Network Working Group Chris Newman Request for Comments: DRAFT Sun Microsystems
draft-ietf-imapext-i18n-04.txtdraft-ietf-imapext-i18n-05.txt Arnt Gulbrandsen Oryx Mail Systems October 2004July 2005 Internet Message Access Protocol Internationalization Status of this Memo By submitting this Internet-Draft, I certifyeach author represents that any applicable patent or other IPR claims of which I amhe or she is aware have been or will be disclosed, and any of which I becomehe or she becomes aware will be disclosed, in accordance with RFC 3668.Section 6 of BCP 79. This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. 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". The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet- Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Copyright Notice Copyright (C) The Internet Society 2004. All Rights Reserved.2005. Abstract Internet Message Access Protocol (IMAP) version 4rev1 has basic support for non-ASCII characters in mailbox names and search substrings. It also supports non-ASCII message headers and content encoded as specified by Multipurpose Internet Mail Extensions (MIME). This specification defines a collection of IMAP extensions which improve international support including comparator negotiation for search, sort and thread, language negotiation for international error text, and translations for namespace prefixes. Internet-draft October 2004July 2005 Table of Contents 1. Conventions Used in this Document . . . . . . . . . . . . . . 3 2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 3. LANGUAGE Extension . . . . . . . . . . . . . . . . . . . . . 3 3.1 LANGUAGE Extension Requirements . . . . . . . . . . . . . . . 3 3.2 LANGUAGE Command . . . . . . . . . . . . . . . . . . . . . . 4 3.3 LANGUAGE Response . . . . . . . . . . . . . . . . . . . . . . 5 3.4 TRANSLATION Extension to the NAMESPACE Response . . . . . . . 6 3.5 Formal Syntax . . . . . . . . . . . . . . . . . . . . . . . . 6 4. COMPARATOR Extension . . . . . . . . . . . . . . . . . . . . 7 4.1 COMPARATOR Extension Requirements . . . . . . . . . . . . . . 8 4.2 Comparators and Charsets . . . . . . . . . . . . . . . . . . 9 4.3 COMPARATOR Command . . . . . . . . . . . . . . . . . . . . . 9 4.4 COMPARATOR Response . . . . . . . . . . . . . . . . . . . . . 10 4.5 Formal Syntax . . . . . . . . . . . . . . . . . . . . . . . . 10 5. Other IMAP Internationalization Issues . . . . . . . . . . . 11 5.1 UTF-8 Userids and Passwords . . . . . . . . . . . . . . . . . 11 5.2 UTF-8 Mailbox Names . . . . . . . . . . . . . . . . . . . . . 11 5.3 UTF-8 Domains, Addresses and Mail Headers . . . . . . . . . . 12 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12 7. Security Considerations . . . . . . . . . . . . . . . . . . . 12 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 13 9. Relevant Standards for i18n IMAP Implementations . . . . . . 13 Normative References . . . . . . . . . . . . . . . . . . . . 14 Informative References . . . . . . . . . . . . . . . . . . . 14 Author's Address . . . . . . . . . . . . . . . . . . . . . . 15 Intellectual Property and Copyright Statements . . . . . . . 16 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" . The formal syntax use the Augmented Backus-Naur Form (ABNF)  notation including the core rules defined in Appendix A of RFC 2234. The UTF8-related productions are defined in RFC 3629 . 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. Internet-draft October 2004July 2005 2. Introduction This specification defines two IMAP4rev1  extensions to enhance international support. These extensions can be advertised and implemented separately. The LANGUAGE extension allows the client to request a suitable language for protocol error messages and in combination with the NAMESPACE extension  enables namespace translations. The COMPARATOR extension allows the client to request a suitable comparator which will modify the behavior of the base specification's SEARCH command as well as the SORT and THREAD extensions . This leverages the comparator registry . 