Internet Engineering Task Force P. Resnick, Ed.
Internet-Draft Qualcomm Incorporated
Obsoletes: RFC5738 (if approved) C. Newman, Ed.
Intended status: Standards Track Oracle
Expires: February 2, 2013 S. Shen, Ed.
CNNIC
August 1, 2012
IMAP Support for UTF-8
draft-ietf-eai-5738bis-07
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
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This Internet-Draft will expire on February 2, 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
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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 and UTF-8 in IMAP Quoted
Strings . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. IMAP UTF8 Append Data Extension . . . . . . . . . . . . . . . 5
5. LOGIN Command and UTF-8 . . . . . . . . . . . . . . . . . . . 5
6. UTF8=ONLY Capability . . . . . . . . . . . . . . . . . . . . . 6
7. Dealing With Legacy Clients . . . . . . . . . . . . . . . . . 6
8. Issues with UTF-8 Header Mailstore . . . . . . . . . . . . . . 7
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
10. Security Considerations . . . . . . . . . . . . . . . . . . . 8
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
11.1. Normative References . . . . . . . . . . . . . . . . . . 8
11.2. Informative References . . . . . . . . . . . . . . . . . 9
Appendix A. Design Rationale . . . . . . . . . . . . . . . . . . 9
Appendix B. Acknowledgments . . . . . . . . . . . . . . . . . . . 10
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1. Introduction
This specification forms part of the Email Address
Internationalization protocols described in the Email Address
Internationalization Framework document [RFC6530]. It 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. This
document assumes that the reader will have a reasonably good
understanding of the RFCs above and their update.
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 and UTF-8 in IMAP Quoted Strings
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.
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A client MUST use the "ENABLE" command (defined in [RFC5161]) with
the "UTF8=ACCEPT" option (defined in Section 4 below) to indicate to
the server that the client accepts UTF-8 in 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 additional information in
these sections.)
The IMAP4rev1 [RFC3501] base specification forbids the use of 8-bit
characters in atoms or quoted strings. Thus, a UTF-8 string can only
be sent as a literal. This can be inconvenient from a coding
standpoint, and unless the server offers IMAP4 non-synchronizing
literals [RFC2088], this requires an extra round trip for each UTF-8
string sent by the client. When the IMAP server advertises the
"UTF8=ACCEPT" capability, it informs the client that it supports
UTF-8 in quoted-strings with the following syntax:
quoted =/ DQUOTE *uQUOTED-CHAR DQUOTE
; QUOTED-CHAR is not modified, as it will affect
; other RFC 3501 ABNF non terminal.
uQUOTED-CHAR = QUOTED-CHAR / UTF8-2 / UTF8-3 / UTF8-4
UTF8-2 = <Defined in Section 4 of RFC3629>
UTF8-3 = <Defined in Section 4 of RFC3629>
UTF8-4 = <Defined in Section 4 of RFC3629>
When this extended quoting mechanism is used by the client, then the
server MUST reject octet sequences with the high bit set that fail to
comply with the formal syntax in [RFC3629] with a BAD response. The
IMAP server MUST NOT send UTF-8 in quoted strings to the client
unless the client has indicated support for that syntax by using the
"ENABLE UTF8=ACCEPT" command.
If the server advertises the "UTF8=ACCEPT" capability, the client MAY
use extended quoted syntax with any IMAP argument that permits a
string (including astring and nstring). However, if characters
outside the US-ASCII repertoire are used in an inappropriate place,
the results would be the same as if other syntactically valid but
semantically invalid characters were used. Specific cases where
UTF-8 characters are permitted or not permitted are described in the
following paragraphs.
All IMAP servers that advertise the "UTF8=ACCEPT" capability SHOULD
accept UTF-8 in mailbox names, and those that also support the
"Mailbox International Naming Convention" described in RFC 3501,
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Section 5.1.3 MUST accept utf8-quoted mailbox names and convert them
to the appropriate internal format. Mailbox names MUST comply with
the Net-Unicode Definition (Section 2 of [RFC5198]) with the specific
exception that they MUST NOT contain control characters (0000-001F,
0080-009F), delete (007F), line separator (2028), or paragraph
separator (2029).
An IMAP client MUST NOT issue a SEARCH command using CHARSET after
ENABLE command. If an IMAP server receives such a SEARCH command, it
SHOULD reject the command with a BAD response (due to the conflicting
charset labels).
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 ")"
literal8 = <Defined in RFC 4466>
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.
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Although the use of the IMAP AUTHENTICATE command in this way makes
it syntactically 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), allows all of the capabilities that are allowed by
"UTF8=ACCEPT" (see Section 4), and does not permit 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 "UTF8=ONLY" capability problematic are
encouraged to at least detect the "UTF8=ONLY" capability and provide
an informative error message to the end-user.
If the "UTF8=ONLY" capability is specified, UTF-8 must be accepted as
if the "UTF8=ACCEPT" had been specified. For convenience, the
explicit combination of "UTF8=ONLY" and "UTF8=ACCPET" is not allowed.
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), 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
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(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
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 of publication of this
specification, 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. The
issue of handling messages containing non-ASCII characters in legacy
environments is discussed in Section 7.
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.
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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.
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.
[RFC5198] Klensin, J. and M. Padlipsky,
"Unicode Format for Network
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Interchange", RFC 5198, 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.
[RFC6530] Klensin, J. and Y. Ko, "Overview and
Framework for Internationalized
Email", RFC 6530, February 2012.
11.2. Informative References
[RFC2088] Myers, J., "IMAP4 non-synchronizing
literals", RFC 2088, January 1997.
[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. Design Rationale
This non-normative section discusses the reasons behind some of the
design choices in the above specification.
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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. 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.
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
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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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