my comments on draft-ietf-idnabis-bidi-05

John C Klensin klensin at
Mon Sep 7 17:12:36 CEST 2009

--On Monday, September 07, 2009 4:11 PM +0900 "\"Martin J. 
Dürst\"" <duerst at> wrote:

> Hello Mati,
> On 2009/09/07 15:47, Matitiahu Allouche wrote:
>> On October first, Martin J. Dürst asked:
>> conditions 2/4: Why are BN (control characters) allowed in
>> RTL but not in LTR?
>> BN characters are invisible and should be banned as allowing
>> phishing and violating the Label Uniqueness requirement.
>> However, ZWJ and ZWNJ are classified as BN, and ZWNJ is
>> required for the proper orthography of Persian which is
>> written with the Arabic script, hence BNs are allowed in RTL
>> labels.
> That makes a lot of sense. But then shouldn't BN also be
> allowed for  LTR, because some of these characters are needed
> in Indic scripts?

Remember that ZWJ and ZWNJ are allowed by exception, not because 
they are BN, and that they are classified as CONTEXTJ, not as 
DISALLOWED.  If we continue with that model --and no one has 
argued recently that we should not-- then the relevant question 
for ZWJ/ZWNJ is whether the contextual rules are correctly 
applied to the scripts in which they are needed and not about 
their membership in BN.   If anything in Bidi confuses that, or 
confuses the more general principle that it does not override 
Tables, I would think it needs to be fixed... but I haven't seen 
anything that I read as such confusion.


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