3. LANGUAGE Extension IMAP allows server responses to include human-readable text that in many cases needs to be presented to the user. But that text is limited to US-ASCII by the IMAP specification  in order to preserve backwards compatibility with deployed IMAP implementations. This section specifies a way for an IMAP client to negotiate which language the server should use when sending human-readable text. The LANGUAGE extension only provides a mechanism for altering fixed server strings such as response text and NAMESPACE folder names. Assigning localized language aliases to shared mailboxes would be done with a separate mechanism such as the proposed ANNOTATEMORE extension.  3.1 LANGUAGE Extension Requirements IMAP servers that support this extension MUST list the keyword LANGUAGE in their CAPABILITY response as well as in the greeting CAPABILITY data. A server that advertises this extension MUST use the language "i- default" as described in  as its default language until another supported language is negotiated by the client. A server MUST include "i-default" as one of its supported languages. A client that supports this extension MUST be prepared for a possible NAMESPACE response  from the server. The LANGUAGE command is valid in all states. Internet-draft October 2004July 2005 3.2 LANGUAGE Command Arguments: Optional language range argument. Response: A possible LANGUAGE response (see Section 3.3). A possible NAMESPACE response as defined by . Result: OK - Command completed NO - Could not complete command BAD - arguments invalid The LANGUAGE command requests that human-readable text emitted by the server be localized to a language matching the language range argument as described by section 2.5 of RFC 3066. If the command succeeds, the server will return human-readable responses in the specified language starting with the tagged OK response to the LANGUAGE command. These responses will be in UTF-8 . If the command fails, the server will continue to return human- readable responses in the language it was previously using. The client MUST NOT use MUL (Multiple languages) or UND (Undetermined) language tags and the server MUST return BAD if either tag is used. The special "*" language range argument indicates a request to use a language designated as preferred by the server administrator. The preferred language MAY vary based on the currently active user. If the language range does not match a known language tag exactly but does match a language by the rules of section 2.5 of , the server MUST send an untagged LANGUAGE response indicating the language selected. If the language range argument is omitted, the server SHOULD send an untagged LANGUAGE response listing the languages it supports. If the server is unable to enumerate the list of languages it supports it MAY return a tagged NO response to the enumeration request. < The server defaults to using English i-default responses until the user explicitly changes the language. > C: A001 LOGIN KAREN PASSWORD S: A001 OK LOGIN completed < Client requested MUL language. Server MUST reply with BAD. > Internet-draft October 2004July 2005 C: A002 LANGUAGE MUL S: A002 BAD Invalid language MUL < A LANGUAGE command with no arguments is a request to enumerate the list of languages the server supports. > C: A003 LANGUAGE S: * LANGUAGE (EN DE IT i-default) S: A003 OK Supported languages have been enumerated C: B001 LANGUAGE S: B001 NO Server is unable to enumerate supported languages < Once the client changes the language, all responses will be in that language starting with the tagged OK to the LANGUAGE command. Because RFCs are in US-ASCII, this document uses an ASCII transcription rather than UTF-8 text, e.g. ue in the word "ausgefuehrt" > C: A004 LANGUAGE DE S: A004 OK Sprachwechsel durch LANGUAGE-Befehl ausgefuehrt < If a server does not support the requested primary language, responses will continue to be returned in the current language the server is using. > C: A005 LANGUAGE FR S: A005 NO Diese Sprache ist nicht unterstuetzt C: A006 LANGUAGE DE-IT S: * LANGUAGE (DE-IT) S: A006 OK Sprachwechsel durch LANGUAGE-Befehl ausgefuehrt C: A007 LANGUAGE "*" S: * LANGUAGE (DE) S: A007 OK LANGUAGE-Befehl erfolgreich ausgefuehrt 3.3 LANGUAGE Response Contents: A list of one or more language tags. The LANGUAGE response occurs as a result of a LANGUAGE command. A LANGUAGE response with a list containing a single language tag indicates that the server is now using that language. A LANGUAGE response with a list containing multiple language tags indicates the server is communicating a list of available languages to the