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+Network Working Group                                          C. Newman
+Request for Comments: 5255                              Sun Microsystems
+Category: Standards Track                                 A. Gulbrandsen
+                                                  Oryx Mail Systems GmhH
+                                                             A. Melnikov
+                                                           Isode Limited
+                                                               June 2008
+
+
+         Internet Message Access Protocol Internationalization
+
+Status of This Memo
+
+   This document specifies an Internet standards track protocol for the
+   Internet community, and requests discussion and suggestions for
+   improvements.  Please refer to the current edition of the "Internet
+   Official Protocol Standards" (STD 1) for the standardization state
+   and status of this protocol.  Distribution of this memo is unlimited.
+
+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 that
+   improve international support including language negotiation for
+   international error text, translations for namespace prefixes, and
+   comparator negotiation for search, sort, and thread.
+
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+Newman, et al.              Standards Track                     [Page 1]
+
+RFC 5255               IMAP Internationalization               June 2008
+
+
+Table of Contents
+
+   1. Introduction ....................................................3
+   2. Conventions Used in This Document ...............................3
+   3. LANGUAGE Extension ..............................................3
+      3.1. LANGUAGE Extension Requirements ............................4
+      3.2. LANGUAGE Command ...........................................4
+      3.3. LANGUAGE Response ..........................................6
+      3.4. TRANSLATION Extension to the NAMESPACE Response ............7
+      3.5. Formal Syntax ..............................................8
+   4. I18NLEVEL=1 and I18NLEVEL=2 Extensions ..........................9
+      4.1. Introduction and Overview ..................................9
+      4.2. Requirements Common to Both I18NLEVEL=1 and I18NLEVEL=2 ....9
+      4.3. I18NLEVEL=1 Extension Requirements ........................10
+      4.4. I18NLEVEL=2 Extension Requirements ........................10
+      4.5. Compatibility Notes .......................................11
+      4.6. Comparators and Character Encodings .......................11
+      4.7. COMPARATOR Command ........................................13
+      4.8. COMPARATOR Response .......................................14
+      4.9. BADCOMPARATOR Response Code ...............................14
+      4.10. Formal Syntax ............................................14
+   5. Other IMAP Internationalization Issues .........................15
+      5.1. Unicode Userids and Passwords .............................15
+      5.2. UTF-8 Mailbox Names .......................................15
+      5.3. UTF-8 Domains, Addresses, and Mail Headers ................15
+   6. IANA Considerations ............................................16
+   7. Security Considerations ........................................16
+   8. Acknowledgements ...............................................16
+   9. Relevant Sources of Documents for Internationalized IMAP
+      Implementations ................................................17
+   10. Normative References ..........................................17
+   11. Informative References ........................................18
+
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+Newman, et al.              Standards Track                     [Page 2]
+
+RFC 5255               IMAP Internationalization               June 2008
+
+
+1.  Introduction
+
+   This specification defines two IMAP4rev1 [RFC3501] 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 [RFC2342] enables namespace translations.
+
+   The I18NLEVEL=2 extension allows the client to request a suitable
+   collation that will modify the behavior of the base specification's
+   SEARCH command as well as the SORT and THREAD extensions [SORT].
+   This leverages the collation registry [RFC4790].  The I18NLEVEL=1
+   extension updates SEARCH/SORT/THREAD to use i;unicode-casemap
+   comparator, as defined in [UCM].  I18NLEVEL=1 is a simpler version of
+   I18NLEVEL=2 with no ability to select a different collation.
+
+2.  Conventions Used in This Document
+
+   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
+   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
+   document are to be interpreted as described in [RFC2119].
+
+   The formal syntax uses the Augmented Backus-Naur Form (ABNF)
+   [RFC5234] notation including the core rules defined in Appendix A.
+
+   The UTF-8-related productions are defined in [RFC3629].
+
+   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.  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 [RFC3501] 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.
