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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
(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 2010
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
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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]
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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.
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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.
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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
Melnikov & Sirainen Standards Track [Page 6]