IMAP Support for UTF-8
draft-ietf-eai-5738bis-05
The information below is for an old version of the document.
| Document | Type | Active Internet-Draft (eai WG) | |
|---|---|---|---|
| Authors | Pete Resnick , Chris Newman , Sean Shen | ||
| Last updated | 2012-07-14 | ||
| Replaces | draft-ietf-eai-5378bis | ||
| Stream | Internet Engineering Task Force (IETF) | ||
| Formats | plain text htmlized pdfized bibtex | ||
| Reviews |
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| Stream | WG state | WG Document | |
| Document shepherd | (None) | ||
| IESG | IESG state | AD is watching | |
| Consensus boilerplate | Unknown | ||
| Telechat date | (None) | ||
| Responsible AD | Pete Resnick | ||
| Send notices to | eai-chairs@tools.ietf.org, draft-ietf-eai-5738bis@tools.ietf.org |
draft-ietf-eai-5738bis-05
Internet Engineering Task Force P. Resnick, Ed.
Internet-Draft Qualcomm Incorporated
Obsoletes: RFC5738 (if approved) C. Newman, Ed.
Intended status: Standards Track Oracle
Expires: January 15, 2013 S. Shen, Ed.
CNNIC
July 14, 2012
IMAP Support for UTF-8
draft-ietf-eai-5738bis-05
Abstract
This specification extends the Internet Message Access Protocol
version 4rev1 (IMAP4rev1) to support UTF-8 encoded international
characters in user names, mail addresses and message headers. This
specification replaces RFC 5738.
Status of This Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 15, 2013.
Copyright Notice
Copyright (c) 2012 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
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions Used in this Document . . . . . . . . . . . . . . . 3
3. UTF8=ACCEPT IMAP Capability . . . . . . . . . . . . . . . . . . 3
4. IMAP UTF8 Append Data Extension . . . . . . . . . . . . . . . . 4
5. LOGIN Command and UTF-8 . . . . . . . . . . . . . . . . . . . . 4
6. UTF8=ONLY Capability . . . . . . . . . . . . . . . . . . . . . 4
7. Dealing With Legacy Clients . . . . . . . . . . . . . . . . . . 5
8. Issues with UTF-8 Header Mailstore . . . . . . . . . . . . . . 6
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
10. Security Considerations . . . . . . . . . . . . . . . . . . . . 6
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7
11.1. Normative References . . . . . . . . . . . . . . . . . . . 7
11.2. Informative References . . . . . . . . . . . . . . . . . . 8
Appendix A. Appendix A. Design Rationale . . . . . . . . . . . . . 8
Appendix B. Appendix B. Acknowledgments . . . . . . . . . . . . . 8
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1. Introduction
This specification extends IMAP4rev1 [RFC3501] to permit UTF-8
[RFC3629] in headers as described in "Internationalized Email
Headers" [RFC6532] . It also adds a mechanism to support mailbox
names using the UTF-8 charset. This specification creates two new
IMAP capabilities to allow servers to advertise these new extensions.
Most of this specification assumes that the IMAP server will be
operating in a fully internationalized environment, i.e., one in
which all clients accessing the server will be able to accept non-
ASCII message header fields and other information as specified in
Section 3. At least during a transition period, that assumption will
not be realistic for many environments; the issues involved are
discussed in Section 7 below.
This specification replaces an earlier, experimental, approach to the
same problem [RFC5738].
2. Conventions Used in this Document
The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and "MAY"
in this document are to be interpreted as defined in "Key words for
use in RFCs to Indicate Requirement Levels" [RFC2119].
The formal syntax uses the Augmented Backus-Naur Form (ABNF)
[RFC5234] notation. In addition, rules from IMAP4rev1 [RFC3501],
UTF-8 [RFC3629], "Collected Extensions to IMAP4 ABNF" [RFC4466], and
IMAP4 LIST Command Extensions [RFC5258] are also referenced.
In examples, "C:" and "S:" indicate lines sent by the client and
server, respectively. If a single "C:" or "S:" label applies to
multiple lines, then the line breaks between those lines are for
editorial clarity only and are not part of the actual protocol
exchange.
3. UTF8=ACCEPT IMAP Capability
The "UTF8=ACCEPT" capability indicates that the server supports the
ability to open mailboxes containing internationalized messages with
SELECT and EXAMINE, and UTF-8 responses from the LIST and LSUB
commands.
A client MUST use the "ENABLE UTF8=ACCEPT" command (defined in
[RFC5161]) to indicate to the server that the client accepts UTF-8
quoted-strings. The "ENABLE UTF8=ACCEPT" command MUST only be used
in the authenticated state. (Note that the "UTF8=ONLY" capability
described in Section 6 imply the "UTF8=ACCEPT" capability. See
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additional information in these sections.)
