[lsb@lsb.org: [EAI] (summary) display of RightToLeft chars in localparts and hostnames]

[John C Klensin]

--On Thursday, 07 December, 2006 09:36 +0100 Harald Alvestrand
<harald at alvestrand.no> wrote:

> --On 7. desember 2006 13:01 +0900 Soobok Lee <lsb at lsb.org>
> wrote:
>
>> I found this section in stringprep2003:
>> 
>> <quote from section 5.7>
>>  5.8 Change display properties or are deprecated
>> 
>>    The following characters can cause changes in display or
>>    the order in which characters appear when rendered, or are
>>    deprecated in Unicode.
>> 
>>    200E; LEFT-TO-RIGHT MARK
>>    200F; RIGHT-TO-LEFT MARK
>>    202A; LEFT-TO-RIGHT EMBEDDING
>>    202B; RIGHT-TO-LEFT EMBEDDING
>>    202C; POP DIRECTIONAL FORMATTING
>>    202D; LEFT-TO-RIGHT OVERRIDE
>>    202E; RIGHT-TO-LEFT OVERRIDE
>>    206A; INHIBIT SYMMETRIC SWAPPING
>>    206B; ACTIVATE SYMMETRIC SWAPPING
>>    206C; INHIBIT ARABIC FORM SHAPING
>>    206D; ACTIVATE ARABIC FORM SHAPING
>> </quote>
>> 
>> My suggestion for new stringprep200x is to move these chars
>>   to "mapped to nothing lists". that is, how about deleting
>>   silently them instead of prohibiting them and returning
>>   error ?
> 
> Any string that contains them will (one assumes) depend on
> their correct interpretation for correct display.
> 
> Mapping them out and letting people use the resulting string
> powerfully violates the principle of least astonishment; if I,
> for reasons of my own, choose to send in the string (in
> network order) <RLO> D N A R T S E V L A <RLO>, expecting to
> see the display ALVESTRAND, I will be astonished if the result
> is DNARTSEVLA.

It is easy to get trapped by this very interesting discussion,
which is precisely the reason I have suggested it be taken off
this list.  While my UI intuition causes me to agree with Harald
that the latter would be pretty astonishing (and silly), it is
ultimately up to the delivery MTA (and the associated alias and
other configuration tables) as to whether any or all of the
following are treated as synonyms for "harald.alvestrand":
   Harold.AlvestranD
   harald.dnartsevla
   Harald.Alvestrand
   harald
   חרד
   alvestrand
   former-ietf-chair

Now, clearly some of those are more sensible than others.  Some
are more obvious than others.  Some would cause astonishment.
But it is up to him and the server which ones are accepted.  And
any sending MUA that tries to make a guess, especially a guess
based on transforming one form into another is in violation of
2821.

    john