< draft-gulbrandsen-imap-enable-04.txt		draft-gulbrandsen-imap-enable-05.txt >
	Network Working Group Arnt Gulbrandsen		Network Working Group Arnt Gulbrandsen
	Internet-Draft Oryx Mail Systems GmbH		Internet-Draft Oryx Mail Systems GmbH
	Intended Status: Proposed Standard Alexey Melnikov		Intended Status: Proposed Standard Alexey Melnikov
	Isode Limited		Isode Limited
	December 14, 2007		December 16, 2007
	The IMAP ENABLE Extension		The IMAP ENABLE Extension
	draft-gulbrandsen-imap-enable-04.txt		draft-gulbrandsen-imap-enable-05.txt
	Status of this Memo		Status of this Memo
	By submitting this Internet-Draft, each author represents that any		By submitting this Internet-Draft, each author represents that any
	applicable patent or other IPR claims of which he or she is aware		applicable patent or other IPR claims of which he or she is aware
	have been or will be disclosed, and any of which he or she becomes		have been or will be disclosed, and any of which he or she becomes
	aware will be disclosed, in accordance with Section 6 of BCP 79.		aware will be disclosed, in accordance with Section 6 of BCP 79.
	Internet-Drafts are working documents of the Internet Engineering		Internet-Drafts are working documents of the Internet Engineering
	Task Force (IETF), its areas, and its working groups. Note that		Task Force (IETF), its areas, and its working groups. Note that
	skipping to change at page 2, line 21 ¶		skipping to change at page 2, line 21 ¶
	document are to be interpreted as described in [RFC2119].		document are to be interpreted as described in [RFC2119].
	Formal syntax is defined by [RFC4234] as modified by [RFC3501].		Formal syntax is defined by [RFC4234] as modified by [RFC3501].
	Example lines prefaced by "C:" are sent by the client and ones		Example lines prefaced by "C:" are sent by the client and ones
	prefaced by "S:" by the server. The five characters [...] means that		prefaced by "S:" by the server. The five characters [...] means that
	something has been elided.		something has been elided.
	2. Overview		2. Overview
	CONSTORE ([RFC4551]), ANNOTATE ([ANNOTATE]) and some extensions		Several IMAP extensions allow the server to return unsolicited
	under consideration at the moment use various commands to enable		responses specific to these extensions in certain circumstances.
	server extensions. (CONDSTORE uses a SELECT or FETCH parameter, and		However, servers cannot send those unsolicited responses until they
	ANNOTATE uses a side effect of FETCH.) This extension adds a		know that the clients support such extensions and thus won't choke
	command, ENABLE, which enables such extensions without causing any		on the extension response data.
	other effect.
			Up until now extensions have typically stated that a server cannot
			send the unsolicited responses until after the client has used a
			command with the extension data (i.e. at that point the server knows
			the client is aware of the extension). CONSTORE ([RFC4551]),
			ANNOTATE ([ANNOTATE]) and some extensions under consideration at the
			moment use various commands to enable server extensions. For example
			CONDSTORE uses a SELECT or FETCH parameter, and ANNOTATE uses a side
			effect of FETCH.
			The ENABLE extension provides an explicit indication from the client
			that it supports particular extensions. This is done using a new
			ENABLE command.
	An IMAP server which supports ENABLE advertises this by including		An IMAP server which supports ENABLE advertises this by including
	the word ENABLE in its capability list.		the word ENABLE in its capability list.
	Most IMAP extensions do not require the client to enable the		Most IMAP extensions do not require the client to enable the
	extension in any way.		extension in any way.
	3. Protocol changes		3. Protocol changes
	3.1 The ENABLE Command		3.1 The ENABLE Command
	Arguments: capability names		Arguments: capability names
	Result: OK: Relevant capabilities enabled		Result: OK: Relevant capabilities enabled
	BAD: No arguments, or syntax error in an argument		BAD: No arguments, or syntax error in an argument
			Internet-draft December 2007
	The ENABLE command takes a list of capability names, and requests		The ENABLE command takes a list of capability names, and requests
	the server to enable the named extensions. Once enabled using		the server to enable the named extensions. Once enabled using
	ENABLE, each extension remains active until the IMAP connection is		ENABLE, each extension remains active until the IMAP connection is
	closed. For each argument, the server does the following:		closed. For each argument, the server does the following:
	- If the argument is not an extension known to the server, the		- If the argument is not an extension known to the server, the
	server MUST ignore the argument.		server MUST ignore the argument.
