Simplified POP and IMAP Downgrading for Internationalized Email

Versions: 00 01 02 03 04 05 06 07 rfc6858                               
Network Working Group                                   Arnt Gulbrandsen
Internet-Draft                                                March 2012
Intended Status: Proposed Standard
Updates: 3501

                  EAI: Simplified POP/IMAP downgrading

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   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

   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 <>", the rendering might be
   " <invalid@eai.invalid>" or "Fred

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Internet-draft                                                March 2012


   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;", 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

   Fax: +49 89 4502 9758


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Internet-draft                                                March 2012

          (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"

Changes since -00

   Added a rule to handle Subject

   Removed the sentence about unknown:;

   Terminology fixes

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