RFC6855: IMAP Support for UTF-8
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Obsoletes:
RFC5738
Internet Engineering Task Force (IETF) P. Resnick, Ed.
Request for Comments: 6855 Qualcomm Incorporated
Obsoletes: 5738 C. Newman, Ed.
Category: Standards Track Oracle
ISSN: 2070-1721 S. Shen, Ed.
CNNIC
March 2013
IMAP Support for UTF-8
Abstract
This specification extends the Internet Message Access Protocol
(IMAP) 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 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/rfc6855.
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.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions Used in This Document . . . . . . . . . . . . . . 2
3. "UTF8=ACCEPT" IMAP Capability and UTF-8 in IMAP
Quoted-Strings . . . . . . . . . . . . . . . . . . . . . . . . 3
4. IMAP UTF8 "APPEND" Data Extension . . . . . . . . . . . . . . 4
5. "LOGIN" Command and UTF-8 . . . . . . . . . . . . . . . . . . 5
6. "UTF8=ONLY" Capability . . . . . . . . . . . . . . . . . . . . 5
7. Dealing with Legacy Clients . . . . . . . . . . . . . . . . . 6
8. Issues with UTF-8 Header Mailstore . . . . . . . . . . . . . . 7
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
10. Security Considerations . . . . . . . . . . . . . . . . . . . 8
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
11.1. Normative References . . . . . . . . . . . . . . . . . . 9
11.2. Informative References . . . . . . . . . . . . . . . . . 10
Appendix A. Design Rationale . . . . . . . . . . . . . . . . . . 11
Appendix B. Acknowledgments . . . . . . . . . . . . . . . . . . . 11
1. Introduction
This specification forms part of the Email Address
Internationalization protocols described in the Email Address
Internationalization Framework document [RFC6530]. It extends IMAP
[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.
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].
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The formal syntax uses the Augmented Backus-Naur Form (ABNF)
[RFC5234] notation. In addition, rules from IMAP [RFC3501], UTF-8
[RFC3629], Extensions to IMAP ABNF [RFC4466], and IMAP "LIST" command
extensions [RFC5258] are also referenced. This document assumes that
the reader will have a reasonably good understanding of these RFCs.
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
the "SELECT" and "EXAMINE" commands, and the server can provide UTF-8
responses to the "LIST" and "LSUB" commands. This capability also
affects other IMAP extensions that can return mailbox names or their
prefixes, such as NAMESPACE [RFC2342] and ACL [RFC4314].
The "UTF8=ONLY" capability, described in Section 6, implies the
"UTF8=ACCEPT" capability. A server is said to support "UTF8=ACCEPT"
if it advertises either "UTF8=ACCEPT" or "UTF8=ONLY".
A client MUST use the "ENABLE" command [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 and supports
the "UTF8=ACCEPT" extension. The "ENABLE UTF8=ACCEPT" command is
only valid in the authenticated state.
The IMAP base specification [RFC3501] 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 IMAP non-synchronizing
literals [RFC2088], this requires an extra round trip for each UTF-8
string sent by the client. When the IMAP server supports
"UTF8=ACCEPT", 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-terminals.
uQUOTED-CHAR = QUOTED-CHAR / UTF8-2 / UTF8-3 / UTF8-4
UTF8-2 = <Defined in Section 4 of RFC 3629>
UTF8-3 = <Defined in Section 4 of RFC 3629>
UTF8-4 = <Defined in Section 4 of RFC 3629>
When this extended quoting mechanism is used by the client, the
server MUST reject, with a "BAD" response, any octet sequences with
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the high bit set that fail to comply with the formal syntax
requirements of UTF-8 [RFC3629]. 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 supports "UTF8=ACCEPT", 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 support "UTF8=ACCEPT" SHOULD accept UTF-8 in
mailbox names, and those that also support the Mailbox International
Naming Convention described in RFC 3501, 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 ([RFC5198], Section 2) with the specific exception that
they MUST NOT contain control characters (U+0000-U+001F and U+0080-U+
009F), a delete character (U+007F), a line separator (U+2028), or a
paragraph separator (U+2029).
