Network Working Group Arnt Gulbrandsen
Internet-Draft Oryx Mail Systems GmbH
Intended Status: Proposed Standard Alexey Melnikov
Isode Limited
December 14, 2007
The IMAP ENABLE Extension
draft-gulbrandsen-imap-enable-04.txt
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Copyright Notice
Copyright (C) The IETF Trust (2007).
Abstract
Most IMAP extensions are used by the client when it wants to and the
server supports it. However, a few extensions require the server to
know whether a client supports that extension. The ENABLE extension
allows an IMAP client to say which extensions it supports.
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1. 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].
Formal syntax is defined by [RFC4234] as modified by [RFC3501].
Example lines prefaced by "C:" are sent by the client and ones
prefaced by "S:" by the server. The five characters [...] means that
something has been elided.
2. Overview
CONSTORE ([RFC4551]), ANNOTATE ([ANNOTATE]) and some extensions
under consideration at the moment use various commands to enable
server extensions. (CONDSTORE uses a SELECT or FETCH parameter, and
ANNOTATE uses a side effect of FETCH.) This extension adds a
command, ENABLE, which enables such extensions without causing any
other effect.
An IMAP server which supports ENABLE advertises this by including
the word ENABLE in its capability list.
Most IMAP extensions do not require the client to enable the
extension in any way.
3. Protocol changes
3.1 The ENABLE Command
Arguments: capability names
Result: OK: Relevant capabilities enabled
BAD: No arguments, or syntax error in an argument
The ENABLE command takes a list of capability names, and requests
the server to enable the named extensions. Once enabled using
ENABLE, each extension remains active until the IMAP connection is
closed. For each argument, the server does the following:
- If the argument is not an extension known to the server, the
server MUST ignore the argument.
- If the argument is an extension known to the server, and it is not
specifically permitted to be enabled using ENABLE, the server MUST
ignore the argument. (Note that knowing about an extension doesn't
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necessarily imply supporting that extension.)
- If the argument is an extension is supported by the server and
which needs to be enabled, the server MUST enable the extension
for the duration of the connection. At present this applies only
to CONDSTORE ([RFC4551]). Note that once an extension is enabled,
there is no way to disable it.
If the ENABLE command is successful, the server MUST result in the
ENABLED response (see Section 3.2).
Clients SHOULD only include extensions that need to be enabled in
the server. At the time this RFC is published CONDSTORE is the only
such extension (ie. ENABLE CONDSTORE is an additional "Condstore
enabling command" as defined in [RFC4551]). Future RFCs may add to
this list. [Note to the RFC Editor: If the IMAP ANNOTATE document
has been published already, ANNOTATE should be mentioned as well as
CONDSTORE.]
The ENABLE command is only valid in Authenticated state (see
[RFC3501]), before any mailbox is selected. Clients MUST NOT issue
ENABLE once they select a mailbox, however server implementations
don't have to check that no mailbox is selected or was previously
selected during the duration of a connection.
The ENABLE command can be issued multiple times in a session. It is
additive, i.e. "ENABLE a b", followed by "ENABLE c" is the same as a
single command "ENABLE a b c".
There are no limitations on pipelining ENABLE. For example, it is
possible to send ENABLE and then immediately AUTHENTICATE.
The server MUST NOT change the CAPABILITY list as a result of
executing ENABLE, i.e. a CAPABILITY command issued right after an
ENABLE command MUST list the same capabilities as a CAPABILITY
command issued before the ENABLE command. The following example
demonstrates that:
C: t1 CAPABILITY
S: * CAPABILITY IMAP4rev1 AUTH=CRAM-MD5 AUTH=DIGEST-MD5 ID
LITERAL+ ENABLE
S: t1 OK foo
C: t2 ENABLE CONDSTORE X-GOOD-IDEA
S: * ENABLED X-GOOD-IDEA
S: t2 OK foo
C: t3 CAPABILITY
S: * CAPABILITY IMAP4rev1 AUTH=CRAM-MD5 AUTH=DIGEST-MD5 ID
LITERAL+ ENABLE
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S: t3 OK foo again
In the example above the server chose not to advertise support for
some IMAP extensions until after authentication. This is so, because
X-GOOD-IDEA was successfully enabled but is not advertised in the
CAPABILITY response.
In the following example, the client enables CONDSTORE:
C: a1 ENABLE CONDSTORE
S: * ENABLED CONDSTORE
S: a1 OK Conditional Store enabled
Note that according to [RFC3501] the list of advertised capabilities
MAY change after a STARTTLS and/or AUTHENTICATE/LOGIN command. The
ENABLE command doesn't change that.
In the next example, the client asks about the server capabilities,
the server tells the client only what's usable prior to login, the
client enables CONDSTORE and X-GOOD-IDEA, then it logs in.
