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

Gunnar Hellström <> Sun, 25 August 2019 10:17 UTC

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To: Christer Holmberg <>, "" <>
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Date: Sun, 25 Aug 2019 12:17:26 +0200
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Subject: Re: [MMUSIC] draft-holmberg-mmusic-t140-usage-data-channel - language indication
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Hi Christer,

Please see inline,

Den 2019-08-24 kl. 12:25, skrev Christer Holmberg:
> 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.

Whoops, there was a copy-paste error there. the second dsca attribute 
was intended to be:

a=dsca:id hlang-recv:language
also, the value may be a list of space separated language tags, so it would be more correct to say:

a=dsca:id hlang-send:language(s)
a=dsca:id hlang-recv:language(s)

This is a bit informal way to specify the format. Do you think it is acceptable? The reader should anyway look into RFC 8373 for the complete specification.

> 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?
See below
> ---
>> 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.
Yes, it may be sufficient with a clear statement that for m=application 
/ webrtc-datachannel subprotocol T140, language indications according to 
RFC 8373 are allowed and indicate written language media. But that is a 
kind of extension over what RFC 8373 specified because the use for 
a=application was excluded there. Do we need to make IANA aware of that 
extension in some way?
> ---
> 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.

Yes, the sdpneg draft, section 9.2.2 seems to indicate that attributes 
already specified for media level are allowed to be used at dsca level 
without specific registration if the use is not modified from its 
original use at media level.

But rfc4566bis, section about updates to existing attributes 
requires us to register if there is a change to the semantics of the 
original attribute. Maybe we should view as modified semantics the use 
together with m=application and the need to look at the subprotocol T140 
to understand that it is about written language indications.

If so, there is a simplified registration format in rfc4566bis 
that we could use.



> Regards,
> Christer
> _______________________________________________
> mmusic mailing list

Gunnar Hellström
+46 708 204 288