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 © 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.

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].

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 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

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".

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 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.

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.

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.
   
   [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.

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.

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