U-labels, NFC, and symmetry

Andrew Sullivan ajs at shinkuro.com
Fri Apr 8 14:07:03 CEST 2011

On Thu, Apr 07, 2011 at 03:59:05PM -0600, Peter Saint-Andre wrote:

> 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.

Yes.  U

> 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.

I suppose in principle it could have used NFD instead, but it needed
to be one or the other, because the result in principle ought to be
binary equivalent.  I forget, however, exactly why we decided to
prefer NFC (if I ever knew).

> In the meantime, I shall pursue a way to specify XMPP domainparts
> independently of the term U-label.

Hrm.  Don't the domainparts have to be usable in domain name slots?
If so, then specifying as NFD means that they must _always_ be
transformed to be used as part of an IDNA lookup (or to go into the
IDNA2008 transformation, because before you get to that you have to
have a U-label).  Are you sure that's what you want?


Andrew Sullivan
ajs at shinkuro.com
Shinkuro, Inc.

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