Re: [I18ndir] [art] Modern Network Unicode

John C Klensin <john-ietf@jck.com> Fri, 12 July 2019 02:36 UTC

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Date: Thu, 11 Jul 2019 22:36:28 -0400
From: John C Klensin <john-ietf@jck.com>
To: Carsten Bormann <cabo@tzi.org>, Ira McDonald <blueroofmusic@gmail.com>
cc: art@ietf.org, "Asmus Freytag (c)" <asmusf@ix.netcom.com>, i18ndir@ietf.org
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Subject: Re: [I18ndir] [art] Modern Network Unicode
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--On Friday, July 12, 2019 01:46 +0200 Carsten Bormann
<cabo@tzi.org>; wrote:

> This is a great discussion.
> 
> To me, it seems to converge on the following.
> 
> (1) Sending sane data is the job of the data originator.  
> 
> (2) Do not include gratuitous normalization steps in your
> processing, once the data have been originated in a sane form.
> 
> (2a) If you broke it, you fix it (as far as possible): If your
> processing steps did involve gratuitous normalization, you
> have to renormalize to NFC before sending.
> 
> Here, "sane" is defined as:
> 
> (0) Data SHOULD be originated in NFC, unless that would be
> inappropriate for the specific script, in which case the
> community consensus rules for the script govern.
> 
> For Latin script, this happens to collapse to what 5198 says.
> 
> This set of rules places the onus on the place where the data
> is generated, which is usually the place that knows most about
> the specific script and about the intent of the originator.
> If you know that place isn't doing its job, add the rule:
> 
> (1a) If the data originator does not do (0), the software
> placing the data on the network may need to sanitize
> (normalize towards sane).
> 
> 1a is similar to 2a in that it doesn't create perfect
> results, so both SHOULD be avoided — there is no way to,
> after the fact, perfectly sanitize data that weren't
> originated sane or that were gratuitously normalized on the
> way.
> 
> With these definitions, MNU can direct towards: 
> (A) Senders: send sane data
> (B) Recipients: break as little as reasonable when data
> received isn't sane 
> (C) B is not a valid excuse not to do A,
> and specifically: recipients are not expected to clean up
> after senders (because there is no correct way to do that).
> 
> (Rule C is the often forgotten third rule of the Postel
> principle. It also means that an entity that is a recipient of
> MNU and then sends the data on as MNU has no need to
> gratuitously normalize, but it does not entirely get rid of
> rule 1a for recipients of data from places known not to be
> sane.)

I think this is exactly right.  And, fwiw, I agree with your
observation about the Postel principle.

   john