Network Working Group P. Resnick
Internet-Draft Qualcomm Incorporated
Updates: 3501 (if approved) C. Newman
Intended status: Experimental Sun Microsystems
Expires: December 27, 2009 June 25, 2009
IMAP Support for UTF-8
draft-ietf-eai-imap-utf8-06
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Abstract
This specification extends the Internet Message Access Protocol
version 4rev1 (IMAP4rev1) to support unencoded international
characters in user names, mail addresses and message headers. This
is an early draft and intended as a framework for discussion. Please
do not deploy implementations of this draft.
Table of Contents
1. Conventions Used in this Document . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. UTF8 IMAP Capability . . . . . . . . . . . . . . . . . . . . . 3
3.1. IMAP UTF-8 Quoted Strings . . . . . . . . . . . . . . . . 3
3.2. UTF8 Parameter to SELECT and EXAMINE . . . . . . . . . . . 4
3.3. UTF-8 LIST and LSUB Responses . . . . . . . . . . . . . . 5
3.4. UTF-8 Interaction with IMAP4 LIST Command Extensions . . . 6
3.4.1. UTF8 and UTF8ONLY LIST Selection Options . . . . . . . 6
3.4.2. UTF8 LIST Return Option . . . . . . . . . . . . . . . 6
4. UTF8=APPEND Capability . . . . . . . . . . . . . . . . . . . . 6
5. UTF8=USER Capability . . . . . . . . . . . . . . . . . . . . . 7
6. UTF8=ALL Capability . . . . . . . . . . . . . . . . . . . . . 7
7. UTF8=ONLY Capability . . . . . . . . . . . . . . . . . . . . . 7
8. Up-Conversion Server Requirements . . . . . . . . . . . . . . 8
9. Issues with UTF-8 Header Mailstore . . . . . . . . . . . . . . 8
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
11. Security Considerations . . . . . . . . . . . . . . . . . . . 10
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
12.1. Normative References . . . . . . . . . . . . . . . . . . . 11
12.2. Informative References . . . . . . . . . . . . . . . . . . 12
Appendix A. Design Rationale . . . . . . . . . . . . . . . . . . 12
Appendix B. Acknowledgments . . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13
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1. 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 use the Augmented Backus-Naur Form (ABNF) [RFC5234]
notation including the core rules defined in Appendix B of RFC 4234.
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.
2. Introduction
This specification extends IMAP4rev1 [RFC3501] to permit unencoded
UTF-8 [RFC3629] in headers as described in Internationalized Email
Headers [RFC5335]. It also adds a mechanism to support mailbox
names, login names and passwords using the UTF-8 charset.
3. UTF8 IMAP Capability
The basic "UTF8" capability indicates the server supports UTF-8
quoted strings, the "UTF8" parameter to SELECT and EXAMINE, and UTF-8
responses from the LIST and LSUB commands.
A client MUST use the "ENABLE UTF8" command (defined in [RFC5161]) to
indicate to the server that the client accepts UTF-8 quoted-strings.
The "ENABLE UTF8" command MUST only be used in the authenticated
state.
3.1. IMAP UTF-8 Quoted Strings
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" capability, it informs the client that it supports native
UTF-8 quoted-strings with the following syntax:
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string =/ utf8-quoted
utf8-quoted = "*" DQUOTE *UQUOTED-CHAR DQUOTE
UQUOTED-CHAR = QUOTED-CHAR / UTF8-2 / UTF8-3 / UTF8-4
; UTF8-2, UTF8-3, and UTF8-4 are as defined in RFC 3629
When this quoting mechanism is used by the client (specifically an
octet sequence beginning with *" and ending with "), then the server
MUST reject octet sequences with the high bit set which fail to
comply with the formal syntax in [RFC3629] with a BAD response.
The IMAP server MUST NOT send utf8-quoted syntax to the client unless
the client has indicated support for that syntax by using the "ENABLE
UTF8" command.
If the UTF8 capability is advertised, then utf8-quoted syntax MAY be
used with any IMAP argument that permits a string or an astring.
However, if characters outside the US-ASCII repertoire are used in an
inappropriate place, the results would be the same as if other
syntacticly valid but semantically invalid characters were used. For
example, if the client includes UTF-8 characters in the user or
password arguments (and the server has not advertised UTF8-USER), the
LOGIN command will fail as it would with any other invalid user name
or password. Specific cases where UTF-8 characters are permitted or
not permitted are described in the following paragraphs.
All IMAP servers SHOULD accept UTF-8 in mailbox names and IMAP
servers which 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 (section 2 of
[RFC5198]) with the specific exception that they may not contain
control characters (0000-001F, 0080-009F), delete (007F), line
separator (2028) or paragraph separator (2029).
