wdiff rfc6858.original rfc6858v6.txt

Network Working Group Arnt
Internet Engineering Task Force (IETF) A. Gulbrandsen

Intended Status: Proposed Standard
Request for Comments: 6858 February 2013
Updates: 3501
Category: Standards Track
ISSN: 2070-1721

Simplified POP/IMAP POP and IMAP Downgrading for Internationalized Email
draft-ietf-eai-simpledowngrade-07.txt

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 This Memo

This Internet-Draft is submitted to 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 full conformance with Section 2 of RFC 5741.

Information about the
provisions current status of BCP 78 this document, any errata,
and BCP 79. how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6858.

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This Internet-Draft expires in January 2013.

Internet-draft August 2012

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.

1. Overview

It may happen that a

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

Still, security.

However, the sender's name, the message subject, body text of text, and
attachments can easily be displayed, so a helpful IMAP/POP IMAP or POP server
may prefer to provide access to what it can 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
[RFC5721] POP
[RFC6856] and/or [RFC5738]. IMAP [RFC6855].

The server is assumed to be internationalized internally, internally and to store
messages that are internationalized messages natively. When it needs
to present an internationalized message to a conventional client, it 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 (the "synthetic message"). sizes be exact and adding a reporting
requirement as discussed in Section 3 below.

2. Information preserved Preserved and lost Lost

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

The synthetic 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 [RFC5738].

Internet-draft August 2012

Upper case POP [RFC6856]
and/or IMAP [RFC6855] support for UTF-8.

Uppercase letters in examples represents non-ASCII. represent non-ASCII characters.
example.com is a plain
domain, domain; EXAMPLE.com represents a non-ASCII
domain in the .com top-
level top-level domain.

2.1

2.1. Email addresses 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
that tells the user what happened is good. Anything which that produces an
email address which that 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 one of these examples:

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

(The

The .invalid top-level domain is reserved by [RFC2606], therefore 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 will need [RFC2047]
encoding.) 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 "Bcc:", "Cc:", "From:", "Reply-To:",
"Resent-Bcc:", "Resent-Cc:", "Resent-From:", "Resent-Sender:",
"Resent-To:", "Return-Path:", "Sender:", and To. "To:". Any addresses
present in other header fields, such as
Received, "Received:", are not regarded
as addresses by this specification.

2.2

2.2. MIME parameters Parameters

Any MIME parameter [RFC2045] (whether in the message header or a
bodypart body
part header) which that cannot be presented as-is 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

Internet-draft August 2012

2.3 "Subject"

2.3. Subject Field

If the Subject field cannot be presented as-is, to the client exactly as it
appears in the incoming message, the server presents a representation
encoded as specified in [RFC2047].

2.4 RFC 2047.

2.4. Remaining header fields Header Fields

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

3. IMAP-specific details IMAP-Specific Details

IMAP allows clients to retrieve the message size without downloading
it,
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: 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 of the surrogate message differs.

When an IMAP server carries out performs downgrading as part of generating FETCH
responses, it reports which messages were synthesised 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
From, To the
"From:", "To:", and Cc. "Cc:" fields. It may may, for example, contain an
internationalized Subject, etc. "Subject:".

4. POP-specific details POP-Specific Details

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

Internet-draft August 2012

5. Security Considerations

If the internationalized message uses any sort of signature, signature that
covers header fields, the
synthetic message's signature of the surrogate message almost
certainly is invalid. invalid and may be invalid in other cases. This is a
necessary limitation of displaying internationalized messages in
conventional
legacy clients, since the client does those clients do not support internationalized addresses.
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 "Message-Id:",
"In-Reference-To:", and References "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 versions on the server. This may lead to permanent loss
of information when the only remaining version of a message is the
synthetic
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 will does not use an old
synthetic
surrogate 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 has added DOWNGRADED to the IMAP "IMAP Response Code Codes" registry.

7. References

7.1. Normative References

[RFC1939] Myers, J 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.

Internet-draft August 2012

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

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

[RFC3501] Crispin, "Internet Message Access Protocol M., "INTERNET MESSAGE ACCESS PROTOCOL - Version VERSION
4rev1", RFC 3501, June March 2003.

7.2. Informative References

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

[RFC5721] Gellens, R., and C.

[RFC6855] Resnick, P., Ed., Newman, "POP3 C., Ed., and S. Shen, Ed., "IMAP
Support for UTF-8", RFC
5721, 6855, February 2010.
[RFC5738] Resnick, P. and C. 2013.

[RFC6856] Gellens, R., Newman, "IMAP C., Yao, J., and K. Fujiwara, "Post
Office Protocol Version 3 (POP3) Support for UTF-8", RFC
5738, March 2010.

10.
6856, February 2013.

[RFC6857] Fujiwara, K., "Post-Delivery Message Downgrading for
Internationalized Email Messages", RFC 6857, February
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:
EMail: arnt@gulbrandsen.priv.no

Internet-draft August 2012

(RFC Editor: Please delete everything after this point)

Open Issues

Should Kazunori Fujiwara's downgrade document also mention
DOWNGRADED?

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
it.

Internet-draft August 2012

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
caching.

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.