U-labels, NFC, and symmetry
stpeter at stpeter.im
Fri Apr 15 16:33:07 CEST 2011
On 4/15/11 7:54 AM, Bjoern Hoehrmann wrote:
> * Peter Saint-Andre wrote:
>> To be valid, U-labels and A-labels must obey an important symmetry
>> constraint. While that constraint may be tested in any of several
>> ways, an A-label A1 must be capable of being produced by conversion
>> from a U-label U1, and that U-label U1 must be capable of being
>> produced by conversion from A-label A1. Among other things, this
>> implies that both U-labels and A-labels must be strings in Unicode
>> NFC [Unicode-UAX15] normalized form.
>> How is it that NFC meets the symmetry requirement, but NFD does not?
> This is saying that conversion always produces output that is in NFC. If
> your question is why you can't replace NFC by NFD in the text above then
> that is because conversion can produce output that is in NFC but not in
> NFD. So, "because IDNA requires NFC".
Right. It's not that all internationalization technologies require NFC
based on some abstract symmetry requirements, only that IDNA2008
requires NFC. (However, other technologies might want to use NFC for
consistency with IDNA2008 and RFC 5198 -- that's a separate question.)
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