Comments on draft-ietf-idnabis-defs-10

Vint Cerf vint at
Tue Sep 1 17:10:27 CEST 2009

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?


On Sep 1, 2009, at 11:05 AM, Andrew Sullivan wrote:

> On Wed, Sep 02, 2009 at 12:45:05AM +1000, Wil Tan wrote:
>> 2. If A-labels are not allowed to have uppercase ASCII characters,  
>> why do we
>> define case-insensitive comparison for them? An invalid A-label  
>> should not
>> be equivalent to a valid one. It should be as Paul suggested, i.e.
>> "case-preserving" (why not just "case-sensitive" as it's more
>> straightforward?)
> This one's easy: we're stuck with what DNS does.  A-labels are all LDH
> labels, and LDH labels compare for equivalence according to the
> case-insensitive matching rules defined as part of DNS (see STD13).
> Attempting to invent special rules for A-label matching will have the
> bizarre result that an IDNA-aware application will not match two
> "identical" DNS labels, even though an IDNA-oblivious application will
> match them.
>> 3. We are "violating" (may be too strong a term, "contradicting"  
>> perhaps)
>> the underlying assumptions that DNS labels are case insensitive.  
>> There are
>> lots of deployed software that relies on that assumption. Domain  
>> names are
>> often presented (and perhaps stored) in uppercase by some  
>> registries in
>> Whois and EPP. I'm also worried about potential security issues  
>> that may
>> arise if the case insensitivity property is not preserved.
> Well, we haven't violated that, precisely because no valid A-label
> has, at least, an ASCII capital letter that will remain in the
> translation back to a U-label.  But the upper-cased version (or any
> mixed-case version) of the same A-label will still be DNS equivalent.
> It just won't be an A-label, it seems.
> A
> -- 
> Andrew Sullivan
> ajs at
> Shinkuro, Inc.
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