If an IMAP client issues a SEARCH command which uses a mixture of
utf8-quoted syntax and a SEARCH CHARSET other than UTF-8, then the
IMAP server SHOULD reject the command with a BAD response (due to the
conflicting charset labels).
3.2. UTF8 Parameter to SELECT and EXAMINE
The "UTF8" capability also indicates the server supports the UTF8
parameter to SELECT and EXAMINE. When a mailbox is selected with the
UTF8 parameter, it alters the behavior of all IMAP commands related
to message sizes, message headers and MIME body headers so they refer
to the message with UTF-8 headers. If the mailstore is not UTF-8
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header native and the SELECT or EXAMINE command with UTF-8 header
modifier succeeds, then the server MUST return results as if the
mailstore was UTF-8 header native with upconversion requirements as
described in Section 8. The server MAY reject the SELECT or EXAMINE
command with the [NOT-UTF-8] response code, unless the UTF8=ALL or
UTF8=ONLY capability is advertised.
Servers MAY include mailboxes which can only be selected or examined
if the UTF8 parameter is provided. However, such mailboxes MUST NOT
be included in the output of an unextended LIST, LSUB or equivalent
command. If a client attempts to SELECT or EXAMINE such mailboxes
without the UTF8 parameter, the server MUST reject the command with a
[UTF-8-ONLY] response code. As a result, such mailboxes will not
accessible by IMAP clients written prior to this specification and
are discouraged unless the server advertises UTF8=ONLY or the server
implements IMAP4 LIST Command Extensions [RFC5258].
utf8-select-param = "UTF8"
;; Conforms to <select-param> from RFC 4466
C: a SELECT newmailbox (UTF8)
S: ...
S: a OK SELECT completed
C: b FETCH 1 (SIZE ENVELOPE BODY)
S: ... < UTF-8 header native results >
S: b OK FETCH completed
C: c EXAMINE legacymailbox (UTF8)
S: c NO [NOT-UTF-8] Mailbox does not support UTF-8 access
C: d SELECT funky-new-mailbox
S: d NO [UTF-8-ONLY] Mailbox requires UTF-8 client
3.3. UTF-8 LIST and LSUB Responses
After an IMAP client successfully issues an "ENABLE UTF8" command,
the server MUST NOT return in LIST results any mailbox names to the
client following the IMAP4 Mailbox International Naming Convention.
Instead, the server MUST return any mailbox names with characters
outside the US-ASCII repertorie using utf8-quoted syntax. (The IMAP4
Mailbox International Naming Convention has proved problematic in the
past, so the desire is to make this syntax obsolete as quickly as
possible.)
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3.4. UTF-8 Interaction with IMAP4 LIST Command Extensions
When an IMAP server advertises both the "UTF8" capability and the
"LIST-EXTENEDED" [RFC5258] capability, the server MUST support the
LIST extensions described in this section.
3.4.1. UTF8 and UTF8ONLY LIST Selection Options
The UTF8 LIST selection option tells the server to include mailboxes
that only support UTF-8 headers in the output of the list command.
The UTF8ONLY LIST selection option tells the server to include all
mailboxes that support UTF-8 headers and to exclude mailboxes that
don't support UTF-8 headers. Note that UTF8ONLY implies UTF8 so it
is not necessary for the client to request both. Use of either
selection option will also result in UTF-8 mailbox names in the
result as described in Section 3.3 and implies the UTF8 List return
option described in Section 3.4.2.
3.4.2. UTF8 LIST Return Option
If the client supplies the UTF8 LIST return option, then the server
MUST include either the \NoUTF8 or the \UTF8Only mailbox attribute as
appropriate. The \NoUTF8 mailbox attribute indicates an attempt to
SELECT or EXAMINE that mailbox with the UTF8 parameter will fail with
a [NOT-UTF-8] response code. The \UTF8Only mailbox attribute
indicates an attempt to SELECT or EXAMINE that mailbox without the
UTF8 parameter will fail with a [UTF-8-ONLY] response code. Note
that computing this information may be expensive on some server
implementations so this return option should not be used unless
necessary.
The ABNF [RFC5234] for these LIST extensions follows:
list-select-independent-opt =/ "UTF8"
list-select-base-opt =/ "UTF8ONLY"
mbx-list-oflag =/ "\NoUTF8" / "\UTF8Only"
return-option =/ "UTF8"
resp-text-code =/ "NOT-UTF-8" / "UTF-8-ONLY"
4. UTF8=APPEND Capability
If the UTF8=APPEND capability is advertised, then the server accepts
UTF-8 headers in the APPEND command message argument. A client which
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sends a message with UTF-8 headers to the server MUST include the
UTF8 APPEND parameter. The ABNF for this APPEND parameter follows:
append-ext =/ "UTF8"
A server which advertises UTF8=APPEND has to comply with the
requirements of the IMAP base specification and [RFC5322] for message
fetching. Mechanisms for 7-bit downgrading to help comply with the
standards are discussed in Downgrading mechanism for
Internationalized eMail Address (IMA) [RFC5504].
