Re: [MMUSIC] Draft new version: T140-data-channel -02 - Use direction attributes on dcsa level

[Gunnar Hellström <[e-mail]>]

Hi,

The issue here was my view:

I think that session level direction attributes should work on T.140 
data channels where no direction attribute is set on the dcsa(t140) level.

Den 2019-09-05 kl. 03:20, skrev Christer Holmberg:
>
> Hi,
>
> What part sdpneg-23 do you think is trying to say that?
>
It is in sdpneg-28 and RFC4566bis

It requires some steps of logic that you may find weak:

1. sdpneg talks a lot about a "subprotocol" concept as if it exists also 
outside of the data channels. I assume that we can see "t140" as the 
subprotocol in our case, and that it was defined in RFC 4103 that is RTP 
transport of the t140 subprotocol and MIME media declared as text/t140.

2. In section 5.2 and 5.2.1, there is a lot of wording trying to 
describe how already specified general attributes that are used for the 
subprotocol on media level are automatically accepted with the same 
semantics when used on dsca(subprotocol) level. No IANA registration 
would be needed.

It is for example section 5.2.1 "DCSA Syntax" in 
draft-ietf-mmusic-data-channel-sdpneg

saying:
"

It is assumed that in general the usages of subprotocol related media
    level attributes are independent from the subprotocol's transport
    protocol.  Such transport protocol independent subprotocol related
    attributes are used in the same way as defined in the original
    subprotocol specification, also if the subprotocol is transported
    over a data channel and if the attribute is correspondingly embedded
    in a "a=dcsa" attribute.

"
3. RFC 4566bis, section 6.7 moves the direction attributes from session 
level to be applied to the media description if no direction attribute 
was declared on media level.

6.7.  Media Direction Attributes

    At most one occurrence of recvonly, sendrecv, sendonly, or inactive
    MAY appear at session level, and at most one MAY appear in each media
    description.

    If any one of these appears in a media description then it applies
    for that media description.  If none appear in a media description
    then the one from session level, if any, applies to that media
    description.

4. Combining 2. and 3. results in the session level attributes are 
applied automatically on the dsca(subprotocol) level, e.g. the direction 
attributes on the t140 subprotocol.

5. This deduction also has the side effect that we do not need to IANA 
specify all usage of general SDP attributes which already were possible 
on the RTP/TEXT/t140 subprotocol, as long as it is used without change 
in semantics.

Regards
Gunnar
>
> Regards,
>
> Christer
>
> *Lähettäjä:*Gunnar Hellström <[e-mail]>
> *Lähetetty:* torstai 5. syyskuuta 2019 0.23
> *Vastaanottaja:* Christer Holmberg <[e-mail]>; 
> [e-mail]
> *Aihe:* Re: [MMUSIC] Draft new version: T140-data-channel -02 - Use 
> direction attributes on dcsa level
>
> (sorry, I forgot to change topic)
>
> Den 2019-09-04 kl. 23:20, skrev Gunnar Hellström:
>
>     Hi,
>
>     This is another topic for discussion:
>
>     I think that session level direction attributes should work on
>     T.140 data channels where no direction attribute is set on the
>     dcsa(t140) level.
>     RFC 4566bis seems to expect that. It is written as if it was
>     defining that session level attributes if specified should govern
>     in the whole session.
>
>     And I think that the sdpneg-23 draft tries to say that attributes
>     defined for general session and media level use should be allowed
>     to work on DCSA level for any subprotocol without further
>     specification or registration.
>
>     Regards
>
>     Gunnar
>
>     It should be discussed what happens if there is a direction
>     attribute on media level in the media declaration for the SCTP
>     declaration.. Probably a similar statement as for session level is
>     needed.
>
>     Den 2019-09-04 kl. 23:03, skrev Gunnar Hellström:
>
>         Hi,
>
>         I have created a number of pull requests to the GitHub
>         repository for draft-holmberg-mmusic-t140-usage-data-channel-02
>
>         Many are editorial. A few require discussion in the list.
>
>         Regards
>
>         Gunnar
>
>         Den 2019-09-03 kl. 14:10, skrev Christer Holmberg:
>
>             Hi,
>
>             I have mereged the following PR:
>
>             https://github.com/cdh4u/draft-datachannel-t140/pull/6
>             <https://protect2.fireeye.com/url?k=bb2745e9-e7b548b5-bb270572-0cc47ad93d46-f9e326b54116c066&q=1&u=https%3A%2F%2Fgithub.com%2Fcdh4u%2Fdraft-datachannel-t140%2Fpull%2F6>
>
>             …and submitted a new version (-02) of
>             draft-holmberg-mmusic-t140-usage-data-channel.
>
>             The main changes are:
>
>               * IANA registry update for SDP attributes that can be
>                 included in dcsa attributes
>               * Detailed procedures on usage of the direction
>                 attributes inside dcsa attributes
>
>             Regards,
>
>             Christer
>
>
>
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>         Gunnar Hellström
>
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