+
+
+
+
+
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+
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+Newman, et al.              Standards Track                     [Page 3]
+
+RFC 5255               IMAP Internationalization               June 2008
+
+
+   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 METADATA
+   extension (see [METADATA]).
+
+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 [RFC2277] 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.  IMAP
+   servers SHOULD NOT advertise the LANGUAGE extension if they discover
+   that they only support "i-default".
+
+   Clients and servers that support this extension MUST also support the
+   NAMESPACE extension [RFC2342].
+
+   The LANGUAGE command is valid in all states.  Clients SHOULD issue
+   LANGUAGE before authentication, since some servers send valuable user
+   information as part of authentication (e.g., "password is correct,
+   but expired").  If a security layer (such as SASL or TLS) is
+   subsequently negotiated by the client, it MUST re-issue the LANGUAGE
+   command in order to make sure that no previous active attack (if any)
+   on LANGUAGE negotiation has effect on subsequent error messages.
+   (See Section 7 for a more detailed explanation of the attack.)
+
+3.2.  LANGUAGE Command
+
+   Arguments: Optional language range arguments.
+
+   Response:  A possible LANGUAGE response (see Section 3.3).
+              A possible NAMESPACE response (see Section 3.4).
+
+   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 one of the language range
+   argument as described by Section 2 of [RFC4647].
+
+
+
+
+
+
+Newman, et al.              Standards Track                     [Page 4]
+
+RFC 5255               IMAP Internationalization               June 2008
+
+
+   If the command succeeds, the server will return human-readable
+   responses in the first supported language specified.  These responses
+   will be in UTF-8 [RFC3629].  The server MUST send a LANGUAGE response
+   specifying the language used, and the change takes effect immediately
+   after the LANGUAGE response.
+
+   If the command fails, the server continues to return human-readable
+   responses in the language it was previously using.
+
+   The special "default" 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 a language range does not match a known language tag exactly but
+   does match a language by the rules of [RFC4647], the server MUST send
+   an untagged LANGUAGE response indicating the language selected.
+
+   If there aren't any arguments, 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.  If, after
+   receiving a LANGUAGE request, the server discovers that it doesn't
+   support any language other than i-default, it MUST 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, which no server supports. >
+
+      C: A002 LANGUAGE MUL
+      S: A002 NO Unsupported 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
+
+
+
+
+
+
+Newman, et al.              Standards Track                     [Page 5]
+
+RFC 5255               IMAP Internationalization               June 2008
+
+
+      < Once the client changes the language, all responses will be in
+        that language starting after the LANGUAGE response.  Note that
+        this includes the NAMESPACE response.  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: C001 LANGUAGE DE
+      S: * LANGUAGE (DE)
+      S: * NAMESPACE (("" "/")) (("Other Users/" "/" "TRANSLATION"
+            ("Andere Ben&APw-tzer/"))) (("Public Folders/" "/"
+            "TRANSLATION" ("Gemeinsame Postf&AM8-cher/")))
+      S: C001 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: D001 LANGUAGE FR
+      S: D001 NO Diese Sprache ist nicht unterstuetzt
+      C: D002 LANGUAGE DE-IT
+      S: * LANGUAGE (DE-IT)
+      S: * NAMESPACE (("" "/"))(("Other Users/" "/" "TRANSLATION"
+            ("Andere Ben&APw-tzer/"))) (("Public Folders/" "/"
+            "TRANSLATION" ("Gemeinsame Postf&AM8-cher/")))
+      S: D002 OK Sprachwechsel durch LANGUAGE-Befehl ausgefuehrt
+      C: D003 LANGUAGE "default"
+      S: * LANGUAGE (DE)
+      S: D003 OK Sprachwechsel durch LANGUAGE-Befehl ausgefuehrt
+
+      < Server does not speak French, but does speak English.  User
+        speaks Canadian French and Canadian English. >
+
+      C: E001 LANGUAGE FR-CA EN-CA
+      S: * LANGUAGE (EN)
+      S: E001 OK Now speaking English
+
+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.
