Comments on draft-ietf-idnabis-defs-10

Mark Andrews marka at
Wed Sep 2 04:06:31 CEST 2009

In message <20090901165041.GK14929 at>, Andrew Sullivan writes:
> On Wed, Sep 02, 2009 at 02:30:08AM +1000, Wil Tan wrote:
> > >
> > Agreed. I'm not actually advocating special rules for A-label matching, jus
> t
> > pointing an inconsistency where label1 is a valid A-label, and is equivalen
> t
> > to label2 which is an invalid A-label.
> Yes, this is a strange property of A-labels, but it's ok: A-labels are
> a subset of LDH-labels.  Until you drew attention to this, it wasn't
> obvious to me that even if different LDH-labels matched, one of them
> being an A-label was not enough to make the rest of them A-labels.
> It's still ok, but it is a subtle point, and the sort of sharp corner
> that can snag when people implement for sure.
> > If the A-label qualification is just a definition, it wouldn't matter much.
> > But it's how we define registration and lookup behavior where A-label is
> > concerned that I'm afraid this could cause unintended consequences in
> > software implementations.
> > 
> > As it is currently defined, IDNA2008 protocol allows a conforming
> > applications to behave in different ways (even without mapping).
> This is exactly what I am denying.  I think that the definitions are
> complete enough that truly conforming applications will never have
> this situation.  (There is, however, a nasty corner case as a result
> of the 0x20 stratagy: as currently defined, no IDNA2008 domain is
> compatible with the 0x20 strategy, which is an important thing for
> DNSEXT to hear.)

I don't draw that conclusion.  The upper layers *already* have to
lower case the domain names in the RR's to correct reverse the
mapping even is 0x20 is not being used as there is no requirement
for DNS servers to lowercase domain names the responses they return.

0x20 is not a issue here.   Current DNS servers should do more case
preservation than they currently do but that is not a issue for
this WG.

> Of course, it is quite likely that, working from the
> current text, an implementation might not be "truly conforming",
> because this subtle point (matching LDH-labels, one of which is an
> A-label, need not all be A-labels) could get overlooked.  I claim that
> conforming applications won't behave in different ways because upper
> case ASCII characters turn out not to be allowed in A-labels.  That's
> pretty amazing, but it appears to be true.
> A
> -- 
> Andrew Sullivan
> ajs at
> Shinkuro, Inc.
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> Idna-update at
Mark Andrews, ISC
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PHONE: +61 2 9871 4742                 INTERNET: marka at

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