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

      Simplified POP/IMAP Downgrading for Internationalized Email

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

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Internet-draft                                               August 2012


   This document specifies a method for IMAP and POP servers to serve
   internationalized messages to conventional clients. The specification
   is simple, easy to implement and provides only rudimentary results.

1. Overview

   It may happen that a conventional IMAP or POP client opens a mailbox
   containing internationalized messages, or even attempt to read
   internationalized messages, for instance when a user has both
   internationalized and conventional MUAs.

   Some operations cannot be performed by conventional clients. Most
   importantly, an internationalized message usually contains at least
   one internationalized address, so address-based operations are only
   rarely possible. This includes displaying the addresses, replying,
   and most types of address-based signature or security processing.

   Still, the sender's name, the message subject, body text and
   attachments can easily be displayed, so a helpful IMAP/POP server may
   prefer to provide access to what it can rather than hide the message

   This document specifies a way to present such messages to the client.
   It values simplicity of implementation over fidelity of
   representation, since implementing a high-fidelity downgrade
   algorithm is likely more work than implementing proper support for
   [RFC5721] and/or [RFC5738].

   The server is assumed to be internationalized internally, and to
   store messages internationalized messages natively. When it needs to
   present an internationalized message to a conventional client, it
   synthesizes a conventional 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 information to
   the client software that would require or enable it to apply special
   handling to the message. Client authors who wish to handle
   internationalized messages are encouraged to implement [RFC5738].

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   Upper case in examples represents non-ASCII. example.com is a plain
   domain, EXAMPLE.com represents a non-ASCII domain in the .com top-
   level domain.

2.1 Email addresses

   Each internationalized email address in the 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. Anything
   which tells the user what happened is good. Anything which produces
   an email address which might belong to someone else is bad.

   Given an internationalized address "Fred Foo <fred@EXAMPLE.com>", an
   implementation may choose to render it e.g. as these examples:

      "fred@EXAMPLE.com" <invalid@internationalized-address.invalid>
      Fred Foo <invalid@internationalized.invalid>

   (The .invalid top-level domain is reserved by [RFC2606], therefore
   the first two examples are syntactically valid, but will never belong
   to anyone. Note that the display-name often will need [RFC2047]

   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, such as
   Received, are not regarded as addresses by this specification.

2.2 MIME parameters

   Any MIME parameter [RFC2045] (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

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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 2.1-2.3 is silently excised.

3. IMAP-specific details

   IMAP allows clients to retrieve the message size without downloading
   it, using RFC822.SIZE, BODY.SIZE[] and so on. [RFC3501] requires that
   the returned size be exact.

   This specification relaxes that requirement: When a conventional
   client requests size information for a message, the IMAP server is
   permitted to return size information for the internationalized
   message, even though the synthetic message's size differs.

   When an IMAP server carries out downgrading as part of generating
   FETCH responses, it reports which messages were synthesised using a
   response code and attendant UID set. This can be helpful to humans
   debugging the server and/or client.

        S: 1 FETCH (UID 65 [...]
        S: 2 FETCH (UID 70 [...]
        S: a OK [DOWNGRADED 70,105,108,109] Done

   The message-set argument to DOWNGRADED contains UIDs.

   Note that DOWNGRADED does not necessarily mention all the
   internationalized messages in the mailbox. In the example above, we
   know that UID 65 does not contain internationalized addresses in
   From, To and Cc. It may contain an internationalized Subject, etc.

4. POP-specific details

   The number of lines specified in the TOP command (see [RFC1939])
   refers to the synthetic message. The message size reported by e.g.
   LIST may refer to either the internationalized or the synthetic

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Internet-draft                                               August 2012

5. Security Considerations

   If the internationalized message uses any sort of signature, the
   synthetic message's signature almost certainly is invalid.  This is a
   necessary limitation of displaying internationalized messages in
   conventional clients, since the client does not support
   internationalized addresses.