+
+
+
+
+
+Newman, et al.              Standards Track                     [Page 6]
+
+RFC 5255               IMAP Internationalization               June 2008
+
+
+3.4.  TRANSLATION Extension to the NAMESPACE Response
+
+   If localized representations of the namespace prefixes are available
+   in the selected language, 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 [RFC3501] 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 Postf&AM8-cher/")))
+      S: A001 OK LANGUAGE-Befehl ausgefuehrt
+
+
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+Newman, et al.              Standards Track                     [Page 7]
+
+RFC 5255               IMAP Internationalization               June 2008
+
+
+3.5.  Formal Syntax
+
+   The following syntax specification inherits ABNF [RFC5234] rules from
+   IMAP4rev1 [RFC3501], IMAP4 Namespace [RFC2342], Tags for the
+   Identifying Languages [RFC4646], UTF-8 [RFC3629], and Collected
+   Extensions to IMAP4 ABNF [RFC4466].
+
+    command-any       =/ language-cmd
+        ; LANGUAGE command is valid in all states
+
+    language-cmd      = "LANGUAGE" *(SP lang-range-quoted)
+
+    response-payload  =/ language-data
+
+    language-data     = "LANGUAGE" SP "(" lang-tag-quoted *(SP
+                      lang-tag-quoted) ")"
+
+    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 [RFC4466]
+
+    lang-range-quoted = astring
+        ; Once any literal wrapper or quoting is removed, this
+        ; follows the language-range rule in [RFC4647]
+
+    lang-tag-quoted   = astring
+        ; Once any literal wrapper or quoting is removed, this follows
+        ; the Language-Tag rule in [RFC4646]
+
+    resp-text         = ["[" resp-text-code "]" SP ] UTF8-TEXT-CHAR
+                        *(UTF8-TEXT-CHAR / "[")
+        ; After the server is changed to a language other than
+        ; i-default, this resp-text rule replaces the resp-text
+        ; rule from [RFC3501].
+
+    UTF8-TEXT-CHAR    = %x20-5A / %x5C-7E / UTF8-2 / UTF8-3 / UTF8-4
+        ; UTF-8 excluding 7-bit control characters and "["
+
+
+
+
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+
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+Newman, et al.              Standards Track                     [Page 8]
+
+RFC 5255               IMAP Internationalization               June 2008
+
+
+4.  I18NLEVEL=1 and I18NLEVEL=2 Extensions
+
+4.1.  Introduction and Overview
+
+   IMAP4rev1 [RFC3501] includes the SEARCH command that can be used to
+   locate messages matching criteria including human-readable text.  The
+   SORT extension [SORT] 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.
+
+   Section 4 defines two IMAP extensions for internationalizing IMAP
+   SEARCH, SORT, and THREAD [SORT] using the comparator framework
+   [RFC4790].
+
+   The I18NLEVEL=1 extension updates SEARCH/SORT/THREAD to use
+   i;unicode-casemap comparator, as defined in [UCM].  See Sections 4.2
+   and 4.3 for more details.
+
+   The I18NLEVEL=2 extension is a superset of the I18NLEVEL=1 extension.
+   It adds to I18NLEVEL=1 extension the ability to determine the active
+   comparator (see definition below) and to negotiate use of comparators
+   using the COMPARATOR command.  It also adds the COMPARATOR response
+   that indicates the active comparator and possibly other available
+   comparators.  See Sections 4.2 and 4.4 for more details.
+
+4.2.  Requirements Common to Both I18NLEVEL=1 and I18NLEVEL=2
+
+   The term "default comparator" refers to the comparator that is used
+   by SEARCH and SORT absent any negotiation using the COMPARATOR
+   command (see Section 4.7).  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.
