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

[Paul Kyzivat <[e-mail]>]

On 12/13/19 3:30 PM, Christer Holmberg wrote:
> 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... :)

IMO using "webrtc-datachannel" would be wrong and confusing.

Using the subprotocol would make sense, but is then redundant. Using "-" 
seems reasonable to me.

	Thanks,
	Paul

> 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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