IMAP Support for UTF-8
draft-ietf-eai-5738bis-03

Internet Engineering Task Force                          P. Resnick, Ed.
Internet-Draft                                     Qualcomm Incorporated
Obsoletes: RFC5738 (if approved)                          C. Newman, Ed.
Intended status: Standards Track                                  Oracle
Expires: June 28, 2012                                      S. Shen, Ed.
                                                                   CNNIC
                                                       December 26, 2011

                         IMAP Support for UTF-8
                       draft-ietf-eai-5738bis-03

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.  This
   specification replaces RFC 5738.

Status of This Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on June 28, 2012.

Copyright Notice

   Copyright (c) 2011 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
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   (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
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   include Simplified BSD License text as described in Section 4.e of

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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  . . . . . . . . . . . . . . . .  4
     3.2.  UTF8 Parameter to SELECT and EXAMINE . . . . . . . . . . .  5
     3.3.  UTF-8 LIST and LSUB Responses  . . . . . . . . . . . . . .  6
     3.4.  UTF-8 Interaction with IMAP4 LIST Command Extensions . . .  6
       3.4.1.  UTF8 LIST Selection Option . . . . . . . . . . . . . .  6
       3.4.2.  UTF8 LIST Return Option  . . . . . . . . . . . . . . .  7
   4.  IMAP UTF8 Append Data Extension  . . . . . . . . . . . . . . .  8
   5.  LOGIN Command and UTF-8  . . . . . . . . . . . . . . . . . . .  9
   6.  UTF8=ONLY Capability . . . . . . . . . . . . . . . . . . . . .  9
   7.  Issues with UTF-8 Header Mailstore . . . . . . . . . . . . . .  9
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 11
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 11
     10.2. Informative References . . . . . . . . . . . . . . . . . . 12
   Appendix A.  Appendix A. Design Rationale  . . . . . . . . . . . . 13
   Appendix B.  Appendix B. Acknowledgments . . . . . . . . . . . . . 13

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1.  Introduction

   This specification extends IMAP4rev1 [RFC3501] to permit UTF-8
   [RFC3629] in headers as described in "Internationalized Email
   Headers" [I-D.ietf-eai-rfc5335bis] .  It also adds a mechanism to
   support mailbox names, login names, and passwords using the UTF-8
   charset.  This specification creates two new IMAP capabilities to
   allow servers to advertise these new extensions, along with one new
   IMAP LIST selection option and a new IMAP LIST return option.

   This specification permits implementation of an IMAP server that
   hides mailboxes with internationalized email messages from IMAP
   clients that do not support this extension.  Implementation of "Post-
   delivery Message Downgrading for Internationalized Email Messages"
   [popimap-downgrade] is necessary for an MDA to make mailboxes with
   internationalized email messages visible to IMAP clients that do not
   support this extension.

   This specification replaces an earlier, experimental, approach to the
   same problem [RFC5738].

2.  Conventions Used in this Document

   The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and "MAY"
   in this document are to be interpreted as defined in "Key words for
   use in RFCs to Indicate Requirement Levels" [RFC2119].

   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

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   in the authenticated state.  (Note that the "UTF8=ONLY" 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
   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        =/ uQuoted

        uQuoted       = "*" DQUOTE *uQUOTED-CHAR DQUOTE
              ; referred as 'utf8-quote' in this document

        DQUOTE        = <Defined in appendix B.1 of RFC 5234>

        uQUOTED-CHAR  = QUOTED-CHAR / UTF8-2 / UTF8-3 / UTF8-4

        UTF8-2        =   <Defined in Section 4 of RFC3629>

        UTF8-3        =   <Defined in Section 4 of RFC3629>

        UTF8-4        =   <Defined in Section 4 of RFC3629>

   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.  Specific cases where UTF-8 characters
   are permitted or not permitted are described in the following
   paragraphs.

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   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).

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.

   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 LIST, LSUB, or equivalent command
   unless the "UTF8" parameter is present.  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".

   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)

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   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 using either utf8-quoted or literal 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.)

3.4.  UTF-8 Interaction with IMAP4 LIST Command Extensions

   When an IMAP server advertises the "UTF8=ACCEPT" capability the
   server MUST implement a "UTF8" LIST selection option and LIST return
   option.  These options are compatible with the "LIST-EXTENDED"
   [RFC5258] capability.

