Internet Engineering Task Force (IETF)                    A. Gulbrandsen
Request for Comments: 6858                                    March 2013
Updates: 3501
Category: Standards Track
ISSN: 2070-1721

    Simplified POP and IMAP Downgrading for Internationalized Email

Abstract

   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.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6858.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   publication of this document.  Please review these documents
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

1.  Overview

   A conventional IMAP or POP client may open a mailbox containing
   internationalized messages or may even attempt to read
   internationalized messages, for instance, when a user has both
   internationalized and conventional Mail User Agents (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 rarely
   possible.  This includes displaying the addresses, replying to
   messages, and the processing of most types of address-based signature
   or security.

   However, the sender's name, message subject, body of text, and
   attachments can easily be displayed, so a helpful IMAP or POP server
   may prefer to display as much of the message as possible, rather than
   hide the message entirely.

   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 such as the one specified in a companion document [RFC6857]
   is likely more work than implementing proper UTF-8 support for POP
   [RFC6856] and/or IMAP [RFC6855].

   The server is assumed to be internationalized internally and to store
   messages that are internationalized messages natively.  When it needs
   to present an internationalized message to a conventional client, the
   server synthesizes a conventional message containing most of the
   information and presents the "surrogate message".

   This specification modifies the base IMAP specification [RFC3501] by
   relaxing a requirement that sizes be exact and adding a reporting
   requirement as discussed in Section 3 below.

2.  Information Preserved and Lost

   The surrogate message is intended to convey the most important
   information to the user.  Where information is lost, the user should
   consider the message incomplete rather than modified.

   The surrogate 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 POP [RFC6856]
   and/or IMAP [RFC6855] support for UTF-8.

   Uppercase letters in examples represent non-ASCII characters.
   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
   that tells the user what happened is good.  Anything that produces an
   email address that might belong to someone else is bad.

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

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

   The .invalid top-level domain is reserved as a Top Level DNS Name
   [RFC2606]; therefore, the first two examples are syntactically valid,
   but they will never belong to anyone.  Note that the display-name
   often needs encoding (see the Message Header Extensions document
   [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 body
   part header) that cannot be presented to the client exactly as it
   appears in the incoming message is silently excised.

   Given a field such as

      Content-Disposition: attachment; filename=FOO

   the field is presented as

      Content-Disposition: attachment

2.3.  Subject Field

   If the Subject field cannot be presented to the client exactly as it
   appears in the incoming message, the server presents a representation
   encoded as specified in RFC 2047.

2.4.  Remaining Header Fields

   Any header field that cannot be presented to the client, even with
   the modifications listed in Sections 2.1-2.3, is silently excised.

3.  IMAP-Specific Details

   IMAP allows clients to retrieve the message size without downloading
   the message, using RFC822.SIZE, BODY.SIZE[] and so on.  The IMAP
   specification [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 size of the surrogate message differs.

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

      C: a UID FETCH 1:* BODY.PEEK[HEADER.FIELDS(To From Cc)]
      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 the
   "From:", "To:", and "Cc:" fields.  It may, for example, contain an
   internationalized "Subject:".

4.  POP-Specific Details

   The number of lines specified in the TOP command [RFC1939] refers to
   the surrogate message.  The message size reported by, for example,
   LIST may refer to either the internationalized or the surrogate
   message.

5.  Security Considerations

   If the internationalized message uses any sort of signature that
   covers header fields, the signature of the surrogate message almost
   certainly is invalid and may be invalid in other cases.  This is a
   necessary limitation of displaying internationalized messages in
   legacy clients, since those clients do not support internationalized
   header fields.  These cases are discussed in more detail in the POP
   or IMAP Downgrade document [RFC6857].  Even though invalid, these
   signatures should not be removed from the surrogate message, to
   preserve as much of the information as possible from the original
   message.

   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 versions on the server.  This may lead to permanent loss
   of information when the only remaining version of a message is the
   surrogate 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 does not use an old
   surrogate message from its cache rather than retrieve the real
   message from the server.

6.  IANA Considerations

   IANA has added DOWNGRADED to the "IMAP Response Codes" registry.

7.  References

7.1.  Normative References

   [RFC1939]  Myers, J. and M. Rose, "Post Office Protocol - Version 3",
              STD 53, 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.

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

   [RFC2606]  Eastlake, D., 3rd and A. Panitz, "Reserved Top Level DNS
              Names", BCP 32, RFC 2606, June 1999.

   [RFC3501]  Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
              4rev1", RFC 3501, March 2003.

7.2.  Informative References

   [RFC1925]  Callon, R., "The Twelve Networking Truths", RFC 1925,
              April 1 1996.

   [RFC6855]  Resnick, P., Ed., Newman, C., Ed., and S. Shen, Ed., "IMAP
              Support for UTF-8", RFC 6855, March 2013.

   [RFC6856]  Gellens, R., Newman, C., Yao, J., and K. Fujiwara, "Post
              Office Protocol Version 3 (POP3) Support for UTF-8", RFC
              6856, March 2013.

   [RFC6857]  Fujiwara, K., "Post-Delivery Message Downgrading for
              Internationalized Email Messages", RFC 6857, March 2013.

8.  Acknowledgements

   Claudio Allocchio, Ned Freed, Kazunori Fujiwara, Ted Hardie, John
   Klensin, Barry Leiba, John Levine, Alexey Melnikov, Chris Newman, and
   Joseph Yee.  This specification was inspired by the principle stated
   in Rule 12 of "The Twelve Networking Truths" [RFC1925].

Author's Address

   Arnt Gulbrandsen
   Schweppermannstr. 8
   D-81671 Muenchen
   Germany

   Fax: +49 89 4502 9758
   EMail: arnt@gulbrandsen.priv.no