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-
-
-
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-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
- have been or will be disclosed, and any of which he or she becomes
- aware will be disclosed, in accordance with Section 6 of BCP 79.
-
- Internet-Drafts are working documents of the Internet Engineering
- Task Force (IETF), its areas, and its working groups. Note that
- other groups may also distribute working documents as Internet-
- Drafts.
-
- Internet-Drafts are draft documents valid for a maximum of six
- months and may be updated, replaced, or obsoleted by other documents
- at any time. It is inappropriate to use Internet-Drafts as
- reference material or to cite them other than as "work in progress".
-
- The list of current Internet-Drafts can be accessed at
- http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-
- Draft Shadow Directories can be accessed at
- http://www.ietf.org/shadow.html.
-
- 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.
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- (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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-Intellectual Property Statement
-
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- pertain to the implementation or use of the technology described in
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- 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.
-
-
-Full Copyright Statement
-
- Copyright (C) The IETF Trust (2008). This document is subject to
- the rights, licenses and restrictions contained in BCP 78, and
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-
- This document and the information contained herein are provided on
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-
-Acknowledgment
-
- Funding for the RFC Editor function is currently provided by the
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