Once an IMAP client has enabled UTF-8 support with the "ENABLE
UTF8=ACCEPT" command, it MUST NOT issue a "SEARCH" command that
contains a charset specification. If an IMAP server receives such a
"SEARCH" command in that situation, it SHOULD reject the command with
a "BAD" response (due to the conflicting charset labels).
4. IMAP UTF8 "APPEND" Data Extension
If the server supports "UTF8=ACCEPT", 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" data extension to the "APPEND" command. If the server
also advertises the "CATENATE" capability [RFC4469], the client can
use the same data extension to include such a message in a catenated
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
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If an IMAP server supports "UTF8=ACCEPT" and the IMAP client has not
issued the "ENABLE UTF8=ACCEPT" command, the server MUST reject, with
a "NO" response, an "APPEND" command that includes any 8-bit
character in message header fields.
5. "LOGIN" Command and UTF-8
This specification does not extend the IMAP "LOGIN" command [RFC3501]
to support UTF-8 usernames and passwords. Whenever a client needs to
use UTF-8 usernames or passwords, it MUST use the IMAP "AUTHENTICATE"
command, which is already capable of passing UTF-8 usernames and
credentials.
Although using the IMAP "AUTHENTICATE" command in this way makes it
syntactically legal to have a UTF-8 username or password, there is no
guarantee that the user provisioning system utilized 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 indicates that the server supports
"UTF8=ACCEPT" (see Section 4) and that it requires support for UTF-8
from clients. In particular, this means that the server will send
UTF-8 in quoted-strings, and it will not accept the older
international mailbox name convention (modified UTF-7 [RFC3501]).
Because these are incompatible changes to IMAP, explicit server
announcement and client confirmation is necessary: clients MUST use
the "ENABLE UTF8=ACCEPT" command before using this server. A server
that advertises "UTF8=ONLY" will reject, with a "NO [CANNOT]"
response [RFC5530], any command that might require UTF-8 support and
is not preceded by an "ENABLE UTF8=ACCEPT" command.
IMAP clients that find support for a server that announces
"UTF8=ONLY" problematic are encouraged to at least detect the
announcement and provide an informative error message to the
end-user.
Because the "UTF8=ONLY" server capability includes support for
"UTF8=ACCEPT", the capability string will include, at most, one of
those and never both. For the client, "ENABLE UTF8=ACCEPT" is always
used -- never "ENABLE UTF8=ONLY".
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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 that 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.
When a message that requires SMTPUTF8 is encountered and the client
does not enable UTF-8 capability, choices available to the server
include hiding the problematic message(s), creating in-band or
out-of-band notifications or error messages, or somehow trying to
create a surrogate of the message with the intention of providing
useful information to that client about what has occurred. Such
surrogate messages cannot be actual substitutes for the original
message: they will almost always be impossible to reply to (either at
all or without loss of information) and the new header fields or
specialized constructs for server-client communications may go beyond
the requirements of current email specifications (e.g., [RFC5322]).
Consequently, such messages may confuse some legacy mail user agents
(including IMAP clients) or not provide expected information to
users. There are also trade-offs in constructing surrogates 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 "Post-Delivery Message Downgrading for
Internationalized Email Messages" [RFC6857]) and trying to minimize
those costs while still providing useful information (for example, in
"Simplified POP and IMAP Downgrading for Internationalized Email"
[RFC6858]).
Implementations that choose to perform downgrading SHOULD use one of
the standardized algorithms provided in RFC 6857 or RFC 6858.
Getting downgrade algorithms right, and minimizing the risk of
operational problems and harm to the email system, is tricky and
requires careful engineering. These two algorithms are well
understood and carefully designed.
