Comments on IDNA Bidi

Harald Alvestrand harald at
Sat Jan 12 09:43:15 CET 2008

Mark Davis skrev:

(deleting matters already covered in the exchange with Ken)
>     >
>     > Bidi-5.
>     >    One particular example of the last case is if a program
>     chooses to
>     >    examine the last character (in network order) of a string in
>     order to
>     >    determine its directionality, rather than its first; if it
>     finds an
>     >
>     >    NSM character and tries to display the string as if it was a
>     left-to-
>     >    right string, the resulting display may be interesting, but not
>     >    useful.
>     >
>     > I don't understand this paragraph. When and why would this
>     happen with
>     > IDNA-conformant programs?
>     >
>     I think the text is clear enough - if you get a label "ALEF BET <some
>     NSM character>", an IDNA2003 program can look at the last character in
>     the string and say "this is not a RTL string", and treat it as if
>     it was
>     LTR. In IDNA2003, that will be a safe assumption. In IDNAx, it
>     will not
>     be a safe assumption.
> I find that a bit odd. The case you are taking is
> A program is looking at an IDNAbis URL, and thinks that it is a valid
> IDNA2003 URL, and makes some assumptions about it, and things break.
> This case that you mention is just a tip of a iceberg. There are a
> *very* large number of assumptions that a program can make about
> IDNA2003 that will completely break under IDNAbis (as currently
> drafted). Many, many things would break, not just this, and not just
> this in BIDI. So I don't see why you are just calling out this one.
Mark, this is not helpful.

Speaking for IDNAbis-bidi ONLY:

This is the *one* concrete example that people have come up with where
an implementor could make a choice that might be reasonable to make in
some context and actually have a concrete difference in behaviour
between IDNA2003 and IDNAbis-bidi apart from the obvious one (that more
characters are permitted). The concern was raised, and the text got added.

If you can come up with another reasonable implementation choice that
people could make because of IDNA2003 that would cause a difference in
behaviour under IDNAbis-bidi that is not obvious, state it.


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