U-labels, NFC, and symmetry

Bjoern Hoehrmann derhoermi at gmx.net
Fri Apr 15 15:54:13 CEST 2011

* 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".
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