Re: Case sensitivity on the LHS

Roy Badami <[email protected]> wrote:

> So does IDNA permit mixed-case annotation?  I was under the impression
> that at one point the draft forbade the use of mixed-case annotation
> with IDNA, but I can't find that prohibition in the current documents.

IDNA makes no mention of mixed-case annotation or case preservation.

> How exactly does mixed-case annotation work in IDNA?

Officially, it doesn't.  It was too contentious to be mentioned.
The prevailing opinion was that IDNA is complex enough already, and
shouldn't saddle implementors with nonessential complications.

However, IDNA is architected so that mixed-case annotations could be
added in the future in a backward-compatible way.  The ToASCII operation
already has almost enough flexibility for this, because the case of the
letters used to encode the non-ASCII characters is unconstrained.  The
ASCII letters, however, are always lowercased by the Nameprep step,
and Punycode simply copies them, so a strict implementation of ToASCII
cannot preserve the case of ASCII letters for inputs containing some
non-ASCII characters.  And a strict implementation of ToUnicode has no
flexibility at all; the exact Unicode output is completely determined
regardless of any mixed-case in the ACE input.

But ToASCII and ToUnicode don't really need to be that strict to ensure
interoperability.  As long as they output something equivalent to what
the official versions output, that's good enough.  A future update
of IDNA could therefore relax the specification enough to allow for
mixed-case annotations.  Even if such an update never comes, gutsy
implementors could go ahead and do it, and still interoperate with the
strictly conformant implementations and with each other.  That last
sentence is probably heresy.  :)

> You have to somehow pass case information through the nameprep
> process; is the precise algorithm actually spelled out anywhere?

Nope.  Since it's not officially supported, working out all the details
wasn't a priority, and hasn't yet been done.

AMC

P.S. Roy, have you subscribed to the list yet?  You appear to be
participating as actively as anyone.