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Internet Engineering Task Force (IETF)                       A. MelnikovRequest for Comments: 5819                                 Isode LimitedCategory: Standards Track                                    T. SirainenISSN: 2070-1721                                             Unaffiliated                                                              March 2010IMAP4 Extension for Returning STATUS Information in Extended LISTAbstract   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 inSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://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 toBCP 78 and the IETF Trust's Legal   Provisions Relating to IETF Documents   (http://trustee.ietf.org/license-info) in effect on the date of   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.Melnikov & Sirainen          Standards Track                    [Page 1]

RFC 5819                         TITLE*                       March 2010Table of Contents1. Introduction ....................................................21.1. Conventions Used in This Document ..........................22. STATUS Return Option to LIST Command ............................23. Examples ........................................................34. Formal Syntax ...................................................45. Security Considerations .........................................46. IANA Considerations .............................................47. Acknowledgements ................................................58. Normative References ............................................51.  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 inRFC 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 selectableMelnikov & Sirainen          Standards Track                    [Page 2]

RFC 5819                         TITLE*                       March 2010   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.Melnikov & Sirainen          Standards Track                    [Page 3]

RFC 5819                         TITLE*                       March 20104.  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.Melnikov & Sirainen          Standards Track                    [Page 4]

RFC 5819                         TITLE*                       March 2010   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.org7.  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.Melnikov & Sirainen          Standards Track                    [Page 5]

RFC 5819                         TITLE*                       March 2010Authors' 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.fiMelnikov & Sirainen          Standards Track                    [Page 6]