IMAP servers which do not advertise the UTF8=APPEND or UTF8=ONLY
capability SHOULD reject an APPEND command which includes any 8-bit
in the message headers with a "NO" response.
5. UTF8=USER Capability
If the UTF8=USER capability is advertised, that indicates the server
accepts UTF-8 user names and passwords and applies SASLprep [RFC4013]
to both arguments of the LOGIN command. The server MUST reject UTF-8
which fails to comply with the formal syntax in RFC 3629 [RFC3629].
6. UTF8=ALL Capability
This capability indicates all server mailboxes support UTF-8 headers.
Specifically, SELECT and EXAMINE with the UTF8 parameter will never
fail with a [NOT-UTF-8] response token.
7. UTF8=ONLY Capability
This capability permits an IMAP server to advertise that it does not
support the international mailbox name convention (modified UTF-7),
and does not permit selection or examination of any mailbox unless
the UTF8 parameter is provided. As this is an incompatible change to
IMAP, a clear warning is necessary. IMAP clients which find
implementation of the UTF8 capability problematic are encouraged to
at least detect the UTF8=ONLY capability and provide an informative
error message to the end-user.
When an IMAP mailbox internally uses UTF-8 header native storage, the
down-conversion step necessary to permit selection or examination of
the mailbox in a backwards compatible fashion will become more
difficult to support. Although it is hoped deployed IMAP servers do
not advertise UTF8=ONLY for some years, this capability is intended
to minimize the disruption when legacy support finally goes away.
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The UTF8=ONLY capability implies the UTF8 base capability, the
UTF8=ALL capability and the UTF8=APPEND capability. A server which
advertises UTF8=ONLY need not advertise the three implicit
capabilities.
8. Up-Conversion Server Requirements
When an IMAP4 server uses a traditional mailbox format that includes
7-bit headers and it chooses to permit access to that mailbox with
the UTF8 parameter, it MUST support minimal up-conversion as
described in this section.
The server MUST support up-conversion of the following address
header-fields in the message header: From, Sender, To, CC, Bcc,
Resent-From, Resent-Sender, Resent-To, Resent-CC, Resent-Bcc, and
Reply-To. This up-conversion MUST include address local-parts in
fields downgraded according to [RFC5504], address domains encoded
according to IDNA [RFC3490], and MIME header encoding [RFC2047] of
display-names and any [RFC5322] comments.
The following charsets MUST be supported for up-conversion of MIME
header encoding [RFC2047]: UTF-8, US-ASCII, ISO-8859-1, ISO-8859-2,
ISO-8859-3, ISO-8859-4, ISO-8859-5, ISO-8859-6, ISO-8859-7,
ISO-8859-8, ISO-8859-9, ISO-8859-10, ISO-8859-14, and ISO-8859-15.
Other widely deployed MIME charsets SHOULD be supported.
Up-conversion of MIME header encoding of the following headers MUST
also be implemented: Subject, Date ([RFC5322] comments only),
Comments, Keywords, Content-Description.
Server implementations also SHOULD up-convert all MIME body headers,
SHOULD up-convert or remove the deprecated (and misused) name
parameter [RFC1341] on Content-Type and MUST up-convert the Content-
Disposition filename parameter. These parameters can be encoded
using the standard MIME parameter encoding [RFC2231] mechanism, or
via non-standard use of MIME header encoding [RFC2047] in quoted
strings.
The IMAP server MUST NOT perform up-conversion of headers and content
of multipart/signed, as well as Original-Recipient and Return-Path.
9. 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
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the IMAP4rev1 [RFC3501] base specification 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
Downgrading mechanism for Internationalized eMail Address (IMA)
[RFC5504].
An IMAP server with a mailbox that supports UTF-8 headers MUST comply
with the protocol requirements implicit from Section 8. However, the
code necessary for such compliance need not be part of the IMAP
server itself in this case. For example, the minimal required up-
conversion could be performed when a message is inserted into the
IMAP-accessible mailbox.
10. IANA Considerations
This adds five new capabilities ("UTF8", "UTF8=USER", "UTF8=APPEND",
"UTF8=ALL", "UTF8=ONLY") to the IMAP4rev1 capability registry
[RFC3501].
This adds two new IMAP4 list selection options and one new IMAP4 list
return option.
1. LIST-EXTENDED option name: UTF8
LIST-EXTENDED option type: SELECTION
Implied return options(s): UTF8
LIST-EXTENDED option description: Causes the LIST response to
include mailboxes which mandate the UTF8 SELECT/EXAMINE
parameter.
