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IMAP Extensions Working Group                                   B. Leiba
Internet Draft                           IBM T.J. Watson Research Center
                                                             A. Melnikov
                                                           Isode Limited
Expires August 2005                                        February 2005



                     IMAP4 LIST Command Extensions
                draft-ietf-imapext-list-extensions-11.txt

Status of this Document

   By submitting this Internet-Draft, I certify that any applicable
   patent or other IPR claims of which I am aware have been disclosed, or
   will be disclosed, and any of which I become aware will be disclosed,
   in accordance with RFC 3668.

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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 ietf-imapext@imc.org.

   This documents obsoletes RFC 3348 and updates RFC 2193.


Abstract

   IMAP4 has two commands for listing mailboxes: LIST and LSUB.  As we
   have added extensions that have required specialized lists (see
   [MboxRefer] for an example) we have had to expand the number of list
   commands, since each extension must add its function to both LIST and
   LSUB, and these commands are not, as they are defined, extensible.
   If we've needed the extensions to work together, we've had to add a
   set of commands to mix the different options, the set increasing in
   size with each new extension.  This document describes an extension
   to the base LIST command that will allow these additions to be done
   with mutually compatible options to the LIST command, avoiding the
   exponential increase in specialized list commands.


1. 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].

   The term "canonical LIST pattern" refers to
   the canonical pattern constructed internally by the server from
   the reference and mailbox name arguments (Section 6.3.8 of [IMAP4]).
   The [IMAP4] LIST command returns only mailboxes that match the
   canonical LIST pattern.

   Other terms are introduced where they are referenced for the first time.

   <<Other editorial comments/questions are enclosed like this.>>


2. Introduction and overview

   The LIST command is extended by amending the syntax to allow
   options and multiple patterns to be specified.  The list of options
   replaces the several commands that are currently used to mix and
   match the information requested.  The new syntax is backward-
   compatible, with no ambiguity: the new syntax is being used if one of
   the following conditions is true:
    1). if the first word after the command name begins with a
        parenthesis ("LIST selection options");
    2). if the second word after the command name begins with a
        parenthesis ("multiple mailbox patterns");
    3). if the LIST command has more than 2 parameters
        ("LIST return options");

   Otherwise the original syntax is used.

   By adding options to the LIST command, we are announcing the intent
   to phase out and eventually to deprecate 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
   "Child Mailbox Extension" [ChildMbox].  The base
   LSUB command is not deprecated by this extension; rather, this
   extension adds a way to obtain subscription information with more
   options, with those server implementations that support it.  Clients
   that simply need a list of subscribed mailboxes, as provided by the
   LSUB command, SHOULD continue to use that command.

   This document defines an IMAP4 extension that is identified by the
   capability string "X-DRAFT-W12-LISTEXT" <<Note to the RFC Editor:
   the capability name will change upon publication as an RFC>>.
   The X-DRAFT-W12-LISTEXT extension makes the
   following changes to the IMAP4 protocol, which are described in
   more details in sections 3 and 4 of this document:

   a.  defines new syntax for LIST command options.
   b.  extend LIST to allow for multiple mailbox patterns.
   c.  adds LIST command selection options: SUBSCRIBED, REMOTE and
       RECURSIVEMATCH.
   d.  adds LIST command return options: SUBSCRIBED and CHILDREN.
   e.  adds new mailbox attributes: "\NonExistent", "\Subscribed",
       "\Remote", "\HasChildren" and "\HasNoChildren".
   f.  adds CHILDINFO extended data item.


3. Extended LIST Command

   This extension updates the syntax of the LIST command to allow for multiple
   mailbox patterns to be specified, if they are enclosed in parantheses.
   A mailbox name match a list of mailbox patterns if it matches at least
   one mailbox pattern. Note that if a mailbox name matches multiple mailbox
   patterns from the list, the server should return only a single LIST
   response.

   Note that the non-extended LIST command is required to treat an empty
   ("" string) mailbox name argument as a special request to return the
   hierarchy delimiter and the root name of the name given in the
   reference parameter (as per [IMAP4]). However ANY extended LIST command
   (extended in any of 3 ways specified in section 2, or any combination of
   therof) MUST NOT treat the empty mailbox name as such special request
   and any regular processing described in this document applies.
   In particular, if an extended LIST command has multiple mailbox names
   and one (or more) of them is the empty string, the empty string MUST be
   ignored for the purpose of matching.

   Some servers might restrict which patterns are allowed in a LIST command.
   If a server doesn't accept a particular pattern, it MUST silently ignore
   it.

   The LIST command syntax is also extended in two additional ways: by adding a
   parenthesized list of command options between the command name and the
   reference name (LIST selection options) and an optional list of options at
   the end that control what kind of information should be returned (LIST return
   options). See the formal syntax in section 6 for specific details.

   A LIST selection option tells the server which mailbox names should be
   selected by the LIST operation.
   The server should return information about all mailbox names that match any
   of the "canonical LIST pattern" (as described above) and satisfy additional
   selection criteria (if any) specified by the LIST selection options. Let's
   call any such mailbox name a "matched mailbox name".
   When multiple selection options are specified, the server MUST return
   information about mailbox names that satisfy every selection option, unless
   a description of a particular specified option prescribes special rules.
   An example of an option prescribing special rules is the RECURSIVEMATCH
   selection option described later in this section.
   We will use the term "selection criteria" when referring collectively to all
   selection options specified in a LIST command.

