Network Working Group C. Newman
Internet-Draft Sun Microsystems
Expires: November 7, 2003 May 9, 2003
Internet Message Access Protocol Internationalization
draft-ietf-imapext-i18n-00.txt
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Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
Internet Message Access Protocol (IMAP) version 4rev1 has basic
support for non-ASCII characters in mailbox names and search
substrings. It also supports non-ASCII message headers and content
encoded as specified by Multipurpose Internet Mail Extensions (MIME).
This specification defines a collection of IMAP extensions which
improve international support including comparator negotiation for
search, sort and thread, language negotiation for international error
text, and translations for namespace prefixes.
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Table of Contents
1. Conventions Used in this Document . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. LANGUAGE Extension . . . . . . . . . . . . . . . . . . . . . . 3
3.1 LANGUAGE Extension Requirements . . . . . . . . . . . . . . . 3
3.2 LANGUAGE Command . . . . . . . . . . . . . . . . . . . . . . . 4
3.3 LANGUAGE Response . . . . . . . . . . . . . . . . . . . . . . 6
3.4 TRANSLATION Extension to the NAMESPACE Response . . . . . . . 6
3.5 Formal Syntax . . . . . . . . . . . . . . . . . . . . . . . . 7
4. COMPARATOR Extension . . . . . . . . . . . . . . . . . . . . . 7
4.1 COMPARATOR Extension Requirements . . . . . . . . . . . . . . 8
4.2 Comparators and Charsets . . . . . . . . . . . . . . . . . . . 9
4.3 COMPARATOR Command . . . . . . . . . . . . . . . . . . . . . . 9
4.4 COMPARATOR Response . . . . . . . . . . . . . . . . . . . . . 10
4.5 COMPARATOR SEARCH and SORT Key . . . . . . . . . . . . . . . . 10
4.6 Formal Syntax . . . . . . . . . . . . . . . . . . . . . . . . 11
5. Other IMAP Internationalization Issues . . . . . . . . . . . . 11
5.1 UTF-8 Userids and Passwords . . . . . . . . . . . . . . . . . 11
5.2 UTF-8 Mailbox Names . . . . . . . . . . . . . . . . . . . . . 12
5.3 UTF-8 Domains, Addresses and Mail Headers . . . . . . . . . . 12
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
7. Security Considerations . . . . . . . . . . . . . . . . . . . 13
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13
9. Relevant Standards for i18n IMAP Implementations . . . . . . . 14
10. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . 14
Normative References . . . . . . . . . . . . . . . . . . . . . 14
Informative References . . . . . . . . . . . . . . . . . . . . 15
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 16
Intellectual Property and Copyright Statements . . . . . . . . 17
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1. Conventions Used in this Document
The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and "MAY"
in this document are to be interpreted as defined in "Key words for
use in RFCs to Indicate Requirement Levels" [1].
The formal syntax use the Augmented Backus-Naur Form (ABNF) [2]
notation including the core rules defined in Appendix A of RFC 2234.
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 [6] 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 [4] enables namespace translations.
The COMPARATOR extension allows the client to request a suitable
comparator which will modify the behavior of the base specification's
SEARCH command as well as the SORT and THREAD extensions [15]. This
leverages the comparator registry [8].
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 [6] 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.
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 [3] as its default language until another
supported language is negotiated by the client. A server MUST include
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"i-default" as one of its supported languages.
A client that supports this extension MUST be prepared for a possible
NAMESPACE response [4] from the server.
The LANGUAGE command is valid in not-authenticated, authenticated and
selected states.
3.2 LANGUAGE Command
Arguments: Optional language range argument.
Response: A possible LANGUAGE response (see Section 3.3).
A possible NAMESPACE response as defined by [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 the language matching the language range
argument as described by section 2.5 of RFC 3066.
If the command succeeds, the server will return human-readable
responses in the specified language starting with the tagged OK
response to the LANGUAGE command. These responses will be in UTF-8
[7].
If the command fails, the server will continue to return
human-readable responses in the language it was previously using.
