn't seem to add anything.



There is nothing to add, and that's why we are using "-" to indicate that. I believe there are cases where we use "-" for the fmt in the m= line, aren't there?

Having said that, if the community prefers "webrtc-datachannel" I can live with that. I think it is a waste of characters, but the ship to keep SDP bodies small and compact sailed a long time ago, so... :)

I found the permission last in sdpneg section 5.2.1 to override what is specified for fmtp and fmt in RFC4566bis.

 "If
   existing media attributes are used in a datachannel subprotocol
   specific way, then a new dcsa usage level MUST be defined for the
   existing media attribute.  Where the SDP attribute is applicable to a
   particular subprotocol/s this SHALL also be registered by indicating
   the applicable subprotocol identifiers (see
   /[I-D.ietf-mmusic-rfc4566bis<https://tools.ietf.org/html/draft-ietf-mmusic-data-channel-sdpneg-28#ref-I-D.ietf-mmusic-rfc4566bis>] section-8.5) along with the dcsa usage
   level."

The use of fmtp is specified and registered in t140-usage draft section 9.2 as required.
Maybe the specific syntax of fmt=- should be mentioned in the registration (?).

Regards

Gunnar







Regards,

Christer







Den 2019-12-12 kl. 20:12, skrev Christer Holmberg:
Hi,

When the chairs reviewed the T.140 data channel usage draft, the following issue was raised:

Currently, when we send an fmtp attribute encapsulated in a dcsa attribute, we use "-" as the fmt value.

     Example: a=dcsa:2 fmtp:- cps=20

However, 4566bis says the following about the fmtp attribute:

"The format must be one of the formats specified for the media."

Now, the format is 'webrtc-datachannel', so one could claim that should be used.

     Example: a=dcsa:2 fmtp:webrtc-datachannel cps=20


 But, that applies to the whole SCTP association, and the dcsa attribute is associated with a specific data channel usage. In addition, the usage and syntax of the fmtp attribute may vary depending on data channel usage.

Another option could be to include the sub-protocol:

     Example: a=dcsa:2 fmtp:t140 cps=20


But, the dcsa attribute already maps the fmtp attribute to the sub-protocol.


So, what to do?


My personal suggestion would be to keep "-", and perhaps add a note somewhere. Not sure we would need to update 4566bis, but if people want to do that we could for sure do it.

Anyone having a different opinion?


Regards,


Christer




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Gunnar Hellström
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