Re: [MMUSIC] draft-holmberg-mmusic-t140-usage-data-channel - language indication

Christer Holmberg <> Sat, 24 August 2019 10:26 UTC

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From: Christer Holmberg <>
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Thread-Topic: [MMUSIC] draft-holmberg-mmusic-t140-usage-data-channel - language indication
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Date: Sat, 24 Aug 2019 10:25:55 +0000
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Subject: Re: [MMUSIC] draft-holmberg-mmusic-t140-usage-data-channel - language indication
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Hi Gunnar,

Please see inline.

12. At the end of 3.2, add the following about language negotiation

> The users may be interested to negotiate language to use during the 
> real-time text session. This is done by including specifications of 
> language capabilities in the media descriptors for the t140 data 
> channels by use of dsca attributes hlang-send and hlang-recv according 
> to RFC 8373 [RFC8373]. The format is as follows:
> a=dsca:id hlang-send:language
> a=dsca:id hlang-send:language
> The negotiation is performed as indicated in RFC8373.

I may modify it a little, but I will add the text.

There will be a pull request, so you will be able to comment on the text (and all other changes based on your comments) before a new draft version is submitted :)

> -EDITORs NOTE - There may be a need for IANA registration of this way 
> to use the human language attributes.
> RFC 8373 was created partly to satisfy a dependency from 3GPP requiring negotiation of language per media. We knew 
> that the T140 text data channel was about to be specified, but we could not request IANA to register the use of the 
> hlang- attributes because of the state of the t140-usage-data channel draft, so the registration request was deleted from 
> RFC 8373. It needs to be completed now.

Not sure I follow. The attributes have been registered with IANA, and BCP47 is referenced for the attribute values. What else is needed?


>12a: Allow use of hlang- attributes together with m=application media declarations
> RFC 8373 has a statement in section 5.3 saying:
> "This document does not define the use of language tags in media other than interactive streams of audio, video, and 
> text (such as "message" or "application"). Such use could be supported by future work or by application agreement."
> The reason for this limitation is that it must be clear for what modality (spoken or written or signed ) the language is intended 
> to be used. For audio, text and video, RFC 8373 defines the implicit relation audio=spoken, text=written, video=signed, but 
> for "application" there is no such implicit relation and it is in our case the subprotocol T140 that we can be sure carries written 
> language modality. So, we just need to specify in the draft that for media "application  / webrtc-datachannel" and subprotocol "t140", 
> the language modality is "written".  It may in fact be evident from the context.

I will look into it, but it seems like a useful clarification to make.


12b: Allow use of hlang-attributes in dsca parameter values.

> RFC 8373 registers SDP parameter att-values hlang-send and hlang-recv only for media-level use. The registration requirements in rfc4566bis
> require us to register the use also on dsca(subprotocol) level for the t140 subprotocol.
> I am not sure how to express that registration. The attributes and values are as defined in RFC 8373. We just need to add a usage level
> for SDP att-values hlang-send and hlang-recv for media "application  / webrtc-datachannel"
>    o  Usage Level: Add the following
>     dcsa(subprotocol).
>         dcsa(t140)  for written media.
> I would appreciate guidance on how this should be expressed in IANA considerations.

I don't think that is needed. We haven't registered other attributes that are carried within a dsca attribute either. From an SDP perspective, the attributes will still be used for media-level.