+
+   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 THREAD=ORDEREDSUBJECT,
+   then the active comparator applies to the ORDEREDSUBJECT threading
+   algorithm.  If the server advertises THREAD=REFERENCES, then the
+   active comparator applies to the subject field comparisons done by
+   REFERENCES threading algorithm.  Future extensions may choose to
+   apply the active comparator to their SEARCH keys.
+
+
+
+
+
+
+Newman, et al.              Standards Track                     [Page 9]
+
+RFC 5255               IMAP Internationalization               June 2008
+
+
+   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.
+
+   A server that advertises I18NLEVEL=1 or I18NLEVEL=2 extension MUST
+   implement the i;unicode-casemap comparator, as defined in [UCM].
+
+   A server that advertises I18NLEVEL=1 or I18NLEVEL=2 extension MUST
+   support UTF-8 as a SEARCH charset.
+
+4.3.  I18NLEVEL=1 Extension Requirements
+
+   An IMAP server that satisfies all requirements specified in Sections
+   4.2 and 4.6 (and that doesn't support/advertise any other
+   I18NLEVEL=<n> extension, where n > 1) MUST list the keyword
+   I18NLEVEL=1 in its CAPABILITY data once IMAP enters the authenticated
+   state, and MAY list that keyword in other states.
+
+4.4.  I18NLEVEL=2 Extension Requirements
+
+   An IMAP server that satisfies all requirements specified in Sections
+   4.2, 4.4, and 4.6-4.10 (and that doesn't support/advertise any other
+   I18NLEVEL=<n> extension, where n > 2) MUST list the keyword
+   I18NLEVEL=2 in its CAPABILITY data once IMAP enters the authenticated
+   state, and MAY list that keyword in other states.
+
+   A server that advertises this extension MUST implement the
+   i;unicode-casemap comparator, as defined in [UCM].  It MAY implement
+   other comparators from the IANA registry established by [RFC4790].
+   See also Section 4.5 of this document.
+
+   A server that advertises this extension SHOULD use i;unicode-casemap
+   as the default comparator.  (Note that i;unicode-casemap is the
+   default comparator for I18NLEVEL=1, but not necessarily the default
+   for I18NLEVEL=2.) 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.
+
+   Note that since SEARCH uses the substring operation, IMAP servers can
+   only implement collations that offer the substring operation (see
+   [RFC4790], Section 4.2.2).  Since SORT uses the ordering operation
+   (which in turn uses the equality operation), IMAP servers that
+   advertise the SORT extension can only implement collations that offer
+   all three operations (see [RFC4790], Sections 4.2.2-4.2.4).
+
+
+
+
+
+Newman, et al.              Standards Track                    [Page 10]
+
+RFC 5255               IMAP Internationalization               June 2008
+
+
+   If the active collation does not provide the operations needed by an
+   IMAP command, the server MUST respond with a tagged BAD.
+
+4.5.  Compatibility Notes
+
+   Several server implementations deployed prior to the publication of
+   this specification comply with I18NLEVEL=1 (see Section 4.3), but do
+   not advertise that.  Other legacy servers use the i;ascii-casemap
+   comparator (see [RFC4790]).
+
+   There is no good way for a client to know which comparator a legacy
+   server uses.  If the client has to assume the worst, it may end up
+   doing expensive local operations to obtain i;unicode-casemap
+   comparisons even though the server implements it.
+
+   Legacy server implementations which comply with I18NLEVEL=1 should be
+   updated to advertise I18NLEVEL=1.  All server implementations should
+   eventually be updated to comply with the I18NLEVEL=2 extension.
+
+4.6.  Comparators and Character Encodings
+
+   RFC 3501, Section 6.4.4, says:
+
+         In all search keys that use strings, a message matches the key
+         if the string is a substring of the field.  The matching is
+         case-insensitive.
+
+   When performing the SEARCH operation, the active comparator is
+   applied instead of the case-insensitive matching specified above.