client, and no change in the active language has been made. Internet-draft October 2004July 2005 3.4 TRANSLATION Extension to the NAMESPACE Response If the server supports the IMAP4 NAMESPACE command , the server MUST return an untagged NAMESPACE response when a language is negotiated. However the server MUST NOT return a NAMESPACE response if it is in not-authenticated state. If as a result of the newly negotiated language, localized representations of the namespace prefixes are available, the server SHOULD include these in the TRANSLATION extension to the NAMESPACE response. The TRANSLATION extension to the NAMESPACE response returns a single string, containing the modified UTF-7  encoded translation of the namespace prefix. It is the responsibility of the client to convert between the namespace prefix and the translation of the namespace prefix when presenting mailbox names to the user. In this example a server supports the IMAP4 NAMESPACE command. It uses no prefix to the user's Personal Namespace, a prefix of "Other Users" to its Other Users' Namespace and a prefix of "Public Folders" to its only Shared Namespace. Since a client will often display these prefixes to the user, the server includes a translation of them that can be presented to the user. C: A001 LANGUAGE DE-IT S: * NAMESPACE (("" "/"))(("Other Users/" "/" "TRANSLATION" ("Andere Ben&APw-tzer/"))) (("Public Folders/" "/" "TRANSLATION" ("Gemeinsame Mailboxen/"))) S: A001 OK La Language commande a ete executee avec successLANGUAGE-Befehl ausgefuehrt 3.5 Formal Syntax The following syntax specification inherits ABNF  rules from IMAP4rev1 , IMAP4 Namespace , Tags for the Identification of Languages , and UTF-8 . command-any =/ language-cmd ; LANGUAGE command is valid in all states language-cmd = "LANGUAGE" [SP lang-range-quoted] language-data = "LANGUAGE" SP "(" lang-tag-quoted *(SP lang- tag-quoted) ")" Internet-draft October 2004July 2005 namespace-trans = SP DQUOTE "TRANSLATION" DQUOTE SP "(" string ")" ; the string is encoded in Modified UTF-7. ; this is a subset of the syntax permitted by ; the Namespace_Response_Extension rule in RFC 2342 lang-range-quoted = astring ; Once any literal wrapper or quoting is removed, this ; follows the language-range rule in section 2.5 of RFC 3066 lang-tag-quoted = astring ; Once any literal wrapper or quoting is removed, this follows ; the Language-Tag rule in section 2.1 of RFC 3066 ; After the server is changed to a language other than ; i-default, the resp-text rule from RFC 3501 is replaced ; with the following: resp-text = ["[" resp-text-code "]" SP ] UTF8-TEXT-CHAR *(UTF8-TEXT-CHAR / "[") UTF8-TEXT-CHAR = %x20-%x5A / %x5C-%x7E / UTF8-2 / UTF8-3 / UTF8-4 ; UTF-8 excluding 7-bit control characters and "[" 4. COMPARATOR Extension IMAP4rev1  includes the SEARCH command which can be used to locate messages matching criteria including human-readable text. The SORT extension  to IMAP allows the client to ask the server to determine the order of messages based on criteria including human-readable text. These mechanisms require the ability to support non-English search and sort functions. This section defines an IMAP extension to negotiate use of comparators  to internationalize IMAP SEARCH, SORT and THREAD. The IMAP extension consists of a new command to determine or change the active comparator and a new response to indicate the active comparator and possibly other available comparators. The term "default comparator" refers to the comparator which is used by SEARCH and SORT absent any negotiation using the COMPARATOR command. The term "active comparator" refers to the comparator which will be used within a session e.g. by SEARCH and SORT. The COMPARATOR command is used to change the active comparator. Internet-draft October 2004July 2005 The active comparator applies to the following SEARCH keys: "BCC", "BODY", "CC", "FROM", "SUBJECT", "TEXT", "TO" and "HEADER". If the server also advertises the "SORT" extension, then the active comparator applies to the following SORT keys: "CC", "FROM", "SUBJECT" and "TO". If the server advertises the THREAD=ORDEREDSUBJECT, then the active comparator applies to the ORDEREDSUBJECT threading algorithm. Future extensions may choose to apply the active comparator to their SEARCH keys. For SORT and THREAD, the pre-processing necessary to extract the base subject text from a Subject header occurs prior to the application of a comparator. 