Re: [ietf-smtp] Stray <LF> in the middle of messages

John C Klensin <john-ietf@jck.com> Wed, 10 June 2020 15:16 UTC

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Date: Wed, 10 Jun 2020 11:15:59 -0400
From: John C Klensin <john-ietf@jck.com>
To: Paul Smith <paul@pscs.co.uk>, ietf-smtp@ietf.org
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Subject: Re: [ietf-smtp] Stray <LF> in the middle of messages
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Paul, Keith,

A few small additions (no disagreement)...

--On Wednesday, June 10, 2020 14:10 +0100 Paul Smith
<paul@pscs.co.uk> wrote:

> On 10/06/2020 12:33, Keith Moore wrote:
>> 
>> The CRLF convention for transfer of Internet text files (in
>> FTP, which  was the first protocol used to send email on the
>> ARPAnet), predates  the existence of both PC-DOS and MacOS by
>> several years.   I am  guessing that FTP got its
>> end-of-line convention for text files from  TELNET.

It also, and it is probably relevant, predates Unix, certainly
any general availability of Unix, by some years.

> Strictly speaking, CRLF is the "right" one to use given those
> control codes' definitions.

And those definitions were very clear in the first version(s) of
ASCII, including the one against RFC 20 was written.   They got
somewhat fuzzier as assorted systems had conventions of their
own and the language of the standard was revised and adapted.
 
> CR takes you back to the start of the current line, and LF
> takes you to the next line (at the current position), so to
> get to the start of the next line, you need either CRLF or
> LFCR (AIUI CRLF was chosen to give the print head more time to
> go back to the start of the line).
> 
> I remember sending CR (without LF) to a printer, and then
> reprinting the last line to make it print 'bold'.

Also approximating certain Latin script diacritical and other
markings by printing the line or ASCII letters and then going
back and printing accent marks, tilde, "/", comma as an
approximation to cedilla, and so on over them.  That particular
mechanism was also the source of the non-destructive backspace
(about which the standard was also very clear even if common
practice differed).

   john