Internet DRAFT - draft-ietf-eai-5738bis
draft-ietf-eai-5738bis
Internet Engineering Task Force P. Resnick, Ed.
Internet-Draft Qualcomm Incorporated
Obsoletes: RFC5738 (if approved) C. Newman, Ed.
Intended status: Standards Track Oracle
Expires: May 20, 2013 S. Shen, Ed.
CNNIC
November 16, 2012
IMAP Support for UTF-8
draft-ietf-eai-5738bis-12
Abstract
This specification extends the Internet Message Access Protocol
version 4rev1 (IMAP4rev1) to support UTF-8 encoded international
characters in user names, mail addresses and message headers. This
specification replaces RFC 5738.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on May 20, 2013.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions Used in this Document . . . . . . . . . . . . . . 3
3. UTF8=ACCEPT IMAP Capability and UTF-8 in IMAP Quoted
Strings . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. IMAP UTF8 Append Data Extension . . . . . . . . . . . . . . . 5
5. LOGIN Command and UTF-8 . . . . . . . . . . . . . . . . . . . 5
6. UTF8=ONLY Capability . . . . . . . . . . . . . . . . . . . . . 6
7. Dealing With Legacy Clients . . . . . . . . . . . . . . . . . 6
8. Issues with UTF-8 Header Mailstore . . . . . . . . . . . . . . 8
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
10. Security Considerations . . . . . . . . . . . . . . . . . . . 9
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
11.1. Normative References . . . . . . . . . . . . . . . . . . 9
11.2. Informative References . . . . . . . . . . . . . . . . . 10
Appendix A. Design Rationale . . . . . . . . . . . . . . . . . . 11
Appendix B. Acknowledgments . . . . . . . . . . . . . . . . . . . 11
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1. Introduction
This specification forms part of the Email Address
Internationalization protocols described in the Email Address
Internationalization Framework document [RFC6530]. It extends
IMAP4rev1 [RFC3501] to permit UTF-8 [RFC3629] in headers as described
in "Internationalized Email Headers" [RFC6532]. It also adds a
mechanism to support mailbox names using the UTF-8 charset. This
specification creates two new IMAP capabilities to allow servers to
advertise these new extensions.
This specification assumes that the IMAP server will be operating in
a fully internationalized environment, i.e., one in which all clients
accessing the server will be able to accept non-ASCII message header
fields and other information as specified in Section 3. At least
during a transition period, that assumption will not be realistic for
many environments; the issues involved are discussed in Section 7
below.
This specification replaces an earlier, experimental, approach to the
same problem [RFC5738].
2. 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" [RFC2119].
The formal syntax uses the Augmented Backus-Naur Form (ABNF)
[RFC5234] notation. In addition, rules from IMAP4rev1 [RFC3501],
UTF-8 [RFC3629], "Collected Extensions to IMAP4 ABNF" [RFC4466], and
IMAP4 LIST Command Extensions [RFC5258] are also referenced. This
document assumes that the reader will have a reasonably good
understanding of the RFCs above.
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. UTF8=ACCEPT IMAP Capability and UTF-8 in IMAP Quoted Strings
The "UTF8=ACCEPT" capability indicates that the server supports the
ability to open mailboxes containing internationalized messages with
SELECT and EXAMINE, and can provide UTF-8 responses to the LIST and
LSUB commands. This capability also affects other IMAP extensions
that can return mailbox names or their prefixes, such as NAMESPACE
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[RFC2342] and ACL [RFC4314].
The "UTF8=ONLY" capability described in Section 6 implies the
"UTF8=ACCEPT" capability. A server is said to "support UTF8=ACCEPT"
if it advertises either "UTF8=ACCEPT" or "UTF8=ONLY".
A client MUST use the "ENABLE" command (defined in [RFC5161]) with
the "UTF8=ACCEPT" option (defined in Section 4 below) to indicate to
the server that the client accepts UTF-8 in quoted-strings and
supports UTF8=ACCEPT extension. The "ENABLE UTF8=ACCEPT" command is
only valid in the authenticated state.
The IMAP4rev1 [RFC3501] base specification forbids the use of 8-bit
characters in atoms or quoted strings. Thus, a UTF-8 string can only
be sent as a literal. This can be inconvenient from a coding
standpoint, and unless the server offers IMAP4 non-synchronizing
literals [RFC2088], this requires an extra round trip for each UTF-8
string sent by the client. When the IMAP server supports
"UTF8=ACCEPT" it supports UTF-8 in quoted-strings with the following
syntax:
quoted =/ DQUOTE *uQUOTED-CHAR DQUOTE
; QUOTED-CHAR is not modified, as it will affect
; other RFC 3501 ABNF non terminal.
uQUOTED-CHAR = QUOTED-CHAR / UTF8-2 / UTF8-3 / UTF8-4
UTF8-2 = <Defined in Section 4 of RFC3629>
UTF8-3 = <Defined in Section 4 of RFC3629>
UTF8-4 = <Defined in Section 4 of RFC3629>
When this extended quoting mechanism is used by the client, then the
server MUST reject with a "BAD" response any octet sequences with the
high bit set that fail to comply with the formal syntax in [RFC3629].
