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Network Working Group P. Resnick
Request for Comments: 5738 Qualcomm Incorporated
Updates: 3501 C. Newman
Category: Experimental Sun Microsystems
March 2010
IMAP Support for UTF-8
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.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for examination, experimental implementation, and
evaluation.
This document defines an Experimental Protocol for the Internet
community. 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). Not
all documents approved by the IESG are a candidate for any level of
Internet Standard; see 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/rfc5738.
Copyright Notice
Copyright (c) 2010 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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RFC 5738 IMAP Support for UTF-8 March 2010
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions Used in This Document . . . . . . . . . . . . . . 3
3. UTF8=ACCEPT IMAP Capability . . . . . . . . . . . . . . . . . 3
3.1. IMAP UTF-8 Quoted Strings . . . . . . . . . . . . . . . . 3
3.2. UTF8 Parameter to SELECT and EXAMINE . . . . . . . . . . . 5
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 . . . . . . . . . . . . . . . . . . . . 7
5. UTF8=USER Capability . . . . . . . . . . . . . . . . . . . . . 7
6. UTF8=ALL Capability . . . . . . . . . . . . . . . . . . . . . 7
7. UTF8=ONLY Capability . . . . . . . . . . . . . . . . . . . . . 8
8. Up-Conversion Server Requirements . . . . . . . . . . . . . . 8
9. Issues with UTF-8 Header Mailstore . . . . . . . . . . . . . . 9
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
11. Security Considerations . . . . . . . . . . . . . . . . . . . 11
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
12.1. Normative References . . . . . . . . . . . . . . . . . . . 11
12.2. Informative References . . . . . . . . . . . . . . . . . . 13
Appendix A. Design Rationale . . . . . . . . . . . . . . . . . . 14
Appendix B. Examples Demonstrating Relationships between
UTF8= Capabilities . . . . . . . . . . . . . . . . . 15
Appendix C. Acknowledgments . . . . . . . . . . . . . . . . . . . 15
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1. Introduction
This specification extends IMAP4rev1 [RFC3501] to permit 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. This
specification creates five new IMAP capabilities to allow servers to
advertise these new extensions, along with two new IMAP LIST
selection options and a new IMAP LIST return option.
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 including the core rules defined in Appendix B of
[RFC5234]. 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 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=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 7 and the "UTF8=ALL" capability described in
Section 6 imply the "UTF8=ACCEPT" capability. See additional
information in these sections.)
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
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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
native UTF-8 quoted-strings with the following syntax:
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 that 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=ACCEPT" command.
If the server advertises the "UTF8=ACCEPT" capability, the client MAY
use utf8-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. 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 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,
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 that uses a mixture of
utf8-quoted syntax and a SEARCH CHARSET other than UTF-8. If an IMAP
server receives such a SEARCH command, it SHOULD reject the command
with a BAD response (due to the conflicting charset labels).
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3.2. UTF8 Parameter to SELECT and EXAMINE
The "UTF8=ACCEPT" capability also indicates that 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 header native and the SELECT or EXAMINE
command with UTF-8 header modifier succeeds, then the server MUST
return results as if the mailstore were 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 that 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 be 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=ACCEPT"
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 repertoire using utf8-quoted syntax.
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(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.)
3.4. UTF-8 Interaction with IMAP4 LIST Command Extensions
When an IMAP server advertises both the "UTF8=ACCEPT" capability and
the "LIST-EXTENDED" [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
that 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 that 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"
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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 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
A server that 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 that do not advertise the "UTF8=APPEND" or "UTF8=ONLY"
capability SHOULD reject an APPEND command that includes any 8-bit in
the message headers with a "NO" response.
Note that the "UTF8=ONLY" capability described in Section 7 implies
the "UTF8=APPEND" capability. See additional information in that
section.
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 that fails to comply with the formal syntax in RFC 3629
[RFC3629] or if it encounters Unicode characters listed in Section
2.3 of SASLprep RFC 4013 [RFC4013].
