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IMAP Support for UTF-8

The information below is for an old version of the document.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 6855.
Authors Pete Resnick , Chris Newman , Sean Shen
Last updated 2012-08-24 (Latest revision 2012-08-01)
Replaces draft-ietf-eai-5378bis
RFC stream Internet Engineering Task Force (IETF)
Additional resources Mailing list discussion
Stream WG state WG Document
Document shepherd Dr. John C. Klensin
Shepherd write-up Show Last changed 2012-08-24
IESG IESG state AD Evaluation::Revised I-D Needed
Consensus boilerplate Unknown
Telechat date (None)
Responsible AD Barry Leiba
Send notices to,
Internet Engineering Task Force                          P. Resnick, Ed.
Internet-Draft                                     Qualcomm Incorporated
Obsoletes: RFC5738 (if approved)                          C. Newman, Ed.
Intended status: Standards Track                                  Oracle
Expires: February 2, 2013                                   S. Shen, Ed.
                                                          August 1, 2012

                         IMAP Support for UTF-8


   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

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   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 February 2, 2013.

Copyright Notice

   Copyright (c) 2012 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
   ( 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

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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 and UTF-8 in IMAP Quoted
       Strings  . . . . . . . . . . . . . . . . . . . . . . . . . . .  3
   4.  IMAP UTF8 Append Data Extension  . . . . . . . . . . . . . . .  5
   5.  LOGIN Command and UTF-8  . . . . . . . . . . . . . . . . . . .  5
   6.  UTF8=ONLY Capability . . . . . . . . . . . . . . . . . . . . .  6
   7.  Dealing With Legacy Clients  . . . . . . . . . . . . . . . . .  6
   8.  Issues with UTF-8 Header Mailstore . . . . . . . . . . . . . .  7
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  7
   10. Security Considerations  . . . . . . . . . . . . . . . . . . .  8
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . .  8
     11.1.  Normative References  . . . . . . . . . . . . . . . . . .  8
     11.2.  Informative References  . . . . . . . . . . . . . . . . .  9
   Appendix A.  Design Rationale  . . . . . . . . . . . . . . . . . .  9
   Appendix B.  Acknowledgments . . . . . . . . . . . . . . . . . . . 10

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1.  Introduction

   This specification forms part of the Email Address
   Internationalization protocols described in the Email Address
   Internationalization Framework document [RFC6530].  It extends
   IMAP4rev1 [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.

   Most of 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].

   The formal syntax uses the Augmented Backus-Naur Form (ABNF)
   [RFC5234] notation.  In addition, rules from IMAP4rev1 [RFC3501],
   UTF-8 [RFC3629], "Collected Extensions to IMAP4 ABNF" [RFC4466], and
   IMAP4 LIST Command Extensions [RFC5258] are also referenced.  This
   document assumes that the reader will have a reasonably good
   understanding of the RFCs above and their update.

   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

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
   SELECT and EXAMINE, and UTF-8 responses from the LIST and LSUB

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   A client MUST use the "ENABLE" command (defined in [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.  The
   "ENABLE UTF8=ACCEPT" command MUST only be used 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.)

   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
   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 terminal.

            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 extended quoting mechanism is used by the client, 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 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 advertises the "UTF8=ACCEPT" capability, 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 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,

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   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 using CHARSET after
   ENABLE command.  If an IMAP server receives such a SEARCH command, it
   SHOULD reject the command with a BAD response (due to the conflicting
   charset labels).

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 ")"

            literal8   = <Defined in RFC 4466>

        append-data    =/ utf8-literal

        cat-part       =/ utf8-literal

   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

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

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

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   Although the use of the IMAP AUTHENTICATE command in this way makes
   it syntactically 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

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), allows all of the capabilities that are allowed by
   "UTF8=ACCEPT" (see Section 4), and does not permit the international
   mailbox name convention (modified UTF-7).  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.

   If the "UTF8=ONLY" capability is specified, UTF-8 must be accepted as
   if the "UTF8=ACCEPT" had been specified.  For convenience, the
   explicit combination of "UTF8=ONLY" and "UTF8=ACCPET" is not allowed.

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 who 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.

   Choices available to the server when a message that requires SMTPUTF8
   is encountered and the client doesn't enable UTF-8 capability include
   hiding the problematic message(s), creating in band or out of band
   notifications or error messages, or somehow trying to create a
   variation on the message with the intention of providing useful
   information to that client about what has occurred.  Such variant
   messages cannot be actual substitutes for the original message: it
   will rarely be possible to reply to (either at all or without loss of
   information), new header fields or specialized constructs for server-
   client communication may go beyond the requirements of, e.g., RFC
   5322 and may consequently confuse some legacy mail user agents

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   (including IMAP clients) or otherwise not provide the expected
   information to users.  There are also tradeoffs in constructing
   variants 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 [popimap-downgrade]) and
   trying to minimize those costs while still providing useful
   information (for example, in [I-D.ietf-eai-simpledowngrade]).

   Because such messages are really variations on the original ones, not
   really "downgraded ones" (although that terminology is often used for
   convenience), they inevitably have relationships to the original ones
   that the IMAP specification [RFC3501] did not anticipate.  In
   particular, digital signatures computed over the original message
   will often not be applicable to the variant version and 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 behaves as the user would expect.  Those issues
   may be especially sensitive if the server caches the variant message
   or computes and stores it when the message arrives with the intent of
   making either form available depending on client capabilities.

   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 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.  The
   issue of handling messages containing non-ASCII characters in legacy
   environments is discussed in Section 7.

9.  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.

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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 user names and passwords.  Otherwise, this is not believed
   to alter the security considerations of IMAP4rev1.

   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 and 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 above.

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

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                                   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.

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

   [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.

11.2.  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.

   [I-D.ietf-eai-simpledowngrade]  Gulbrandsen, A., "EAI: Simplified
                                   POP/IMAP downgrading",
                                   (work in progress), June 2012.

   [popimap-downgrade]             Fujiwara, K., "Post-delivery Message
                                   Downgrading for Internationalized
                                   Email Messages",
                                   (work in progress), July 2012.

Appendix A.  Design Rationale

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

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   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.  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

   Phone: +1 858 651 4478

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


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

   Phone: +86 10-58813038

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