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rfc:rfc5161

Network Working Group A. Gulbrandsen, Ed. Request for Comments: 5161 Oryx Mail Systems GmbH Category: Standards Track A. Melnikov, Ed.

                                                         Isode Limited
                                                            March 2008
                     The IMAP ENABLE Extension

Status of This Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Abstract

 Most IMAP extensions are used by the client when it wants to and the
 server supports it.  However, a few extensions require the server to
 know whether a client supports that extension.  The ENABLE extension
 allows an IMAP client to say which extensions it supports.

1. Overview

 Several IMAP extensions allow the server to return unsolicited
 responses specific to these extensions in certain circumstances.
 However, servers cannot send those unsolicited responses until they
 know that the clients support such extensions and thus won't choke on
 the extension response data.
 Up until now, extensions have typically stated that a server cannot
 send the unsolicited responses until after the client has used a
 command with the extension data (i.e., at that point the server knows
 the client is aware of the extension).  CONDSTORE ([RFC4551]),
 ANNOTATE ([ANNOTATE]), and some extensions under consideration at the
 moment use various commands to enable server extensions.  For
 example, CONDSTORE uses a SELECT or FETCH parameter, and ANNOTATE
 uses a side effect of FETCH.
 The ENABLE extension provides an explicit indication from the client
 that it supports particular extensions.  This is done using a new
 ENABLE command.
 An IMAP server that supports ENABLE advertises this by including the
 word ENABLE in its capability list.

Gulbrandsen & Melnikov Standards Track [Page 1] RFC 5161 The IMAP ENABLE Extension March 2008

 Most IMAP extensions do not require the client to enable the
 extension in any way.

2. Conventions Used in This Document

 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 [RFC2119].
 Formal syntax is defined by [RFC5234] and [RFC3501].
 Example lines prefaced by "C:" are sent by the client and ones
 prefaced by "S:" by the server.  The five characters [...] means that
 something has been elided.

3. Protocol Changes

3.1. The ENABLE Command

 Arguments: capability names
 Result:    OK: Relevant capabilities enabled
            BAD: No arguments, or syntax error in an argument
 The ENABLE command takes a list of capability names, and requests the
 server to enable the named extensions.  Once enabled using ENABLE,
 each extension remains active until the IMAP connection is closed.
 For each argument, the server does the following:
  1. If the argument is not an extension known to the server, the server

MUST ignore the argument.

  1. If the argument is an extension known to the server, and it is not

specifically permitted to be enabled using ENABLE, the server MUST

   ignore the argument.  (Note that knowing about an extension doesn't
   necessarily imply supporting that extension.)
  1. If the argument is an extension that is supported by the server and

that needs to be enabled, the server MUST enable the extension for

   the duration of the connection.  At present, this applies only to
   CONDSTORE ([RFC4551]).  Note that once an extension is enabled,
   there is no way to disable it.
 If the ENABLE command is successful, the server MUST send an untagged
 ENABLED response (see Section 3.2).

Gulbrandsen & Melnikov Standards Track [Page 2] RFC 5161 The IMAP ENABLE Extension March 2008

 Clients SHOULD only include extensions that need to be enabled by the
 server.  At the time of publication, CONDSTORE is the only such
 extension (i.e., ENABLE CONDSTORE is an additional "CONDSTORE
 enabling command" as defined in [RFC4551]).  Future RFCs may add to
 this list.
 The ENABLE command is only valid in the authenticated state (see
 [RFC3501]), before any mailbox is selected.  Clients MUST NOT issue
 ENABLE once they SELECT/EXAMINE a mailbox; however, server
 implementations don't have to check that no mailbox is selected or
 was previously selected during the duration of a connection.
 The ENABLE command can be issued multiple times in a session.  It is
 additive; i.e., "ENABLE a b", followed by "ENABLE c" is the same as a
 single command "ENABLE a b c".  When multiple ENABLE commands are
 issued, each corresponding ENABLED response SHOULD only contain
 extensions enabled by the corresponding ENABLE command.
 There are no limitations on pipelining ENABLE.  For example, it is
 possible to send ENABLE and then immediately SELECT, or a LOGIN
 immediately followed by ENABLE.
 The server MUST NOT change the CAPABILITY list as a result of
 executing ENABLE; i.e., a CAPABILITY command issued right after an
 ENABLE command MUST list the same capabilities as a CAPABILITY
 command issued before the ENABLE command.  This is demonstrated in
 the following example:
    C: t1 CAPABILITY
    S: * CAPABILITY IMAP4rev1 ID LITERAL+ ENABLE X-GOOD-IDEA
    S: t1 OK foo
    C: t2 ENABLE CONDSTORE X-GOOD-IDEA
    S: * ENABLED X-GOOD-IDEA
    S: t2 OK foo
    C: t3 CAPABILITY
    S: * CAPABILITY IMAP4rev1 ID LITERAL+ ENABLE X-GOOD-IDEA
    S: t3 OK foo again
 In the following example, the client enables CONDSTORE:
    C: a1 ENABLE CONDSTORE
    S: * ENABLED CONDSTORE
    S: a1 OK Conditional Store enabled

