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rfc:rfc6858

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
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 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.

Gulbrandsen Standards Track [Page 1] RFC 6858 POP and IMAP for Internationalized Email March 2013

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.

Gulbrandsen Standards Track [Page 2] RFC 6858 POP and IMAP for Internationalized Email March 2013

 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

Gulbrandsen Standards Track [Page 3] RFC 6858 POP and IMAP for Internationalized Email March 2013

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.

Gulbrandsen Standards Track [Page 4] RFC 6858 POP and IMAP for Internationalized Email March 2013

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.

Gulbrandsen Standards Track [Page 5] RFC 6858 POP and IMAP for Internationalized Email March 2013

 [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

Gulbrandsen Standards Track [Page 6]

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