Comments on draft-ietf-idnabis-defs-10

Vint Cerf vint at
Tue Sep 1 22:01:26 CEST 2009

let's go with the "make sure the XN-label string is in
lowercase before converting to U-label"


On Sep 1, 2009, at 3:58 PM, John C Klensin wrote:

> --On Tuesday, September 01, 2009 12:08 -0400 Andrew Sullivan
> <ajs at> wrote:
>> On Tue, Sep 01, 2009 at 11:10:27AM -0400, Vint Cerf wrote:
>>> andrew, the problem with that last point is that the two
>>> labels will   match in DNS but produce different U-label on
>>> conversion. I think that   is not a good outcome.
>>> downcasing would solve that wouldn't it?
>> I am really reluctant to add a normative downcasing step
>> anywhere, because it's a very late change to the protocol.  I
>> can't think of anything it would break, but I didn't think of
>> this problem before, either, and mostly nobody else did
>> either.  I'd have grave concerns about adding another
>> normative step to the protocol during WGLC, and then sending
>> the result on to the IESG.
> Agreed.
>> The important thing we have exposed in this discussion is that
>> there are _several_ possible matches in the DNS to a given
>> valid U-label, but exactly one of those candidate DNS matches
>> is itself a valid A-label.  That suggests that saying, "making
>> sure you downcase before converting the U-label is probably a
>> good idea," may be enough -- roughly what John suggested, I
>> think, but I'm not totally sure.  We first need to resolve the
>> discussion of uppercase/lowercase form from Punycode.
> What I'm suggesting, refined by reading a lot of additional
> messages and getting a little sleep, is either the above or the
> nearly equivalent "make sure that the string is already in lower
> case before converting to a U-label".   Neither results in a
> change to the protocol; they are just recommendations of an
> extra validation and precaution against nonsense.
>   john
> _______________________________________________
> Idna-update mailing list
> Idna-update at

More information about the Idna-update mailing list