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IETF RFC 5255
Internet Message Access Protocol Internationalization
Last modified on Monday, June 2nd, 2008
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Network Working Group C. Newman
Request for Comments: 5255 Sun Microsystems
Category: Standards Track A. Gulbrandsen
Oryx Mail Systems GmhH
A. Melnikov
Isode Limited
June 2008
Internet Message Access Protocol Internationalization
 Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Abstract
Internet Message Access Protocol (IMAP) version 4rev1 has basic
support for non-ASCII characters in mailbox names and search
substrings. It also supports non-ASCII message headers and content
encoded as specified by Multipurpose Internet Mail Extensions (MIME).
This specification defines a collection of IMAP extensions that
improve international support including language negotiation for
international error text, translations for namespace prefixes, and
comparator negotiation for search, sort, and thread.
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 [RFC 3501] 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 [RFC 2342] 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 [RFC 4790]. 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 [RFC 2119].
The formal syntax uses the Augmented Backus-Naur Form (ABNF)
[RFC 5234] notation including the core rules defined in Appendix A.
The UTF-8-related productions are defined in [RFC 3629].
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 [RFC 3501] 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 [RFC 2277] 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 [RFC 2342].
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 [RFC 4647].
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 [RFC 3629]. 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 [RFC 4647], 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 [RFC 3501] 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 [RFC 5234] rules from
IMAP4rev1 [RFC 3501], IMAP4 Namespace [RFC 2342], Tags for the
Identifying Languages [RFC 4646], UTF-8 [RFC 3629], and Collected
Extensions to IMAP4 ABNF [RFC 4466].
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 [RFC 4466]
lang-range-quoted = astring
; Once any literal wrapper or quoting is removed, this
; follows the language-range rule in [RFC 4647]
lang-tag-quoted = astring
; Once any literal wrapper or quoting is removed, this follows
; the Language-Tag rule in [RFC 4646]
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 [RFC 3501].
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 [RFC 3501] 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
[RFC 4790].
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 [RFC 4790].
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
[RFC 4790], 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 [RFC 4790], 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 [RFC 4790]).
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 [RFC 2047] for headers and
[RFC 2045] 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 [RFC 4790]).
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 [RFC 4790]).
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
[RFC 5234].
(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
[RFC 4790]) 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 [RFC 4790].
< 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 [RFC 5234] rules from
IMAP4rev1 [RFC 3501] and the Internet Application Protocol Comparator
Registry [RFC 4790].
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 [RFC 4790]
comp-order-quoted = astring
; Once any literal wrapper or quoting is removed, this
; follows the collation-order rule from [RFC 4790]
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 [RFC 4790]
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 [RFC 3490] since SASL (see [RFC 4422]) 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)" [RFC 3490]. 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
[RFC 2047]). 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 [RFC 3629] 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 [RFC 2045] for message bodies.
o MIME header encoding [RFC 2047] for message headers.
o The IETF EAI working group.
o MIME Parameter Value and Encoded Word Extensions [RFC 2231] 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 [RFC 3490] and punycode [RFC 3492] for domain names
(currently only relevant to IMAP clients).
o The UTF-8 charset [RFC 3629].
o The IETF policy on Character Sets and Languages [RFC 2277].
10. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 2277] Alvestrand, H., "IETF Policy on Character Sets and
Languages", BCP 18, RFC 2277, January 1998.
[RFC 2342] Gahrns, M. and C. Newman, "IMAP4 Namespace", RFC 2342, May
1998.
[RFC 3501] 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
[RFC 3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.
[RFC 5234] Crocker, D., Ed., and P. Overell, "Augmented BNF for
Syntax Specifications: ABNF", STD 68, RFC 5234, January
2008.
[RFC 4422] Melnikov, A., Ed., and K. Zeilenga, Ed., "Simple
Authentication and Security Layer (SASL)", RFC 4422, June
2006.
[RFC 4466] Melnikov, A. and C. Daboo, "Collected Extensions to IMAP4
ABNF", RFC 4466, April 2006.
[RFC 4646] Phillips, A. and M. Davis, "Tags for Identifying
Languages", BCP 47, RFC 4646, September 2006.
[RFC 4647] Phillips, A. and M. Davis, "Matching of Language Tags",
BCP 47, RFC 4647, September 2006.
[RFC 4790] 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.
[RFC 2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, November 1996.
[RFC 2047] Moore, K., "MIME (Multipurpose Internet Mail Extensions)
Part Three: Message Header Extensions for Non-ASCII Text",
RFC 2047, November 1996.
11. Informative References
[RFC 2231] Freed, N. and K. Moore, "MIME Parameter Value and Encoded
Word Extensions: Character Sets, Languages, and
Continuations", RFC 2231, November 1997.
[RFC 3490] 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
[RFC 3492] 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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Newman, et al. Standards Track PAGE 20
Internet Message Access Protocol Internationalization
RFC TOTAL SIZE: 41403 bytes
PUBLICATION DATE: Monday, June 2nd, 2008
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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