+
+   An IMAP server which performs collation operations (e.g., as part of
+   commands such as SEARCH, SORT, and THREAD) does so according to the
+   following procedure:
+
+   (a) MIME encoding (for example, see [RFC2047] for headers and
+       [RFC2045] for body parts) MUST be removed in the texts being
+       collated.
+
+       If MIME encoding removal fails for a message (e.g., a body part
+       of the message has an unsupported Content-Transfer-Encoding, uses
+       characters not allowed by the Content-Transfer-Encoding, etc.),
+       the collation of this message is undefined by this specification,
+       and is handled in an implementation-dependent manner.
+
+   (b) The decoded text from (a) MUST be converted to the charset
+       expected by the active comparator.
+
+
+
+
+
+Newman, et al.              Standards Track                    [Page 11]
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+RFC 5255               IMAP Internationalization               June 2008
+
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+   (c) For the substring operation:
+
+       If step (b) failed (e.g., the text is in an unknown charset,
+       contains a sequence that is not valid according in that charset,
+       etc.), the original decoded text from (a) (i.e., before the
+       charset conversion attempt) is collated using the i;octet
+       comparator (see [RFC4790]).
+
+       If step (b) was successful, the converted text from (b) is
+       collated according to the active comparator.
+
+       For the ordering operation:
+
+       All strings that were successfully converted by step (b) are
+       separated from all strings that failed step (b).  Strings in each
+       group are collated independently.  All strings successfully
+       converted by step (b) are then validated by the active
+       comparator.  Strings that pass validation are collated using the
+       active comparator.  All strings that either fail step (b) or fail
+       the active collation's validity operation are collated (after
+       applying step (a)) using the i;octet comparator (see [RFC4790]).
+       The resulting sorted list is produced by appending all collated
+       "failed" strings after all strings collated using the active
+       comparator.
+
+       Example: The following example demonstrates ordering of 4
+       different strings using the i;unicode-casemap [UCM] comparator.
+       Strings are represented using hexadecimal notation used by ABNF
+       [RFC5234].
+
+       (1) %xD0 %xC0 %xD0 %xBD %xD0 %xB4 %xD1 %x80 %xD0 %xB5
+           %xD0 %xB9 (labeled with charset=UTF-8)
+       (2) %xD1 %x81 %xD0 %x95 %xD0 %xA0 %xD0 %x93 %xD0 %x95
+           %xD0 %x99 (labeled with charset=UTF-8)
+       (3) %xD0 %x92 %xD0 %xB0 %xD1 %x81 %xD0 %xB8 %xD0 %xBB
+           %xD0 %xB8 %xFF %xB9 (labeled with charset=UTF-8)
+       (4) %xE1 %xCC %xC5 %xCB %xD3 %xC5 %xCA (labeled with
+           charset=KOI8-R)
+
+       Step (b) will convert string (4) to the following sequence of
+       octets (in UTF-8):
+
+       %xD0 %x90 %xD0 %xBB %xD0 %xB5 %xD0 %xBA %xD1 %x81 %xD0
+       %xB5 %xD0 %xB9
+
+       and will reject strings (1) and (3), as they contain octets not
+       allowed in charset=UTF-8.
+
+
+
+
+Newman, et al.              Standards Track                    [Page 12]
+
+RFC 5255               IMAP Internationalization               June 2008
+
+
+       After that, using the i;unicode-casemap collation, string (4)
+       will collate before string (2).  Using the i;octet collation on
+       the original strings, string (3) will collate before string (1).
+       So the final ordering is as follows: (4) (2) (3) (1).
+
+   If the substring operation (e.g., IMAP SEARCH) of the active
+   comparator returns the "undefined" result (see Section 4.2.3 of
+   [RFC4790]) for either the text specified in the SEARCH command or the
+   message text, then the operation is repeated on the result of step
+   (a) using the i;octet comparator.
