EAI: Simplified POP/IMAP downgrading
draft-ietf-eai-simpledowngrade-00

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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