4.1 COMPARATOR Extension Requirements IMAP servers that support this extension MUST list the keyword COMPARATOR in their CAPABILITY data once IMAP enters authenticated state, and MAY list that keyword in other states. A server that advertises this extension MUST implement the en;ascii- casemap and i;octet comparators, as defined in . A server intended to be deployed globally MUST implement the i;basic;uca=3.1.1;uv=3.2 comparator. A server that advertises this extension MUST use a registered case- insensitive comparator which supports the substring matching function as the default comparator. If the server also advertises the SORT or THREAD=ORDEREDSUBJECT extensions, then the default comparator MUST also support the ordering function. The selection of the default comparator MAY be adjustable by the server administrator, and MAY be sensitive to the current user. Once the IMAP connection enters authenticated state, the default comparator MUST remain static for the remainder of that connection. A server that advertises this extension MUST support UTF-8 as a SEARCH charset. The COMPARATOR command is valid in authenticated and selected states. Internet-draft October 2004July 2005 4.2 Comparators and Charsets For SEARCH, SORT and THREAD operations that apply to message headers, the server is responsible for removing the MIME header encoding  and converting the text of any known charsets to UTF-8 prior to applying the comparator algorithm. Unknown charsets should never match when using the SEARCH command, and should sort together with invalid comparator input for the SORT and THREAD commands. When message text is in a known charset other than UTF-8, the server is responsible for converting that text to UTF-8 prior to applying the comparator. When message text is in an unknown charset, then the text should be skipped by the SEARCH command unless the comparator is i;octet. 4.3 COMPARATOR Command Arguments: Optional comparator order arguments. Response: A possible COMPARATOR response (see Section 4.4). Result: OK - Command completed NO - No matching comparator found BAD - arguments invalid The COMPARATOR command is used to determine or change the active comparator. When issued with no arguments, it will resultresults in a COMPARATOR response indicating the currently active comparator. When issued with one or more comparator order argument, it will change the active comparator if any comparator matches any argument. The COMPARATOR response will list other matching comparators if more than one matches the specified patterns. The argument "*" refers to the server's default comparator. Otherwise each argument is an comparator specification as defined in the Internet Application Protocol Comparator Registry . < The client requests activating a Czech comparator if possible, or else a generic international comparator which it considers suitable for Czech. The server picks the first supported comparator. > C: A001 COMPARATOR cz;* i;basic* S: * COMPARATOR i;basic;uca=3.1.1;uv=3.2 S: A001 OK Will use i;basic;uca=3.1.1;uv=3.2 for collation Internet-draft October 2004July 2005 < The client requests pure octet matching, then does a search for potential GIF files, then switches back to its usual comparator. > C: B123 COMPARATOR i;octet S: * COMPARATOR i;octet S: B123 OK C: B124 SEARCH OR BODY GIF87A BODY GIF89A S: * SEARCH 42 69 S: B124 OK C: B125 COMPARATOR cz;* i;basic* S: * COMPARATOR i;basic;uca=3.1.1;uv=3.2 S: B125 OK 4.4 COMPARATOR Response Contents: The active comparator. An optional list of available matching comparators The COMPARATOR response occurs as a result of a COMPARATOR command. The first argument in the comparator response is the name of the active comparator. The second argument is a list of comparators which matched any of the arguments to the COMPARATOR command and is present only if more than one match is found. 