IMAP Support for UTF-8
RFC 6855

Internet Engineering Task Force (IETF)                   P. Resnick, Ed.
Request for Comments: 6855                         Qualcomm Incorporated
Obsoletes: 5738                                           C. Newman, Ed.
Category: Standards Track                                         Oracle
ISSN: 2070-1721                                             S. Shen, Ed.
                                                                   CNNIC
                                                              March 2013

                         IMAP Support for UTF-8

Abstract

   This specification extends the Internet Message Access Protocol
   (IMAP) 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 is an Internet Standards Track document.

   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).  Further information on
   Internet Standards is available in 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/rfc6855.

Copyright Notice

   Copyright (c) 2013 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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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
   2.  Conventions Used in This Document  . . . . . . . . . . . . . .  2
   3.  "UTF8=ACCEPT" IMAP Capability and UTF-8 in IMAP
       Quoted-Strings . . . . . . . . . . . . . . . . . . . . . . . .  3
   4.  IMAP UTF8 "APPEND" Data Extension  . . . . . . . . . . . . . .  4
   5.  "LOGIN" Command and UTF-8  . . . . . . . . . . . . . . . . . .  5
   6.  "UTF8=ONLY" Capability . . . . . . . . . . . . . . . . . . . .  5
   7.  Dealing with Legacy Clients  . . . . . . . . . . . . . . . . .  6
   8.  Issues with UTF-8 Header Mailstore . . . . . . . . . . . . . .  7
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  8
   10. Security Considerations  . . . . . . . . . . . . . . . . . . .  8
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     11.1.  Normative References  . . . . . . . . . . . . . . . . . .  9
     11.2.  Informative References  . . . . . . . . . . . . . . . . . 10
   Appendix A.  Design Rationale  . . . . . . . . . . . . . . . . . . 11
   Appendix B.  Acknowledgments . . . . . . . . . . . . . . . . . . . 11

1.  Introduction

   This specification forms part of the Email Address
   Internationalization protocols described in the Email Address
   Internationalization Framework document [RFC6530].  It extends IMAP
   [RFC3501] to permit UTF-8 [RFC3629] in headers, as described in
   "Internationalized Email Headers" [RFC6532].  It also adds a
   mechanism to support mailbox names using the UTF-8 charset.  This
   specification creates two new IMAP capabilities to allow servers to
   advertise these new extensions.

   This specification assumes that the IMAP server will be operating in
   a fully internationalized environment, i.e., one in which all clients
   accessing the server will be able to accept non-ASCII message header
   fields and other information, as specified in Section 3.  At least
   during a transition period, that assumption will not be realistic for
   many environments; the issues involved are discussed in Section 7
   below.

   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].

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   The formal syntax uses the Augmented Backus-Naur Form (ABNF)
   [RFC5234] notation.  In addition, rules from IMAP [RFC3501], UTF-8
   [RFC3629], Extensions to IMAP ABNF [RFC4466], and IMAP "LIST" command
   extensions [RFC5258] are also referenced.  This document assumes that
   the reader will have a reasonably good understanding of these RFCs.

3.  "UTF8=ACCEPT" IMAP Capability and UTF-8 in IMAP Quoted-Strings

   The "UTF8=ACCEPT" capability indicates that the server supports the
   ability to open mailboxes containing internationalized messages with
   the "SELECT" and "EXAMINE" commands, and the server can provide UTF-8
   responses to the "LIST" and "LSUB" commands.  This capability also
   affects other IMAP extensions that can return mailbox names or their
   prefixes, such as NAMESPACE [RFC2342] and ACL [RFC4314].

   The "UTF8=ONLY" capability, described in Section 6, implies the
   "UTF8=ACCEPT" capability.  A server is said to support "UTF8=ACCEPT"
   if it advertises either "UTF8=ACCEPT" or "UTF8=ONLY".

