IMAP Extensions Working Group                                   B. Leiba
Internet Draft                           IBM T.J. Watson Research Center
Document: draft-ietf-imapext-list-extensions-00.txt         October 2000
                                                      Expires April 2001

                     IMAP4 LIST Command Extensions

Status of this Document

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.  Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
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   A revised version of this draft document will be submitted to the RFC
   editor as an Proposed Standard for the Internet Community.
   Discussion and suggestions for improvement are requested, and should
   be sent to  This document will expire before 30
   April 2001.  Distribution of this memo is unlimited.

1. Abstract

   IMAP4 has two commands for listing mailboxes: LIST and LSUB.  As we
   add extensions that require specialized lists (see [MboxRefer] for an
   example) we expand the number of list commands, as each extension
   must add its function to both LIST and LSUB.  This document describes
   extensions to the LIST command that allow these additions to be done
   in mutually compatible options to the LIST command, avoiding the
   exponential increase in specialized list commands.

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2. Conventions used in this document

   In examples, "C:" indicates lines sent by a client that is connected
   to a server.  "S:" indicates lines sent by the server to the client.

   The words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and "MAY" are
   used in this document as specified in RFC 2119 [Keywords].

3. Introduction

   The extensions to the LIST command will be accomplished by amending
   the syntax to allow options to be specified.  The list of options
   will replace the several commands that are currently used to mix and
   match the information requested.  The new syntax is backward-
   compatible, with no ambiguity: if the first word after the command
   name begins with a parenthesis, the new syntax is being used; if it
   does not, it's in the original syntax.

   By adding options to the LIST command, we are announcing the intent
   to phase out and eventually to deprecate the base LSUB command as
   well as the RLIST and RLSUB commands described in [MboxRefer].  We
   are also defining the mechanism to request extended mailbox
   information, such as is described in the now-expired "Child Mailbox
   Extension" draft [ChildMbox].

4. LIST Command Options

   The LIST command syntax is extended by adding a parenthesized list of
   command options between the command name and the reference name (see
   the formal syntax in section 6 for specific details).  Command
   options will be defined in this document and in approved extension
   documents; each option will be enabled by a capability string (one
   capability may enable multiple options), and a client MUST NOT send
   an option for which the server has not advertised support.  A server
   MAY ignore options it does not recognize.

   This extension is identified by the capability string "LISTEXT", and
   support for it is a prerequisite for any future extensions that
   require specialized forms of the LIST command.  Such extensions MUST
   refer to this document and MUST add their function through command
   options as described herein.

   The options defined in this specification are
       SUBSCRIBED - causes the LIST command to list subscribed
          mailboxes, rather than the actual mailboxes.  This will often
          be a subset of the actual mailboxes.  It's also possible for
          this list to contain the names of mailboxes that don't exist.

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          In any case, the list MUST include exactly those mailbox names
          that match the selection criteria and are subscribed to.  This
          option is intended to replace the LSUB command, and MUST be
          supported by all servers.
       REMOTE - causes the LIST command to show remote mailboxes as
          well as local ones, as described in [MboxRefer].  This option
          is intended to replace the RLIST command and, in conjunction
          with the SUBSCRIBED option, the RLSUB command.  This option is
          only available on servers that also support RFC-2193.
       CHILDREN - Requests mailbox child information as originally
          proposed in [ChildMbox].  See section 5, below, for details.
          Support for this is optional, but this option MUST be accepted
          by all servers (though it MAY be ignored).

5. The CHILDREN Option

   The CHILDREN option implements the Child Mailbox Extension,
   originally proposed by Mike Gahrns and Raymond Cheng, of Microsoft
   Corporation.  Most of the information in this section is taken
   directly from their original specification [ChildMbox].  The CHILDREN
   option is simply an indication that the client wants this
   information; a server MAY provide it even if the option is not
   specified, or MAY ignore the option entirely.

   Many IMAP4 [IMAP4] 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 Child Mailbox 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.

   The Child Mailbox Extension defines two new attributes that MAY be
   returned within a LIST response: \HasChildren and \HasNoChildren.
   While these attributes MAY be returned in response to any LIST
   command, the CHILDREN option is provided to indicate that the client
   particularly wants this information.  If the CHILDREN option is
   present, the server SHOULD return these attributes even if their
   computation is expensive.

\HasChildren - The presence of this attribute indicates that the mailbox

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       has child mailboxes.

       A server SHOULD NOT set this attribute if there are child
       mailboxes, and the user does not have permissions to access any
       of 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.  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
       the LIST response.

\HasNoChildren - The presence of this attribute indicates that the
       mailbox has NO child mailboxes that are accessible to the
       currently authenticated user.

   In some instances a server that supports the Child Mailbox Extension
   might 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.  In particular, some
   servers may not be able to combine the SUBSCRIBED and CHILDREN
   options.  Such servers MUST honour the SUBSCRIBED option, and they
   will simply ignore the CHILDREN option if both are requested.

   It is an error for the server to return both a \HasChildren and a
   \HasNoChildren attribute in a LIST response.

