U-labels, NFC, and symmetry

Peter Saint-Andre stpeter at stpeter.im
Fri Apr 15 17:52:34 CEST 2011

On 4/15/11 9:50 AM, Andrew Sullivan wrote:
> On Fri, Apr 15, 2011 at 07:19:34AM -0600, Peter Saint-Andre wrote:
>> How is it that NFC meets the symmetry requirement, but NFD does not?
> NFC meets the symmetry requirement because a string in NFD just isn't
> a U-label, period.  It might be equivalent to a string in NFC that is
> a U-label, but it is not itself a U-label.
> This is really no different from the other mapping cases.  Given
> locale L and a string in ISO8859-N, you might know how to map string
> S(L,ISO8859-N) into a U-label, at which point you could start doing
> work on it.  But IDNA2008 is silent on that.
> Similarly, given unnormalized Unicode, you could run it through NFC
> (and maybe some other stuff) and get it to be a U-label, but it isn't
> a U-label before that.
> If what you're saying is that you want a definition of D-compatible
> U-label, I am not sure whether that is practical.

I (well, the XMPP folks) *might* want a D-compatible domaineything. I
think we've already determined that such a thing would not be a U-label.


Peter Saint-Andre

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