Network Working Group Arnt Gulbrandsen
Internet-Draft March 2012
Intended Status: Proposed Standard
Updates: 3501
EAI: Simplified POP/IMAP downgrading
draft-ietf-eai-simpledowngrade-01.txt
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Abstract
This document specifies a method for IMAP and POP servers to serve
EAI messages to non-EAI clients. The specification is simple, easy to
implement and provides only rudimentary results.
1. Overview
It may happen that an EAI-ignorant IMAP or POP client opens a mailbox
containing EAI messages, or even read EAI messages, for instance when
a user has both EAI-capable and EAI-ignorant MUAs.
While the server can hide the existence of such messages entirely,
doing that can be both tricky to implement and not very friendly to
the user.
This document specifies a way to present such messages to the client.
It values simplicity of implementation over fidelity of
representation, on the theory that anyone who wants accuracy should
use EAI, and implementers' time should be used for implementing EAI
proper.
The server is assumed to be EAI-capable internally. When it needs to
present an EAI message (the "real message") to a non-EAI client, it
synthesizes a non-EAI message containing most of the information and
presents that (the "synthetic message").
2. Information preserved and lost
The synthetic message is intended to convey the most important
information to the user. Where information is lost, the user should
see the message as incomplete rather than modified.
The synthetic message is not intended to convey any EAI information
to the MUA. Nothing parsable is added.
2.1 Email addresses
Each EAI-specific email address in the 14 header fields listed below
is replaced with an invalid email address whose display-name tells
the user what happened.
The format of the display-name is explicitly unspecified. Given an
EAI address "Fred <fred@EXAMPLE.com>", the rendering might be
"fred@EXAMPLE.com <invalid@eai.invalid>" or "Fred
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<invalid@eai.invalid>".
The affected header fields are Bcc, Cc, From, Reply-To, Resent-Bcc,
Resent-Cc, Resent-From, Resent-Sender, Resent-To, Return-Path, Sender
and To. Any addresses present in other header fields are not
regarded as addresses by this specification.
2.2 Mime parameters
Any mime atttribute/value pair (whether in the message header or a
bodypart header) which cannot be presented as-is to the client is
silently excised.
Given a field such as "Content-Disposition: attachment; filename=foo;
signed-off-by=fred@EXAMPLE.com", the field is presented as "Content-
Disposition: attachment; filename=foo".
2.3 "Subject"
If the Subject field cannot be presented as-is, the server presents a
representation encoded as specified in [RFC2047].
2.4 Remaining header fields
Any header field which cannot be presented to the client even after
the modifications in sections 3.1 and 3.2 is silently excised.
3. IMAP-specific details
IMAP offers a way to retrieve the message size without downloading
it, RFC822.SIZE. [RFC3501] requires that this size be exact.
This specification relaxes that requirement: An IMAP server is
permitted to send the size of the real message as RFC822.SIZE, even
though the synthetic message's size differs.
4. POP-specific details
None appear to be needed.
5. Security Considerations
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If the real message contains signed body parts, the synthetic message
may contain an invalid signature.
If any excised information is significant, then that information does
not arrive at the recipient. Notably, the message-id, in-reference-to
and/or references fields may be excised, which might cause a lack of
context when the recipient reads the message.
6. Acknowledgements
John Levine, Kazunori Fujiwara and Chris Newman helped with this
document. I think someone else did too, but cannot find the relevant
mail. Speak up or be forgotten.
9. Normative References
[RFC2047] Moore, "MIME (Multipurpose Internet Mail Extensions) Part
Three: Message Header Extensions for Non-ASCII Text", RFC
2047, University of Tennessee, November 1996.
[RFC3501] Crispin, "Internet Message Access Protocol - Version
4rev1", RFC 3501, University of Washington, June 2003.
10. Author's Address
Arnt Gulbrandsen
Schweppermannstr. 8
D-81671 Muenchen
Germany
Fax: +49 89 4502 9758
Email: arnt@gulbrandsen.priv.no
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(RFC Editor: Please delete everything after this point)
IANA Considerations
This document has no actions for IANA.
Open Issues
Should the message be marked somehow? E.g. by adding a "owngraded"
flag?
Changes since -00
Added a rule to handle Subject
Removed the sentence about unknown:;
Terminology fixes
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