   Note: the \HasNoChildren attribute should not be confused with the
   IMAP4 [IMAP4] defined attribute \NoInferiors which indicates that no
   child mailboxes exist now and none can be created in the future.

6. Examples

   The first example shows the complete local hierarchy that will be
   used for the other examples.

       C: A01 LIST "" "*"
       S: * LIST (\Marked \NoInferiors) "/" "inbox"
       S: * LIST () "/" "Fruit"
       S: * LIST () "/" "Fruit/Apple"
       S: * LIST () "/" "Fruit/Banana"

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       S: * LIST () "/" "Tofu"
       S: * LIST () "/" "Vegetable"
       S: * LIST () "/" "Vegetable/Broccoli"
       S: A01 OK done

   In the next example, we'll see the subscribed mailboxes.  This is
   equivalent to <LSUB "" "*">.  Note that the mailbox called
   "Fruit/Peach" is subscribed to, but does not actually exist (perhaps
   it was deleted while still subscribed).

       C: A02 LIST (SUBSCRIBE) "" "*"
       S: * LIST (\Marked \NoInferiors) "/" "inbox"
       S: * LIST () "/" "Fruit"
       S: * LIST () "/" "Fruit/Banana"
       S: * LIST () "/" "Fruit/Peach"
       S: A02 OK done

   The next example shows the use of the CHILDREN option.  The client,
   without having to list the second level of hierarchy, now knows which
   of the top-level mailboxes have sub-mailboxes (children) and which do
   not.  Note that it's not necessary for the server to return the
   \HasNoChildren flag for the inbox, because the \NoInferiors flag
   already implies that, and has a stronger meaning.

       C: A03 LIST (CHILDREN) "" "%"
       S: * LIST (\Marked \NoInferiors) "/" "inbox"
       S: * LIST (\HasChildren) "/" "Fruit"
       S: * LIST (\HasNoChildren) "/" "Tofu"
       S: * LIST (\HasChildren) "/" "Vegetable"
       S: A03 OK done

   In this example we see more mailboxes, which reside on another server
   to which we may obtain referrals.  This is similar to the command
   <RLIST "" "%">.  We also see the mixing of two options.  Note that in
   the case of the remote mailboxes, the server might or might not be
   able to include CHILDREN information; it includes it if it can, and
   omits it if it can't.

       C: A04 LIST (REMOTE CHILDREN) "" "%"
       S: * LIST (\Marked \NoInferiors) "/" "inbox"
       S: * LIST (\HasChildren) "/" "Fruit"
       S: * LIST (\HasNoChildren) "/" "Tofu"
       S: * LIST (\HasChildren) "/" "Vegetable"
       S: * LIST () "/" "Bread"
       S: * LIST (\HasChildren) "/" "Meat"
       S: A04 OK done

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7. Formal Syntax

   The following syntax specification uses the augmented Backus-Naur
   Form (BNF) as described in [ABNF].  Terms not defined here are taken
   from [IMAP4].

   child-mbox-flag = "\HasChildren" / "\HasNoChildren"
                      ; flags for Child Mailbox Extension, at most one
                      ; possible per LIST response

   list            = "LIST" [SP list-options] SP mailbox SP list-mailbox

   list-options     = "(" [option *(SP option)] ")"

   mbox-list-oflag := child-mbox-flag

   option           = "SUBSCRIPTIONS" / "CHILDREN" / "REMOTE" /

   option-extension = atom

8. Security Considerations

   This document describes syntactic changes to the specification of the
   IMAP4 commands LIST, LSUB, RLIST, and RLSUB, and the modified LIST
   command has the same security considerations as those commands.  They
   are described in [IMAP4] and [MboxRefer].

   The Child Mailbox 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.   In most situations this will not be a
   security concern, because if information regarding whether a mailbox
   has children is considered sensitive, a user would not be granted
   access to that mailbox in the first place.

9. References

   [IMAP4]; Crispin, M.; "Internet Message Access Protocol - Version
   4rev1"; RFC 2060; University of Washington; December 1996.

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   [MboxRefer]; Gahrns, M.; "IMAP4 Mailbox Referrals"; RFC 2193;
   Microsoft Corporation; September 1997.

   [ChildMbox]; Gahrns, M. & Cheng, R.; "IMAP4 Child Mailbox Extension";
   draft-gahrns-imap-child-mailbox-03.txt; Microsoft Corporation;
   November 1997 -- expired, listed for historical reference.

   [Keywords]; Bradner, S.; "Key words for use in RFCs to Indicate
   Requirement Levels"; RFC 2119; Harvard University; March 1997.

   [ABNF]; Crocker, D., and Overell, P. "Augmented BNF for Syntax
   Specifications: ABNF", RFC 2234, November 1997.

10. Acknowledgements

   Mike Gahrns and Raymond Cheng of Microsoft Corporation originally
   devised the Child Mailbox Extension and proposed it in 1997; the
   idea, as well as most of the text in section 5, is theirs.

11. Author's Address

   Barry Leiba
   IBM T.J. Watson Research Center
   30 Saw Mill River Road
   Hawthorne, NY  10532

   Phone: 1-914-784-7941

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