GENWiki

Premier IT Outsourcing and Support Services within the UK

User Tools

Site Tools


rfc:rfc6855

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.

Resnick, et al. Standards Track [Page 1] RFC 6855 IMAP Support for UTF-8 March 2013

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].

Resnick, et al. Standards Track [Page 2] RFC 6855 IMAP Support for UTF-8 March 2013

 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

Resnick, et al. Standards Track [Page 3] RFC 6855 IMAP Support for UTF-8 March 2013

 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

Resnick, et al. Standards Track [Page 4] RFC 6855 IMAP Support for UTF-8 March 2013

 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".

Resnick, et al. Standards Track [Page 5] RFC 6855 IMAP Support for UTF-8 March 2013

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

Resnick, et al. Standards Track [Page 6] RFC 6855 IMAP Support for UTF-8 March 2013

 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.

Resnick, et al. Standards Track [Page 7] RFC 6855 IMAP Support for UTF-8 March 2013

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.

Resnick, et al. Standards Track [Page 8] RFC 6855 IMAP Support for UTF-8 March 2013

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.

Resnick, et al. Standards Track [Page 9] RFC 6855 IMAP Support for UTF-8 March 2013

 [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.

Resnick, et al. Standards Track [Page 10] RFC 6855 IMAP Support for UTF-8 March 2013

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.

Resnick, et al. Standards Track [Page 11] RFC 6855 IMAP Support for UTF-8 March 2013

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

Resnick, et al. Standards Track [Page 12]

/data/webs/external/dokuwiki/data/pages/rfc/rfc6855.txt · Last modified: 2013/03/12 21:39 by 127.0.0.1

Donate Powered by PHP Valid HTML5 Valid CSS Driven by DokuWiki