   A LIST return option controls which information is returned for each matched
   mailbox name. Note that return options MUST NOT cause the server to report
   information about additional mailbox names. If the client has not specified
   any return option, only information about attributes should be returned by
   the server. (Of course the server is allowed to include any other information
   at will)

   Both selection and return 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 MUST respond to options it does not recognize with a BAD response.
   The client SHOULD NOT specify any option more than once, however if the
   client does this, the server MUST act as if it received the option only once.
   The order in which options are specified by the client is not significant.

   <<Each option (except for RECURSIVEMATCH) should have a corresponding
     mailbox attribute?>>

   This extension is identified by the capability string "X-DRAFT-W12-LISTEXT"
   <<Note to the RFC editor: please update upon publication as above>>, 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.
   Note that extensions that don't require support for an extended LIST
   command, but use extended LIST responses (see below), don't need to advertise
   the "X-DRAFT-W12-LISTEXT" capability string. <<Note to the RFC Editor: fix
   upon publication as above>>

   This extension also defines extensions to the LIST response, allowing
   a series of extended fields at the end, a parenthesized list of tagged
   data (also referred to as "extended data item"). The first element of
   an extended field is a tag, which identifies type of the data. Tags
   MUST be registered with IANA, as described in section 8.5 of this
   document. An example of such extended set might be

       ((tablecloth (("fringe" "lacy")("color" "white")))(X-Sample
       "text"))

   or...

       ((tablecloth ("fringe" "lacy"))(X-Sample "text" "and even more text"))

   See the formal grammar, below, for the full syntactic details.
   The server MUST NOT return any extended data item, unless the client has
   expressed its ability to support extended LIST responses, for example by
   using an extended LIST command.
   The server MAY return data in the extended fields that was not solicited
   by the client. The client MUST ignore all extended fields it doesn't
   recognize.

   The X-DRAFT-W12-LISTEXT <<Note to the RFC Editor: fix upon publication>>
   capability also defines several new mailbox attributes.

   The "\NonExistent" attribute indicates that a mailbox does not actually
   exist. Note that this attribute is not meaningful by itself, as mailboxes
   that match the canonical LIST pattern but don't exist must not be returned
   unless one of the two conditions listed below is also satisfied:

   a) the mailbox also satisfies the selection criteria

   b) the mailbox has at least one child mailbox that satisfies the selection
      criteria<<, but doesn't match the canonical LIST pattern.>>

   In practice this means that the "\NonExistent" attribute is usually returned
   with one or more of "\Subscribed", "\Remote" or the CHILDINFO extended data
   item (see their description below).

   The "\NonExistent" attribute implies "\NoSelect". The "\NonExistent"
   attribute MUST be supported and MUST be accurately computed.

   The selection options defined in this specification are

       SUBSCRIBED - causes the LIST command to list subscribed
          names, rather than the existing 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.
          In any case, the list MUST include exactly those mailbox names
          that match the canonical list pattern and are subscribed to.  This
          option is intended to supplement the LSUB command.
          Of particular note are the mailbox attributes as returned by this
          option, compared with what is returned by LSUB. With the
          latter, the attributes returned may not reflect the actual attribute
          status on the mailbox name, and the \NoSelect attribute has a special
          meaning (it indicates that this mailbox is not, itself,
          subscribed, but that it has child mailboxes that are).  With
          the SUBSCRIBED selection option described here, the attributes are
          accurate, complete, and have no special meanings.
          "LSUB" and "LIST (SUBSCRIBED)" are, thus, not the same thing,
          and some servers must do significant extra work to respond to
          "LIST (SUBSCRIBED)".  Because of this, clients SHOULD continue
          to use "LSUB" unless they specifically want the additional
          information offered by "LIST (SUBSCRIBED)".

          This option defines a new mailbox attribute, "\Subscribed", that
          indicates that a mailbox name is subscribed to. The "\Subscribed"
          attribute MUST be supported and MUST be accurately computed
          when the SUBSCRIBED selection option is specified.

          Note that the SUBSCRIBED selection option implies the SUBSCRIBED
          return option (see below).

       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 selection option, the RLSUB command.

          This option defines a new mailbox attribute, "\Remote", that
          indicates that a mailbox is a remote mailbox.  The "\Remote"
          attribute MUST be accurately computed when the REMOTE option is
          specified.

          Note that a server implementation that doesn't support
          any remote mailboxes is compliant with this specification
          as long as it accepts and ignores the REMOTE selection option.
          Note that if the server choses to ignore the REMOTE selection
          option, it still has to treat RECURSIVEMATCH REMOTE as a valid
          combination of selection options (see also the description of
          the RECURSIVEMATCH option below).

       RECURSIVEMATCH - this option forces the server to return
          information about parent mailboxes that don't match other
          selection options, but have some submailboxes that do.
          Information about children is returned in the CHILDINFO
          extended data item, as described in section 3.3.

          Note 1: In order for a parent mailbox to be returned, it still
           has to match the canonical LIST pattern.

          Note 2: When returning the CHILDINFO extended data item,
           it doesn't matter if the submailbox matches
           the canonical LIST pattern or not. See also example 9 in
           section 5.

