IMAP Extensions Working Group                      M. Gahrns, Microsoft
                                                    R. Cheng, Microsoft
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

   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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   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",
   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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   /*** Consider a server that has the following mailbox hierarchy:


   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

   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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   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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   [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
   One Microsoft Way
   Redmond, WA, 98072

   Phone: (425) 936-9833

   Raymond Cheng
   One Microsoft Way
   Redmond, WA, 98072

   Phone: (425) 703-4913

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