< draft-gahrns-imap-child-mailbox-03.txt		draft-gahrns-imap-child-mailbox-04.txt >
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	IMAP Extensions Working Group M. Gahrns, Microsoft		RFC 3348
	R. Cheng, Microsoft
	INTERNET-DRAFT
	Document: draft-gahrns-imap-child-mailbox-03 November 2000
	IMAP4 Child Mailbox Extension
	Status of this Memo
	This document is an Internet-Draft and is in full conformance with
	all provisions of Section 10 of RFC 2026.
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	1. Abstract
	Many IMAP4 [RFC-2060] clients present to the user a hierarchical
	view of the mailboxes that a user has access to. Rather than
	initially presenting to the user the entire mailbox hierarchy, it is
	often preferable to show to the user a collapsed outline list of the
	mailbox hierarchy (particularly if there is a large number of
	mailboxes). The user can then expand the collapsed outline
	hierarchy as needed. It is common to include within the collapsed
	hierarchy a visual clue (such as a "+") to indicate that there are
	child mailboxes under a particular mailbox. When the visual clue
	is clicked the hierarchy list is expanded to show the child
	mailboxes.
	The CHILDREN extension provides a mechanism for a client to
	efficiently determine if a particular mailbox has children, without
	issuing a LIST "" * or a LIST "" % for each mailbox name.
	Several IMAP vendors have implemented this proposal and this
	documents the expected behavior of existing implementations. The
	IMAPEXT Working Group, is currently considering a LIST extension
	where it has been proposed to incorporate similar functionality to
	the \HasChildren and \HasNoChildren flags. With this new LIST
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	IMAP4 Child Mailbox Extension November 2000
	Extension, the client would have an opportunity to request whether
	or not the server should return child mailbox information. This
	would be an advantage for servers where this information is
	expensive to compute, since the server would only need to compute
	the information when it knew that the client requesting the
	information would be able to use it.
	As such, it is expected that this internet-draft will move to
	informational status, as child mailbox information is absorbed into
	a more general LIST extension.
	2. Conventions used in this document
	In examples, "C:" and "S:" indicate lines sent by the client and
	server respectively. If such lines are wrapped without a new "C:"
	or "S:" label, then the wrapping is for editorial clarity and is not
	part of the command.
	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].
	3. Requirements
	IMAP4 servers that support this extension MUST list the keyword
	CHILDREN in their CAPABILITY response.
	The CHILDREN extension defines two new attributes that MAY be
	returned within a LIST response.
	\HasChildren - The presence of this attribute indicates that the
	mailbox has child mailboxes.
	Servers SHOULD NOT return \HasChildren if child mailboxes exist, but
	none will be displayed to the current user in a LIST response (as
	should be the case where child mailboxes exist, but a client does
	not have permissions to access them.) In this case, \HasNoChildren
	SHOULD be used.
	In many cases, however, a server may not be able to efficiently
	compute whether a user has access to all child mailboxes, or
	multiple users may be accessing the same account and simultaneously
	changing the mailbox hierarchy. As such a client MUST be prepared
	to accept the \HasChildren attribute as a hint. That is, a mailbox
	MAY be flagged with the \HasChildren attribute, but no child
	mailboxes will appear in a subsequent LIST response.
	Example 3.1:
	============
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	IMAP4 Child Mailbox Extension November 2000
	/*** Consider a server that has the following mailbox hierarchy:
	INBOX
	ITEM_1
	ITEM_1A
	ITEM_2
	TOP_SECRET
	Where INBOX, ITEM_1 and ITEM_2 are top level mailboxes. ITEM_1A is
	a child mailbox of ITEM_1 and TOP_SECRET is a child mailbox of
	ITEM_2 that the currently logged on user does NOT have access to.
	Note that in this case, the server is not able to efficiently
	compute access rights to child mailboxes and responds with a
	\HasChildren attribute for mailbox ITEM_2, even though
	ITEM_2/TOP_SECRET does not appear in the list response. ***/
	C: A001 LIST "" *
	S: * LIST (\HasNoChildren) "/" INBOX
	S: * LIST (\HasChildren) "/" ITEM_1
	S: * LIST (\HasNoChildren) "/" ITEM_1/ITEM_1A
	S: * LIST (\HasChildren) "/" ITEM_2
	S: A001 OK LIST Completed
	\HasNoChildren - The presence of this attribute indicates that the
	mailbox has NO child mailboxes that are accessible to the currently
	authenticated user. If a mailbox has the \Noinferiors attribute,
	the \HasNoChildren attribute is redundant and SHOULD be omitted in
	the LIST response.