4. IMAP UTF8 Append Data Extension
If the "UTF8=ACCEPT" capability is advertised, then the server
accepts UTF-8 headers in the APPEND command message argument. A
client that sends a message with UTF-8 headers to the server MUST
send them using the "UTF8" APPEND data extension. If the server also
advertises the CATENATE capability (as specified in [RFC4469]), the
client can use the same data extension to include such a message in a
CATENATE message part. The ABNF for the APPEND data extension and
CATENATE extension follows:
utf8-literal = "UTF8" SP "(" literal8 ")"
append-data =/ utf8-literal
cat-part =/ utf8-literal
IMAP servers that advertise support for "UTF8=ACCEPT" or "UTF8=ONLY"
MUST reject an APPEND command that includes any 8-bit in the message
headers with a "NO" response, when IMAP clients do not issue "ENABLE
UTF8=ACCEPT" or "ENABLE UTF8=ONLY".
Note that the "UTF8=ONLY" capability described in Section 6 implies
the "UTF8=ACCEPT" capability. See additional information in that
section.
5. LOGIN Command and UTF-8
This specification doesn't extend the IMAP LOGIN command [RFC3501] to
support UTF-8 usernames and passwords. Whenever a client needs to
use UTF-8 username/passwords, it MUST use the IMAP AUTHENTICATE
command which is already capable of passing UTF-8 user names and
credentials.
Although this makes it syntacically legal to have a UTF-8 user name
or password, there is no guarantee the user provisioning system used
by the IMAP server will allow such identities. This is an
implementation decision and MAY depend on what identity system the
IMAP server is configured to use.
6. UTF8=ONLY Capability
The "UTF8=ONLY" capability permits an IMAP server to advertise that
it does not support the international mailbox name convention
(modified UTF-7). As this is an incompatible change to IMAP, a clear
warning is necessary. IMAP clients that find implementation of the
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"UTF8=ONLY" capability problematic are encouraged to at least detect
the "UTF8=ONLY" capability and provide an informative error message
to the end-user.
The "UTF8=ONLY" capability implies the "UTF8=ACCEPT" capability.
UTF8=ACCEPT and UTF8=ONLY SHOULD be mutually exclusive. An IMAP
server can advertise one of them, but never both.
7. Dealing With Legacy Clients
In most situations, it will be difficult or impossible for the
implementer or operator of an IMAP (or POP) server to know whether
all of the clients that might access it, or the associated mail store
more generally, will be able to support the facilities defined in
this document. In almost all cases, servers who conform to this
specification will have to be prepared to deal with clients that do
not enable the relevant capabilities. Unfortunately, there is no
completely satisfactory way to do so other than for systems that wish
to receive email that requires SMTPUTF8 capabilities to be sure that
all components of those systems -- including IMAP and other clients
selected by users -- are upgraded appropriately.
Choices available to the server when a message that requires SMTPUTF8
is encountered and the client doesn't enable UTF-8 capability include
hiding the problematic message(s) as outlined elsewhere in this
specification, creating in band or out of band notifications or error
messages, or somehow trying to create a variation on the message with
the intention of providing useful information to that client about
what has occurred. Such variant messages cannot be actual
substitutes for the original message: it will rarely be possible to
reply to (either at all or without loss of information), new header
fields or specialized constructs for server-client communication may
go beyond the requirements of, e.g., RFC 5322 and may consequently
confuse some legacy mail user agents (including IMAP clients) or
otherwise not provide the expected information to users. There are
also tradeoffs in constructing variants of the original message
between accepting complexity and additional computation costs in
order to try to preserve as much information as possible (for
example, in [popimap-downgrade]) and trying to minimize those costs
while still providing useful information (for example, in
[I-D.ietf-eai-simpledowngrade]).
Because such messages are really variations on the original ones, not
really "downgraded" (ones although that terminology is often used for
convenience), they inevitably have relationships to the original ones
that the IMAP specification [RFC3501] did not anticipate. In
particular, digital signatures computed over the original message
will often not be applicable to the variant version and servers that
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may be accessed by the same user with different clients or methods
(e.g., POP or webmail systems in addition to IMAP or IMAP clients
with different capabilities) will need to exert extreme care to be
sure that UIDVALIDITY behaves as the user would expect. Those issues
may be especially sensitive if the server caches the variant message
or computes and stores it when the message arrives with the intent of
making either form available depending on client capabilities.
The best (or "least bad") approach for any given environment will
depend on local conditions, local assumptions about user behavior,
the degree of control the server operator has over client usage and
upgrading, the options that are actually available, and so on. It is
impossible, at least at the time, to give good advice that will apply
to all situations, or even particular profiles of situations, other
than "upgrade legacy clients as soon as possible".
8. Issues with UTF-8 Header Mailstore
When an IMAP server uses a mailbox format that supports UTF-8 headers
and it permits selection or examination of that mailbox without the
"UTF8" parameter, it is the responsibility of the server to comply
with the IMAP4rev1 base specification [RFC3501] and [RFC5322] with
respect to all header information transmitted over the wire.
Mechanisms for 7-bit downgrading to help comply with the standards
are discussed in [popimap-downgrade].