   If any excised information is significant, then that information does
   not arrive at the recipient. Notably, the Message-Id, In-Reference-To
   and References fields may be excised, which might cause a lack of
   context when the recipient reads the message.

   Some POP or IMAP clients, such as Fetchmail, download messages and
   delete the version on the server.  This may lead to permanent loss of
   information when the only remaining version of a message is the
   synthetic message.

   Other clients cache messages for a very long time, even across client
   upgrades, such as the stock Android client. When such a client is
   internationalized, care must be taken so that it will not use an old
   synthetic message from its cache rather than retrieve the real
   message from the server.

6. Acknowledgements

   Claudio Allocchio, Ned Freed, Kazunori Fujiwara, Ted Hardie, John
   Klensin, Barry Leiba, John Levine, Alexey Melnikov, Chris Newman,
   Joseph Yee and the originator of rule 12 in [RFC1925] helped with
   this document.

7. IANA Considerations

   The IANA is requested to add DOWNGRADED to the IMAP Response Code

8. Normative References

   [RFC1939]  Myers, J and M. Rose, "Post Office Protocol - Version 3",
              RFC 1939, May 1996.

   [RFC2045]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part One: Format of Internet Message
              Bodies", RFC 2045, November 1996.

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   [RFC2047]  Moore, "MIME (Multipurpose Internet Mail Extensions) Part
              Three: Message Header Extensions for Non-ASCII Text", RFC
              2047, November 1996.

   [RFC2606]  Eastlake, D. and A. Panitz, "Reserved Top Level DNS
              Names", BCP 32, RFC 2606, June 1999.

   [RFC3501]  Crispin, "Internet Message Access Protocol - Version
              4rev1", RFC 3501, June 2003.

9. Informative References

   [RFC1925]  Callon, R., "Fundamental Truths of Networking", RFC 1925,
              April 1996.

   [RFC5721]  Gellens, R., and C. Newman, "POP3 Support for UTF-8", RFC
              5721, February 2010.
   [RFC5738]  Resnick, P. and C. Newman, "IMAP Support for UTF-8", RFC
              5738, March 2010.

10. Author's Address

   Arnt Gulbrandsen
   Schweppermannstr. 8
   D-81671 Muenchen

   Fax: +49 89 4502 9758

   Email: arnt@gulbrandsen.priv.no

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Internet-draft                                               August 2012

          (RFC Editor: Please delete everything after this point)

Open Issues

   Should Kazunori Fujiwara's downgrade document also mention

   RFC Editor: IF 5721 and/or 5738 have been superseded by new RFCs at
   this time, please change the references to those RFCs throughout this
   document. Well, except in the previous sentence. I'm such a pedant.

   RFC Editor: I do not know the difference between that and which. Will
   and shall outnumber me too. Please fix all that. Thank you.

Changes since -00

   Added a rule to handle Subject

   Removed the sentence about unknown:;

   Terminology fixes

Changes since -01

   Nits from Joseph Yee.

   Clarified the address rendering and added non-.invalid examples,
   based on suggestions from Kazunori Fujiwara.

   Many changes from Barry Leiba: Simplified and better terminology,
   reformatted examples, more references, etc.

   Specified POP TOP. A bit of a no-op specification.

   Mention BODY.SIZE[] as well as RFC822.SIZE. Wave hands so
   BODY.SIZE[1] sneaks past.

   http://rant.gulbrandsen.priv.no/good-bad-rfc fwiw

Changes since -02

   Added the DOWNGRADED response code, since both Barry and Alexey wants

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Changes since -03

   Added/changed text in response to appsdir reviews from Ted Hardie and
   Claudio Allocchio.

Changes since -04

   Closed two open issues; the interest in them was clearly negligible.

   "Updates: 3501" because of the SIZE relaxation.

   Security considerations about download-and-delete and long-term

   Bring on the WGLC!

Changes since -05

   Text changes from John Klensin

Changes since -06

   Text changes from Barry Leiba. I hate case sensitivity in human
   language, but right now I need to pack my suitcases, not argue.

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