M-label definition

[Paul Hoffman]

At 3:41 PM -0400 4/8/09, John C Klensin wrote:
>The above assumes something that I don't think has been
>explicitly discussed, which is that there are two ways to do
>this "lookup mapping" job:

This has been discussed a few times in the last month.

>(1) View it strictly as a backward-compatibility mechanism, to
>be applied only if IDNA2008 lookup (with no mapping) fails.
>That guarantees two lookups for any string that is valid under
>IDNA2008 but not found and that is at least syntax-valid under
>IDNA2003.  Even here, the mappings that are permitted need to be
>designed so that the targets of the mappings don't un-do
>IDNA2008 decisions.
>
>(2) Try to devise a new mapping table that can be applied before
>conversion to A-label form and actual DNS resolution.  This
>would have to be an entirely new mapping table constructed more
>or less along the lines Martin has outlined -- lower-case, width
>corrections, and a few other things, but not fonts, superscripts
>or subscripts, boxed or circled characters, etc.

There are more options than that. One that has already been discussed is "map all the time, not using a table". Another is "Do a modified second lookup if the IDNA2008 lookup fails", where the modification is "don't follow the IDNA2003 rules for eszett and final sigma".

It may be a good time for some to start listing the proposals. Or, it might be premature. However, it is clear that some of the active participants are not able to keep all the informal proposals in our heads.