9. IANA Considerations
This document adds two new capabilities ("UTF8=ACCEPT" and
"UTF8=ONLY") to the IMAP4rev1 Capabilities registry [RFC3501]. Three
other IMAP capabilites that were described in the experimental
predecessor to this document (UTF8=ALL, UTF8=APPEND, UTF8=USER) are
to be marked OBSOLETE in the registry.
10. Security Considerations
The security considerations of UTF-8 [RFC3629] and SASLprep [RFC4013]
apply to this specification, particularly with respect to use of
UTF-8 in user names and passwords. Otherwise, this is not believed
to alter the security considerations of IMAP4rev1.
Special considerations, some of them with security implications,
occur if a server that conforms to this specification is accessed by
a client that does not and in some more complex situations in which a
given message is accessed by multiple clients that might use
different protocols and/or support different capabilities. Those
issues are discussed in Section 7 above.
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11. References
11.1. Normative References
[RFC2119] Bradner, S., "Key words for use in
RFCs to Indicate Requirement Levels",
BCP 14, RFC 2119, March 1997.
[RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS
PROTOCOL - VERSION 4rev1", RFC 3501,
March 2003.
[RFC3629] Yergeau, F., "UTF-8, a transformation
format of ISO 10646", STD 63,
RFC 3629, November 2003.
[RFC4013] Zeilenga, K., "SASLprep: Stringprep
Profile for User Names and
Passwords", RFC 4013, February 2005.
[RFC4466] Melnikov, A. and C. Daboo, "Collected
Extensions to IMAP4 ABNF", RFC 4466,
April 2006.
[RFC4469] Resnick, P., "Internet Message Access
Protocol (IMAP) CATENATE Extension",
RFC 4469, April 2006.
[RFC5161] Gulbrandsen, A. and A. Melnikov, "The
IMAP ENABLE Extension", RFC 5161,
March 2008.
[RFC5234] Crocker, D. and P. Overell,
"Augmented BNF for Syntax
Specifications: ABNF", STD 68,
RFC 5234, January 2008.
[RFC5258] Leiba, B. and A. Melnikov, "Internet
Message Access Protocol version 4 -
LIST Command Extensions", RFC 5258,
June 2008.
[RFC6532] Yang, A., Steele, S., and N. Freed,
"Internationalized Email Headers",
RFC 6532, February 2012.
[RFC5322] Resnick, P., Ed., "Internet Message
Format", RFC 5322, October 2008.
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11.2. Informative References
[RFC5738] Resnick, P. and C. Newman, "IMAP
Support for UTF-8", RFC 5738,
March 2010.
[I-D.ietf-eai-simpledowngrade] Gulbrandsen, A., "EAI: Simplified
POP/IMAP downgrading",
draft-ietf-eai-simpledowngrade-05
(work in progress), June 2012.
[popimap-downgrade] Fujiwara, K., "Post-delivery Message
Downgrading for Internationalized
Email Messages",
draft-ietf-eai-popimap-downgrade-06
(work in progress), July 2012.
Appendix A. Appendix A. Design Rationale
This non-normative section discusses the reasons behind some of the
design choices in the above specification.
The basic approach of advertising the ability to access a mailbox in
UTF-8 mode is intended to permit graceful upgrade, including servers
that support multiple mailbox formats. In particular, it would be
undesirable to force conversion of an entire server mailstore to
UTF-8 headers, so being able to phase-in support for new mailboxes
and gradually migrate old mailboxes is permitted by this design.
The "UTF8=ONLY" mechanism simplifies diagnosis of interoperability
problems when legacy support goes away. In the situation where
backwards compatibility is broken anyway, just-send-UTF-8 IMAP has
the advantage that it might work with some legacy clients. However,
the difficulty of diagnosing interoperability problems caused by a
just-send-UTF-8 IMAP mechanism is the reason the "UTF8=ONLY"
capability mechanism was chosen.
Appendix B. Appendix B. Acknowledgments
The authors wish to thank the participants of the EAI working group
for their contributions to this document with particular thanks to
Harald Alvestrand, David Black, Randall Gellens, Arnt Gulbrandsen,
Kari Hurtta, John Klensin, Xiaodong Lee, Charles Lindsey, Alexey
Melnikov, Subramanian Moonesamy, Shawn Steele, Daniel Taharlev, and
Joseph Yee for their specific contributions to the discussion.
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Authors' Addresses
Pete Resnick (editor)
Qualcomm Incorporated
5775 Morehouse Drive
San Diego, CA 92121-1714
US
Phone: +1 858 651 4478
EMail: presnick@qualcomm.com
Chris Newman (editor)
Oracle
800 Royal Oaks
Monrovia, CA 91016
USA
Phone:
EMail: chris.newman@oracle.com
Sean Shen (editor)
CNNIC
No.4 South 4th Zhongguancun Street
Beijing, 100190
China
Phone: +86 10-58813038
EMail: shenshuo@cnnic.cn
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