	- If the argument is an extension known to the server, and it is not		- If the argument is an extension known to the server, and it is not
	specifically permitted to be enabled using ENABLE, the server MUST		specifically permitted to be enabled using ENABLE, the server MUST
	ignore the argument. (Note that knowing about an extension doesn't		ignore the argument. (Note that knowing about an extension doesn't
	Internet-draft December 2007
	necessarily imply supporting that extension.)		necessarily imply supporting that extension.)
	- If the argument is an extension is supported by the server and		- If the argument is an extension is supported by the server and
	which needs to be enabled, the server MUST enable the extension		which needs to be enabled, the server MUST enable the extension
	for the duration of the connection. At present this applies only		for the duration of the connection. At present this applies only
	to CONDSTORE ([RFC4551]). Note that once an extension is enabled,		to CONDSTORE ([RFC4551]). Note that once an extension is enabled,
	there is no way to disable it.		there is no way to disable it.
	If the ENABLE command is successful, the server MUST result in the		If the ENABLE command is successful, the server MUST send an
	ENABLED response (see Section 3.2).		untagged ENABLED response (see Section 3.2).
	Clients SHOULD only include extensions that need to be enabled in		Clients SHOULD only include extensions that need to be enabled by
	the server. At the time this RFC is published CONDSTORE is the only		the server. At the time this RFC is published CONDSTORE is the only
	such extension (ie. ENABLE CONDSTORE is an additional "Condstore		such extension (ie. ENABLE CONDSTORE is an additional "Condstore
	enabling command" as defined in [RFC4551]). Future RFCs may add to		enabling command" as defined in [RFC4551]). Future RFCs may add to
	this list. [Note to the RFC Editor: If the IMAP ANNOTATE document		this list. [Note to the RFC Editor: If the IMAP ANNOTATE document
	has been published already, ANNOTATE should be mentioned as well as		has been published already, ANNOTATE should be mentioned as well as
	CONDSTORE.]		CONDSTORE.]
	The ENABLE command is only valid in Authenticated state (see		The ENABLE command is only valid in Authenticated state (see
	[RFC3501]), before any mailbox is selected. Clients MUST NOT issue		[RFC3501]), before any mailbox is selected. Clients MUST NOT issue
	ENABLE once they select a mailbox, however server implementations		ENABLE once they SELECT/EXAMINE a mailbox, however server
	don't have to check that no mailbox is selected or was previously		implementations don't have to check that no mailbox is selected or
	selected during the duration of a connection.		was previously selected during the duration of a connection.
	The ENABLE command can be issued multiple times in a session. It is		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		additive, i.e. "ENABLE a b", followed by "ENABLE c" is the same as a
	single command "ENABLE a b c".		single command "ENABLE a b c". When multiple ENABLE commands are
			issued, each corresponding ENABLED response SHOULD only contain
			extensions enabled by the corresponding ENABLE command.
	There are no limitations on pipelining ENABLE. For example, it is		There are no limitations on pipelining ENABLE. For example, it is
	possible to send ENABLE and then immediately AUTHENTICATE.		possible to send ENABLE and then immediately SELECT, or a LOGIN
			immediately followed by ENABLE.