4.5 Formal Syntax The following syntax specification inherits ABNF  rules from IMAP4rev1 , and Internet Application Protocol Comparator Registry . command-auth =/ comparator-cmd resp-text-code =/ "BADCOMPARATOR" / "BADMATCH" comparator-cmd = "COMPARATOR" *(SP comp-order-quoted) comparator-data = "COMPARATOR" SP comp-sel-quoted [SP "(" comp-name-quoted *(SP comp-name-quoted) ")"] comp-name-quoted = astring ; Once any literal wrapper or quoting is removed, ; this follows the comparator-name rule Internet-draft October 2004July 2005 comp-order-quoted = astring ; Once any literal wrapper or quoting is removed, ; this follows the comparator-order rule comp-sel-quoted = astring ; Once any literal wrapper or quoting is removed, ; this follows the comparator-sel rule 5. Other IMAP Internationalization Issues The following sections provide an overview of various other IMAP internationalization issues. These issues are not resolved by this specification, but could be resolved by future standards work. 5.1 UTF-8Unicode Userids and Passwords IMAP4rev1 presently restricts the userid and password fields of the LOGIN command to US-ASCII. Because the ability to enter a userid and password is necessary to use IMAP at all for most authentication mechanisms, the potential inability to enter userid and password with some clients is a serious interoperability concern. However, because of the visibility of these fields to end-users, it is expected that supporting unicode login names and passwords will eventually be practical and necessary. This has been left out of this document, because the SASL-related profile of stringprep  has not yet been published as an RFC, and the impact on ACLs and email addresses has not been assessed.The "userid" and "password" fields of the IMAP LOGIN command are restricted to US-ASCII only until a future standards track RFC states otherwise. Servers are encouraged to validate both fields to make sure they conform to the formal syntax of UTF-8 and to reject the LOGIN command if that syntax is violated. Servers MAY reject the use of any 8-bit in the "userid" or "password" field. When AUTHENTICATE is used, some servers may support userids and passwords in Unicode . However, such userids cannot be used as email addresses, and at present also seem to be incompatible with the current latest ACL drafts. Unless the ACL drafts resolve this, server authors are cautioned against supporting ACL and unicode userids simultaneously. 5.2 UTF-8 Mailbox Names The modified UTF-7 mailbox naming convention described in section 5.1.3 of RFC 3501 is best viewed as an transition from the status quo in 1996 when modified UTF-7 was first specified. At that time, there was widespread unofficial use of local character sets such as ISO-8859-1 and Shift-JIS for non-ASCII mailbox names, with resultant non-interoperability. Internet-draft October 2004The requirements in section 5.1 of RFC 3501 are very important if we're ever going to be able to deploy UTF-8 mailbox names. Servers are encourated to enforce them. Internet-draft July 2005 5.3 UTF-8 Domains, Addresses and Mail Headers There is now an IETF standard for Internationalizing Domain Names in Applications . While IMAP clients are free to support this standard, an argument can be made that it would be helpful to simple clients if the IMAP server could perform this conversion (the same argument would apply to MIME header encoding ). However, it would be unwise to move forward with such work until the work in progress to define the format of international email addresses is complete. 6. IANA Considerations When this is published as an RFC, the IMAP extensions LANGUAGE and COMPARATOR are registered. 7. Security Considerations The LANGUAGE extension makes a new command available in "Not Authenticated" state in IMAP. Some IMAP implementations run with root privilege when the server is in "Not Authenticated" state and do not revoke that privilege until after authentication is complete. Such implementations are particularly vulnerable to buffer overflow security errors at this stage and need to implement parsing of this command with extra care. A LANGUAGE command issued prior to activation of a security layer is subject to an active attack which suppresses or modifies the negotiation and thus makes STARTTLS or authentication error messages more difficult to interpret. This is not a new attack as the error messages themselves are subject to active attack. Clients MUST re- issue the LANGUAGE command once a security layer is active, so this does not impact subsequent protocol operations. Both the LANGUAGE and COMPARATOR extensions use the UTF-8 charset, thus the security considerations for UTF-8  are relevent. However, neither uses UTF-8 for