   A client MUST use the "ENABLE" command [RFC5161] with the
   "UTF8=ACCEPT" option (defined in Section 4 below) to indicate to the
   server that the client accepts UTF-8 in quoted-strings and supports
   the "UTF8=ACCEPT" extension.  The "ENABLE UTF8=ACCEPT" command is
   only valid in the authenticated state.

   The IMAP base specification [RFC3501] 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 IMAP non-synchronizing
   literals [RFC2088], this requires an extra round trip for each UTF-8
   string sent by the client.  When the IMAP server supports
   "UTF8=ACCEPT", it supports UTF-8 in quoted-strings with the following
   syntax:

            quoted        =/ DQUOTE *uQUOTED-CHAR DQUOTE
                   ; QUOTED-CHAR is not modified, as it will affect
                   ; other RFC 3501 ABNF non-terminals.

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

            UTF8-2        =   <Defined in Section 4 of RFC 3629>

            UTF8-3        =   <Defined in Section 4 of RFC 3629>

            UTF8-4        =   <Defined in Section 4 of RFC 3629>

   When this extended quoting mechanism is used by the client, the
   server MUST reject, with a "BAD" response, any octet sequences with

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   the high bit set that fail to comply with the formal syntax
   requirements of UTF-8 [RFC3629].  The IMAP server MUST NOT send UTF-8
   in quoted-strings to the client unless the client has indicated
   support for that syntax by using the "ENABLE UTF8=ACCEPT" command.

   If the server supports "UTF8=ACCEPT", the client MAY use extended
   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.

   All IMAP servers that support "UTF8=ACCEPT" 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 ([RFC5198], Section 2) with the specific exception that
   they MUST NOT contain control characters (U+0000-U+001F and U+0080-U+
   009F), a delete character (U+007F), a line separator (U+2028), or a
   paragraph separator (U+2029).

   Once an IMAP client has enabled UTF-8 support with the "ENABLE
   UTF8=ACCEPT" command, it MUST NOT issue a "SEARCH" command that
   contains a charset specification.  If an IMAP server receives such a
   "SEARCH" command in that situation, it SHOULD reject the command with
   a "BAD" response (due to the conflicting charset labels).

4.  IMAP UTF8 "APPEND" Data Extension

   If the server supports "UTF8=ACCEPT", 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" data extension to the "APPEND" command.  If the server
   also advertises the "CATENATE" capability [RFC4469], the client can
   use the same data extension to include such a message in a catenated
   message part.  The ABNF for the "APPEND" data extension and
   "CATENATE" extension follows:

        utf8-literal   = "UTF8" SP "(" literal8 ")"

        literal8       = <Defined in RFC 4466>

        append-data    =/ utf8-literal

        cat-part       =/ utf8-literal

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   If an IMAP server supports "UTF8=ACCEPT" and the IMAP client has not
   issued the "ENABLE UTF8=ACCEPT" command, the server MUST reject, with
   a "NO" response, an "APPEND" command that includes any 8-bit
   character in message header fields.

5.  "LOGIN" Command and UTF-8

   This specification does not extend the IMAP "LOGIN" command [RFC3501]
   to support UTF-8 usernames and passwords.  Whenever a client needs to
   use UTF-8 usernames or passwords, it MUST use the IMAP "AUTHENTICATE"
   command, which is already capable of passing UTF-8 usernames and
   credentials.

   Although using the IMAP "AUTHENTICATE" command in this way makes it
   syntactically legal to have a UTF-8 username or password, there is no
   guarantee that the user provisioning system utilized 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 indicates that the server supports
   "UTF8=ACCEPT" (see Section 4) and that it requires support for UTF-8
   from clients.  In particular, this means that the server will send
   UTF-8 in quoted-strings, and it will not accept the older
   international mailbox name convention (modified UTF-7 [RFC3501]).
   Because these are incompatible changes to IMAP, explicit server
   announcement and client confirmation is necessary: clients MUST use
   the "ENABLE UTF8=ACCEPT" command before using this server.  A server
   that advertises "UTF8=ONLY" will reject, with a "NO [CANNOT]"
   response [RFC5530], any command that might require UTF-8 support and
   is not preceded by an "ENABLE UTF8=ACCEPT" command.

