Protocol Action: 'Right-to-left scripts for IDNA' to Proposed Standard

Slim Amamou slim at
Tue Feb 16 17:05:59 CET 2010

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On Mon, Feb 15, 2010 at 8:16 PM, Patrik Fältström <patrik at> wrote:
> This is not possible to achieve as we have a requirement that the domain names are to always be passed around in logical order, as we do not know what (in free flowing text for example) what is the beginning and end of a domain name.

This is nice. It looks like I can mathematically demonstrate that
labels can only be in network order given that assertion.

> 3.  The requirement set for the BIDI rule
> (...)
> Label Uniqueness: No two labels, when presented in display order
>      in the same paragraph, should have the same sequence of characters
>      without also having the same sequence of characters in network
>      order, both when the paragraph has LTR direction and when the
>      paragraph has RTL direction.

let ABC 3 different labels in network order A != B !=C
let JKL their LTR display order J != K != L
let MNO their RTL display order M != N != O

given the requirement above :

J = M           J = M = A
and       =>
K = N           K = N = B           JKL = ABC
K = N           K = N = B           MNO = ABC
and       =>
L = O           L = O = C

                            L = N => L = K which is absurd
suppose L != O =>  or
                            L = M => L = J which is absurd

Note that this is a recursive demonstration : I demonstrated for N+1
labels which means that a domain name with any number of labels must
be displayed in network order to satisfy the requirement.

Please review this demonstration carefully, as I did not do maths
since 1996 (and I was not really good at it).

> And, this is one of the reasons various recommendations say that one should not mix RTL and LTR in domain names, labels etc. If you do not mix directionality, you will not get any problems, right?

Right. But this will be problematic since current CC-TLDs are RTL
including those of countries using RTL languages.

Slim Amamou | سليم عمامو

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