Lookup for reserved LDH labels

[Marcos Sanz]

John,

additional remarks inline:

> > On my side, I claim that that restriction simply does not
> > apply because  "ad--acta.de" is not a "putative U-label", in
> > fact it is no U-label at all  (cf. U-Label definition in RFC
> > 5890, Section 2.3.2.1).
> 
> The term "putative U-label" was used precisely to cover the cast
> of things that could not possibly be "U-labels".  That includes
> not only [valid] U-labels (if it isn't valid, it isn't a
> U-label), but XN-- labels that aren't U-labels _and_ anything
> else with "--" in the third and forth positions.  The intent
> --which I think actually goes back to IDNA2003 although I don't
> have time to check right now-- is to reserve the entire family

IDNA2003's ToASCII("ad--acta.de") did not fail. It was an aware decission 
built-in in the design of the algorithm; cf RFC 3490 Section 4.1, 1st and 
4th algorithm steps:

1. If the sequence contains any code points outside the ASCII range 
(0..7F) then proceed to step 2, otherwise skip to step 3.
4. If the sequence contains any code points outside the ASCII range 
(0..7F) then proceed to step 5, otherwise skip to step 8.

These steps short-circuit the algorithm for all ASCII labels.

> > Thus, the protocol should never fail on lookup for
> > "ad--acta.de". Is that  correct?
> 
> It is not.  See above.

I still disagree with that. See my other mail.

Best,
Marcos