   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 LIST Selection Option

   The "UTF8" LIST selection option tells the server to include
   mailboxes that only support UTF-8 headers in the output of the list
   command.  Use of this 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.

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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-independent-opt is defined in RFC 5258 Section 6

        list-select-base-opt        =/ "UTF8ONLY"
              ; list-select-base-opt is defined in RFC 5258 Section 6

        return-option               =/ "UTF8"
              ; return-option is defined in RFC 5258 Section 6

        mbx-list-oflag              =/ "\NoUTF8" / "\UTF8Only"
              ; mbx-list-oflag is defined in RFC 3501 Section 9

        resp-text-code              =/ "NOT-UTF-8" / "UTF-8-ONLY"
                      ; resp-text-code is defined in RFC 3501 Section 9

   In the event that the server advertises UTF8=ACCEPT and does not also
   advertise LIST-EXTENDED [RFC 5258], then the first four of the above
   five ABNF rules are replaced with the following ABNF:

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      list =      "LIST" [SP list-select-opts] SP mailbox SP list-mailbox
                  [SP list-return-opts]
                  ; replaces "list" rule in RFC 3501 Section 9

      list-select-opts = "(" [*(list-select-opt SP) list-select-opt] ")"

      list-select-opt  = "SUBSCRIBED" / "UTF8"

      list-return-opts =  "RETURN" SP
                  "(" [return-option *(SP return-option)] ")"

      return-option =  "SUBSCRIBED" / "UTF8"

      mbx-list-sflag =/ "\NonExistent"

      mbx-list-oflag =/ "\NoUTF8" / "\UTF8Only" / "\Subscribed"
                 ; mbx-list-oflag is defined in RFC 3501 Section 9

   In this case, the "SUBSCRIBED" LIST select option and "SUBSCRIBED"
   LIST return option work as described in LIST Command Extensions [RFC
   5258].  Any IMAP client that uses LIST selection options or LIST
   return options (including "UTF8" and "SUBSCRIBED") MUST be able to
   interoperate with a server that advertises both LIST-EXTENDED and
   UTF8=ACCEPT.  Such servers MAY include LIST response extensions as
   described in LIST Command Extensions [RFC 5258].

4.  IMAP UTF8 Append Data Extension

   If the "UTF8=ACCEPT" 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=ACCEPT" MAY fail for \NotUTF8
   mailboxes with a NOT-UTF-8 response code.  If this command does not
   fail, it MAY follow 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

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   [popimap-downgrade].

   IMAP servers that advertise support for "UTF8=ACCEPT" or "UTF8=ONLY"
   MUST reject an APPEND command that includes any 8-bit in the message
   headers with a "NO" response, when IMAP clients do not issue "ENABLE
   UTF8=ACCEPT" or "ENABLE UTF8=ONLY".

   Note that the "UTF8=ONLY" capability described in Section 6 implies
   the "UTF8=ACCEPT" capability.  See additional information in that
   section.

5.  LOGIN Command and UTF-8

   This specification doesn't extend the IMAP LOGIN command [RFC3501] to
   support UTF-8 usernames and passwords.  Whenever a client needs to
   use UTF-8 username/passwords, it MUST use the IMAP AUTHENTICATE
   command which is already capable of passing UTF-8 user names and
   credentials.

   Although this makes it syntacically legal to have a UTF-8 user name
   or password, there is no guarantee the user provisioning system used
   by the IMAP server will allow such identities.  This is an
   implementation decision and MAY depend on what identity system the
   IMAP server is configured to use.

6.  UTF8=ONLY Capability

   The "UTF8=ONLY" capability 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.

   The "UTF8=ONLY" capability implies the "UTF8=ACCEPT" capability.
   UTF8=ACCEPT and UTF8=ONLY SHOULD be mutually exclusive.  An IMAP
   server can advertise one of them, but never both.

7.  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

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   are discussed in [popimap-downgrade].

8.  IANA Considerations

   This document adds two new capabilities ("UTF8=ACCEPT" and
   "UTF8=ONLY") to the IMAP4rev1 Capabilities registry [RFC3501].  Three
   other IMAP capabilites that were described in the experimental
   predecessor to this document (UTF8=ALL, UTF8=APPEND, UTF8=USER) are
   to be marked OBSOLETE in the registry.

