Re: [xmpp] [Technical Errata Reported] RFC7622 (5769)

[Florian Schmaus <[e-mail]>]

On 09.07.19 14:02, Florian Zeitz wrote:
> Am 02.07.19 um 09:25 schrieb Florian Schmaus:
>>> c) I don't think we want A-Labels. It's just going to cause confusion -
>>> is 商业.中国 the same as xn--vhqr8o.xn--fiqs8s? They always look up the
>>> same in DNS, after all, by definition. I can't see why we'd think
>>> allowing them wasn't going to cause problems.
>>
>> I can not remember that I ever ran into problems caused by A-labels in
>> domainparts. And hence I assume that the cost of explicitly disallowing
>> them isn't worth the gain. If we can, we should not make the PRECIS
>> operations on the domainpart to require anything besides the
>> "inspection" of every code point, with as less as possible context
>> around the inspected code point involved.
>>
>> Note that instead of forbidding A-labels / ACE in domainparts, RFC7622
>> currently states in § 5.
>>
>>   XMPP applications MUST support IDNA2008 for domainparts
>>
>> which I interpret as "use U-labels of your DNS name in the domainpart
>> (if applicable)".
>>
>> - Florian
>>
=
> 
> The intend of 7622 (in particular Section 3.2.1) as I recall it was that
> A-labels would generally be valid in JIDs.

The question here is: Do you mean A-label or "a string which is (also) a
valid A-label"?

> However, software would be
> required to process (user) input containing A-labels so that any
> JID-slots would only contain U-labels on the wire.

Exactly. This is an important insight: Any string which is a valid
A-label may be valid in the domainpart of a Jid. However, if this string
got derived from an U-label, then you did something wrong. As JID
domainparts constructed from DNS names which contain U-label(s) should
conserve the U-label presentation (and not use the ACE/A-label form).

- Florian