Network Working Group Arnt Gulbrandsen
Internet-Draft Oryx Mail Systems GmbH
Intended Status: Proposed Standard March 2007
The IMAP ENABLE Extension
draft-gulbrandsen-imap-enable-01.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 to. 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.
3. 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 does
not make sense to enable the extension in this way, the server
MUST ignore the argument. (For example, ENABLE ID does nothing
because ID (see [RFC2971]) does not need to be enabled in the
server prior to being used.)
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- 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.
Clients SHOULD only include extensions that need to be enabled in
the server. At present CONDSTORE is the only such extension.
In this example, the client enables CONDSTORE:
C: a ENABLE CONDSTORE
S: a OK Conditional Store enabled
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: a CAPABILITY
S: * CAPABILITY IMAP4rev1 AUTH=CRAM-MD5 AUTH=DIGEST-MD5 ID
LITERAL+ ENABLE
S: a OK foo
C: b ENABLE CONDSTORE X-GOOD-IDEA
S: b OK foo
C: c LOGIN d e
S: c OK foo C: d CAPABILITY
S: * CAPABILITY IMAP4rev1 ID LITERAL+ CONDSTORE
S: d OK foo
After command b, the client does not know whether CONDSTORE and X-
GOOD-IDEA are enabled. After command d, the client learns that the
server supports CONDSTORE but not X-GOOD-IDEA, so it knows that
CONDSTORE is enabled.
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)
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5. Security considerations
This document does not add any new security considerations to IMAP.
6. IANA considerations
The IANA is requested to add ENABLE to the list of IMAP extensions,
http://www.iana.org/assignments/imap4-capabilities.
7. Credits
The idea came from Randy Gellens. Alexey Melnikov thought it was a
good idea. The author of this document typed it down and added the
open issues section.
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.
8. Informative References
[RFC2971] Showalter, "IAMP4 ID extension", RFC 2971, Mirapoint
Inc., October 2000.
[RFC4551] Melnikov, Hole, "IMAP Extension for Conditional STORE
Operation or Quick Flag Changes Resynchronization", RFC
4551, Isode Ltd., June 2006.
[ANNOTATE] Daboo, Gellens, "IMAP ANNOTATE Extension", draft-ietf-
imapext-annotate, August 2006.
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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
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Full Copyright Statement
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Acknowledgment
Funding for the RFC Editor function is currently provided by the
Internet Society.
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(RFC Editor: Please delete everything after this point)
Open Issues
Should ENABLE be a command-nonauth, so clients have to declare their
desires right at the start? That might simplify some server
implementations, particularly proxies. Servers would have to ignore
any capability names they don't know.
At the moment a client can "enable" any capability, even ones not
advertised by the server. This allows a client to enable all it can
support right at the start, even though the server won't advertise
its capabilities until after LOGIN/STARTTLS.
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.
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