John C Klensin klensin at
Thu Sep 3 20:15:51 CEST 2009

--On Thursday, September 03, 2009 11:41 -0400 Andrew Sullivan
<ajs at> wrote:

> Given the results of the previous discussion, I'm still fairly
> certain that the existing definitions prohibit A-labels that
> have any upper case letter in them.  But I concede that this
> depends on certain non-obvious premises, and therefore an
> implementer who really wanted to get upper case ASCII into an
> apparent A-label would have an argument that they were
> conforming with the protocol as it stands.  I therefore think
> it would be nice to add a sentence to Defs that reminds the
> reader that A-labels turn out always to be lower case, (maybe
> it ought to go in Rationale instead).  My suggestion is
> something like the following: "A consequence of the
> restrictions on valid characters in U-labels turns out to be
> that mixed-case annotation, of the sort outlined in [RFC3492]
> Appendix A, is never useful.  Therefore, since a valid A-label
> is the result of Punycode encoding of a U-label, A-labels
> always happen to be only lower case, despite matching other
> (mixed- or upper-case) potential labels in the DNS."

This works for me if others find it agreeable.  If we are trying
to warn implementers, it should go into Defs because our
hypothesis when we split up the document was that implementers
do not need to read Rationale closely and may not do so at all.


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