          The RECURSIVEMATCH option MUST NOT occur as the only selection
          option, as it only makes sense when other selection options are
          also used. The server MUST return BAD tagged response in such case.

          Note that even if RECURSIVEMATCH option is specified, the client
          MUST still be able to handle a case when a CHILDINFO extended
          data item is returned and there are no submailboxes
          that meet the selection criteria of the given LIST command,
          as they can be deleted/renamed after the LIST response was sent,
          but before the client had a chance to access them.

   The return options defined in this specification are

       SUBSCRIBED - causes the LIST command to return subscription
          state for all matching mailbox names. The "\Subscribed"
          attribute MUST be supported and MUST be accurately computed
          when the SUBSCRIBED return option is specified.

       CHILDREN - Requests mailbox child information as originally
          proposed in [ChildMbox].  See section 4, below, for details.
          This option MUST be accepted by all servers. <<Should the last
          sentence be deleted?>>


3.1. General principles for returning LIST responses

   This section outlines several principles that can be used by server
   implementations of this document to decide if a LIST response should be
   returned, as well as how many responses and what kind of information
   they may contain.

   1) Exactly one LIST response should be returned for each mailbox
      name which matches the canonical LIST pattern.
      Server implementors must not assume that clients will be able to
      assemble mailbox attributes and other information returned in multiple
      LIST responses.

   2) There are only two reasons for including a matching mailbox name
      in the responses to the LIST command (Note that the server is allowed
      to return unsolicited responses at any time. Such responses are not
      governed by this rule):

      a) the mailbox name also satisfies the selection criteria;

      b) the mailbox name doesn't satisfy the selection criteria, but
         it has at least one child mailbox name that satisfies the
         selection criteria and that doesn't match the canonical LIST
         pattern.
         For more information on this case see the CHILDINFO extended data
         item described in section 3.3.  Note that the CHILDINFO extended
         data item can only be returned when the RECURSIVEMATCH selection
         option is specified.

   3) Attributes returned in the same LIST response must be treated additively.
      For example the following response

       S: * LIST (\Subscribed \NonExistent) "/" "Fruit/Peach"

      means that the "Fruit/Peach" mailbox doesn't exist, but it is
      subscribed.


3.2. Additional requirements on LISTEXT clients

   All clients that support this extension MUST treat an attribute with
   a stronger meaning, as implying any attribute that can be inferred from it.
   For example, the client must treat presence of the \NoInferiors attribute
   as if the \HasNoChildren attribute was also sent by the server.

   The following table summarizes inference rules described in section 3.

           +--------------------+-------------------+
           | returned attribute | implied attribute |

           +--------------------+-------------------+
           |  \NoInferiors      |   \HasNoChildren  |
           |  \NonExistent      |   \NoSelect       |
           +--------------------+-------------------+


3.3. CHILDINFO extended data item

   The CHILDINFO extended data item MUST only be returned when the client
   has specified the RECURSIVEMATCH selection option.

   The CHILDINFO extended data item in a LIST response describes the
   selection criteria that has caused it to be returned and indicates that
   the mailbox has one or more child mailbox that match the selection
   criteria.
   <<The LSUB command indicates this condition by using the "\NoSelect"
   attribute, but the LIST (SUBSCRIBED) command MUST NOT do that, since
   "\NoSelect" retains its original meaning here.  Further, the CHILDINFO
   extended data item is more general, in that it can be used with any
   extended set of selection criteria.>>

    Note: When returning the CHILDINFO extended data item,
          it doesn't matter if a child mailbox matches
          the canonical LIST pattern or not. See also example 9 in
          section 5.

   The returned selection criteria allows the client to distinguish
   a solicited response from an unsolicited, as well as distinguish
   between solicited responses caused by multiple different<<?>> LIST
   commands.

  <<Suppressing redundant CHILDINFO when parent doesn't match the
    selection criteria but has a child that does AND the child also
    matches the canonical LIST pattern?>>

  <<Add example for (REMOTE RECURSIVEMATCH), when REMOTE is ignored by
    the server (i.e. the client has effectively specified CHILDREN)?>>

   Examples 8 and 10 in section 5 demonstrates the difference between
   present CHILDINFO extended data item and the "\HasChildren" attribute.

   The following table summarizes interaction between "\NonExistent"
   attribute and CHILDINFO (the first collumn describes if the parent
   mailbox exists):

 +------+------------+---------------------+--------------------------+
 |exists|  meets the | has a child that    |         returned         |

 |      |  selection | meets the selection |    LISTEXT attributes    |
 |      |  criteria  | criteria            |      and CHILDINFO       |
 +------+------------+---------------------+--------------------------+
 |  no  |    no      |      no             |no LIST response returned |
 | yes  |    no      |      no             |no LIST response returned |
 |  no  |   yes      |      no             |(\NonExistent <attr>)     |
 | yes  |   yes      |      no             |(<attr>)                  |
 |  no  |    no      |     yes             |(\NonExistent) + CHILDINFO|
 | yes  |    no      |     yes             |() + CHILDINFO            |
 |  no  |   yes      |     yes             |(\NonExistent <attr>) +   |
 |      |            |                     |      CHILDINFO           |
 | yes  |   yes      |     yes             |(<attr>) + CHILDINFO      |
 +------+------------+---------------------+--------------------------+

   where <attr> is one or more attributes that correspond to the
   selection criteria, for example for the SUBSCRIBED option the <attr>
   is \Subscribed. <<REMOTE is a special case?>>

   <<Is this table still needed?>>


4. The CHILDREN return Option

   The CHILDREN return 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
   return option is simply an indication that the client wants this
   information; a server MAY provide it even if the option is not
   specified.
   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 CHILDREN return option 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 CHILDREN return option 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 return option is provided to indicate that the
   client particularly wants this information.  If the CHILDREN return
   option is present, the server MUST return these attributes even
   if their computation is expensive.