The IMAP server MUST NOT send UTF-8 in quoted strings to the client
unless the client has indicated support for that syntax by using the
"ENABLE UTF8=ACCEPT" command.
If the server supports "UTF8=ACCEPT", the client MAY use extended
quoted syntax with any IMAP argument that permits a string (including
astring and nstring). However, if characters outside the US-ASCII
repertoire are used in an inappropriate place, the results would be
the same as if other syntactically valid but semantically invalid
characters were used. Specific cases where UTF-8 characters are
permitted or not permitted are described in the following paragraphs.
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All IMAP servers that support "UTF8=ACCEPT" SHOULD accept UTF-8 in
mailbox names, and those that also support the "Mailbox International
Naming Convention" described in RFC 3501, Section 5.1.3, MUST accept
utf8-quoted mailbox names and convert them to the appropriate
internal format. Mailbox names MUST comply with the Net-Unicode
Definition ([RFC5198], Section 2) with the specific exception that
they MUST NOT contain control characters (0000-001F, 0080-009F),
delete (007F), line separator (2028), or paragraph separator (2029).
Once an IMAP client has enabled UTF-8 support with the "ENABLE
UTF8=ACCEPT" command, it MUST NOT issue a SEARCH command that
contains a CHARSET specification. If an IMAP server receives such a
SEARCH command in that situation, it SHOULD reject the command with a
BAD response (due to the conflicting charset labels).
4. IMAP UTF8 Append Data Extension
If the server supports "UTF8=ACCEPT", then the server accepts UTF-8
headers in the APPEND command message argument. A client that sends
a message with UTF-8 headers to the server MUST send them using the
"UTF8" APPEND data extension. If the server also advertises the
CATENATE capability (as specified in [RFC4469]), the client can use
the same data extension to include such a message in a CATENATE
message part. The ABNF for the APPEND data extension and CATENATE
extension follows:
utf8-literal = "UTF8" SP "(" literal8 ")"
literal8 = <Defined in RFC 4466>
append-data =/ utf8-literal
cat-part =/ utf8-literal
If an IMAP server supports "UTF8=ACCEPT" and the IMAP client has not
issued the "ENABLE UTF8=ACCEPT" command, the server MUST reject with
a "NO" response an APPEND command that includes any 8-bit character
in message header fields.
5. LOGIN Command and UTF-8
This specification doesn't extend the IMAP LOGIN command [RFC3501] to
support UTF-8 usernames and passwords. Whenever a client needs to
use UTF-8 username/passwords, it MUST use the IMAP AUTHENTICATE
command which is already capable of passing UTF-8 user names and
credentials.
Although the use of the IMAP AUTHENTICATE command in this way makes
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it syntactically legal to have a UTF-8 user name or password, there
is no guarantee the user provisioning system used by the IMAP server
will allow such identities. This is an implementation decision and
may depend on what identity system the IMAP server is configured to
use.
6. UTF8=ONLY Capability
The "UTF8=ONLY" capability indicates that the server supports
"UTF8=ACCEPT" (see Section 4), and also that it requires support for
UTF-8 from clients. In particular, this means that it will send
UTF-8 in quoted strings, and it will not accept the older
international mailbox name convention (modified UTF-7). Because
these are incompatible changes to IMAP, explicit server announcement
and client confirmation is necessary: clients MUST use the "ENABLE
UTF8=ACCEPT" command before using this server. A server that
advertises "UTF8=ONLY" will reject with a "NO [CANNOT]" response any
command that might require UTF-8 support and is not preceded by an
"ENABLE UTF8=ACCEPT" command.
IMAP clients that find support for a server that announces
"UTF8=ONLY" problematic are encouraged to at least detect the
announcement and provide an informative error message to the end-
user.
Because the "UTF8=ONLY" server capability includes support for
"UTF8=ACCEPT", the capability string will include at most one of
those and never both. For the client, "ENABLE UTF8=ACCEPT" is always
used -- never "ENABLE UTF8-ONLY".
7. Dealing With Legacy Clients
In most situations, it will be difficult or impossible for the
implementer or operator of an IMAP (or POP) server to know whether
all of the clients that might access it, or the associated mail store
more generally, will be able to support the facilities defined in
this document. In almost all cases, servers who conform to this
specification will have to be prepared to deal with clients that do
not enable the relevant capabilities. Unfortunately, there is no
completely satisfactory way to do so other than for systems that wish
to receive email that requires SMTPUTF8 capabilities to be sure that
all components of those systems -- including IMAP and other clients
selected by users -- are upgraded appropriately.