Because such messages are really surrogates of the original ones, not
really "downgraded" ones (although that terminology is often used for
convenience), they inevitably have relationships to the originals
that the IMAP specification [RFC3501] did not anticipate. This
brings up two concerns in particular: First, digital signatures
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computed over and intended for the original message will often not be
applicable to the surrogate message, and will often fail signature
verification. (It will be possible for some digital signatures to be
verified, if they cover only parts of the original message that are
not affected in the creation of the surrogate.) Second, 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 [RFC3501] behaves as the user would expect.
Those issues may be especially sensitive if the server caches the
surrogate message or computes and stores it when the message arrives
with the intent of making either form available depending on client
capabilities. Additionally, in order to cope with the case when a
server compliant with this extension returns the same UIDVALIDITY to
both legacy and "UTF8=ACCEPT"-aware clients, a client upgraded from
being non-"UTF8=ACCEPT"-aware MUST discard its cache of messages
downloaded from the server.
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
issuing "ENABLE UTF8=ACCEPT" first, it is the responsibility of the
server to comply with the IMAP base specification [RFC3501] and the
Internet Message Format [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.
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9. IANA Considerations
This document redefines two capabilities ("UTF8=ACCEPT" and
"UTF8=ONLY") in the "IMAP 4 Capabilities" registry [RFC3501]. Three
other capabilities that were described in the experimental
predecessor to this document ("UTF8=ALL", "UTF8=APPEND", "UTF8=USER")
are now OBSOLETE. IANA has updated the registry as follows:
OLD:
+--------------+-----------------+
| UTF8=ACCEPT | [RFC5738] |
| UTF8=ALL | [RFC5738] |
| UTF8=APPEND | [RFC5738] |
| UTF8=ONLY | [RFC5738] |
| UTF8=USER | [RFC5738] |
+--------------+-----------------+
NEW:
+------------------------+---------------------+
| UTF8=ACCEPT | [RFC6855] |
| UTF8=ALL (OBSOLETE) | [RFC5738] [RFC6855]|
| UTF8=APPEND (OBSOLETE) | [RFC5738] [RFC6855]|
| UTF8=ONLY | [RFC6855] |
| UTF8=USER (OBSOLETE) | [RFC5738] [RFC6855]|
+------------------------+---------------------+
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 usernames and passwords. Otherwise, this is not believed to
alter the security considerations of IMAP.
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, as well as 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.
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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.
[RFC5198] Klensin, J. and M. Padlipsky, "Unicode Format for Network
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.
[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.
[RFC6532] Yang, A., Steele, S., and N. Freed, "Internationalized
Email Headers", RFC 6532, February 2012.
[RFC6857] Fujiwara, K., "Post-Delivery Message Downgrading for
Internationalized Email Messages", RFC 6857, March 2013.
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RFC 6855 IMAP Support for UTF-8 March 2013
[RFC6858] Gulbrandsen, A., "Simplified POP and IMAP Downgrading for
Internationalized Email", RFC 6858, March 2013.
11.2. Informative References
[RFC2088] Myers, J., "IMAP4 non-synchronizing literals", RFC 2088,
January 1997.
[RFC2342] Gahrns, M. and C. Newman, "IMAP4 Namespace", RFC 2342,
May 1998.
[RFC4314] Melnikov, A., "IMAP4 Access Control List (ACL) Extension",
RFC 4314, December 2005.
[RFC5530] Gulbrandsen, A., "IMAP Response Codes", RFC 5530,
May 2009.
[RFC5738] Resnick, P. and C. Newman, "IMAP Support for UTF-8",
RFC 5738, March 2010.
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Appendix A. Design Rationale
This non-normative section discusses the reasons behind some of the
design choices in this specification.
The "UTF8=ONLY" mechanism simplifies diagnosis of interoperability
problems when legacy support goes away. In the situation where
backwards compatibility is not working anyway, the non-conforming
"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.
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Authors' Addresses
Pete Resnick (editor)
Qualcomm Incorporated
5775 Morehouse Drive
San Diego, CA 92121-1714
USA
Phone: +1 858 651 4478
EMail: presnick@qti.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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