The client MUST NOT use MUL (Multiple languages) or UND
(Undetermined) language tags and the server MUST return BAD if either
tag is used. The special "*" language range argument indicates a
request to use a language designated as preferred by the server
administrator. The preferred language MAY vary based on the
currently active user.
If the language range does not match a known language tag exactly but
does match a language by the rules of section 2.5 of [5], the server
MUST send an untagged LANGUAGE response indicating the language
selected.
If the language range argument is omitted, the server SHOULD send an
untagged LANGUAGE response listing the languages it supports. If the
server is unable to enumerate the list of languages it supports it
MAY return a tagged NO response to the enumeration request.
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< The server defaults to using English i-default responses until
the user explicitly changes the language. >
C: A001 LOGIN KAREN PASSWORD
S: A001 OK LOGIN completed
< Client requested MUL language. Server MUST reply with BAD >
C: A002 LANGUAGE MUL
S: A002 BAD Invalid language MUL
< A LANGUAGE command with no arguments is a request to enumerate
the list of languages the server supports. >
C: A003 LANGUAGE
S: * LANGUAGE (EN DE IT i-default)
S: A003 OK Supported languages have been enumerated
C: B001 LANGUAGE
S: B001 NO Server is unable to enumerate supported languages
< Once the client changes the language, all responses will be in
that language starting with the tagged OK to the LANGUAGE
command. >
C: A004 LANGUAGE FR
S: A004 OK La Language commande a ete execute avec success
< If a server does not support the requested primary language,
responses will continue to be returned in the current language
the server is using. >
C: A005 LANGUAGE DE
S: A005 NO Ce Language n'est pas supporte
C: A006 LANGUAGE FR-CA
S: * LANGUAGE (FR)
S: A006 OK La Language commande a ete execute avec success
C: A007 LANGUAGE "*"
S: * LANGUAGE (FR)
S: A007 OK La Language commande a ete execute avec success
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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 the server supports the IMAP4 NAMESPACE command [4], the server
MUST return an untagged NAMESPACE response when a language is
negotiated. However the server MUST NOT return a NAMESPACE response
if it is in not-authenticated state.
If as a result of the newly negotiated language, localized
representations of the namespace prefixes are available, the server
SHOULD include these in the TRANSLATION extension to the NAMESPACE
response.
The TRANSLATION extension to the NAMESPACE response returns a single
string, containing the modified UTF-7 [6] 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 FR-CA
S: * NAMESPACE (("" "/"))(("Other Users/" "/" "TRANSLATION"
("Autres Utilisateurs/"))) (("Public Folders/" "/"
"TRANSLATION" ("R&Aok-pertoires Publics/")))
S: A001 OK La Language commande a ete executee avec success
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3.5 Formal Syntax
The following syntax specification inherits ABNF [2] rules from
IMAP4rev1 [6], IMAP4 Namespace [4], Tags for the Identification of
Languages [5], and UTF-8 [7].
command-any =/ language-cmd
; LANGUAGE command is valid in all states
mailbox-data =/ language-data
language-cmd = "LANGUAGE" [SP lang-range-quoted]
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 RFC 2342
lang-range-quoted = astring
; Once any literal wrapper or quoting is removed, this follows
; the language-range rule in section 2.5 of RFC 3066
lang-tag-quoted = astring
; Once any literal wrapper or quoting is removed, this follows
; the Language-Tag rule in section 2.1 of RFC 3066
; After the server is changed to a language other than i-default,
; the resp-text rule from RFC 3501 is replaced with the following:
resp-text = ["[" resp-text-code "]" SP ]
UTF8-TEXT-CHAR *(UTF8-TEXT-CHAR / "[")
UTF8-TEXT-CHAR = %x20-%x5A / %x5C-%x7E / UTF8-2 / UTF8-3 / UTF8-4
; UTF-8 excluding 7-bit control characters and "["
4. COMPARATOR Extension
IMAP4rev1 [6] includes the SEARCH command which can be used to locate
messages matching criteria including human-readable text. The SORT
extension [15] 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.
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This section defines an IMAP extension to negotiate use of
comparators [8] to internationalize IMAP SEARCH, SORT and THREAD.