	In some instances a server that supports the CHILDREN extension MAY
	NOT be able to determine whether a mailbox has children. For
	example it may have difficulty determining whether there are child
	mailboxes when LISTing mailboxes while operating in a particular
	namespace.
	In these cases, a server MAY exclude both the \HasChildren and
	\HasNoChildren attributes in the LIST response. As such, a client
	can not make any assumptions about whether a mailbox has children
	based upon the absence of a single attribute.
	It is an error for the server to return both a \HasChildren and a
	\HasNoChildren attribute in a LIST response.
	It is an error for the server to return both a \HasChildren and a
	\NoInferiors attribute in a LIST response.
	Note: the \HasNoChildren attribute should not be confused with the
	IMAP4 [RFC-2060] defined attribute \Noinferiors which indicates that
	no child mailboxes exist now and none can be created in the future.
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	IMAP4 Child Mailbox Extension November 2000
	The \HasChildren and \HasNoChildren attributes might not be returned
	in response to a LSUB response. Many servers maintain a simple
	mailbox subscription list that is not updated when the underlying
	mailbox structure is changed. A client MUST NOT assume that
	hierarchy information will be maintained in the subscription list.
	RLIST is a command defined in [RFC-2193] that includes in a LIST
	response mailboxes that are accessible only via referral. That is,
	a client must explicitly issue an RLIST command to see a list of
	these mailboxes. Thus in the case where a mailbox has child
	mailboxes that are available only via referral, the mailboxes would
	appear as \HasNoChildren in response to the LIST command, and
	\HasChildren in response to the RLIST command.
	5. Formal Syntax
	The following syntax specification uses the augmented Backus-Naur
	Form (BNF) as described in [ABNF].
	Two new mailbox attributes are defined as flag_extensions to the
	IMAP4 mailbox_list response:
	HasChildren = "\HasChildren"
	HasNoChildren = "\HasNoChildren"
	6. Security Considerations
	This extension provides a client a more efficient means of
	determining whether a particular mailbox has children. If a mailbox
	has children, but the currently authenticated user does not have
	access to any of them, the server SHOULD respond with a
	\HasNoChildren attribute. In many cases, however, a server may not
	be able to efficiently compute whether a user has access to all
	child mailboxes. If such a server responds with a \HasChildren
	attribute, when in fact the currently authenticated user does not
	have access to any child mailboxes, potentially more information is
	conveyed about the mailbox than intended. A server designed with
	such levels of security in mind SHOULD NOT attach the \HasChildren
	attribute to a mailbox unless the server is certain that the user
	has access to at least one of the child mailboxes.
	7. References
	[RFC-2060], Crispin, M., "Internet Message Access Protocol - Version
	4rev1", RFC 2060, University of Washington, December 1996.
	[RFC-2119], Bradner, S, "Key words for use in RFCs to Indicate
	Requirement Levels", RFC 2119, Harvard University, March 1997
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	IMAP4 Child Mailbox Extension November 2000
	[RFC-2234], D. Crocker and P. Overell, Editors, "Augmented BNF for
	Syntax Specifications: ABNF", RFC 2234, Internet Mail Consortium,
	November 1997
	[RFC-2193], Gahrns, M, "IMAP4 Mailbox Referrals", RFC 2193,
	Microsoft Corporation, September 1997
	8. Acknowledgments
	The authors would like to thank the participants of several IMC Mail
	Connect events for their input when this idea was originally
	presented and refined.
	9. Author's Address
	Mike Gahrns
	Microsoft
	One Microsoft Way
	Redmond, WA, 98072
	Phone: (425) 936-9833		Title: The Internet Message Action Protocol (IMAP4) Child
	Email: mikega@microsoft.com		Mailbox Extension
			Author(s): M. Gahrns, R. Cheng
			Status: Informational
			Date: July 2002
			Mailbox: mikega@microsoft.com, raych@microsoft.com
			Pages: 6
			Characters: 11868
			Updates/Obsoletes/SeeAlso: None
	Raymond Cheng		I-D Tag: draft-gahrns-imap-child-mailbox-04.txt
	Microsoft
	One Microsoft Way
	Redmond, WA, 98072
	Phone: (425) 703-4913		URL: ftp://ftp.rfc-editor.org/in-notes/rfc3348.txt
	Email: raych@microsoft.com
	Gahrns and Cheng Expires May 2001 5		The Internet Message Action Protocol (IMAP4) CHILDREN extension
	IMAP4 Child Mailbox Extension November 2000		provides a mechanism for a client to efficiently determine if a
			particular mailbox has children, without issuing a LIST "" * or a LIST
			"" % for each mailbox.
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