identifiers so the most serious concerns do not apply. Internet-draft October 2004July 2005 8. Acknowledgements The LANGUAGE extension is based on a previous Internet draft by Mike Gahrns and Alexey Melnikov, a substantial portion of the text in that section was written by them. Many people have participated in discussions about an IMAP Language extension in the various fora of the IETF and Internet working groups, so any list of contributors is bound to be incomplete. However, the authors would like to thank Andrew McCown for early work on the original proposal, John Myers for suggestions regarding the namespace issue, along with Jutta Degener, Mark Crispin, Mark Pustilnik, Larry Osterman and Martin Duerst for their many suggestions that have been incorporated into this document. Initial discussion of the COMPARATOR extension involved input from Mark Crispin and other participants of the IMAP Extensions WG. 9. Relevant Standards for i18n IMAP Implementations This is a non-normative list of standards to consider when implementing i18n aware IMAP software. o The LANGUAGE and COMPARATOR extensions to IMAP (this specification). o The 8-bit rules for mailbox naming in section 5.1 of RFC 3501. o The Mailbox International Naming Convention in section 5.1.3 of RFC 3501. o MIME  for message bodies. o MIME header encoding  for message headers. o MIME Parameter Value and Encoded Word Extensions  for filenames. Quality IMAP server implementations will automatically combine multipart parameters when generating the BODYSTRUCTURE. There is also some deployed non-standard use of MIME header encoding inside double-quotes for filenames. o IDNA  and punycode  for domain names (presently only relevant to IMAP clients). o The UTF-8 charset . o The IETF policy on Character Sets and Languages . Internet-draft October 2004July 2005 Normative References  Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.  Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", RFC 2234, November 1997.  Alvestrand, H., "IETF Policy on Character Sets and Languages", BCP 18, RFC 2277, January 1998.  Gahrns, M. and C. Newman, "IMAP4 Namespace", RFC 2342, May 1998.  Alvestrand, H., "Tags for the Identification of Languages", BCP 47, RFC 3066, January 2001.  Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4rev1", RFC 3501, March 2003.  Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD 63, RFC 3629, November 2003.  Newman, C., "Internet Application Protocol Comparator Registry", draft-newman-i18n-comparator-02draft-newman-i18n-comparator-05 (work in progress), July 2004.May 2005. Informative References  Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies", RFC 2045, November 1996.  Moore, K., "MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text", RFC 2047, November 1996.  Freed, N. and K. Moore, "MIME Parameter Value and Encoded Word Extensions: Character Sets, Languages, and Continuations", RFC 2231, November 1997.  Hoffman, P. and M. Blanchet, "Preparation of Internationalized Strings ("stringprep")", RFC 3454, December 2002.  Faltstrom, P., Hoffman, P. and A. Costello, "Internationalizing Domain Names in Applications (IDNA)", RFC 3490, March 2003. Internet-draft October 2004July 2005  Costello, A., "Punycode: A Bootstring encoding of Unicode for Internationalized Domain Names in Applications (IDNA)", RFC 3492, March 2003.  Crispin, M. and K. Murchison, "INTERNET MESSAGE ACCESS PROTOCOL - SORT AND THREAD EXTENSION", draft-ietf-imapext-sort-17 (work in progress), May 2004.  Daboo, C., "IMAP ANNOTATEMORE Extension", draft-daboo-imap- annotatemore-05annotatemore-07 (work in progress), April 2004.February 2005. Authors' Addresses Chris Newman Sun Microsystems 1050 Lakes Drive West Covina, CA 91790 US Email: firstname.lastname@example.org Arnt Gulbrandsen Oryx Mail Systems GmbH Joseph-Dollinger-Bogen 14 D-80807Schweppermannstr. 8 D-81781 Muenchen Germany Email: email@example.com Phone: +49 89 32356-4014502 9757 Fax: +49 89 32356-4094502 9758 Internet-draft October 2004July 2005 Intellectual Property Statement The IETF takes no position regarding the validity or scope of any intellectual propertyIntellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; neithernor does it represent that it has made any independent effort to identify any such rights. 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