The IMAP extension consists of a new command to determine or change
the active comparator, a new response to indicate the active
comparator and possibly other available comparators, SEARCH and SORT
keys which can be used to change comparators on-the-fly, and an
additional response code to indicate that the failure of a SEARCH or
SORT command was due to an invalid comparator.
The term "default comparator" refers to the comparator which is used
by SEARCH and SORT absent any negotiation using the COMPARATOR
command or SEARCH/SORT key. The term "active comparator" refers to
the comparator which will be used within a session by SEARCH and SORT
absent use of the COMPARATOR SEARCH/SORT key. The COMPARATOR command
is used to change the active comparator. The term "selected
comparator" refers to the comparator selected by the most recent
COMPARATOR SEARCH/SORT key within the context of the current SEARCH/
SORT program or the active comparator if there is no COMPARATOR
SEARCH/SORT key yet seen in context.
The selected 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 selected
comparator applies to the following SORT keys: "CC", "FROM",
"SUBJECT" and "TO". If the server advertises the
THREAD=ORDEREDSUBJECT, then the active comparator applies to the
ORDEREDSUBJECT threading algorithm.
For SORT and THREAD, the pre-processing necessary to extract the base
subject text from a Subject header occurs prior to the application of
a comparator.
4.1 COMPARATOR Extension Requirements
IMAP servers that support this extension MUST list the keyword
COMPARATOR in their CAPABILITY data once IMAP enters authenticated
state.
A server that advertises this extension MUST implement the
en;ascii-casemap and i;octet comparators. A server intended to be
deployed globally MUST implement the i;basic-uca=3.1.1-uv=3.2
comparator.
A server that advertises this extension MUST use a registered
case-insensitive comparator which supports the substring matching
function as the default comparator. If the server also advertises
the SORT or THREAD=ORDEREDSUBJECT extensions, then the default
comparator MUST also support the ordering function. The selection of
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the default comparator MAY be adjustable by the server administrator,
and MAY be sensitive to the current user. Once the IMAP connection
enters authenticated state, the default comparator MUST remain static
for the remainder of that connection.
A server that advertises this extension MUST support UTF-8 as a
SEARCH charset.
The COMPARATOR command is valid in authenticated and selected states.
4.2 Comparators and Charsets
For SEARCH, SORT and THREAD operations that apply to message headers,
the server is responsible for removing the MIME header encoding [10]
and converting the text of any known charsets to UTF-8 prior to
applying the comparator algorithm. Unknown charsets should never
match when using the SEARCH command, and will sort together with
invalid comparator input for the SORT and THREAD commands.
When message text is in a known charset other than UTF-8, the server
is responsible for converting that text to UTF-8 prior to applying
the comparator. When message text is in an unknown charset, then the
text should be skipped by the SEARCH command unless the comparator is
i;octet.
4.3 COMPARATOR Command
Arguments: Optional comparator order arguments.
Response: A possible COMPARATOR response (see Section 4.4).
Result: OK - Command completed
NO - No matching comparator found
BAD - arguments invalid
The COMPARATOR command is used to determine or change the active
comparator. When issued with no arguments, it will result in a
COMPARATOR response indicating the currently active comparator. When
issued with one or more comparator order argument, it will change the
active comparator if any comparator matches any argument. The
COMPARATOR response will list other matching comparators if more than
one matches the specified patterns.
When the single argument "*" is used with the COMPARATOR command, it
will set the active comparator to be the default comparator.
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4.4 COMPARATOR Response
Contents: The active comparator.
An optional list of available matching comparators
The COMPARATOR response occurs as a result of a COMPARATOR command.
The first argument in the comparator response is the name of the
active comparator. The second argument is a list of comparators
which matched any of the arguments to the COMPARATOR command and is
present only if more than one match is found.
4.5 COMPARATOR SEARCH and SORT Key
The COMPARATOR SEARCH key takes a comparator order argument. That
argument will select the comparator to use for subsequent SEARCH keys
in the search program. The COMPARATOR SORT key works in a similar
fashion except that it applies to subsequent SORT keys in the SORT
criterion.