	The server MUST NOT change the CAPABILITY list as a result of		The server MUST NOT change the CAPABILITY list as a result of
	executing ENABLE, i.e. a CAPABILITY command issued right after an		executing ENABLE, i.e. a CAPABILITY command issued right after an
			Internet-draft December 2007
	ENABLE command MUST list the same capabilities as a CAPABILITY		ENABLE command MUST list the same capabilities as a CAPABILITY
	command issued before the ENABLE command. The following example		command issued before the ENABLE command. The following example
	demonstrates that:		demonstrates that:
	C: t1 CAPABILITY		C: t1 CAPABILITY
	S: * CAPABILITY IMAP4rev1 AUTH=CRAM-MD5 AUTH=DIGEST-MD5 ID		S: * CAPABILITY IMAP4rev1 ID LITERAL+ ENABLE X-GOOD-IDEA
	LITERAL+ ENABLE
	S: t1 OK foo		S: t1 OK foo
	C: t2 ENABLE CONDSTORE X-GOOD-IDEA		C: t2 ENABLE CONDSTORE X-GOOD-IDEA
	S: * ENABLED X-GOOD-IDEA		S: * ENABLED X-GOOD-IDEA
	S: t2 OK foo		S: t2 OK foo
	C: t3 CAPABILITY		C: t3 CAPABILITY
	S: * CAPABILITY IMAP4rev1 AUTH=CRAM-MD5 AUTH=DIGEST-MD5 ID		S: * CAPABILITY IMAP4rev1 ID LITERAL+ ENABLE X-GOOD-IDEA
	LITERAL+ ENABLE
	Internet-draft December 2007
	S: t3 OK foo again		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:		In the following example, the client enables CONDSTORE:
	C: a1 ENABLE CONDSTORE		C: a1 ENABLE CONDSTORE
	S: * ENABLED CONDSTORE		S: * ENABLED CONDSTORE
	S: a1 OK Conditional Store enabled		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		3.2 The ENABLED Response
	Contents: capability listing		Contents: capability listing
	The ENABLED response occurs as a result of an ENABLE command. The		The ENABLED response occurs as a result of an ENABLE command. The
	capability listing contains a space-separated listing of capability		capability listing contains a space-separated listing of capability
	names that the server supports and that were successfully enabled.		names that the server supports and that were successfully enabled.
	The ENABLED response may contain no capabilities, which means that		The ENABLED response may contain no capabilities, which means that
	no extensions listed by the client were successfully enabled.		no extensions listed by the client were successfully enabled.
	Internet-draft December 2007		3.3 Note to designers of extensions that may use the ENABLE command
			Designers of IMAP extensions are discouraged from creating
			extensions that require ENABLE unless there is no good alternative
			design. Specifically, extensions that cause potentially
			incompatible behavior changes to deployed server responses (and thus
			benefit from ENABLE) have a higher complexity cost than extensions
			that do not.
	4. Formal Syntax		4. Formal Syntax
	The following syntax specification uses the Augmented Backus-Naur		The following syntax specification uses the Augmented Backus-Naur
	Form (ABNF) notation as specified in [RFC4234]. [RFC3501] defines		Form (ABNF) notation as specified in [RFC4234] including the core
	the non-terminals "capability" and "command-any".		rules in Appendix B.1. [RFC3501] defines the non-terminals
			"capability" and "command-any".
			Internet-draft December 2007
	Except as noted otherwise, all alphabetic characters are case-		Except as noted otherwise, all alphabetic characters are case-
	insensitive. The use of upper or lower case characters to define		insensitive. The use of upper or lower case characters to define
	token strings is for editorial clarity only. Implementations MUST		token strings is for editorial clarity only. Implementations MUST
	accept these strings in a case-insensitive fashion.		accept these strings in a case-insensitive fashion.
	capability =/ "ENABLE"		capability =/ "ENABLE"
	command-any =/ "ENABLE" 1*(SP capability)		command-any =/ "ENABLE" 1*(SP capability)
	response-data =/ "*" SP enable-data CRLF		response-data =/ "*" SP enable-data CRLF
	enable-data = "ENABLED" *(SP capability)		enable-data = "ENABLED" *(SP capability)
	5. Security considerations		5. Security considerations
	The ENABLE command may be executed before authentication, so it can		It is believed that this extension doesn't add any new security
	be used by malevolent clients. Servers should parse and implement it		considerations that are not already present in the base IMAP
	with particular care.		protocol [RFC3501].