Gulbrandsen & Melnikov Standards Track [Page 3] RFC 5161 The IMAP ENABLE Extension March 2008

3.2. The ENABLED Response

 Contents:   capability listing
 The ENABLED response occurs as a result of an ENABLE command.  The
 capability listing contains a space-separated listing of capability
 names that the server supports and that were successfully enabled.
 The ENABLED response may contain no capabilities, which means that no
 extensions listed by the client were successfully enabled.

3.3. Note to Designers of Extensions That May Use the ENABLE Command

 Designers of IMAP extensions are discouraged from creating extensions
 that require ENABLE unless there is no good alternative design.
 Specifically, extensions that cause potentially incompatible behavior
 changes to deployed server responses (and thus benefit from ENABLE)
 have a higher complexity cost than extensions that do not.

4. Formal Syntax

 The following syntax specification uses the Augmented Backus-Naur
 Form (ABNF) notation as specified in [RFC5234] including the core
 rules in Appendix B.1.  [RFC3501] defines the non-terminals
 "capability" and "command-any".
 Except as noted otherwise, all alphabetic characters are
 case-insensitive.  The use of upper or lower case characters to
 define token strings is for editorial clarity only.  Implementations
 MUST accept these strings in a case-insensitive fashion.
    capability    =/ "ENABLE"
    command-any   =/ "ENABLE" 1*(SP capability)
    response-data =/ "*" SP enable-data CRLF
    enable-data   = "ENABLED" *(SP capability)

5. Security Considerations

 It is believed that this extension doesn't add any security
 considerations that are not already present in the base IMAP protocol
 [RFC3501].

6. IANA Considerations

 The IANA has added ENABLE to the IMAP4 Capabilities Registry.

Gulbrandsen & Melnikov Standards Track [Page 4] RFC 5161 The IMAP ENABLE Extension March 2008

7. Acknowledgments

 The editors would like to thank Randy Gellens, Chris Newman, Peter
 Coates, Dave Cridland, Mark Crispin, Ned Freed, Dan Karp, Cyrus
 Daboo, Ken Murchison, and Eric Burger for comments and corrections.
 However, this doesn't necessarily mean that they endorse this
 extension, agree with all details, or are responsible for errors
 introduced by the editors.

8. Normative References

 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC3501]  Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
            4rev1", RFC 3501, March 2003.
 [RFC5234]  Crocker, D., Ed., and P. Overell, "Augmented BNF for
            Syntax Specifications: ABNF", STD 68, RFC 5234, January
            2008.
 [RFC4551]  Melnikov, A. and S. Hole, "IMAP Extension for Conditional
            STORE Operation or Quick Flag Changes Resynchronization",
            RFC 4551, June 2006.

9. Informative References

 [ANNOTATE] Daboo, C. and R. Gellens, "IMAP ANNOTATE Extension", Work
            in Progress, August 2006.

Gulbrandsen & Melnikov Standards Track [Page 5] RFC 5161 The IMAP ENABLE Extension March 2008

Editors' Addresses

 Arnt Gulbrandsen
 Oryx Mail Systems GmbH
 Schweppermannstr. 8
 D-81671 Muenchen
 Germany
 Fax: +49 89 4502 9758
 EMail: arnt@oryx.com
 Alexey Melnikov
 Isode Ltd
 5 Castle Business Village
 36 Station Road
 Hampton, Middlesex  TW12 2BX
 UK
 EMail: Alexey.Melnikov@isode.com

Gulbrandsen & Melnikov Standards Track [Page 6] RFC 5161 The IMAP ENABLE Extension March 2008

Full Copyright Statement

 Copyright (C) The IETF Trust (2008).
 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.
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 OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
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Gulbrandsen & Melnikov Standards Track [Page 7]

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