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

[Christer Holmberg <[e-mail]>]

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