6. UTF8=ALL Capability
The "UTF8=ALL" 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 code.
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Note that the "UTF8=ONLY" capability described in Section 7 implies
the "UTF8=ALL" capability. See additional information in that
section.
Note that the "UTF8=ALL" capability implies the "UTF8=ACCEPT"
capability.
7. 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), 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 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.
When an IMAP mailbox internally uses UTF-8 header native storage, the
down-conversion step is necessary to permit selection or examination
of the mailbox in a backwards compatible fashion will become more
difficult to support. Although it is hoped that deployed IMAP
servers will not advertise "UTF8=ONLY" for some years, this
capability is intended to minimize the disruption when legacy support
finally goes away.
The "UTF8=ONLY" capability implies the "UTF8=ACCEPT" capability, the
"UTF8=ALL" capability, and the "UTF8=APPEND" capability. A server
that 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 Internationalizing Domain Names in Applications (IDNA)
[RFC3490], and MIME header encoding [RFC2047] of display-names and
any [RFC5322] comments.
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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.
If the server supports other charsets in IMAP SEARCH or IMAP CONVERT
[RFC5259], it SHOULD also support those charsets in this conversion.
Up-conversion of MIME header encoding of the following headers MUST
also be implemented: Subject, Date ([RFC5322] comments only),
Comments, Keywords, and Content-Description.
Server implementations also SHOULD up-convert all MIME body headers
[RFC2045], SHOULD up-convert or remove the deprecated (and misused)
"name" parameter [RFC1341] on Content-Type, and MUST up-convert the
Content-Disposition [RFC2183] "filename" parameter, except when any
of these are contained within a multipart/signed MIME body part (see
below). 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 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 "Downgrading Mechanism for Email Address
Internationalization" [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=ACCEPT", "UTF8=USER",
"UTF8=APPEND", "UTF8=ALL", and "UTF8=ONLY") to the IMAP4rev1
Capabilities registry [RFC3501].
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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 that mandate the UTF8 SELECT/EXAMINE parameter.
Published specification: RFC 5738, Section 3.4.1
Security considerations: RFC 5738, Section 11
Intended usage: COMMON
Person and email address to contact for further information: see
the 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
Implied return options(s): UTF8
LIST-EXTENDED option description: Causes the LIST response to
include mailboxes that mandate the UTF8 SELECT/EXAMINE parameter
and exclude mailboxes that do not support the UTF8 SELECT/EXAMINE
parameter.
Published specification: RFC 5738, Section 3.4.1
Security considerations: RFC 5738, Section 11
Intended usage: COMMON
Person and email address to contact for further information: see
the Authors' Addresses at the end of this specification
Owner/Change controller: iesg@ietf.org
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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 5738, Section 3.4.1
Security considerations: RFC 5738, Section 11
Intended usage: COMMON
Person and email address to contact for further information: see
the 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
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.
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RFC 5738 IMAP Support for UTF-8 March 2010
[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.
[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.
[RFC5259] Melnikov, A. and P. Coates, "Internet Message Access
Protocol - CONVERT Extension", RFC 5259, July 2008.
[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
October 2008.
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RFC 5738 IMAP Support for UTF-8 March 2010
[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.
[RFC5721] Gellens, R. and C. Newman, "POP3 Support for UTF-8",
RFC 5721, February 2010.
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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.
"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.
"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 [RFC5721].
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.
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RFC 5738 IMAP Support for UTF-8 March 2010
Appendix B. Examples Demonstrating Relationships between UTF8=
Capabilities
UTF8=ACCEPT UTF8=USER UTF8=APPEND
UTF8=ACCEPT UTF8=ALL
UTF8=ALL ; Note, same as above
UTF8=ACCEPT UTF8=USER UTF8=APPEND UTF8=ALL UTF8=ONLY
UTF8=USER UTF8=ONLY ; Note, same as above
Appendix C. 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
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/
Chris Newman
Sun Microsystems
800 Royal Oaks
Monrovia, CA 91016
US
EMail: chris.newman@sun.com
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