Comments on draft-ietf-idnabis-defs-10

Wil Tan wil at cloudregistry.net
Mon Aug 31 08:09:37 CEST 2009


Agreed. I just came to the same conclusion too.

2009/8/31 John C Klensin <klensin at jck.com>

>
>
> --On Monday, August 31, 2009 07:10 +0200 Patrik Fältström
> <patrik at frobbit.se> wrote:
>
> >
> > On 31 aug 2009, at 07.05, Patrik Fältström wrote:
> >
> >> So, casefold of the ascii in the A-label only result in
> >> casefold of the ascii in the U-label.
> >
> > Ok, after a gulp of coffee, I see your point.
> >
> > In the tables document we have the following:
> >
> >> 2.5. LDH (E) E:
> >>
> >>     cp is in {002D, 0030..0039, 0061..007A}
> >
> > That make for example 'F' illegal in an U-label.
> >
> > By changing this rule to the following, uppercase ASCII would
> > be ok to have in an U-label.
> >
> > 2.5. LDH (E) E:
> >
> >      cp is in {002D, 0030..0039, 0041..005A, 0061..007A}
>
> But that would be a mistake, IMO, because it would result in
> U-labels containing uppercase ASCII that produce different
> A-labels from the same U-label with lowercase ASCII.  Those
> A-labels would match in the DNS, which uses case-insensitive
> comparison, but not in ordinary string comparisons (because they
> are different).
>
> Put differently, that would give us a pair of U-labels that do
> not compare equal on bitstring comparison (the only kind of
> comparison defined for U-labels) but which produce equivalent
> (although not identical) A-labels.  And, because of the
> compression "feature", we would lose unambiguous symmetry of
> A-labels and U-labels because, depending on where it came from,
> a given A-label (and its DNS-equivalents) could produce
> different U-labels.
>
> This would also imply that
>   "Fältström" and "fältström"
> would be valid U-labels but that "FÄLTSTRÖM" would not be
> because Ä and Ö are both DISALLOWED.  I don't think we want to
> go there.
>
>    john
>
>
>
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-- 
Wil Tan
CTO
Cloud Registry <http://www.cloudregistry.net>
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