[idn] Last call comments to nameprep/stringprep: BIDI

[Martin Duerst <duerst@w3.org>]

Currently, neither draft-ietf-idn-nameprep-07.txt nor
draft-hoffman-stringprep-00.txt deal with bidirectionality
(mixing right-to-left (Arabic/Hebrew) and left-to-right
writing directions) issues. This should be changed as soon
as possible.

If a label can contain both right-to-left and left-to-right
characters, how it will be displayed, and how displayed
labels will be entered and looked up in the DNS, is highly
context-dependent. This is obviously very undesirable.

The following is a proposal written up by Mark Davis,
based on input from others:


 >>>>
A. Characters are classified into RTL, LTR, DIGIT, OTHER.

These categories are drawn from the BIDI algorithm. The precise lists of 
characters in each category would be added to NamePrep as an appendix. The 
composition is as follows (See 
<http://www.unicode.org/reports/tr9/#Bidirectional_Character_Types>http://ww 
w.unicode.org<http://www.unicode.org/reports/tr9/#Bidirectional_Character_Ty 
pes>/reports/tr9/#Bidirectional_Character_Types).

LTR   := L ; # including LRM

RTL   := R | AL ;

DIG   := EN | AN ;

OTH := all other characters: NSM, ON, etc.

Note: The characters in categories LRM, RLM, LRO, RLO, LRE, RLE, PDF, B, S, 
and some other BIDI categories are prohibited anyway.


B. In any field that contains any RTL characters:
B0. no LTR characters can occur.
C1. a sequence of characters of type DIG can only occur at the end.
C2. a sequence of characters of type OTHER can occur only between 
characters of type RTL.
 >>>>

I propose that this be added as an additional step after the current
'prohibition' step.

Regards,    Martin.