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rfc:rfc5738

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.

Resnick & Newman Experimental [Page 1] 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

Resnick & Newman Experimental [Page 2] RFC 5738 IMAP Support for UTF-8 March 2010

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

Resnick & Newman Experimental [Page 3] RFC 5738 IMAP Support for UTF-8 March 2010

 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).

Resnick & Newman Experimental [Page 4] RFC 5738 IMAP Support for UTF-8 March 2010

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.

Resnick & Newman Experimental [Page 5] RFC 5738 IMAP Support for UTF-8 March 2010

 (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"

Resnick & Newman Experimental [Page 6] RFC 5738 IMAP Support for UTF-8 March 2010

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.

Resnick & Newman Experimental [Page 7] RFC 5738 IMAP Support for UTF-8 March 2010

 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.

Resnick & Newman Experimental [Page 8] RFC 5738 IMAP Support for UTF-8 March 2010

 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].

Resnick & Newman Experimental [Page 9] RFC 5738 IMAP Support for UTF-8 March 2010

 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

Resnick & Newman Experimental [Page 10] RFC 5738 IMAP Support for UTF-8 March 2010

 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.

Resnick & Newman Experimental [Page 11] 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.

Resnick & Newman Experimental [Page 12] 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.

Resnick & Newman Experimental [Page 13] RFC 5738 IMAP Support for UTF-8 March 2010

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.

Resnick & Newman Experimental [Page 14] 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

Resnick & Newman Experimental [Page 15]

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