If no comparator matches the pattern specified by the COMPARATOR
SEARCH or SORT key, then the SEARCH or SORT command will fail with a
[BADCOMPARATOR] response code. This error code is also returned if a
comparator is found, but it does not support the necessary function
(substring matching for SEARCH, or ordering for SORT).
If an input string provided as part of a SEARCH program causes an
error when used with the selected comparator, the SEARCH command will
fail with a [BADMATCH] response code.
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4.6 Formal Syntax
The following syntax specification inherits ABNF [2] rules from
IMAP4rev1 [6], and Internet Application Protocol Comparator Registry
[8].
command-auth =/ comparator-cmd
mailbox-data =/ comparator-data
search-key =/ comparator-key
sort-criterion =/ comparator-key
; this only applies to servers which advertise both
; the COMPARATOR and SORT extensions.
resp-text-code =/ "BADCOMPARATOR" / "BADMATCH"
comparator-cmd = "COMPARATOR" *(SP comp-order-quoted)
comparator-data = "COMPARATOR" SP comp-sel-quoted [SP "("
comp-name-quoted *(SP comp-name-quoted) ")"]
comparator-key = "COMPARATOR" SP comp-order-quoted
comp-name-quoted = astring
; Once any literal wrapper or quoting is removed,
; this follows the comparator-name rule
comp-order-quoted = astring
; Once any literal wrapper or quoting is removed,
; this follows the comparator-order rule
comp-sel-quoted = astring
; Once any literal wrapper or quoting is removed,
; this follows the comparator-sel rule
5. Other IMAP Internationalization Issues
The following sections provide an overview of various other IMAP
internationalization issues. These issues are not resolved by this
specification, but could be resolved by future standards work.
5.1 UTF-8 Userids and Passwords
IMAP4rev1 presently restricts the userid and password fields of the
LOGIN command to US-ASCII. Because the ability to enter a userid and
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password is necessary to use IMAP at all for most authentication
mechanisms, the potential lack of input methods for non-ASCII text is
a serious interoperability concern. However, because of the
visibility of these fields to end-users, it is expected that pressure
to support UTF-8 login names and passwords will eventually become
irresistable. This specification defers such work until the
SASL-related profile of stringprep [12] is published as an RFC, and
the impact on ACLs and email addresses has been assessed.
The "userid" and "password" fields of the IMAP LOGIN command are
restricted to US-ASCII only until a future standards track RFC states
otherwise. Servers are encouraged to validate both fields to make
sure they conform to the formal syntax of UTF-8 and to reject the
LOGIN command if that syntax is violated. Servers MAY reject the use
of any 8-bit in the "userid" or "password" field.
5.2 UTF-8 Mailbox Names
There appears to be rough concensus in the IMAP community that the
modified UTF-7 mailbox naming convention was a mistake and we should
have used UTF-8 instead. However, the preliminary design discussions
to create a transitional mechanism for UTF-8 mailbox names suggests
that the cost of supporting both a UTF-8 mechanism and a modified
UTF-7 convention for mailbox names during a transition period might
exceed the benefit of the eventual goal. Because of this
controversy, UTF-8 mailbox name mechanisms are not included in this
specification.
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.
5.3 UTF-8 Domains, Addresses and Mail Headers
There is now an IETF standard for Internationalizing Domain Names in
Applications [13]. While IMAP clients are free to support this
standard and convert punycode [14] to UTF-8 at display time, 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 [10]). 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.
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6. IANA Considerations
When this is published as an RFC, the following IMAP extensions will
be registered:
+-----------------+------------+
| Capability Name | Reference |
+-----------------+------------+
| LANGUAGE | [RFC XXXX] |
| COMPARATOR | [RFC XXXX] |
+-----------------+------------+
7. Security Considerations
The LANGUAGE extension makes a new command available in "Not
Authenticated" state in IMAP. Some IMAP implementations run with
root privilege when the server is in "Not Authenticated" state and do
not revoke that privilege until after authentication is complete.
Such implementations are particularly vulnerable to buffer overflow
security errors at this stage and need to implement parsing of this
command with extra care.
