Re: [idn] opting out of SC/TC equivalence

[David Hopwood <david.hopwood@zetnet.co.uk>]

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tsenglm@cc.ncu.edu.tw wrote:
> Patrik Fältström wrote:
> > When the matching algorithm you envision encounters a character where the
> > tables do not tell what to do about it, what will the algorithm do to it?
> > Pass it through unmodified? Reject it? Does something else which you have
> > not defined yet?
>
> [pass it through unmodified]
> 
> > If you tell us this, and claim that this is acceptable treatment of the
> > 20-40.000 characters not in the tables, we can at least discuss this
> > proposal (whether such a partial TC/SC mapping is acceptable), instead of
> > asking again and again what to do about them.
>
>            As Patrik Fältström described, if it is a mechanism in
> UNICODE consortium, then they have the version update procedures and IDNA
> also treat the version update in UNICODE, why it can not be updated step
> by step to increase it?

Because that could cause existing domain names to break (i.e. fail to resolve
when processed by a new version of nameprep). This would happen when a
name contains a source character for one of the new mappings. In order to
avoid that problem, all authoritative servers with names in the affected
scripts would have to be upgraded before any resolvers or applications. That
is at least undesirable, and may be impractical.

Note that there are rules for updating nameprep and NFKC that ensure the
same problem doesn't happen for canonicalization or case mappings (roughly
speaking, no new mapping can be added from a previously assigned character;
see UAX #15 for a more precise statement of the update policy for NFKC).

- -- 
David Hopwood <david.hopwood@zetnet.co.uk>

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