Lookup for reserved LDH labels

"Martin J. Dürst" duerst at it.aoyama.ac.jp
Wed Nov 7 10:37:13 CET 2012

Hello Marcos, others,

On 2012/11/07 16:14, John C Klensin wrote:

> The term "putative U-label" was used precisely to cover the cast
> of things that could not possibly be "U-labels".  That includes
> not only [valid] U-labels (if it isn't valid, it isn't a
> U-label), but XN-- labels that aren't U-labels _and_ anything
> else with "--" in the third and forth positions.  The intent
> --which I think actually goes back to IDNA2003 although I don't
> have time to check right now-- is to reserve the entire family
> of labels with hyphens in that position for special purpose use
> with this and special protocols.

My recollection of the discussions that lead up to IDNA2003 was that 
there were quite extensive searches for actually existing "two letters 
followed by two hyphens followed by something else" domain names, 
leading up to a "random selection" of the now well-known xn-- from a 
short list of only 10 or 20 remaining candidates, most if not all of 
them starting with "x". At that time, there was quite a strong concern 
concern to not step on anybody's feet. So the restriction as we now have 
it in IDNA 2008 may not have been as prominent or clear-cut in IDNA 2003.

> The conformance rule is
> essentially that one can either not support IDNA at all or, if
> IDNA is supported, must not look up those labels with other
> prefixes unless the prefix is standardized for some other use
> that is also supported.
>> Thus, the protocol should never fail on lookup for
>> "ad--acta.de". Is that  correct?
> It is not.  See above.
>> * FWIW idnkit-2.2 works according to my expectations, ICU does
>> not.

Can you make sure they get a bug report?

On a related topic, I'm wondering what the situation with respect to "ß" 
is these days in .de. There were lots of discussions, and quite some 
plans, and I'm wondering how far they have proceeded.

Regards,   Martin.

> That is a bug in the former.
> best regards,
>     john
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