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rfc:rfc5255

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.

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

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.

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

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 "["

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] RFC 5255 IMAP Internationalization June 2008

 (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

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 contained in BCP 78, and except as set forth therein, the authors
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Newman, et al. Standards Track [Page 20]

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