+
+   The ordering operation (e.g., IMAP SORT and THREAD) SHOULD collate
+   the following together: strings encoded using unknown or invalid
+   character encodings, strings in unrecognized charsets, and invalid
+   input (as defined by the active collation).
+
+4.7.  COMPARATOR Command
+
+   Arguments: Optional comparator order arguments.
+
+   Response:  A possible COMPARATOR response (see Section 4.8).
+
+   Result:    OK - Command completed
+              NO - No matching comparator found
+              BAD - Arguments invalid
+
+   The COMPARATOR command is valid in authenticated and selected states.
+
+   The COMPARATOR command is used to determine or change the active
+   comparator.  When issued with no arguments, it results in a
+   COMPARATOR response indicating the currently active comparator.
+
+   When issued with one or more comparator arguments, it changes the
+   active comparator as directed.  (If more than one installed
+   comparator is matched by an argument, the first argument wins.) The
+   COMPARATOR response lists all matching comparators if more than one
+   matches the specified patterns.
+
+   The argument "default" refers to the server's default comparator.
+   Otherwise, each argument is a collation specification as defined in
+   the Internet Application Protocol Comparator Registry [RFC4790].
+
+        < 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. >
+
+
+
+
+
+Newman, et al.              Standards Track                    [Page 13]
+
+RFC 5255               IMAP Internationalization               June 2008
+
+
+        C: A001 COMPARATOR "cz;*" i;basic
+        S: * COMPARATOR i;basic
+        S: A001 OK Will use i;basic for collation
+
+4.8.  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.
+
+4.9.  BADCOMPARATOR Response Code
+
+   This response code SHOULD be returned as a result of server failing
+   an IMAP command (returning NO), when the server knows that none of
+   the specified comparators match the requested comparator(s).
+
+4.10.  Formal Syntax
+
+   The following syntax specification inherits ABNF [RFC5234] rules from
+   IMAP4rev1 [RFC3501] and the Internet Application Protocol Comparator
+   Registry [RFC4790].
+
+    command-auth      =/ comparator-cmd
+
+    resp-text-code    =/ "BADCOMPARATOR"
+
+    comparator-cmd    = "COMPARATOR" *(SP comp-order-quoted)
+
+    response-payload  =/ comparator-data
+
+    comparator-data   = "COMPARATOR" SP comp-sel-quoted [SP "("
+                        comp-id-quoted *(SP comp-id-quoted) ")"]
+
+    comp-id-quoted    = astring
+        ; Once any literal wrapper or quoting is removed, this
+        ; follows the collation-id rule from [RFC4790]
+
+    comp-order-quoted = astring
+        ; Once any literal wrapper or quoting is removed, this
+        ; follows the collation-order rule from [RFC4790]
+
+
+
+
+
+
+Newman, et al.              Standards Track                    [Page 14]
+
+RFC 5255               IMAP Internationalization               June 2008
+
+
+    comp-sel-quoted   = astring
+        ; Once any literal wrapper or quoting is removed, this
+        ; follows the collation-selected rule from [RFC4790]
+
+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 other standards work, such as
+   that being done by the EAI working group (see [IMAP-EAI]).
+
+5.1.  Unicode Userids and Passwords
+
+   IMAP4rev1 currently restricts the userid and password fields of the
+   LOGIN command to US-ASCII.  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 LOGIN command if either the "userid" or
+   "password" field contains an octet with the highest bit set.
+
+   When AUTHENTICATE is used, some servers may support userids and
+   passwords in Unicode [RFC3490] since SASL (see [RFC4422]) allows
+   that.  However, such userids cannot be used as part of email
+   addresses.
+
+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.
+
+   The 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 encouraged to enforce them.