C: b1 CAPABILITY
S: * CAPABILITY IMAP4rev1 AUTH=CRAM-MD5 AUTH=DIGEST-MD5 ID
LITERAL+ ENABLE
S: b1 OK foo
C: b2 ENABLE CONDSTORE X-GOOD-IDEA
S: * ENABLED CONDSTORE
S: b2 OK foo
C: b3 LOGIN d e
S: b3 OK foo
C: b4 CAPABILITY
S: * CAPABILITY IMAP4rev1 ID LITERAL+ CONDSTORE
S: b4 OK foo
After command b2, the client learns that the server supports and
successfully enabled CONDSTORE but not X-GOOD-IDEA.
3.2 The ENABLED Response
Contents: capability listing
The ENABLED response occurs as a result of an ENABLE command. The
capability listing contains a space-separated listing of capability
names that the server supports and that were successfully enabled.
The ENABLED response may contain no capabilities, which means that
no extensions listed by the client were successfully enabled.
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4. Formal Syntax
The following syntax specification uses the Augmented Backus-Naur
Form (ABNF) notation as specified in [RFC4234]. [RFC3501] defines
the non-terminals "capability" and "command-any".
Except as noted otherwise, all alphabetic characters are case-
insensitive. The use of upper or lower case characters to define
token strings is for editorial clarity only. Implementations MUST
accept these strings in a case-insensitive fashion.
capability =/ "ENABLE"
command-any =/ "ENABLE" 1*(SP capability)
response-data =/ "*" SP enable-data CRLF
enable-data = "ENABLED" *(SP capability)
5. Security considerations
The ENABLE command may be executed before authentication, so it can
be used by malevolent clients. Servers should parse and implement it
with particular care.
It is possible to use ENABLE to find out whether a server implements
certain IMAP extensions. For example, if a server does not advertise
IDLE (see [RFC2177]) in unauthenticated mode, an unauthenticated
client can send ENABLE IDLE and use the response to find out whether
the server supports IDLE. This is believed to be harmless, since the
relevant extensions are necessarily not available to the client.
6. IANA considerations
The IANA is requested to add ENABLE to the IMAP4 Capabilities
Registry. [TO BE REMOVED: This registration should take place at
the following location:
http://www.iana.org/assignments/imap4-capabilities]
7. Acknowledgements
The idea came from Randy Gellens. Alexey Melnikov thought it was a
good idea. The so-called author of this document typed it down and
added the open issues section.
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8. Normative References
[RFC2119] Bradner, "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, Harvard University, March
1997.
[RFC3501] Crispin, "Internet Message Access Protocol - Version
4rev1", RFC 3501, University of Washington, June 2003.
[RFC4234] Crocker, Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 4234, Brandenburg
Internetworking, Demon Internet Ltd, October 2005.
[RFC4551] Melnikov, Hole, "IMAP Extension for Conditional STORE
Operation or Quick Flag Changes Resynchronization", RFC
4551, Isode Ltd., June 2006.
8. Informative References
[RFC2177] Leiba, "IMAP4 IDLE Command", RFC 2177, IBM, June 1997.
[ANNOTATE] Daboo, Gellens, "IMAP ANNOTATE Extension", draft-ietf-
imapext-annotate, August 2006.
10. Author's Address
Arnt Gulbrandsen
Oryx Mail Systems GmbH
Schweppermannstr. 8
D-81671 Muenchen
Germany
Fax: +49 89 4502 9758
Email: arnt@oryx.com
Alexey Melnikov
Isode Ltd
5 Castle Business Village
36 Station Road
Hampton, Middlesex TW12 2BX
UK
Email: Alexey.Melnikov@isode.com
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(RFC Editor: Please delete everything after this point)
Open Issues
None any more.
It would be nice to have some text explaining that well-designed
extensions don't need ENABLE, and why, but my attempts at such text
have not come out well.
Changes since -00
- The IANA asked me to specify the IANA registry exactly
- Say "clients should only use ENABLE when it's really necessary"
- Better abstract
- Wording.
- Refer to RFCs by number, not by topic.
- Boilerplate updates - IETF Trust and so on.
Changes since -01
- Specify that ENABLE ID is BAD, not ignorable.
- Explicitly allow maximum pipelining.
- Security implications.
Changes since -02
- Nits
- Unique tags in examples
- Note specifically that a server can reply BAD to ENABLE ID, even
if it doesn't support ID. All it needs is to know that ID cannot
be ENABLEd.
Changes since -03
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- Added ENABLED response as per discussion on the mailing list
- Changed ENABLE to never return BAD
- Only allow ENABLE in the authenticated state as per consensus in
Vancouver
- Clarified [lack of] interaction with the CAPABILITY response
- Clarified that the ENABLE command is additive
- Added more examples
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