Re: [ietf-822] Question about line folding and maximum line length

Hector Santos <hsantos@isdg.net> Fri, 07 May 2021 00:47 UTC

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Date: Thu, 06 May 2021 20:33:13 -0400
From: Hector Santos <hsantos@isdg.net>
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Organization: Santronics Software, Inc.
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To: Bryan Frimin <bryan=40frimin.fr@dmarc.ietf.org>, ietf-822@ietf.org
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Subject: Re: [ietf-822] Question about line folding and maximum line length
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On 5/6/2021 9:52 AM, Bryan Frimin wrote:
> Hello everyone,
>
> I am implementing an e-mail client. I have to implement the SMTP
> protocol, starting with the internet message format defined by RFC
> 5322[1].
>
> After reading the RFC, I found two points that are not clear to me.
>
> 1. The maximum line length.
> RFC 5322 section 2.1.1[2] describes two possible line length limits. The
> historical one, 78 characters, and the new one, 998 characters. Is the
> historical limit still relevant for an email client in 2021, or can I
> just use the new one?

For SMTP (RFC5321 et al) Internet Mail (RFC5322 et al ) 
transportation, MTA (client/server),  998+EOL is the requirement. EOL 
is <CR><LF> (DOS-based easy to remember). If a line is ending as 
Unix-based <LF> or MAC-based <CR>, then I/O translation must be taken 
into account.

For DISPLAY purposes, the MUA, Mail User Agents, SMTP should at least 
handle old school 80 columns, that meant 78+EOL.  It was always tricky 
to deal with column soft vs hard wraparounds ideas.

> 2. The long line folding mechanism.
> RFC 5322 section 2.2.3[3] defines a folding mechanism for long header
> fields. But it does define whether it is possible to fold a header field
> name longer than the limit (yes it is an edge case, but it could
> happen). And folding can only be done if a lexical token or a space is
> present. So what about values without any separator ?
>

Serious? A Header Field Bame longer than the line limit?   That would 
instantly cause breakage somewhere. So I don't know off hand if the 
doc makes a mention of the Header Field Name limit, but if not, this 
is the group to make that clarification in the ABNF.

The reality is that there are remote hosting limits that are external 
to RFC5822, perhaps even predated RFC5322 header fields, especially 
the top header fields from the Host/User Mail Reader/Writer MUA 
standpoint and for all telecommunications;

From:
To:
Subject:
Date:

All others are network or some meta-related thing,  perhaps, but there 
are legacy and established systems that has limits based on display 
designs. Much shorter than 998,  How will any device display long 
fields over 80 or extended graphically like 132 that the early smarter 
terminals offered?

Overall, there is no standard on displays, yet, because there are so 
many different ways to do it. Smart Watches?

-- 
Hector Santos,
https://secure.santronics.com
https://twitter.com/hectorsantos