IMAP Support for UTF-8
Author(s): Pete Resnick, Chris Newman, Sean Shen
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....
Internet Engineering Task Force P. Resnick, Ed. Internet-Draft Qualcomm Incorporated Obsoletes: RFC5738 (if approved) C. Newman, Ed. Intended status: Standards Track Oracle Expires: May 20, 2013 S. Shen, Ed. CNNIC November 16, 2012 IMAP Support for UTF-8 draft-ietf-eai-5738bis-12 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 RFC5738. Status of This Memo This Internet-Draft is submitted 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 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 May 20, 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 (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 Resnick, et al. Expires May 20, 2013 [Page 1] Internet-Draft IMAP Support for UTF-8 November 2012 the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November 10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s) controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate it into languages other than English. 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 . . . . . . . . . . . . . . 8 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 10. Security Considerations . . . . . . . . . . . . . . . . . . . 9 11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9 11.1. Normative References . . . . . . . . . . . . . . . . . . 9 11.2. Informative References . . . . . . . . . . . . . . . . . 10 Appendix A. Design Rationale . . . . . . . . . . . . . . . . . . 11 Appendix B. Acknowledgments . . . . . . . . . . . . . . . . . . . 11 Resnick, et al. Expires May 20, 2013 [Page 2] Internet-Draft IMAP Support for UTF-8 November 2012 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. 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. 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 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 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 Resnick, et al. Expires May 20, 2013 [Page 3] Internet-Draft IMAP Support for UTF-8 November 2012 [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 (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 and supports UTF8=ACCEPT extension. The "ENABLE UTF8=ACCEPT" command is only valid in the authenticated state. 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 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 RFC3501 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 with a "BAD" response any octet sequences with the high bit set that fail to comply with the formal syntax in [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. Resnick, et al. Expires May 20, 2013 [Page 4] Internet-Draft IMAP Support for UTF-8 November 2012 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 RFC3501, 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 (0000-001F, 0080-009F), delete (007F), line separator (2028), or paragraph separator (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" 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 RFC4466> 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 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 the use of the IMAP AUTHENTICATE command in this way makes Resnick, et al. Expires May 20, 2013 [Page 5] Internet-Draft IMAP Support for UTF-8 November 2012 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 use. 6. UTF8=ONLY Capability The "UTF8=ONLY" capability indicates that the server supports "UTF8=ACCEPT" (see Section 4), and also that it requires support for UTF-8 from clients. In particular, this means that it will send UTF-8 in quoted strings, and it will not accept the older international mailbox name convention (modified UTF-7). 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 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 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 Resnick, et al. Expires May 20, 2013 [Page 6] Internet-Draft IMAP Support for UTF-8 November 2012 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: they will almost always be impossible to reply to (either at all or without loss of information); the new header fields or specialized constructs for server-client communication may go beyond the requirements of, e.g., RFC 5322; they may consequently confuse some legacy mail user agents (including IMAP clients) or otherwise may 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 [I-D.ietf-eai-popimap-downgrade]) and trying to minimize those costs while still providing useful information (for example, in [I-D.ietf-eai-simpledowngrade]). Implementations that choose to do downgrading SHOULD use one of the standardized algorithms, [I-D.ietf-eai-popimap-downgrade] or [I-D.ietf-eai-simpledowngrade]. 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 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. 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 variant 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 variant.) 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 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. 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 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 Resnick, et al. Expires May 20, 2013 [Page 7] Internet-Draft IMAP Support for UTF-8 November 2012 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 IMAP4rev1 base specification [RFC3501] and [RFC 5322] 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 made OBSOLETE. IANA is asked to change the IMAP 4 Capabilities registry as follows: OLD: +--------------+---------------------------+ | UTF8=ACCEPT | [RFC5738] | | UTF8=ALL | [RFC5738] | | UTF8=APPEND | [RFC5738] | | UTF8=ONLY | [RFC5738] | | UTF8=USER | [RFC5738] | +--------------+---------------------------+ NEW: +--------------+---------------------------+ | UTF8=ACCEPT | [[this RFC]] | | UTF8=ALL | OBSOLETE (was [RFC5738]) | | UTF8=APPEND | OBSOLETE (was [RFC5738]) | | UTF8=ONLY | [[this RFC]] | | UTF8=USER | OBSOLETE (was [RFC5738]) | +--------------+---------------------------+ Resnick, et al. Expires May 20, 2013 [Page 8] Internet-Draft IMAP Support for UTF-8 November 2012 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, 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 above. 11. References 11.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC2119, March 1997. [RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4rev1", RFC3501, March 2003. [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD 63, RFC3629, November 2003. [RFC4013] Zeilenga, K., "SASLprep: Stringprep Profile for User Names and Passwords", RFC4013, February 2005. [RFC4466] Melnikov, A. and C. Daboo, "Collected Extensions to IMAP4 ABNF", RFC4466, April 2006. [RFC4469] Resnick, P., "Internet Message Access Protocol (IMAP) CATENATE Extension", RFC4469, April 2006. [RFC5161] Gulbrandsen, A. and A. Melnikov, "The IMAP ENABLE Extension", RFC5161, March 2008. Resnick, et al. Expires May 20, 2013 [Page 9] Internet-Draft IMAP Support for UTF-8 November 2012 [RFC5198] Klensin, J. and M. Padlipsky, "Unicode Format for Network Interchange", RFC5198, March 2008. [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC5234, January 2008. [RFC5258] Leiba, B. and A. Melnikov, "Internet Message Access Protocol version 4 - LIST Command Extensions", RFC5258, June 2008. [RFC6532] Yang, A., Steele, S., and N. Freed, "Internationalized Email Headers", RFC6532, February 2012. [RFC 5322] Resnick, P., Ed., "Internet Message Format", RFC 5322, October 2008. [RFC6530] Klensin, J. and Y. Ko, "Overview and Framework for Internationalized Email", RFC6530, February 2012. [I-D.ietf-eai-popimap-downgrade] Fujiwara, K., "Post-delivery Message Downgrading for Internationalized Email Messages", draft-ietf-eai-popimap-downgrade-08 (work in progress), October 2012. [I-D.ietf-eai-simpledowngrade] Gulbrandsen, A., "Simplified POP/ IMAP Downgrading for Internationalized Email", draft-ietf-eai-simpledowngrade-07 (work in progress), August 2012. 11.2. Informative References [RFC2088] Myers, J., "IMAP4 non-synchronizing literals", RFC2088, January 1997. [RFC5738] Resnick, P. and C. Newman, "IMAP Support for UTF-8", RFC5738, March 2010. [RFC2342] Gahrns, M. and C. Newman, "IMAP4 Namespace", RFC2342, May 1998. Resnick, et al. Expires May 20, 2013 [Page 10] Internet-Draft IMAP Support for UTF-8 November 2012 [RFC4314] Melnikov, A., "IMAP4 Access Control List (ACL) Extension", RFC4314, December 2005. 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. 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: email@example.com Resnick, et al. Expires May 20, 2013 [Page 11] Internet-Draft IMAP Support for UTF-8 November 2012 Chris Newman (editor) Oracle 800 Royal Oaks Monrovia, CA 91016 USA Phone: EMail: firstname.lastname@example.org Sean Shen (editor) CNNIC No.4 South 4th Zhongguancun Street Beijing, 100190 China Phone: +86 10-58813038 EMail: email@example.com Resnick, et al. Expires May 20, 2013 [Page 12]