   \HasChildren - The presence of this attribute indicates that the
       mailbox 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 LISTEXT 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
   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. In particular, some
   servers may not be able to combine the SUBSCRIBED selection option
   and CHILDREN return option.  Such servers MUST honour the SUBSCRIBED
   selection option, and they will simply ignore the CHILDREN return 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.


5. Examples

     Example 1:

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


     Example 2:

   In the next example, we'll see the subscribed mailboxes.  This is
   similar to, but not equivalent with, <LSUB "" "*">.  Note that the mailbox
   called "Fruit/Peach" is subscribed to, but does not actually exist
   (perhaps it was deleted while still subscribed).  The "Fruit"
   mailbox is not subscribed to, but it has two subscribed children.
   The "Vegetable" mailbox is subscribed and has two children, one
   of them is subscribed as well.

       C: A02 LIST (SUBSCRIBED) "" "*"
       S: * LIST (\Marked \NoInferiors \Subscribed) "/" "inbox"
       S: * LIST (\Subscribed) "/" "Fruit/Banana"
       S: * LIST (\Subscribed \NonExistent) "/" "Fruit/Peach"
       S: * LIST (\Subscribed) "/" "Vegetable/Broccoli"
       S: A02 OK done

     Example 3:

   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 submailboxes (children) and which do
   not.  Note that it's not necessary for the server to return the
   \HasNoChildren attribute for the inbox, because the \NoInferiors attribute
   already implies that, and has a stronger meaning.

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

     Example 4:

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

     Example 5:

   The following example also requests the server to include mailboxes,
   which reside on another server. The server returns information about
   all mailboxes which are subscribed. This is similar to the command
   <RLSUB "" "%">.  We also see the use of two selection options.

       C: A05 LIST (REMOTE SUBSCRIBED) "" "*"
       S: * LIST (\Marked \NoInferiors \Subscribed) "/" "inbox"
       S: * LIST (\Subscribed) "/" "Fruit/Banana"
       S: * LIST (\Subscribed \NonExistent) "/" "Fruit/Peach"
       S: * LIST (\Subscribed) "/" "Vegetable"
       S: * LIST (\Subscribed) "/" "Vegetable/Broccoli"
       S: * LIST (\Remote \Subscribed) "/" "Bread"
       S: A05 OK done

     Example 6:

   The following example requests the server to include mailboxes,
   which reside on another server. The server is requested to return
   subscription information for all returned mailboxes. This is different
   from the example above.

   Note that the output of this command is not a superset of the output
   in the previous example, as it doesn't include LIST response for the
   non-existent "Fruit/Peach".

       C: A06 LIST (REMOTE) "" "*" RETURN (SUBSCRIBED)
       S: * LIST (\Marked \NoInferiors \Subscribed) "/" "inbox"
       S: * LIST () "/" "Fruit"
       S: * LIST () "/" "Fruit/Apple"
       S: * LIST (\Subscribed) "/" "Fruit/Banana"
       S: * LIST () "/" "Tofu"
       S: * LIST (\Subscribed) "/" "Vegetable"
       S: * LIST (\Subscribed) "/" "Vegetable/Broccoli"
       S: * LIST () "/" "Vegetable/Corn"
       S: * LIST (\Remote \Subscribed) "/" "Bread"
       S: * LIST (\Remote) "/" "Meat"
       S: A06 OK done

     Example 7:

   In the following example the client has specified multiple mailbox
   patterns. Note that this example doesn't use the mailbox hierarchy
   used in the previous examples.

       C: BBB LIST "" ("INBOX" "Drafts" "Sent/%")
       S: * LIST () "/" "INBOX"
       S: * LIST (\NoInferiors) "/" "Drafts"
       S: * LIST () "/" "Sent/March2004"
       S: * LIST (\Marked) "/" "Sent/December2003"
       S: * LIST () "/" "Sent/August2004"
       S: BBB OK done

     Example 8:

      The following example demonstates the difference between
      \HasChildren attribute and CHILDINFO extended data item.

