Comments on draft-ietf-idnabis-defs-10

James Mitchell james.mitchell at
Tue Sep 1 02:17:54 CEST 2009

> (1) Insert a note into Defs that explicitly points out that no
> valid A-label can contain any upper case characters (at least
> after the "xn" -- does the WG have a preference about the
> prefix?)

XN--KGBECHTV is a valid A-label; xn--Bcher-kva is not.  This behaviour is only with the ASCII characters which are not encoded/decoded as part of the punycode algorithm.
> (2) In Protocol, put in reminders in the Punycode invocation
> steps that indicate that, since the U-label cannot contain upper
> case characters, the A-labels that come out will be all lower
> case and that looking up an A-label that contains upper case
> characters is non-conforming and may produce surprising results.

xn--Bcher-kva is not an A-label, however do not think that looking it up will produce surprising results.  Non-IDN domain names are case-insensitive, IDNA2003 case-folded, and we have a mappings document that recommends that characters are case-folded. 

I thought the issue was the symmetry of XN-labels.  XN--BCHER-KVA, whilst equivalent in the DNS, is not equivalent in IDNA2008 because they produce different Unicode strings.  Perhaps something along the lines of 'If one has an XN-label and wants a putative A-label then they must first lowercase.' is all that is needed?


More information about the Idna-update mailing list