Problem in defs for Fake A-Label

Mark Davis mark at
Mon Mar 23 18:57:02 CET 2009

John, there is a problem in defs with Fake A-Label


   Some labels that are prefixed with "xn--" may not be the output of
   the Punycode algorithm, or may fail the other tests outlined below or
   violate other IDNA restrictions and thus are also not valid IDNA-
   labels.  They are called FAKE-A Labels for convenience.-


Fake A-labels, i.e.,
   invalid strings that appear to be A-labels but are not, MUST NOT be
   placed in DNS zones that support IDNA.

But in ASCII-LABEL for Fake A-Label you have:

             (3) Note that a Fake A-Label has a prefix "xn--"
                    but the remainder of the label is NOT the valid
                    output of the Punycode algorithm.

I think what we should have is: XN-Label (as defined), with a subclass
P-Label (the remainder is valid punycode), with a subclass A-Label
(corresponds to U-Label). Then we have:

   - P-Label corresponds 1:1 with Unicode Label (any Unicode string with at
   least one non-ASCII character)
   - A-Label corresponds 1:1 with U-Label

A Fake A-Label is {XN-Label - A-Label}, and not {XN-Label - P-Label}, which
is what (3) says.

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