A LANGUAGE command issued prior to activation of a security layer is
subject to an active attack which suppresses or modifies the
negotiation and thus makes STARTTLS or authentication error messages
more difficult to interpret. This is not a new attack as the error
messages themselves are subject to active attack. Clients MUST
re-issue the LANGUAGE command once a security layer is active, so
this does not impact subsequent protocol operations.
Both the LANGUAGE and COMPARATOR extensions use the UTF-8 charset,
thus the security considerations for UTF-8 [7] are relevent.
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 Internet draft by Mike
Gahrns and Alexey Melnikov, a substantial portion of the text in that
section was written by them. Many people have participated in
discussions about an IMAP Language extension in the various fora of
the IETF and Internet working groups, so any list of contributors is
bound to be incomplete. However, the authors would like to thank
Andrew McCown for early work on the original proposal, John Myers for
suggestions regarding the namespace issue, along with Jutta Degener,
Mark Crispin, Mark Pustilnik and Larry Osterman for their many
suggestions that have been incorporated into this document.
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Initial discussion of the COMPARATOR extension involved input from
Mark Crispin and other participants of the IMAP Extensions WG.
9. Relevant Standards for i18n IMAP Implementations
This is a non-normative list of standards to consider when
implementing i18n aware IMAP software.
o The LANGUAGE and COMPARATOR extensions to IMAP (this
specification).
o The 8-bit rules for mailbox naming in section 5.1 of RFC 3501.
o The Mailbox International Naming Convention in section 5.1.3 of
RFC 3501.
o MIME [9] for message bodies.
o MIME header encoding [10] for message headers.
o MIME Parameter Value and Encoded Word Extensions [11] 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 [13] and punycode [14] for domain names (presently only
relevant to IMAP clients).
o The UTF-8 charset [7].
o The IETF policy on Character Sets and Languages [3].
10. Open Issues
1. Should there be an explicit request for the list of comparators
in the COMPARATOR response code?
2. Should we add a command to fetch the lookup tables associated
with a comparator?
3. Need examples for COMPARATOR extension.
Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
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[2] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
[3] Alvestrand, H., "IETF Policy on Character Sets and Languages",
BCP 18, RFC 2277, January 1998.
[4] Gahrns, M. and C. Newman, "IMAP4 Namespace", RFC 2342, May 1998.
[5] Alvestrand, H., "Tags for the Identification of Languages", BCP
47, RFC 3066, January 2001.
[6] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4rev1",
RFC 3501, March 2003.
[7] Yergeau, F., "UTF-8, a transformation format of ISO 10646",
draft-yergeau-rfc2279bis-04 (work in progress), February 2003.
[8] Newman, C., "Internet Application Protocol Comparator Registry",
draft-newman-i18n-comparator-00 (work in progress), May 2003.
Informative References
[9] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message Bodies",
RFC 2045, November 1996.
[10] Moore, K., "MIME (Multipurpose Internet Mail Extensions) Part
Three: Message Header Extensions for Non-ASCII Text", RFC 2047,
November 1996.
[11] Freed, N. and K. Moore, "MIME Parameter Value and Encoded Word
Extensions: Character Sets, Languages, and Continuations", RFC
2231, November 1997.
[12] Hoffman, P. and M. Blanchet, "Preparation of Internationalized
Strings ("stringprep")", RFC 3454, December 2002.
[13] Faltstrom, P., Hoffman, P. and A. Costello, "Internationalizing
Domain Names in Applications (IDNA)", RFC 3490, March 2003.
[14] Costello, A., "Punycode: A Bootstring encoding of Unicode for
Internationalized Domain Names in Applications (IDNA)", RFC
3492, March 2003.
[15] Crispin, M. and K. Murchison, "INTERNET MESSAGE ACCESS PROTOCOL
- SORT AND THREAD EXTENSION", draft-ietf-imapext-sort-12 (work
in progress), March 2003.
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Author's Address
Chris Newman
Sun Microsystems
1050 Lakes Drive
West Covina, CA 91790
US
EMail: chris.newman@sun.com
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Internet-Draft IMAP Internationalization May 2003
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