local mappings

[Paul Hoffman]

At 8:26 PM -0500 1/26/09, John C Klensin wrote:
>Reasonable people may disagree, but we've gotten very clear
>signals that all of the mappings of IDNA2003 were bad news.

Reasonable people do disagree. The hyperbole about "all of" is particulary silly.

>They create, rather than reduce confusion, especially since
>information is lost in the reverse mappings. 

That is a very old straw man. The mappings were never meant to be two-way.

>If we prohibit mappings in the protocol completely, we have to
>assume that some people/ implementations will do some "obvious"
>mappings anyway.  Regardless of how it is stated, that is what
>gets us to the "local mapping" text that several people have
>(reasonably, IMO) found to be a matter of concern from the
>standpoint of predictability of behavior.
>
>I'm trying to find a way to get rid of, or severely limit, the
>"local mapping" text. 

You may be missing what people are asking for. I hear many people asking for a protocol that creates interoperable implementations. Reasonable people may disagree that that is a good goal.

>One possible way to do it is to find a
>middle ground on what gets mapped -- one that we can explain in
>a clear way to people who are not experts on Unicode (or scripts
>in general) -- while minimizing the total to preserve a response
>to those who told us that non-reversibility was a big problem
>and that they couldn't figure out what was valid in an IDN and
>what was not... a problem that  is complicated by the
>observation that "valid in an IDN" actually means two separate
>things: strings that can be successfully processed into an
>A-label and strings that can be obtained by decoding A-labels
>(thanks to Patrik for identifying that distinction in this
>latest round).

...at which point you are making the same choices we made in IDNA2003.

>For case-mapping, I know how to define the rule and I know how
>to explain it.  The stopping rule is also clear: conversion to
>lower case is straightforward, even people who don't deal with
>computers understand it, and neither Unicode nor IDNA2003
>confuse case operations with compatibility encodings.   And
>virtually anyone who has looked at or tried to use the
>case-containing scripts (again, with or without computers) has a
>basic understanding of the issue.  

So far so good, although you now have contradicted your "all of" hyperbole above.

>By contrast, these peculiar
>"compatibility" relationships -- the characters that are
>different codings for the same thing except when they aren't --
>seem like a different kettle of fish... differences that exists
>because of design decisions made in Unicode or its predecessors,
>rather than differences that are inherent to the writing system.

Exactly. And yet these are precisely the ones that people want local, usually non-interoperable mappings for.

>Or maybe that distinction doesn't hold up, in which case we
>either need to tell the communities who complained about what is
>and is not valid in a domain name --and about ambiguities in
>"valid"-- to just get used to it or we are back to trying to
>define "local mapping" in a way that at least most of us can
>live with.
>
>If the stopping rule isn't "lower case and lower case only", you
>tell me where it is and how we explain it to someone who doesn't
>want to know about Unicode.

Exactly. What is unclear to me is how you can go down this route and still pretend that the result will be noticeably different than the mappings in IDNA2003. Please elucidate.

> > Before we go to IETF Last Call, it seems to me that we should
>> at least achieve rough consensus within the WG whether global
>> mappings (lower-casing, NFKC, possibly others) will be part of
>> IDNA200X or not.
>
>Sure.  But keep in mind that we reached a consensus about that
>as part of the charter process and it was "no mappings". 

As a previous IESG member, how many changes to the charter do you think we can make before we need to recharter?

>I
>think it is worth exploring whether a very narrow set of
>exceptions would provide high leverage.  But reversibility is
>pretty important, at least in some people's minds, and one
>cannot have both extensive global mapping and even a semblance
>of reversibility...

If you require reversibility, then no Unicode mappings (certainly including case mapping) will make sense.