Published specification: RFC XXXX, Section 3.4.1
Security considerations: RFC XXXX, Section 11
Intended usage: COMMON
Person an email address to contact for further information: see
Authors' Addresses at the end of this specification
Owner/Change controller: iesg@ietf.org
2. LIST-EXTENDED option name: UTF8ONLY
LIST-EXTENDED option type: SELECTION
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Implied return options(s): UTF8
LIST-EXTENDED option description: Causes the LIST response to
include mailboxes which mandate the UTF8 SELECT/EXAMINE parameter
and exclude mailboxes which do not support the UTF8 SELECT/
EXAMINE parameter.
Published specification: RFC XXXX, Section 3.4.1
Security considerations: RFC XXXX, Section 11
Intended usage: COMMON
Person an email address to contact for further information: see
Authors' Addresses at the end of this specification
Owner/Change controller: iesg@ietf.org
3. LIST-EXTENDED option name: UTF8
LIST-EXTENDED option type: RETURN
Implied return options(s): none
LIST-EXTENDED option description: Causes the LIST response to
include \NoUTF8 and \UTF8Only mailbox attributes.
Published specification: RFC XXXX, Section 3.4.1
Security considerations: RFC XXXX, Section 11
Intended usage: COMMON
Person an email address to contact for further information: see
Authors' Addresses at the end of this specification
Owner/Change controller: iesg@ietf.org
11. 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.
12. References
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12.1. Normative References
[RFC1341] Borenstein, N. and N. Freed, "MIME (Multipurpose Internet
Mail Extensions): Mechanisms for Specifying and Describing
the Format of Internet Message Bodies", RFC 1341,
June 1992.
[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.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2183] Troost, R., Dorner, S., and K. Moore, "Communicating
Presentation Information in Internet Messages: The
Content-Disposition Header Field", RFC 2183, August 1997.
[RFC2231] Freed, N. and K. Moore, "MIME Parameter Value and Encoded
Word Extensions:
Character Sets, Languages, and Continuations", RFC 2231,
November 1997.
[RFC3490] Faltstrom, P., Hoffman, P., and A. Costello,
"Internationalizing Domain Names in Applications (IDNA)",
RFC 3490, March 2003.
[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.
[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.
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[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.
[RFC5335] Abel, Y., "Internationalized Email Headers", RFC 5335,
September 2008.
[RFC5504] Fujiwara, K. and Y. Yoneya, "Downgrading Mechanism for
Email Address Internationalization", RFC 5504, March 2009.
12.2. Informative References
[RFC2049] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Five: Conformance Criteria and
Examples", RFC 2049, November 1996.
[RFC2088] Myers, J., "IMAP4 non-synchronizing literals", RFC 2088,
January 1997.
[RFC2277] Alvestrand, H., "IETF Policy on Character Sets and
Languages", BCP 18, RFC 2277, January 1998.
[I-D.ietf-eai-pop]
Gellens, R. and C. Newman, "POP3 Support for UTF-8",
draft-ietf-eai-pop-06 (work in progress), June 2009.
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
which 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.
UTF8=USER is optional because many identity systems are US-ASCII
only, so it's helpful to inform the client up-front that UTF-8 won't
work.
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UTF8=APPEND is optional because it effectively requires IMAP server
support for down-conversion which is a much more complex operation
than up-conversion.
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.
The up-conversion requirements are designed to balance the desire to
deprecate and eventually eliminate complicated encodings (like MIME
header encodings) without creating a significant deployment burden
for servers. As IMAP4 servers already require a MIME parser, this
includes additional server up-conversion requirements not present in
POP3 Support for UTF-8 [I-D.ietf-eai-pop].
The set of mandatory charsets comes from two sources: MIME
requirements [RFC2049] and IETF Policy on Character Sets [RFC2277].
Including a requirement to up-convert widely deployed encoded
ideographic charsets to UTF-8 would be reasonable for most scenarios,
but may require unacceptable table sizes for some embedded devices.
The open-ended recommendation to support widely deployed charsets
avoids the political ramifications of attempting to list such
charsets. The authors believe market forces, existing open-source
software, and public conversion tables are sufficient to deploy the
appropriate charsets.
Appendix B. Acknowledgments
TBD.
Authors' Addresses
Pete Resnick
Qualcomm Incorporated
5775 Morehouse Drive
San Diego, CA 92121-1714
US
Phone: +1 858 651 4478
Email: presnick@qualcomm.com
URI: http://www.qualcomm.com/~presnick/
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Chris Newman
Sun Microsystems
3401 Centrelake Dr., Suite 410
Ontario, CA 91761
US
Email: chris.newman@sun.com
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