    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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       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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       [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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    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.

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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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