   This document 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 5738bis, Section 3.4.1

       Security considerations: RFC 5738bis, Section 9

       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:  Part of previous experiment, no
       longer used.

       Published specification: RFC 5738bis, Section 3.4.1

       Security considerations: RFC 5738bis, section 9

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       Intended usage: OBSOLETE

       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

   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 5738bis, Section 3.4.1

       Security considerations: RFC 5738bis, section 9

       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

9.  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.

   This document does not address downgrading scenarios, the security
   issues are discussed in [popimap-downgrade]

10.  References

10.1.  Normative References

   [RFC2119]                  Bradner, S., "Key words for use in RFCs to
                              Indicate Requirement Levels", BCP 14,
                              RFC 2119, March 1997.

   [RFC3501]                  Crispin, M., "INTERNET MESSAGE ACCESS
                              PROTOCOL - VERSION 4rev1", RFC 3501,
                              March 2003.

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   [RFC3629]                  Yergeau, F., "UTF-8, a transformation
                              format of ISO 10646", STD 63, RFC 3629,
                              November 2003.

   [RFC4013]                  Zeilenga, K., "SASLprep: Stringprep
                              Profile for User Names and Passwords",
                              RFC 4013, February 2005.

   [RFC4466]                  Melnikov, A. and C. Daboo, "Collected
                              Extensions to IMAP4 ABNF", RFC 4466,
                              April 2006.

   [RFC4469]                  Resnick, P., "Internet Message Access
                              Protocol (IMAP) CATENATE Extension",
                              RFC 4469, April 2006.

   [RFC5161]                  Gulbrandsen, A. and A. Melnikov, "The IMAP
                              ENABLE Extension", RFC 5161, March 2008.

   [RFC5198]                  Klensin, J. and M. Padlipsky, "Unicode
                              Format for Network Interchange", RFC 5198,
                              March 2008.

   [RFC5234]                  Crocker, D. and P. Overell, "Augmented BNF
                              for Syntax Specifications: ABNF", STD 68,
                              RFC 5234, January 2008.

   [RFC5258]                  Leiba, B. and A. Melnikov, "Internet
                              Message Access Protocol version 4 - LIST
                              Command Extensions", RFC 5258, June 2008.

   [RFC5322]                  Resnick, P., Ed., "Internet Message
                              Format", RFC 5322, October 2008.

   [I-D.ietf-eai-rfc5335bis]  Yang, A., Steele, S., and N. Freed,
                              "Internationalized Email Headers",
                              draft-ietf-eai-rfc5335bis-13 (work in
                              progress), October 2011.

10.2.  Informative References

   [RFC2088]                  Myers, J., "IMAP4 non-synchronizing
                              literals", RFC 2088, January 1997.

   [RFC5738]                  Resnick, P. and C. Newman, "IMAP Support
                              for UTF-8", RFC 5738, March 2010.

   [popimap-downgrade]        Fujiwara, K., "Post-delivery Message

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                              Downgrading for Internationalized Email
                              Messages",
                              draft-ietf-eai-popimap-downgrade-03 (work
                              in progress), October 2010.

Appendix A.  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.

   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.

Appendix B.  Appendix B. Acknowledgments

   The authors wish to thank the participants of the EAI working group
   for their contributions to this document with particular thanks to
   Harald Alvestrand, David Black, Randall Gellens, Arnt Gulbrandsen,
   Kari Hurtta, John Klensin, Xiaodong Lee, Charles Lindsey, Alexey
   Melnikov, Subramanian Moonesamy, Shawn Steele, Daniel Taharlev, and
   Joseph Yee for their specific contributions to the discussion.

Authors' Addresses

   Pete Resnick (editor)
   Qualcomm Incorporated
   5775 Morehouse Drive
   San Diego, CA  92121-1714
   US

   Phone: +1 858 651 4478
   EMail: presnick@qualcomm.com

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   Chris Newman (editor)
   Oracle
   800 Royal Oaks
   Monrovia, CA 91016
   USA

   Phone:
   EMail: chris.newman@oracle.com

   Sean Shen (editor)
   CNNIC
   No.4 South 4th Zhongguancun Street
   Beijing, 100190
   China

   Phone: +86 10-58813038
   EMail: shenshuo@cnnic.cn

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