	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		6. IANA considerations
	The IANA is requested to add ENABLE to the IMAP4 Capabilities		The IANA is requested to add ENABLE to the IMAP4 Capabilities
	Registry. [TO BE REMOVED: This registration should take place at		Registry. [[Note to RFC-editor: please remove the following before
	the following location:		publication: This registration should take place at the following
	http://www.iana.org/assignments/imap4-capabilities]		location: http://www.iana.org/assignments/imap4-capabilities]]
	7. Acknowledgements		7. Acknowledgements
	The idea came from Randy Gellens. Alexey Melnikov thought it was a		Editors would like to thank Randy Gellens, Chris Newman, Peter
	good idea. The so-called author of this document typed it down and		Coates, Dave Cridland, Mark Crispin, Ned Freed, Dan Karp, Cyrus
	added the open issues section.		Daboo, Ken Murchison and Eric Burger for comments and corrections.
			However this doesn't necessarily mean that they endorse this
	Internet-draft December 2007		extension, agree with all details or responsible for errors
			introduced by editors.
	8. Normative References		8. Normative References
	[RFC2119] Bradner, "Key words for use in RFCs to Indicate		[RFC2119] Bradner, "Key words for use in RFCs to Indicate
	Requirement Levels", RFC 2119, Harvard University, March		Requirement Levels", RFC 2119, Harvard University, March
	1997.		1997.
	[RFC3501] Crispin, "Internet Message Access Protocol - Version		[RFC3501] Crispin, "Internet Message Access Protocol - Version
			Internet-draft December 2007
	4rev1", RFC 3501, University of Washington, June 2003.		4rev1", RFC 3501, University of Washington, June 2003.
	[RFC4234] Crocker, Overell, "Augmented BNF for Syntax		[RFC4234] Crocker, Overell, "Augmented BNF for Syntax
	Specifications: ABNF", RFC 4234, Brandenburg		Specifications: ABNF", RFC 4234, Brandenburg
	Internetworking, Demon Internet Ltd, October 2005.		Internetworking, Demon Internet Ltd, October 2005.
	[RFC4551] Melnikov, Hole, "IMAP Extension for Conditional STORE		[RFC4551] Melnikov, Hole, "IMAP Extension for Conditional STORE
	Operation or Quick Flag Changes Resynchronization", RFC		Operation or Quick Flag Changes Resynchronization", RFC
	4551, Isode Ltd., June 2006.		4551, Isode Ltd., June 2006.
	skipping to change at page 8, line 9 ¶		skipping to change at page 8, line 9 ¶
	Acknowledgment		Acknowledgment
	Funding for the RFC Editor function is currently provided by the		Funding for the RFC Editor function is currently provided by the
	Internet Society.		Internet Society.
	Internet-draft December 2007		Internet-draft December 2007
	(RFC Editor: Please delete everything after this point)		(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		Changes since -00
	- The IANA asked me to specify the IANA registry exactly		- The IANA asked me to specify the IANA registry exactly
	- Say "clients should only use ENABLE when it's really necessary"		- Say "clients should only use ENABLE when it's really necessary"
	- Better abstract		- Better abstract
	- Wording.		- Wording.
	skipping to change at page 9, line 5 ¶		skipping to change at page 8, line 43 ¶
	- Nits		- Nits
	- Unique tags in examples		- Unique tags in examples
	- Note specifically that a server can reply BAD to ENABLE ID, even		- 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		if it doesn't support ID. All it needs is to know that ID cannot
	be ENABLEd.		be ENABLEd.
	Changes since -03		Changes since -03
	Internet-draft December 2007
	- Added ENABLED response as per discussion on the mailing list		- Added ENABLED response as per discussion on the mailing list
	- Changed ENABLE to never return BAD		- Changed ENABLE to never return BAD
	- Only allow ENABLE in the authenticated state as per consensus in		- Only allow ENABLE in the authenticated state as per consensus in
	Vancouver		Vancouver
	- Clarified [lack of] interaction with the CAPABILITY response		- Clarified [lack of] interaction with the CAPABILITY response
			Internet-draft December 2007
	- Clarified that the ENABLE command is additive		- Clarified that the ENABLE command is additive
	- Added more examples		- Added more examples
			Changes since -04
			- Added rationale for the ENABLE extension
			- Fixed several inconsistencies caused by restring ENABLE to
			authentication state only.
End of changes. 25 change blocks.
	86 lines changed or deleted		68 lines changed or added
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