Re: [Slim] Eric Rescorla's No Objection on draft-ietf-slim-negotiating-human-language-19: (with COMMENT)

Eric Rescorla <> Mon, 08 January 2018 01:56 UTC

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From: Eric Rescorla <>
Date: Sun, 07 Jan 2018 17:56:08 -0800
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To: Randall Gellens <>
Cc: Bernard Aboba <>, Bernard Aboba <>, "" <>
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Subject: Re: [Slim] Eric Rescorla's No Objection on draft-ietf-slim-negotiating-human-language-19: (with COMMENT)
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On Sun, Jan 7, 2018 at 5:22 PM, Randall Gellens <>

> At 6:36 AM -0800 1/7/18, Eric Rescorla wrote:
>  On Sat, Jan 6, 2018 at 7:31 PM, Bernard Aboba <<mailto:
>>>> wrote:
>>  On Jan 6, 2018, at 6:55 PM, Eric Rescorla <<>ekr@rtfm
>> .com> wrote:
>>  For disabled users, the capabilities may not be symmetric.
>>  But this is true for ordinary SDP as well. I might be able to receive
>> H.264 but not send it.
>>  [BA] Thanks. The draft should explain the reasoning. IMHO the argument
>> goes sonething like this:
>>  A pure recv/recv negotiation will not necessarily disclose beforehand
>> what special services are needed for the call - services (e.g. ASL
>> interpretation or RTT handling) that could take time to acquire.
>>  Since the actual video media sent is not labelled as ASL even if the
>> answerer has ASL interpreters it can pull in and therefore advertises in
>> SDP ASL reception capability in video, a recv/recv negotiation doesn't tell
>> the Answerer that the Offerer will need them, so the Answerer may need to
>> (frantically) arrange for ASL interpretation after initial receipt of
>> media. In an emergency, that can chew up valuable time.
>>  Thanks. I think it would be helpful to put this logic in the draft.
> I am not clear on what logic we want to add to the draft, or what about
> the draft this logic is explaining.

It would be helpful to explain in the draft why you have deviated from the
otherwise near-universal SDP negotiation pattern of each side advertising
what it accepts.

>  That said, as I noted in my review, it is still possible to get some
>> media (early media) prior to receiving the answer, so this isn't a complete
>> solution.
> The draft provides a useful mechanism that will be helpful.  As an example
> of the fact that others find it useful, NENA has included it in it's
> next-generation emergency call architecture standards.  The draft does not
> try to solve all problems related to human language in real-time calling.

I don't think I claimed it wasn't useful.

The rationale provided for this design is that you wish to have the
answerer notify the offerer of which language it would be providing. The
point I am making is that there is at least one important case where this
design does not provide that, which seems like it's relevant to the design


> --
> Randall Gellens
> Opinions are personal;    facts are suspect;    I speak for myself only
> -------------- Randomly selected tag: ---------------
> (If you can't hear me, it's because I'm in parentheses)