U-labels, NFC, and symmetry

"Martin J. Dürst" duerst at it.aoyama.ac.jp
Fri Apr 8 05:53:58 CEST 2011

On 2011/04/08 6:59, Peter Saint-Andre wrote:

> I'm updating the i18n handling in XMPP, and the XMPP community would
> like to use NFD on the wire for various reasons.

I'm really not sure why NFD would be a good idea. I think it would make 
sense to discuss that seriously, and if possible have various IETF 
technologies stay the same, and preferably NFC. This may not be the list 
for such a discussion, but we should have such a discussion somewhere, 
rather than each protocol going a different way.

Regards,   Martin.

> Ideally we would like
> to do so without requiring a trip through NFC. However, it appears that
> we can do this only by using a term other than U-label, since that is
> tied to NFC. Indeed, it seems that a string in Unicode NFD normalized
> form is not an IDN label at all. This strikes me as unfortunate (I
> thought that normalization was handled only in RFC 5895 along with other
> such mapping issues), but probably because I do not understand how the
> symmetry requirement expressed in RFC 5890 necessitates the use of NFC.
> Would any of the i18n experts on this list care to enlighten me on the
> latter point?
> In the meantime, I shall pursue a way to specify XMPP domainparts
> independently of the term U-label.
> Peter
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