Re: [ietf-dkim] Re: Use of LWSP in ABNF -- consensus call

John C Klensin <john-ietf@jck.com> Fri, 18 May 2007 14:03 UTC

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Date: Fri, 18 May 2007 10:03:31 -0400
From: John C Klensin <john-ietf@jck.com>
To: Tony Finch <dot@dotat.at>
Subject: Re: [ietf-dkim] Re: Use of LWSP in ABNF -- consensus call
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--On Friday, 18 May, 2007 09:00 +0100 Tony Finch <dot@dotat.at>
wrote:

> On Thu, 17 May 2007, John C Klensin wrote:
>> 
>> After all <CRLF> Thing <SPACE><CRLF> could case similar
>> problems if some construction permitted it ...
> 
> This is not news. There have for a long time been problems with
> significant trailing space, which is why CRLF 1*WSP CRLF in a
> header is part of the obs- syntax of 2822, and why
> quoted-printable encodes WSP at the end of a line.
> 
>> ... and defining a grammar that would prohibit any
>> <SPACE><CRLF> construction isn't easy in ABNF for reasons
>> that have nothing to do with LWSP.
> 
> This is simply incorrect. It's trivial to define a whitespace
> construction that only allows CRLF at the beginning of a
> sequence:
> 
> 	NTWSP = [CRLF] 1*WSP ; non-trailing white space

Sure.  Except that much, if not most, of our textual
descriptions of these protocols describes lines, and line-like,
constructions as _ending_ in CRLF.  Moving to  "starting in
CRLF" creates a conceptual difference between prose definition
and formal syntax, which strikes me as a bad idea.   Of course,
in the above, since [CRLF] is optional, "1*WSP" alone satisfies
the production as written and still does not prevent
    <CRLF> space space space 
    <CRLF>
unless _all_ productions are written CRLF first... or one relies
on the comment as a restriction.   And a comment as the
restriction --in the prose-- is exactly what has been suggested,
IMO.

      john