      Let's assume there is the following hierarchy:

       C: C01 LIST "" "*"
       S: * LIST (\Marked \NoInferiors) "/" "inbox"
       S: * LIST () "/" "Foo"
       S: * LIST () "/" "Foo/Bar"
       S: * LIST () "/" "Foo/Baz"
       S: * LIST () "/" "Moo"
       S: C01 OK done

      If the client asks RETURN (CHILDREN) it will get:

       C: CA3 LIST "" "%" RETURN (CHILDREN)
       S: * LIST (\Marked \NoInferiors) "/" "inbox"
       S: * LIST (\HasChildren) "/" "Foo"
       S: * LIST (\HasNoChildren) "/" "Moo"
       S: CA3 OK done

      A). Let's also assume that the mailbox "Foo/Baz" is the only
          subscribed mailbox. Then

       C: C02 LIST (SUBSCRIBED) "" "*"
       S: * LIST (\Subscribed) "/" "Foo/Baz"
       S: C02 OK done

      Now, if the client issues <LIST (SUBSCRIBED) "" "%">, the server will
      return no mailboxes (as the mailboxes "Moo", "Foo" and "Inbox" are NOT
      subscribed). However, if the client issues:

       C: C04 LIST (SUBSCRIBED RECURSIVEMATCH) "" "%"
       S: * LIST () "/" "Foo" (("CHILDINFO" ("SUBSCRIBED")))
       S: C04 OK done

      i.e. the mailbox "Foo" is not subscribed, but it has a child that is.

      A1). If the mailbox "Foo" would have been subscribed instead, the last
           command would return:

       S: * LIST (\Subscribed) "/" "Foo" (("CHILDINFO" ("SUBSCRIBED")))

      or even

       S: * LIST (\Subscribed \HasChildren) "/" "Foo" (("CHILDINFO" ("SUBSCRIBED")))

      A2). If we assume instead that the mailbox "Foo" is not part of the
           original hierarchy and is not subscribed, the last command will
           return

       S: * LIST (\NonExistent) "/" "Foo" (("CHILDINFO" ("SUBSCRIBED")))

      B). Now, let's assume that no mailbox is subscribed. In this case
          the command <LIST (SUBSCRIBED RECURSIVEMATCH) "" "%"> will return
          no responses, as there are no subscribed children (even though
          "Foo" has children).

      C). And finally, let's assume that only the mailboxes "Foo" and "Moo" are
          subscribed. In this case the command:

       C: LIST (SUBSCRIBED RECURSIVEMATCH) "" "%" RETURN (CHILDREN)

          will return:

       S: * LIST (\HasChildren \Subscribed) "/" "Foo"
       S: * LIST (\HasNoChildren \Subscribed) "/" "Moo"

          Which means that the mailbox "Foo" has children, but none of them
          is subscribed.

     Example 9:

      The following example demonstrates that the CHILDINFO extended data item
      is returned whether children mailboxes match the canonical LIST pattern
      or not.

      Let's assume there is the following hierarchy:

       C: D01 LIST "" "*"
       S: * LIST (\Marked \NoInferiors) "/" "inbox"
       S: * LIST () "/" "foo2"
       S: * LIST () "/" "foo2/bar1"
       S: * LIST () "/" "foo2/bar2"
       S: * LIST () "/" "baz2"
       S: * LIST () "/" "baz2/bar2"
       S: * LIST () "/" "baz2/bar22"
       S: * LIST () "/" "baz2/bar222"
       S: * LIST () "/" "eps2"
       S: * LIST () "/" "eps2/mamba"
       S: * LIST () "/" "quux2/bar2"
       S: D01 OK done

      And that the following mailboxes are subscribed:

       C: D02 LIST (SUBSCRIBED) "" "*"
       S: * LIST (\Subscribed) "/" "foo2/bar1"
       S: * LIST (\Subscribed) "/" "foo2/bar2"
       S: * LIST (\Subscribed) "/" "baz2/bar2"
       S: * LIST (\Subscribed) "/" "baz2/bar22"
       S: * LIST (\Subscribed) "/" "baz2/bar222"
       S: * LIST (\Subscribed) "/" "eps2"
       S: * LIST (\Subscribed) "/" "eps2/mamba"
       S: * LIST (\Subscribed) "/" "quux2/bar2"
       S: D02 OK done

      The client issues the following command first:

       C: D03 LIST (RECURSIVEMATCH SUBSCRIBED) "" "*2"
       S: * LIST () "/" "foo2" (("CHILDINFO" ("SUBSCRIBED")))
       S: * LIST (\Subscribed) "/" "foo2/bar2"
       S: * LIST () "/" "baz2" (("CHILDINFO" ("SUBSCRIBED")))
       S: * LIST (\Subscribed) "/" "baz2/bar2"
       S: * LIST (\Subscribed) "/" "baz2/bar22"
       S: * LIST (\Subscribed) "/" "baz2/bar222"
       S: * LIST (\Subscribed) "/" "eps2" (("CHILDINFO" ("SUBSCRIBED")))
       S: * LIST (\Subscribed) "/" "quux2/bar2"
       S: D03 OK done

      and the server may also include

       S: * LIST (\NonExistent) "/" "quux2" (("CHILDINFO" ("SUBSCRIBED")))

      The CHILDINFO extended data item is returned for mailboxes "foo2", "baz2"
      and "eps2", because all of them have subscribed children,
      even though for the mailbox "foo2" only one of the two subscribed
      children match the pattern, for the mailbox "baz2" all the subscribed
      children match the pattern and for the mailbox "eps2" none of the
      subscribed children match the pattern.