+
+5.3.  UTF-8 Domains, Addresses, and Mail Headers
+
+   There is now an IETF standard for "Internationalizing Domain Names in
+   Applications (IDNA)" [RFC3490].  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
+
+
+
+
+Newman, et al.              Standards Track                    [Page 15]
+
+RFC 5255               IMAP Internationalization               June 2008
+
+
+   [RFC2047]).  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
+
+   IANA added LANGUAGE, I18NLEVEL=1, and I18NLEVEL=2 to the IMAP4
+   Capabilities Registry.
+
+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 that 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, in order
+   to prevent this attack from impacting subsequent protocol operations.
+
+   LANGUAGE, I18NLEVEL=1, and I18NLEVEL=2 extensions use the UTF-8
+   charset; thus, the security considerations for UTF-8 [RFC3629] are
+   relevant.  However, neither uses UTF-8 for identifiers, so the most
+   serious concerns do not apply.
+
+8.  Acknowledgements
+
+   The LANGUAGE extension is based on a previous document by Mike
+   Gahrns, a substantial portion of the text in that section was written
+   by him.  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, Cyrus Daboo, Martin Duerst,
+   Timo Sirainen, Ben Campbell, and Magnus Nystrom for their many
+   suggestions that have been incorporated into this document.
+
+   Initial discussion of the I18NLEVEL=2 extension involved input from
+   Mark Crispin and other participants of the IMAP Extensions WG.
+
+
+
+Newman, et al.              Standards Track                    [Page 16]
+
+RFC 5255               IMAP Internationalization               June 2008
+
+
+9.  Relevant Sources of Documents for Internationalized IMAP
+    Implementations
+
+   This is a non-normative list of sources to consider when implementing
+   i18n-aware IMAP software.
+
+      o The LANGUAGE and I18NLEVEL=2 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 [RFC2045] for message bodies.
+
+      o MIME header encoding [RFC2047] for message headers.
+
+      o The IETF EAI working group.
+
+      o MIME Parameter Value and Encoded Word Extensions [RFC2231] 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 [RFC3490] and punycode [RFC3492] for domain names
+        (currently only relevant to IMAP clients).
+
+      o The UTF-8 charset [RFC3629].
+
+      o The IETF policy on Character Sets and Languages [RFC2277].
+
+10.  Normative References
+
+   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
+              Requirement Levels", BCP 14, RFC 2119, March 1997.
+
+   [RFC2277]  Alvestrand, H., "IETF Policy on Character Sets and
+              Languages", BCP 18, RFC 2277, January 1998.
+
+   [RFC2342]  Gahrns, M. and C. Newman, "IMAP4 Namespace", RFC 2342, May
+              1998.
+
+   [RFC3501]  Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
+              4rev1", RFC 3501, March 2003.
+
+
+
+
+
+Newman, et al.              Standards Track                    [Page 17]
+
+RFC 5255               IMAP Internationalization               June 2008
+
+
+   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
+              10646", STD 63, RFC 3629, November 2003.
+
+   [RFC5234]  Crocker, D., Ed., and P. Overell, "Augmented BNF for
+              Syntax Specifications: ABNF", STD 68, RFC 5234, January
+              2008.
+
+   [RFC4422]  Melnikov, A., Ed., and K. Zeilenga, Ed., "Simple
+              Authentication and Security Layer (SASL)", RFC 4422, June
+              2006.
+
+   [RFC4466]  Melnikov, A. and C. Daboo, "Collected Extensions to IMAP4
+              ABNF", RFC 4466, April 2006.
+
+   [RFC4646]  Phillips, A. and M. Davis, "Tags for Identifying
+              Languages", BCP 47, RFC 4646, September 2006.
+
+   [RFC4647]  Phillips, A. and M. Davis, "Matching of Language Tags",
+              BCP 47, RFC 4647, September 2006.
+
+   [RFC4790]  Newman, C., Duerst, M., and A. Gulbrandsen, "Internet
+              Application Protocol Collation Registry", RFC 4790, March
+              2007.
