referencing IDNA2008 (and IDNA2003?)

Adam Barth ietf at
Fri Oct 22 22:21:46 CEST 2010

On Fri, Oct 22, 2010 at 1:18 PM, John C Klensin <klensin at> wrote:
> --On Friday, October 22, 2010 13:05 -0700 Adam Barth
> <ietf at> wrote:
>>> Or, it can take a look at the strings, discover whether there
>>> are non-ASCII characters present, and, if they are, simply
>>> reject that putative label as bogus.  I have a personal
>>> preference (partially based on the "private encoding" theme of
>>> draft-iab-i18n-encoding and the small risk of a false
>>> positive) but, again, that is a decision the WG should make
>>> or leave to the UA.
>> No such sniffing is required.
> You have to sniff a little bit.  The problem is that, if you
> apply "various IDNA algorithms to Y", the effective first step
> under IDNA2008 is to determine whether there are non-ASCII
> characters present.  If there are not, there is no IDNA
> algorithm to apply.  Now you could build a perfectly conforming
> IDNA2008 implementation that would behave pretty much like
> ToASCII does in that regard, i.e., return the plain-ASCII string
> if the input was plain ASCII, return the A-label if the string
> was a U-label, and return a failure indication otherwise.   But,
> as far as the Standard is concerned, you cannot apply IDNA2008
> conversion to an A-label to an all-ASCII string.

Ah, I see.  Is there specific phrase I should use in referencing
IDNA2008 that indicates that UAs should behave that way?


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