Stupid U-label question

Eric Brunner-Williams ebw at abenaki.wabanaki.net
Tue Aug 19 16:50:00 CEST 2008


Is it a prefix if its an infix?

Its early in the morning for me, but if the string fails the "^xn--.*$" 
test (dot in the re sense, not as a label separator, or worse, a leaky 
period), why should anyone care?

Eric

Vint Cerf wrote:
> John,
>
> I can't imagine that anyone would benefit from a more liberal  
> definition of constraint on strings of the form xn--, so I am in  
> agreement with you.
>
> v
>
> On Aug 18, 2008, at 7:16 PM, John C Klensin wrote:
>
>   
>> --On Tuesday, 19 August, 2008 01:12 +0200 Frank Ellermann
>> <hmdmhdfmhdjmzdtjmzdtzktdkztdjz at gmail.com> wrote:
>>
>>     
>>> Hi, triggered by an error - I tried a punycode decode
>>> instead of an IDNA2003 decode - I tested the effect
>>> of xn--xn--4ca-cxa with two tools, both told me "this
>>> is no A-label".
>>>
>>> In other words xn--4caä (ending with an umlauted a)
>>> is no U-label.  I wasn't aware that U-labels must not
>>> start with "xn--", is that mentioned in the drafts ?
>>>       
>> I don't remember what IDNA2003 says, if anything, but the
>> IDNA2008 drafts very clearly (even more clearly in  the versions
>> I'm trying to wrap up and post, I think) say that, if an
>> application is IDNA conforming, and a label appears in a domain
>> name context, then anything starting with "xn--" is required to
>> be an A-label.
>>
>> I think not having such a rule leads to bad places with no real
>> benefits, unless one believes that a someone might actually need
>> a U-label of xn--4caä for a legitimate, non-demonstration,
>> purpose.
>>
>>    john
>>
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>
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