      Note that if the client issues

       C: D03 LIST (RECURSIVEMATCH SUBSCRIBED) "" "*"
       S: * LIST () "/" "foo2" (("CHILDINFO" ("SUBSCRIBED")))
       S: * LIST (\Subscribed) "/" "foo2/bar1"
       S: * LIST (\Subscribed) "/" "foo2/bar2"
       S: * LIST () "/" "baz2" (("CHILDINFO" ("SUBSCRIBED")))
       S: * LIST (\Subscribed) "/" "baz2/bar2"
       S: * LIST (\Subscribed) "/" "baz2/bar22"
       S: * LIST (\Subscribed) "/" "baz2/bar222"
       S: * LIST (\Subscribed) "/" "eps2" (("CHILDINFO" ("SUBSCRIBED")))
       S: * LIST (\Subscribed) "/" "eps2/mamba"
       S: * LIST (\Subscribed) "/" "quux2/bar2"
       S: D03 OK done

      the LIST responses for mailboxes "foo2", "baz2" and "eps2" still have
      the CHILDINFO extended data item, even though this information
      is redundant and the client can determine it by itself.

     Example 10:

      The following example shows usage of multiple mailbox patterns.
      It also demonstrates that the presence of the CHILDINFO extended data item
      doesn't necessarily imply \HasChildren.

       C: a1 LIST "" ("foo" "foo/*")
       S: * LIST () "/" foo
       S: a1 OK done

       C: a2 LIST (SUBSCRIBED) "" "foo/*"
       S: * LIST (\Subscribed \NonExistent) "/" foo/bar
       S: a2 OK done

       C: a3 LIST (SUBSCRIBED RECURSIVEMATCH) "" foo RETURN (CHILDREN)
       S: * LIST (\HasNoChildren) "/" foo (("CHILDINFO" ("SUBSCRIBED")))
       S: a3 OK done


6. 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]. "vendor-token" is defined in [ACAP].

   childinfo-extended-item = "CHILDINFO" SP "(" list-sel-base-opt-quoted
                              *( SP list-sel-base-opt-quoted ) ")"
                        ; Extended data item returned when the RECURSIVEMATCH
                        ; selection option is specified.
                        ; Note 1: the CHILDINFO tag can be returned
                        ;  with and without surrounding quotes, as per
                        ;  mbox-list-extended-item-tag production.
                        ; Note 2: The selection options are returned quoted,
                        ;  unlike their specification in the extended LIST
                        ;  command.

   child-mbox-flag    = "\HasChildren" / "\HasNoChildren"
                        ; attributes for CHILDREN return option, at most one
                        ; possible per LIST response

   eitem-standard-tag = atom
                        ; a tag for extended list data defined in a Standard
                        ; Track or Experimental RFC.

   eitem-vendor-tag   = vendor-tag
                        ; a vendor specific tag for extended list data

   list               = "LIST" [SP list-select-opts] SP mailbox SP mbox-or-pat
                        [SP list-return-opts]

   list-select-opts   = "(" [*(list-select-mod-opt SP) list-select-base-opt
                            *(SP list-select-opt)] ")"
                        ; list selection options, e.g. REMOTE

   list-select-opt    = list-sel-mod-opt / list-sel-base-opt
                        ; An option registration template is described in
                        ; section 8.3 of this document.

   list-sel-base-opt  = "SUBSCRIBED" / "REMOTE" / option-extension
                        ; options that can be used by themselves

   list-sel-base-opt-quoted = <"> list-sel-base-opt <">

   list-sel-mod-opt   = "RECURSIVEMATCH" / option-extension
                        ; options that require a list-select-base-opt
                        ; to also be present

   list-return-opts   = "RETURN" SP "(" [return-option *(SP return-option)] ")"
                        ; list return options, e.g. CHILDREN

   mailbox-list       = "(" [mbx-list-flags] ")" SP
                        (DQUOTE QUOTED-CHAR DQUOTE / nil) SP mailbox
                        [SP mbox-list-extended]

   mbox-list-extended = "(" [mbox-list-extended-item
                        *(SP mbox-list-extended-item)] ")"

   mbox-list-extended-item = "(" mbox-list-extended-item-data ")"

   mbox-list-extended-item-data =  mbox-list-extended-item-tag SP nstring-list

   mbox-list-extended-item-tag  = astring
                        ; The content MUST conform to either "eitem-vendor-tag" or
                        ; "eitem-standard-tag" ABNF productions.
                        ; A tag registration template is described in section
                        ; 8.5 of this document.

   mbox-list-oflag    = child-mbox-flag / "\NonExistent" /
                        / "\Subscribed" / "\Remote"

   mbox-or-pat        = list-mailbox / patterns

   nstring-list       = nstring /
                        "(" [nstring-list *(SP nstring-list)] ")"
                        ;; a recursive list definition

   option-extension   = option-vendor-tag / option-standard-tag

   option-vendor-tag  = vendor-tag
                        ; a vendor specific option

   option-standard-tag= atom
                        ; an option defined in a Standard Track or
                        ; Experimental RFC

   patterns           = "(" list-mailbox *(SP list-mailbox) ")"

   return-option      = "SUBSCRIBED" / "CHILDREN" /
                        option-extension

   vendor-tag         = vendor-token "-" atom


7. 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.

   <<The CHILDINFO extended data item has the same security considerations
   as the \HasChildren attribute described above.>>


8. IANA Considerations

8.1.  Guidelines for IANA


   It is requested that IANA creates two new registries for LISTEXT
   options and LISTEXT extended response data. The templates and
   the initial registrations are detailed below.