Choices available to the server when a message that requires SMTPUTF8
is encountered and the client doesn't enable UTF-8 capability include
hiding the problematic message(s), creating in band or out of band
notifications or error messages, or somehow trying to create a
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variation on the message with the intention of providing useful
information to that client about what has occurred. Such variant
messages cannot be actual substitutes for the original message: they
will almost always be impossible to reply to (either at all or
without loss of information); the new header fields or specialized
constructs for server-client communication may go beyond the
requirements of, e.g., RFC 5322; they may consequently confuse some
legacy mail user agents (including IMAP clients) or otherwise may not
provide the expected information to users. There are also tradeoffs
in constructing variants of the original message between accepting
complexity and additional computation costs in order to try to
preserve as much information as possible (for example, in
[I-D.ietf-eai-popimap-downgrade]) and trying to minimize those costs
while still providing useful information (for example, in
[I-D.ietf-eai-simpledowngrade]).
Implementations that choose to do downgrading SHOULD use one of the
standardized algorithms, [I-D.ietf-eai-popimap-downgrade] or
[I-D.ietf-eai-simpledowngrade]. Getting downgrade algorithms right,
and minimizing the risk of operational problems and harm to the email
system, is tricky and requires careful engineering. These two
algorithms are well understood and carefully designed.
Because such messages are really variations on the original ones, not
really "downgraded ones" (although that terminology is often used for
convenience), they inevitably have relationships to the original ones
that the IMAP specification [RFC3501] did not anticipate. This
brings up two concerns in particular: First, digital signatures
computed over and intended for the original message will often not be
applicable to the variant message, and will often fail signature
verification. (It will be possible for some digital signatures to be
verified, if they cover only parts of the original message that are
not affected in the creation of the variant.) Second, servers that
may be accessed by the same user with different clients or methods
(e.g., POP or webmail systems in addition to IMAP or IMAP clients
with different capabilities) will need to exert extreme care to be
sure that UIDVALIDITY behaves as the user would expect. Those issues
may be especially sensitive if the server caches the variant message
or computes and stores it when the message arrives with the intent of
making either form available depending on client capabilities.
Additionally, in order to cope with the case when a server compliant
with this extension returns the same UIDVALIDITY to both legacy and
UTF8=ACCEPT-aware clients, a client upgraded from non UTF8=ACCEPT
aware MUST discard its cache of messages downloaded from the server.
The best (or "least bad") approach for any given environment will
depend on local conditions, local assumptions about user behavior,
the degree of control the server operator has over client usage and
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upgrading, the options that are actually available, and so on. It is
impossible, at least at the time of publication of this
specification, to give good advice that will apply to all situations,
or even particular profiles of situations, other than "upgrade legacy
clients as soon as possible".
8. Issues with UTF-8 Header Mailstore
When an IMAP server uses a mailbox format that supports UTF-8 headers
and it permits selection or examination of that mailbox without
issuing "ENABLE UTF8=ACCEPT" first, it is the responsibility of the
server to comply with the IMAP4rev1 base specification [RFC3501] and
[RFC5322] with respect to all header information transmitted over the
wire. The issue of handling messages containing non-ASCII characters
in legacy environments is discussed in Section 7.
9. IANA Considerations
This document redefines two capabilities ("UTF8=ACCEPT" and
"UTF8=ONLY") in the IMAP 4 Capabilities registry [RFC3501]. Three
other capabilities that were described in the experimental
predecessor to this document (UTF8=ALL, UTF8=APPEND, UTF8=USER) are
now made OBSOLETE. IANA is asked to change the IMAP 4 Capabilities
registry as follows:
OLD:
+--------------+---------------------------+
| UTF8=ACCEPT | [RFC5738] |
| UTF8=ALL | [RFC5738] |
| UTF8=APPEND | [RFC5738] |
| UTF8=ONLY | [RFC5738] |
| UTF8=USER | [RFC5738] |
+--------------+---------------------------+
NEW:
+--------------+---------------------------+
| UTF8=ACCEPT | [[this RFC]] |
| UTF8=ALL | OBSOLETE (was [RFC5738]) |
| UTF8=APPEND | OBSOLETE (was [RFC5738]) |
| UTF8=ONLY | [[this RFC]] |
| UTF8=USER | OBSOLETE (was [RFC5738]) |
+--------------+---------------------------+
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10. Security Considerations
The security considerations of UTF-8 [RFC3629] and SASLprep [RFC4013]
apply to this specification, particularly with respect to use of
UTF-8 in user names and passwords. Otherwise, this is not believed
to alter the security considerations of IMAP4rev1.
Special considerations, some of them with security implications,
occur if a server that conforms to this specification is accessed by
a client that does not, as well as in some more complex situations in
which a given message is accessed by multiple clients that might use
different protocols and/or support different capabilities. Those
issues are discussed in Section 7 above.