   IMAP clients that find support for a server that announces
   "UTF8=ONLY" problematic are encouraged to at least detect the
   announcement and provide an informative error message to the
   end-user.

   Because the "UTF8=ONLY" server capability includes support for
   "UTF8=ACCEPT", the capability string will include, at most, one of
   those and never both.  For the client, "ENABLE UTF8=ACCEPT" is always
   used -- never "ENABLE UTF8=ONLY".

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7.   Dealing with Legacy Clients

   In most situations, it will be difficult or impossible for the
   implementer or operator of an IMAP (or POP) server to know whether
   all of the clients that might access it, or the associated mail store
   more generally, will be able to support the facilities defined in
   this document.  In almost all cases, servers that conform to this
   specification will have to be prepared to deal with clients that do
   not enable the relevant capabilities.  Unfortunately, there is no
   completely satisfactory way to do so other than for systems that wish
   to receive email that requires SMTPUTF8 capabilities to be sure that
   all components of those systems -- including IMAP and other clients
   selected by users -- are upgraded appropriately.

   When a message that requires SMTPUTF8 is encountered and the client
   does not enable UTF-8 capability, choices available to the server
   include hiding the problematic message(s), creating in-band or
   out-of-band notifications or error messages, or somehow trying to
   create a surrogate of the message with the intention of providing
   useful information to that client about what has occurred.  Such
   surrogate messages cannot be actual substitutes for the original
   message: they will almost always be impossible to reply to (either at
   all or without loss of information) and the new header fields or
   specialized constructs for server-client communications may go beyond
   the requirements of current email specifications (e.g., [RFC5322]).
   Consequently, such messages may confuse some legacy mail user agents
   (including IMAP clients) or not provide expected information to
   users.  There are also trade-offs in constructing surrogates of the
   original message between accepting complexity and additional
   computation costs in order to try to preserve as much information as
   possible (for example, in "Post-Delivery Message Downgrading for
   Internationalized Email Messages" [RFC6857]) and trying to minimize
   those costs while still providing useful information (for example, in
   "Simplified POP and IMAP Downgrading for Internationalized Email"
   [RFC6858]).

   Implementations that choose to perform downgrading SHOULD use one of
   the standardized algorithms provided in RFC 6857 or RFC 6858.
   Getting downgrade algorithms right, and minimizing the risk of
   operational problems and harm to the email system, is tricky and
   requires careful engineering.  These two algorithms are well
   understood and carefully designed.

   Because such messages are really surrogates of the original ones, not
   really "downgraded" ones (although that terminology is often used for
   convenience), they inevitably have relationships to the originals
   that the IMAP specification [RFC3501] did not anticipate.  This
   brings up two concerns in particular: First, digital signatures

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   computed over and intended for the original message will often not be
   applicable to the surrogate message, and will often fail signature
   verification.  (It will be possible for some digital signatures to be
   verified, if they cover only parts of the original message that are
   not affected in the creation of the surrogate.)  Second, servers that
   may be accessed by the same user with different clients or methods
   (e.g., POP or webmail systems in addition to IMAP or IMAP clients
   with different capabilities) will need to exert extreme care to be
   sure that UIDVALIDITY [RFC3501] behaves as the user would expect.
   Those issues may be especially sensitive if the server caches the
   surrogate message or computes and stores it when the message arrives
   with the intent of making either form available depending on client
   capabilities.  Additionally, in order to cope with the case when a
   server compliant with this extension returns the same UIDVALIDITY to
   both legacy and "UTF8=ACCEPT"-aware clients, a client upgraded from
   being non-"UTF8=ACCEPT"-aware MUST discard its cache of messages
   downloaded from the server.

