Casefolding Sigma (was: Re: IDNAbis Preprocessing Draft)

Patrik Fältström patrik at
Mon Jan 28 08:52:32 CET 2008

The tables document explain what codepoints can be in a U-label. After  
reading what all of you have written, I see three different suggestions:

(1) Keep final sigma as it is today, NEVER, as casefold(final sigma) ! 
= final sigma
(2) Have final sigma as an exception, CONTEXT
(3) Have final sigma as an exception, MAYBE NO

I have read the email on this list, and my proposal for conclusion of  
consensus is the following:

Given that some people (and the Unicode Standard) say final sigma in  
some context might be mapped to sigma (casefolding, context dependent  
etc) it would be pretty bad if someone actually register a domain name  
with final sigma. This because people that use clients that "happen"  
to (based on context or whatever else) map this to sigma will not get  
a match when looking up the domain name.

Because of this, and the fact I really want to minimize the amount of  
exceptions, I find the conclusion is that final sigma should stay as a  
non-exception, i.e. alternative (1) above, which imply it will be in  
NEVER and because of that not allowed to be registered in DNS.

That said, any preprocessing, user interface etc, can of course allow  
final sigma and map it to something that is appropriate according to  
whatever application, context, locale or such. Rules that are  
impossible to implement in the global DNS.

Next version of the tables document will because of that NOT say  
anything special about final sigma.


P.S. I have though found some more bugs in my script(s) that generate  
the non-normative tables in the tables document. I have because of  
that now falled back from using my own code to use the Unicode  
libraries in perl. If people know about any problems with that, let me  

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