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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 RFC 5255

Network Working Group                                       Chris Newman
Internet-Draft                                          Sun Microsystems
Intended Status: Proposed Standard                      Arnt Gulbrandsen
                                                  Oryx Mail Systems GmhH
                                                         Alexey Melnikov
                                                           Isode Limited
                                                        February 1, 2008

         Internet Message Access Protocol Internationalization
                     draft-ietf-imapext-i18n-15.txt


Status of this Memo
    By submitting this Internet-Draft, each author represents that any
    applicable patent or other IPR claims of which he or she is aware
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    This Internet-Draft expires in August 2008.


Copyright Notice

    Copyright (C) The IETF Trust (2008).


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



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    which improve international support including comparator negotiation
    for search, sort and thread, language negotiation for international
    error text, and translations for namespace prefixes.


Table of Contents

    1.  Conventions Used in this Document . . . . . . . . . . . . . .  2
    2.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .  3
    3.  LANGUAGE Extension  . . . . . . . . . . . . . . . . . . . . .  3
    3.1 LANGUAGE Extension Requirements . . . . . . . . . . . . . . .  3
    3.2 LANGUAGE Command  . . . . . . . . . . . . . . . . . . . . . .  4
    3.3 LANGUAGE Response . . . . . . . . . . . . . . . . . . . . . .  6
    3.4 TRANSLATION Extension to the NAMESPACE Response . . . . . . .  6
    3.5 Formal Syntax . . . . . . . . . . . . . . . . . . . . . . . .  6
    4.  I18NLEVEL=1 and I18NLEVEL=2 Extensions  . . . . . . . . . . .  7
    4.1 Introduction and Overview . . . . . . . . . . . . . . . . . .  8
    4.2 Requirements common to both I18NLEVEL=1 and I18NLEVEL=2 . . .
    4.3 I18NLEVEL=1 Extension Requirements  . . . . . . . . . . . . .  8
    4.4 I18NLEVEL=2 Extension Requirements  . . . . . . . . . . . . .  8
    4.5 Compatibility Notes
    4.6 Comparators and Charsets  . . . . . . . . . . . . . . . . . .  9
    4.7 COMPARATOR Command  . . . . . . . . . . . . . . . . . . . . .  9
    4.8 COMPARATOR Response . . . . . . . . . . . . . . . . . . . . . 10
    4.9 BADCOMPARATOR Response Code . . . . . . . . . . . . . . . . .
    4.10 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 . . . . . . . . . . 11
    6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
    7.  Security Considerations . . . . . . . . . . . . . . . . . . . 12
    8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . 12
    9.  Relevant Standards for i18n IMAP Implementations  . . . . . . 13
        Normative References  . . . . . . . . . . . . . . . . . . . . 13
        Informative References  . . . . . . . . . . . . . . . . . . . 14
        Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . 15
        Intellectual Property and Copyright Statements  . . . . . . . 16


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 use the Augmented Backus-Naur Form (ABNF)
    [RFC4234] notation including the core rules defined in Appendix A.



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    The UTF8-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.


2.  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 which 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].


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.

    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-



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    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.

    Clients and servers that support this extension MUST also support
    the NAMESPACE extension [RFC2342].

    The LANGUAGE command is valid in all states. Clients are urged to
    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].

    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



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    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.

        < 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

        < 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



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        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.


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



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    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


3.5 Formal Syntax

    The following syntax specification inherits ABNF [RFC4234] 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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4.  I18NLEVEL=1 and I18NLEVEL=2 Extensions


4.1 Introduction and Overview

    IMAP4rev1 [RFC3501] includes the SEARCH command which 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 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 which is used
    by SEARCH and SORT absent any negotiation using the COMPARATOR (see
    Section 4.7) 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.

    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.



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    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 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

    IMAP server that satisfies all requirements specified in sections
    4.2, 4.4, 4.6-4.10 (and 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 ordering operation
    (and by implication equality), IMAP servers which advertise the SORT
    extension can only implement collations that offer all three



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    operations (see [RFC4790] sections 4.2.2-4).

    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
    (see [RFC4790]) comparator.

    There is no good way for a client to know which comparator that 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, 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



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        manner.

    (b) The decoded text from (a) MUST be converted to the charset
        expected by the active comparator.

