Internet Engineering Task Force (IETF)                       A. Melnikov
Request for Comments: 5819                                 Isode Limited
Category: Standards Track                                    T. Sirainen
ISSN: 2070-1721                                             Unaffiliated
                                                              March 2010


   IMAP4 Extension for Returning STATUS Information in Extended LIST

Abstract

   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 and
   to list all 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 command.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc5819.

Copyright Notice

   Copyright (c) 2010 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
   Provisions Relating to IETF Documents
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   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................2
      1.1. Conventions Used in This Document ..........................2
   2. STATUS Return Option to LIST Command ............................2
   3. Examples ........................................................3
   4. Formal Syntax ...................................................4
   5. Security Considerations .........................................4
   6. IANA Considerations .............................................4
   7. Acknowledgements ................................................5
   8. Normative References ............................................5

1.  Introduction

   Many IMAP clients display information about the 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 and
   to list all 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 command.

1.1.  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",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   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 a STATUS command for each mailbox returned by the LIST
   command.  This behavior 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 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 a situation, the LIST command would still return a tagged OK
   reply.

3.  Examples

   C: A01 LIST "" % RETURN (STATUS (MESSAGES UNSEEN))
   S: * LIST () "."  "INBOX"
   S: * STATUS "INBOX" (MESSAGES 17 UNSEEN 16)
   S: * LIST () "." "foo"
   S: * STATUS "foo" (MESSAGES 30 UNSEEN 29)
   S: * LIST (\NoSelect) "." "bar"
   S: A01 OK List completed.

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

   C: A02 LIST (SUBSCRIBED RECURSIVEMATCH)"" % RETURN (STATUS
      (MESSAGES))
   S: * LIST (\Subscribed) "."  "INBOX"
   S: * STATUS "INBOX" (MESSAGES 17)
   S: * LIST () "." "foo" (CHILDINFO ("SUBSCRIBED"))
   S: A02 OK List completed.

   The 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] and [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 in which they are interested.  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.

6.  IANA Considerations

   IMAP4 capabilities are registered by publishing a Standards Track or
   IESG-approved Experimental RFC.  The "IMAP 4 Capabilities" registry
   is available from the IANA webiste:

      http://www.iana.org

   This document defines the LIST-STATUS IMAP capability.  IANA has
   added it to the registry.

   IANA has also added 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: RFC 5819

   Security considerations: RFC 5819

   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.

8.  Normative References

   [ABNF]     Crocker, D., Ed., and P. Overell, "Augmented BNF for
              Syntax Specifications: ABNF", STD 68, RFC 5234, January
              2008.

   [ACL]      Melnikov, A., "IMAP4 Access Control List (ACL) Extension",
              RFC 4314, December 2005.

   [Kwds]     Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [LISTEXT]  Leiba, B. and A. Melnikov, "Internet Message Access
              Protocol version 4 - LIST Command Extensions", RFC 5258,
              June 2008.

   [RFC3501]  Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
              4rev1", RFC 3501, March 2003.

Authors' Addresses

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

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


   Timo Sirainen

   EMail: tss@iki.fi