Stupid U-label question

Vint Cerf vint at google.com
Tue Aug 19 01:55:01 CEST 2008


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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