Comments on IDNA Bidi

Harald Alvestrand harald at
Sun Jan 13 04:26:29 CET 2008

Mark Davis skrev:
> Fair enough.
> If you assume that a URL is valid IDNAbis(draft) when it is actually
> IDNA2003, you will break if
>    1. you assume it is all case folded
>    2. you assume that a non-spacing mark is at the end
>    3. you assume that any of some 3000 characters are not in it
>    4. you assume that ZWJ/NJ is insignificant
>    5. you assume that a trailing combining mark in a label means it is
>       not RTL
>    6. you assume that a
> and so on.
I see where we misunderstand each other now.

The problem identified in bidi is a backwards compatibility problem
because it's a problem that might happen if someone assumes that a
string is IDNA2003 when it's actually IDNAbis, which is the inverse of
the above.

What you are talking about above is what I'd call a *forward*
compatibility problem.


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