Minimal IDNAbis requirements

Erik van der Poel erikv at
Fri Jan 11 18:46:56 CET 2008

On Jan 11, 2008 8:02 AM, Stephane Bortzmeyer <bortzmeyer at> wrote:
> On Tue, Jan 01, 2008 at 12:53:21PM -0800,
>  Erik van der Poel <erikv at> wrote
>  a message of 115 lines which said:
> > For U-labels, string lengths are numbers of codepoints, I suppose. I
> > wonder if it is necessary to explicitly state that. I.e. as opposed
> > to the number of bytes in the UTF-8 encoding of the U-label, or the
> > UTF-16 encoding, etc.
> Since Unicode labels are never put in the DNS itself, do we really
> need to enforce the DNS size limit?

Stephane, John already responded to this:

"No, actually, given the constraints of the other protocols
involved, the limit on a U-label taken alone would be related to

  max(number-of-codepoints, octet-length-of-utf8-form)

which, in practice, would always be the utf-8 string length.
And that number would need to be less than 63 octets per label
and 255 characters per FQDN.  The problem is that, if we don't
want users to see the A-labels any more often in necessary, we
need to recognize the restrictions of applications protocols and
the presentation forms based on them.   in practice, as we are
discovering with email and the EAI work, it is actually easier
to extend the character set and coding of strings than it is to
change length restrictions."

-- end of excerpt from John

In particular, note the email and EAI work.


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