11. References
11.1. Normative References
[RFC2119] Bradner, S., "Key words for use in
RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119,
March 1997.
[RFC3501] Crispin, M., "INTERNET MESSAGE
ACCESS PROTOCOL - VERSION 4rev1",
RFC 3501, March 2003.
[RFC3629] Yergeau, F., "UTF-8, a
transformation format of ISO
10646", STD 63, RFC 3629,
November 2003.
[RFC4013] Zeilenga, K., "SASLprep: Stringprep
Profile for User Names and
Passwords", RFC 4013,
February 2005.
[RFC4466] Melnikov, A. and C. Daboo,
"Collected Extensions to IMAP4
ABNF", RFC 4466, April 2006.
[RFC4469] Resnick, P., "Internet Message
Access Protocol (IMAP) CATENATE
Extension", RFC 4469, April 2006.
[RFC5161] Gulbrandsen, A. and A. Melnikov,
"The IMAP ENABLE Extension",
RFC 5161, March 2008.
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[RFC5198] Klensin, J. and M. Padlipsky,
"Unicode Format for Network
Interchange", RFC 5198, March 2008.
[RFC5234] Crocker, D. and P. Overell,
"Augmented BNF for Syntax
Specifications: ABNF", STD 68,
RFC 5234, January 2008.
[RFC5258] Leiba, B. and A. Melnikov,
"Internet Message Access Protocol
version 4 - LIST Command
Extensions", RFC 5258, June 2008.
[RFC6532] Yang, A., Steele, S., and N. Freed,
"Internationalized Email Headers",
RFC 6532, February 2012.
[RFC5322] Resnick, P., Ed., "Internet Message
Format", RFC 5322, October 2008.
[RFC6530] Klensin, J. and Y. Ko, "Overview
and Framework for Internationalized
Email", RFC 6530, February 2012.
[I-D.ietf-eai-popimap-downgrade] Fujiwara, K., "Post-delivery
Message Downgrading for
Internationalized Email Messages",
draft-ietf-eai-popimap-downgrade-08
(work in progress), October 2012.
[I-D.ietf-eai-simpledowngrade] Gulbrandsen, A., "Simplified POP/
IMAP Downgrading for
Internationalized Email",
draft-ietf-eai-simpledowngrade-07
(work in progress), August 2012.
11.2. Informative References
[RFC2088] Myers, J., "IMAP4 non-synchronizing
literals", RFC 2088, January 1997.
[RFC5738] Resnick, P. and C. Newman, "IMAP
Support for UTF-8", RFC 5738,
March 2010.
[RFC2342] Gahrns, M. and C. Newman, "IMAP4
Namespace", RFC 2342, May 1998.
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[RFC4314] Melnikov, A., "IMAP4 Access Control
List (ACL) Extension", RFC 4314,
December 2005.
Appendix A. Design Rationale
This non-normative section discusses the reasons behind some of the
design choices in the above specification.
The basic approach of advertising the ability to access a mailbox in
UTF-8 mode is intended to permit graceful upgrade, including servers
that support multiple mailbox formats. In particular, it would be
undesirable to force conversion of an entire server mailstore to
UTF-8 headers, so being able to phase-in support for new mailboxes
and gradually migrate old mailboxes is permitted by this design.
The "UTF8=ONLY" mechanism simplifies diagnosis of interoperability
problems when legacy support goes away. In the situation where
backwards compatibility is broken anyway, just-send-UTF-8 IMAP has
the advantage that it might work with some legacy clients. However,
the difficulty of diagnosing interoperability problems caused by a
just-send-UTF-8 IMAP mechanism is the reason the "UTF8=ONLY"
capability mechanism was chosen.
Appendix B. Acknowledgments
The authors wish to thank the participants of the EAI working group
for their contributions to this document with particular thanks to
Harald Alvestrand, David Black, Randall Gellens, Arnt Gulbrandsen,
Kari Hurtta, John Klensin, Xiaodong Lee, Charles Lindsey, Alexey
Melnikov, Subramanian Moonesamy, Shawn Steele, Daniel Taharlev, and
Joseph Yee for their specific contributions to the discussion.
Authors' Addresses
Pete Resnick (editor)
Qualcomm Incorporated
5775 Morehouse Drive
San Diego, CA 92121-1714
US
Phone: +1 858 651 4478
EMail: presnick@qti.qualcomm.com
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Chris Newman (editor)
Oracle
800 Royal Oaks
Monrovia, CA 91016
USA
Phone:
EMail: chris.newman@oracle.com
Sean Shen (editor)
CNNIC
No.4 South 4th Zhongguancun Street
Beijing, 100190
China
Phone: +86 10-58813038
EMail: shenshuo@cnnic.cn
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