Network Working Group Arnt Gulbrandsen
Internet-Draft March 2012
Intended Status: Proposed Standard
EAI: Simplified POP/IMAP downgrading
draft-ietf-eai-simpledowngrade-00.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 regular 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
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be "fred@EXAMPLE.com <invalid@eai.invalid>" or "Fred
<invalid@eai.invalid>".
Irregular email addresses (anything to which one cannot send mail,
such as "unknown:;") are silently excised.
The affected header fields are Bcc, Cc, From, ReplyTo, ResentBcc,
ResentCc, ResentFrom, ResentSender, ResentTo, ReturnPath, Sender and
To. Any addresses present in other header fields are not regarded as
addresses by this specification.
2.2 Mime values and comments
Any mime field (whether in the message header or a bodypart header)
which cannot be presented as-is to the client is silently excised
along with its name.
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 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 exactly
correct.
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.
6. Security Considerations
If the real message contains signed body parts, the synthetic message
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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.
8. Acknowledgements
John Levine and Kazunori Fujiwara helped with this document. I think
a third person did too, but cannot find the relevant mail. Speak up
or be forgotten.
9. Normative References
[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
Only those noted as "fixme" in the text.
Changes since -00
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