    (c) For the substring operation:
        If step (b) failed (e.g., the text is in an unknown charset,
        contains a sequence which 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 i;unicode-casemap [UCM] comparator.
        Strings are represented using hexadecimal notation used by
        ABNF [RFC4234].

        (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):




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        %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.
        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 argument, 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.



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    Otherwise each argument is an 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. >

        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 [RFC4234] rules
    from IMAP4rev1 [RFC3501], and 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) ")"]



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        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]

        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 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 use of any 8-bit in the "userid" or
    "password" field.

    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



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    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 [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 [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

    The IANA is requested to add LANGUAGE, I18NLEVEL=1 and I18NLEVEL=2
    to the IMAP4 Capabilities Registry.  [Note to IANA:
    http://www.iana.org/assignments/imap4-capabilities]


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.

    LANGUAGE, I18NLEVEL=1 and I18NLEVEL=2 extensions use the UTF-8
    charset, thus the security considerations for UTF-8 [RFC3629] are
    relevent.  However, neither uses UTF-8 for identifiers so the most
    serious concerns do not apply.


8.  Acknowledgements



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    The LANGUAGE extension is based on a previous Internet draft 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.


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 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].


Normative References

    [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
               Requirement Levels", BCP 14, RFC 2119, March 1997.

    [RFC2277]  Alvestrand, "IETF Policy on Character Sets and
               Languages", BCP 18, RFC 2277, January 1998.




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    [RFC2342]  Gahrns, Newman, "IMAP4 Namespace", RFC 2342, May 1998.

    [RFC3501]  Crispin, "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
               4rev1", RFC 3501, March 2003.

    [RFC3629]  Yergeau, "UTF-8, a transformation format of ISO 10646",
               STD 63, RFC 3629, November 2003.

    [RFC4234]  Crocker, Overell, "Augmented BNF for Syntax
               Specifications: ABNF", RFC 4234, Brandenburg
               Internetworking, Demon Internet Ltd, October 2005.

    [RFC4422]  Melnikov, Zeilenga, "Simple Authentication and Security
               Layer (SASL)", RFC 4422, June 2006.

    [RFC4466]  Melnikov, Daboo, "Collected Extensions to IMAP4 ABNF",
               RFC 4466, Isode Ltd., April 2006.

    [RFC4646]  Philips, Davis, "Tags for Identifying Languages", BCP 47,
               RFC 4646, September 2006.

    [RFC4647]  Philips, Davis, "Matching of Language Tags", BCP 47, RFC
               4647, September 2006.

    [RFC4790]  Newman, Duerst, Gulbrandsen, "Internet Application
               Protocol Comparator Registry", RFC 4790, February 2007.

    [SORT]     Crispin, M. and K. Murchison, "INTERNET MESSAGE ACCESS
               PROTOCOL - SORT AND THREAD EXTENSION", draft-ietf-
               imapext-sort-19 (work in progress), November 2006.

    [UCM]      Crispin, "i;unicode-casemap - Simple Unicode Collation
               Algorithm", RFC 5051, October 2007.

    [RFC2045]  Freed, Borenstein, "Multipurpose Internet Mail Extensions
               (MIME) Part One: Format of Internet Message Bodies", RFC
               2045, November 1996.

    [RFC2047]  Moore, "MIME (Multipurpose Internet Mail Extensions) Part
               Three: Message Header Extensions for Non-ASCII Text", RFC
               2047, November 1996.


Informative References


    [RFC2231]  Freed, Moore, "MIME Parameter Value and Encoded Word
               Extensions: Character Sets, Languages, and



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               Continuations", RFC 2231, November 1997.

    [RFC3490]  Faltstrom, Hoffman, Costello, "Internationalizing Domain
               Names in Applications (IDNA)", RFC 3490, March 2003.

    [RFC3492]  Costello, "Punycode: A Bootstring encoding of Unicode for
               Internationalized Domain Names in Applications (IDNA)",
               RFC 3492, March 2003.

    [METADATA] Daboo, C., "IMAP METADATA Extension", draft-daboo-imap-
               annotatemore-12 (work in progress), December 2007.

    [IMAP-EAI] Resnick, Newman, "IMAP Support for UTF-8", draft-ietf-
               eai-imap-utf8 (work in progress), May 2006.



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






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