referencing IDNA2008 (and IDNA2003?)

John C Klensin klensin at
Thu Oct 21 23:28:40 CEST 2010

--On Thursday, October 21, 2010 17:08 -0400 Andrew Sullivan
<ajs at> wrote:

> Why not just say that you should use whatever IDNA rules the
> application implements, following all the rules for domain
> comparison in that implementation?  (Note that IDNA2003 domain
> name comparison turned out to be slightly troublesome, because
> people sometimes compared what we came to call U-labels and
> other times compared what we came to call A-labels.  But like
> someone else said up-thread, the implementation is already
> relying on whatever it is relying on.)


And, fwiw, IDNA2003 is absolutely clear that the only valid
comparisons are among what we now call A-labels.  So an IDNA2003
implementation trying to compare U-labels directly is simply
non-conforming or, to use the more precise term, broken.


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