   The best (or "least bad") approach for any given environment will
   depend on local conditions, local assumptions about user behavior,
   the degree of control the server operator has over client usage and
   upgrading, the options that are actually available, and so on.  It is
   impossible, at least at the time of publication of this
   specification, to give good advice that will apply to all situations,
   or even particular profiles of situations, other than "upgrade legacy
   clients as soon as possible".

8.  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
   issuing "ENABLE UTF8=ACCEPT" first, it is the responsibility of the
   server to comply with the IMAP base specification [RFC3501] and the
   Internet Message Format [RFC5322] with respect to all header
   information transmitted over the wire.  The issue of handling
   messages containing non-ASCII characters in legacy environments is
   discussed in Section 7.

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9.  IANA Considerations

   This document redefines two capabilities ("UTF8=ACCEPT" and
   "UTF8=ONLY") in the "IMAP 4 Capabilities" registry [RFC3501].  Three
   other capabilities that were described in the experimental
   predecessor to this document ("UTF8=ALL", "UTF8=APPEND", "UTF8=USER")
   are now OBSOLETE.  IANA has updated the registry as follows:

    OLD:
      +--------------+-----------------+
      | UTF8=ACCEPT  |  [RFC5738]      |
      | UTF8=ALL     |  [RFC5738]      |
      | UTF8=APPEND  |  [RFC5738]      |
      | UTF8=ONLY    |  [RFC5738]      |
      | UTF8=USER    |  [RFC5738]      |
      +--------------+-----------------+

    NEW:
      +------------------------+---------------------+
      | UTF8=ACCEPT            |  [RFC6855]          |
      | UTF8=ALL (OBSOLETE)    |  [RFC5738] [RFC6855]|
      | UTF8=APPEND (OBSOLETE) |  [RFC5738] [RFC6855]|
      | UTF8=ONLY              |  [RFC6855]          |
      | UTF8=USER (OBSOLETE)   |  [RFC5738] [RFC6855]|
      +------------------------+---------------------+

10.  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 usernames and passwords.  Otherwise, this is not believed to
   alter the security considerations of IMAP.

   Special considerations, some of them with security implications,
   occur if a server that conforms to this specification is accessed by
   a client that does not, as well as in some more complex situations in
   which a given message is accessed by multiple clients that might use
   different protocols and/or support different capabilities.  Those
   issues are discussed in Section 7.

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11.  References

11.1.  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.

   [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.

   [RFC6530]  Klensin, J. and Y. Ko, "Overview and Framework for
              Internationalized Email", RFC 6530, February 2012.

   [RFC6532]  Yang, A., Steele, S., and N. Freed, "Internationalized
              Email Headers", RFC 6532, February 2012.

   [RFC6857]  Fujiwara, K., "Post-Delivery Message Downgrading for
              Internationalized Email Messages", RFC 6857, March 2013.

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   [RFC6858]  Gulbrandsen, A., "Simplified POP and IMAP Downgrading for
              Internationalized Email", RFC 6858, March 2013.

11.2.  Informative References

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

   [RFC2342]  Gahrns, M. and C. Newman, "IMAP4 Namespace", RFC 2342,
              May 1998.

   [RFC4314]  Melnikov, A., "IMAP4 Access Control List (ACL) Extension",
              RFC 4314, December 2005.

   [RFC5530]  Gulbrandsen, A., "IMAP Response Codes", RFC 5530,
              May 2009.

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

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Appendix A.  Design Rationale

   This non-normative section discusses the reasons behind some of the
   design choices in this specification.

   The "UTF8=ONLY" mechanism simplifies diagnosis of interoperability
   problems when legacy support goes away.  In the situation where
   backwards compatibility is not working anyway, the non-conforming
   "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.  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.

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Authors' Addresses

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

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

   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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