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Versions: (draft-melnikov-imapext-status-in-list) 00 01 RFC 5819

IMAPEXT                                                      A. Melnikov
Internet-Draft                                             Isode Limited
Intended status: Standards Track                             T. Sirainen
Expires: November 13, 2009                                  May 12, 2009

   IMAP4 Extension for returning STATUS information in extended LIST

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
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Copyright Notice

   Copyright (c) 2009 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   Many IMAP clients display information about total number of messages/
   total number of unseen messages in IMAP mailboxes.  In order to do
   that they are forced to issue a LIST or LSUB command, to list all

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   available mailboxes, followed by a STATUS command for each mailbox
   found.  This document provides an extension to LIST command that
   allows the client to request STATUS information for mailboxes
   together with other information typically returned by the LIST


   A revised version of this draft document will be submitted to the RFC
   editor as a Proposed Standard for the Internet Community.  Discussion
   and suggestions for improvement are requested, and should be sent to

Table of Contents

   1.  Conventions used in this document . . . . . . . . . . . . . . . 3

   2.  STATUS return option to LIST command  . . . . . . . . . . . . . 3

   3.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

   4.  Formal Syntax . . . . . . . . . . . . . . . . . . . . . . . . . 4

   5.  Security Considerations . . . . . . . . . . . . . . . . . . . . 4

   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5

   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 5

   8.  Normative References  . . . . . . . . . . . . . . . . . . . . . 6

       Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . 6

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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 key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [Kwds].

2.  STATUS return option to LIST command

   [RFC3501] explicitly disallows mailbox patterns in the STATUS
   command.  The main reason was to discourage frequent use of the
   STATUS command by clients, as it might be quite expensive for an IMAP
   server to perform.  However this prohibition had resulted in an
   opposite effect: a new generation of IMAP clients appeared, that
   issues STATUS command for each mailbox returned by the LIST command.
   This behaviour is suboptimal to say at least: it wastes extra
   bandwidth and, in the case of a client that doesn't support IMAP
   pipelining, also degrades performance by using too many round trips.
   This document tries to remedy the situation by specifying a single
   command that can be used by the client to request all the necessary
   information.  In order to achieve this goal this document is
   extending the LIST command command with a new return option: STATUS.
   This option takes STATUS data items as parameters.  For each
   selectable mailbox matching the list pattern and selection options,
   the server MUST return an untagged LIST response followed by an
   untagged STATUS response containing the information requested in the
   STATUS return option.

   If an attempted STATUS for a listed mailbox fails because the mailbox
   can't be selected (e.g. if the "l" ACL right [ACL] is granted to the
   mailbox and the "r" right is not granted, or due to a race condition
   between LIST and STATUS changing the mailbox to \NoSelect), the
   STATUS response MUST NOT be returned and the LIST response MUST
   include the \NoSelect attribute.  This means the server may have to
   buffer the LIST reply until it has successfully looked up the
   necessary STATUS information.

   If the server runs into unexpected problems while trying to look up
   the STATUS information, it MAY drop the corresponding STATUS reply.
   In such situation the LIST command would still return a tagged OK

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3.  Examples

   S: * LIST () "."  "INBOX"
   S: * LIST () "." "foo"
   S: * STATUS "foo" (MESSAGES 30 UNSEEN 29)
   S: * LIST (\NoSelect) "." "bar"
   S: A01 OK List completed.

   "bar" mailbox isn't selectable, so it has no STATUS reply.

   S: * LIST (\Subscribed) "."  "INBOX"
   S: * LIST () "." "foo" (CHILDINFO ("SUBSCRIBED"))
   S: A02 OK List completed.

   LIST reply for "foo" is returned because it has matching children,
   but no STATUS reply is returned because "foo" itself doesn't match
   the selection criteria.

4.  Formal Syntax

   The following syntax specification uses the augmented Backus-Naur
   Form (BNF) as described in [ABNF].  Terms not defined here are taken
   from [RFC3501], [LISTEXT].

       return-option =/ status-option

       status-option = "STATUS" SP "(" status-att *(SP status-att) ")"
                       ;; This ABNF production complies with
                       ;; <option-extension> syntax.

5.  Security Considerations

   This extension makes it a bit easier for clients to overload the
   server by requesting STATUS information for a large number of
   mailboxes.  However as already noted in the introduction existing
   clients already try to do that by generating a large number of STATUS
   commands for each mailbox they are interested in.  While performing
   STATUS information retrieval for big lists of mailboxes a server
   implementation needs to make sure that it can still serve other IMAP
   connections and yield execution to other connections, when necessary.

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6.  IANA Considerations

   IMAP4 capabilities are registered by publishing a standards track or
   IESG approved experimental RFC.  The registry is currently located


   This document defines the X-DRAFT-I00-LIST-STATUS [[anchor4: Note to
   RFC Editor: fix before publication]] IMAP capability.  IANA is
   requested to add it to the registry.

   IANA is also requested to add the following new LIST-EXTENDED option
   to the IANA registry established by [LISTEXT]:

   To: iana@iana.org
   Subject: Registration of LIST-EXTENDED option STATUS

   LIST-EXTENDED option name: STATUS

   LIST-EXTENDED option type: RETURN

   LIST-EXTENDED option description: Causes the LIST command to return
   STATUS responses in addition to LIST responses.

   Published specification : XXXX.

   Security considerations: XXXX.

   Intended usage: COMMON

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

   Owner/Change controller: iesg@ietf.org

7.  Acknowledgements

   Thanks to Philip Van Hoof who pointed out that STATUS and LIST
   commands should be combined in order to optimize traffic and number
   of round trips.

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8.  Normative References

   [ABNF]     Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", RFC 5234, January 2008.

   [ACL]      Melnikov, A., "IMAP4 Access Control List (ACL) Extension",
              RFC 4314.

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

   [LISTEXT]  Leiba, B. and A. Melnikov, "IMAP4 LIST Command
              Extensions", RFC 5258, 2008.

              4rev1", RFC 3501, March 2003.

Authors' Addresses

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

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

   Timo Sirainen

   Email: tss@iki.fi

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