Filtering at lookup? (was: Re: Historic scripts as MAYBE?)

Martin Duerst duerst at
Wed Apr 30 08:05:33 CEST 2008

I think I understand most positions in the Historic scripts as MAYBE?
thread now. But I think what hasn't been fully discussed is the assumption
that clients should/must filter domain names before resolving them,
and never resolve domain names containing DISALLOWED or UNASSIGNED
characters. This is quite different from IDNA 2003.

As far as I understand, John and Patrik in particular are very
convinced that this is necessary. But I, for one, am absolutely
not convinced this is necessary, and think that we need to have
a really serious discussion about it.

As far as I understand, there were mainly two kinds of arguments
for why clients must check:
1) It makes sure registries/registrars are kept in check, and
   have a strong incentive to not register disallowed or unassigned
2) It help deal with some edge cases related to normalization and

I personally think 1) has some validity, but I won't be convinced
before I see some reports of real examples that produced something
seriously undesirable.

As for 2), whenever I read such arguments, I always felt that they
were so convolved and exceptional that they didn't really make a
serious engineering argument.

Regards,    Martin.

At 07:24 08/04/29, Harald Tveit Alvestrand wrote:

><curmudgeon hat on>
>as I said in our Mountain View meeting, eliminating MAYBE can very easily lead us to discover that we have turned DISALLOWED into MAYBE.
>The words you used in Mountain View, Mark, were that it would take a "major disaster" before we had to turn a codepoint that was DISALLOWED into ALLOWED. On that common understanding, we agreed that we could live without the MAYBE categories, and just have DISALLOWED, ALLOWED, UNASSIGNED and the context rule categories.
>I agree that we need to document when, how and why we will allow turning DISALLOWED into ALLOWED, and by what procedure we will tell all the users of burned-in-ROM applications that there now exist DNS names they can't look up.
>I hope we don't need to go there. But I'm not optimistic.
>                  Harald
>Idna-update mailing list
>Idna-update at

#-#-#  Martin J. Du"rst, Assoc. Professor, Aoyama Gakuin University
#-#-#       mailto:duerst at     

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