+
+   [SORT]     Crispin, M. and K. Murchison, "Internet Message Access
+              Protocol - SORT and THREAD Extensions", RFC 5256, June
+              2008.
+
+   [UCM]      Crispin, M., "i;unicode-casemap - Simple Unicode Collation
+              Algorithm", RFC 5051, October 2007.
+
+   [RFC2045]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
+              Extensions (MIME) Part One: Format of Internet Message
+              Bodies", RFC 2045, November 1996.
+
+   [RFC2047]  Moore, K., "MIME (Multipurpose Internet Mail Extensions)
+              Part Three: Message Header Extensions for Non-ASCII Text",
+              RFC 2047, November 1996.
+
+11. Informative References
+
+   [RFC2231]  Freed, N. and K. Moore, "MIME Parameter Value and Encoded
+              Word Extensions: Character Sets, Languages, and
+              Continuations", RFC 2231, November 1997.
+
+   [RFC3490]  Faltstrom, P., Hoffman, P., and A. Costello,
+              "Internationalizing Domain Names in Applications (IDNA)",
+              RFC 3490, March 2003.
+
+
+
+Newman, et al.              Standards Track                    [Page 18]
+
+RFC 5255               IMAP Internationalization               June 2008
+
+
+   [RFC3492]  Costello, A., "Punycode: A Bootstring encoding of Unicode
+              for Internationalized Domain Names in Applications
+              (IDNA)", RFC 3492, March 2003.
+
+   [METADATA] Daboo, C., "IMAP METADATA Extension", Work in Progress,
+              April 2008.
+
+   [IMAP-EAI] Resnick, P., and C. Newman, "IMAP Support for UTF-8", Work
+              in Progress, November 2007.
+
+Authors' Addresses
+
+   Chris Newman
+   Sun Microsystems
+   3401 Centrelake Dr., Suite 410
+   Ontario, CA 91761
+   US
+
+   EMail: chris.newman@sun.com
+
+
+   Arnt Gulbrandsen
+   Oryx Mail Systems GmbH
+   Schweppermannstr. 8
+   D-81671 Muenchen
+   Germany
+
+   EMail: arnt@oryx.com
+   Fax: +49 89 4502 9758
+
+
+   Alexey Melnikov
+   Isode Limited
+   5 Castle Business Village, 36 Station Road,
+   Hampton, Middlesex, TW12 2BX, UK
+
+   EMail: Alexey.Melnikov@isode.com
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Newman, et al.              Standards Track                    [Page 19]
+
+RFC 5255               IMAP Internationalization               June 2008
+
+
+Full Copyright Statement
+
+   Copyright (C) The IETF Trust (2008).
+
+   This document is subject to the rights, licenses and restrictions
+   contained in BCP 78, and except as set forth therein, the authors
+   retain all their rights.
+
+   This document and the information contained herein are provided on an
+   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
+   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
+   THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
+   OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
+   THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
+   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
+
+Intellectual Property
+
+   The IETF takes no position regarding the validity or scope of any
+   Intellectual 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; nor does it represent that it has
+   made any independent effort to identify any such rights.  Information
+   on the procedures with respect to rights in RFC documents can be
+   found in BCP 78 and BCP 79.
+
+   Copies of IPR disclosures made to the IETF Secretariat and any
+   assurances of licenses to be made available, or the result of an
+   attempt made to obtain a general license or permission for the use of
+   such proprietary rights by implementers or users of this
+   specification can be obtained from the IETF on-line IPR repository at
+   http://www.ietf.org/ipr.
+
+   The IETF invites any interested party to bring to its attention any
+   copyrights, patents or patent applications, or other proprietary
+   rights that may cover technology that may be required to implement
+   this standard.  Please address the information to the IETF at
+   ietf-ipr@ietf.org.
+
+
+
+
+
+
+
+
+
+
+
+
+Newman, et al.              Standards Track                    [Page 20]
+