8.2.  Registration procedure and Change control


   Registration of a LISTEXT option is done by filling in the template
   in section 8.3 and sending it via electronic mail to <iana@iana.org>.
   Registration of a LISTEXT extended data item is done by filling in the
   template in section 8.5 and sending it via electronic mail to <iana@iana.org>.
   IANA has the right to reject obviously bogus registrations, but will
   perform no review of claims made in the registration form.

   A LISTEXT option/extended data item name that starts with "V-" is reserved
   for vendor specific options/extended data items. All options, whether
   they are vendor specific or global, should be registered with IANA.
   If a LISTEXT extended data item is returned as a result of requesting
   a particular LISTEXT option, the name of the option SHOULD be used
   as the name of the LISTEXT extended data item.

   Each vendor specific options/extended data item MUST start with their
   vendor-token ("vendor prefix"). The vendor-token MUST be registered
   with IANA, using the [ACAP] vendor subtree registry.

   Standard LISTEXT option/extended data item names are case insensitive.
   If the vendor prefix is omitted from a vendor specific LISTEXT
   option/extended data item name, the rest is case insensitive. The vendor
   prefix itself is not case-sensitive, as it might contain non-ASCII
   characters.

   While the registration procedures do not require it, authors of LISTEXT
   options/extended data items are encouraged to seek community review and
   comment whenever that is feasible.  Authors may seek community review by
   posting a specification of their proposed mechanism as an Internet-
   Draft.  LISTEXT options/extended data items intended for widespread use
   should be standardized through the normal IETF process, when appropriate.

   Comments on registered LISTEXT options/extended response data should
   first be sent to the "owner" of the mechanism and/or to the IMAPEXT WG
   mailing list.
   Submitters of comments may, after a reasonable attempt to contact the
   owner, request IANA to attach their comment to the registration itself.
   If IANA approves of this, the comment will be
   made accessible in conjunction with the registration LISTEXT options/
   extended response data itself.

   Once a LISTEXT registration has been published by IANA, the
   author may request a change to its definition.  The change request
   follows the same procedure as the registration request.

   The owner of a LISTEXT registration may pass responsibility for the
   registered option/extended data item to another person or agency by
   informing IANA; this can be done without discussion or review.

   The IESG may reassign responsibility for a LISTEXT option/extended data item.
   The most common case of this will be to enable changes to be made to
   mechanisms where the author of the registration has died, moved out
   of contact or is otherwise unable to make changes that are important
   to the community.

   LISTEXT registrations may not be deleted; mechanisms which are
   no longer believed appropriate for use can be declared OBSOLETE by a
   change to their "intended use" field; such LISTEXT options/extended data
   items will be clearly marked in the lists published by IANA.

   The IESG is considered to be the owner of all LISTEXT options/extended data items
   which are on the IETF standards track.


8.3.  Registration template for LISTEXT options

     To: iana@iana.org
     Subject: Registration of LISTEXT option X

     LISTEXT option name:

     LISTEXT option type: (One of SELECTION or RETURN)

     Implied return options(s), if the option type is SELECTION: (zero or more)

     LISTEXT option description:

     Published specification (optional, recommended):

     Security considerations:

     Intended usage:
     (One of COMMON, LIMITED USE or OBSOLETE)

     Person & email address to contact for further information:

     Owner/Change controller:

     (Any other information that the author deems interesting may be
     added below this line.)



8.4.  Initial LISTEXT option registrations

   It is requested that the LISTEXT option registry is being populated
   with the following entries:

   1)

     To: iana@iana.org
     Subject: Registration of LISTEXT option SUBSCRIBED

     LISTEXT option name: SUBSCRIBED

     LISTEXT option type: SELECTION

     Implied return options(s): SUBSCRIBED

     LISTEXT option description: Causes the LIST command to list
          subscribed mailboxes, rather than the actual mailboxes.

     Published specification : this RFC, section 3.

     Security considerations: this RFC, section 7.

     Intended usage: COMMON

     Person & email address to contact for further information:
        Alexey Melnikov <Alexey.Melnikov@isode.com>

     Owner/Change controller: IESG <iesg@ietf.org>

   2)

     To: iana@iana.org
     Subject: Registration of LISTEXT option REMOTE

     LISTEXT option name: REMOTE

     LISTEXT option type: SELECTION

     Implied return options(s): (none)

     LISTEXT option description: causes the LIST command to return
          remote mailboxes as well as local ones, as described in
          RFC 2193.

     Published specification : this RFC, section 3.

     Security considerations: this RFC, section 7.

     Intended usage: COMMON

     Person & email address to contact for further information:
        Alexey Melnikov <Alexey.Melnikov@isode.com>

     Owner/Change controller: IESG <iesg@ietf.org>

   3)

     To: iana@iana.org
     Subject: Registration of LISTEXT option SUBSCRIBED

     LISTEXT option name: SUBSCRIBED

     LISTEXT option type: RETURN

     LISTEXT option description: Causes the LIST command to return
          subscription state.

     Published specification : this RFC, section 3.

     Security considerations: this RFC, section 7.

     Intended usage: COMMON

     Person & email address to contact for further information:
        Alexey Melnikov <Alexey.Melnikov@isode.com>

     Owner/Change controller: IESG <iesg@ietf.org>

   4)

     To: iana@iana.org
     Subject: Registration of LISTEXT option RECURSIVEMATCH

     LISTEXT option name: RECURSIVEMATCH

     LISTEXT option type: SELECTION

     Implied return options(s): (none)

     LISTEXT option description: Requests that CHILDINFO
       extended data item is to be returned.

     Published specification : this RFC, sections 3.

     Published specification : this RFC

     Security considerations: this RFC, section 7.

     Intended usage: COMMON

     Person & email address to contact for further information:
        Alexey Melnikov <Alexey.Melnikov@isode.com>

     Owner/Change controller: IESG <iesg@ietf.org>

   5)

     To: iana@iana.org
     Subject: Registration of LISTEXT option CHILDREN

     LISTEXT option name: CHILDREN

     LISTEXT option type: RETURN

     LISTEXT option description: Requests mailbox child information.

     Published specification : this RFC, sections 3 and 4.

     Published specification : this RFC

     Security considerations: this RFC, section 7.

     Intended usage: COMMON

     Person & email address to contact for further information:
        Alexey Melnikov <Alexey.Melnikov@isode.com>

     Owner/Change controller: IESG <iesg@ietf.org>


8.5.  Registration template for LISTEXT extended data item

     To: iana@iana.org
     Subject: Registration of X LISTEXT extended data item

     LISTEXT extended data item tag:

     LISTEXT extended data item description:

     Which LISTEXT option(s) (and their types) causes this extended
     data item to be returned (if any):

     Published specification (optional, recommended):

     Security considerations:

     Intended usage:
     (One of COMMON, LIMITED USE or OBSOLETE)

     Person & email address to contact for further information:

     Owner/Change controller:

     (Any other information that the author deems interesting may be
     added below this line.)


8.6.  Initial LISTEXT extended data item registrations

   It is requested that the LISTEXT extended data item registry is
   being populated with the following entries:

   1)

     To: iana@iana.org
     Subject: Registration of CHILDINFO LISTEXT extended data item

     LISTEXT extended data item tag: CHILDINFO

     LISTEXT extended data item description: The CHILDINFO extended data
       item describes the selection criteria that has caused it to be
       returned and indicates that the mailbox has one or more child
       mailbox that match the selection criteria.

     Which LISTEXT option(s) (and their types) causes this extended
     data item to be returned (if any): RECURSIVEMATCH selection
       option

     Published specification : this RFC, section 3.3

     Security considerations: this RFC, section 7.

     Intended usage: COMMON

     Person & email address to contact for further information:
        Alexey Melnikov <Alexey.Melnikov@isode.com>

     Owner/Change controller: IESG <iesg@ietf.org>


9. References

9.1. Normative References

   [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.

   [IMAP4] Crispin, M., "Internet Message Access Protocol - Version
   4rev1", RFC 3501, University of Washington, March 2003.

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

   [ChildMbox] Gahrns, M. & Cheng, R., "IMAP4 Child Mailbox Extension",
   RFC 3348, Microsoft Corporation, July 2002.

   [ACAP] Newman, C. and J. Myers, "ACAP -- Application
   Configuration Access Protocol", RFC 2244, 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 4, is theirs.

   This document is the result of discussions on the IMAP4 and IMAPEXT
   mailing lists and is meant to reflect consensus of those groups.
   In particular, Mark Crispin, Philip Guenther, Cyrus Daboo, Timo Sirainen,
   Ken Murchison, Rob Siemborski, Steve Hole, Arnt Gulbrandsen, Larry
   Greenfield, Dave Cridland and Pete Maclean were active participants
   in those discussions or made suggestions to this document.


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
   Email: leiba@watson.ibm.com


   Alexey Melnikov
   Isode Limited
   5 Castle Business Village
   36 Station Road
   Hampton, Middlesex
   TW12 2BX, UK

   Email: Alexey.Melnikov@isode.com
   Home page:   http://www.melnikov.ca/


12.  IPR Disclosure Acknowledgement

    By submitting this Internet-Draft, we certify that any applicable
    patent or other IPR claims of which I am aware have been disclosed,
    and any of which we become aware will be disclosed, in accordance with
    RFC 3668.


13.   Intellectual Property

    The IETF takes no position regarding the validity or scope of any
    Intellectual Property Rights or other rights that might be claimed to
    pertain to the implementation or use of the technology described in
    this document or the extent to which any license under such rights
    might or might not be available; nor does it represent that it has
    made any independent effort to identify any such rights.  Information
    on the procedures with respect to rights in RFC documents can be
    found in BCP 78 and BCP 79.

    Copies of IPR disclosures made to the IETF Secretariat and any
    assurances of licenses to be made available, or the result of an
    attempt made to obtain a general license or permission for the use of
    such proprietary rights by implementers or users of this
    specification can be obtained from the IETF on-line IPR repository at
    http://www.ietf.org/ipr.

    The IETF invites any interested party to bring to its attention any
    copyrights, patents or patent applications, or other proprietary
    rights that may cover technology that may be required to implement
    this standard.  Please address the information to the IETF at
    ietf-ipr@ietf.org.

14.  Full Copyright Statement

    Copyright (C) The Internet Society (2005).  This document is subject
    to the rights, licenses and restrictions contained in BCP 78, and
    except as set forth therein, the authors retain all their rights.

    This document and the information contained herein are provided on an
    "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
    OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
    ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
    INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
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    WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.


Acknowledgement

    Funding for the RFC Editor function is currently provided by
    the Internet Society.


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