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Network Working Group                                   Arnt Gulbrandsen
Internet-Draft                                    Oryx Mail Systems GmbH
Intended Status: Proposed Standard                       Alexey Melnikov
                                                           Isode Limited
                                                       December 16, 2007


                       The IMAP ENABLE Extension
                  draft-gulbrandsen-imap-enable-05.txt


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Copyright Notice

    Copyright (C) The IETF Trust (2007).


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.




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1.  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 [RFC4234] as modified by [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.


2.  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). CONSTORE ([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 which supports ENABLE advertises this by including
    the word ENABLE in its capability list.

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


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



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    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:

    - If the argument is not an extension known to the server, the
      server MUST ignore the argument.

    - 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.)

    - If the argument is an extension is supported by the server and
      which 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).

    Clients SHOULD only include extensions that need to be enabled by
    the server. At the time this RFC is published CONDSTORE is the only
    such extension (ie. ENABLE CONDSTORE is an additional "Condstore
    enabling command" as defined in [RFC4551]). Future RFCs may add to
    this list. [Note to the RFC Editor: If the IMAP ANNOTATE document
    has been published already, ANNOTATE should be mentioned as well as
    CONDSTORE.]

    The ENABLE command is only valid in 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



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    ENABLE command MUST list the same capabilities as a CAPABILITY
    command issued before the ENABLE command. The following example
    demonstrates that:

         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


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 [RFC4234] including the core
    rules in Appendix B.1.  [RFC3501] defines the non-terminals
    "capability" and "command-any".



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    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 new security
    considerations that are not already present in the base IMAP
    protocol [RFC3501].


6.  IANA considerations

    The IANA is requested to add ENABLE to the IMAP4 Capabilities
    Registry.  [[Note to RFC-editor: please remove the following before
    publication: This registration should take place at the following
    location: http://www.iana.org/assignments/imap4-capabilities]]


7.  Acknowledgements

    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 responsible for errors
    introduced by editors.



8.  Normative References

    [RFC2119]  Bradner, "Key words for use in RFCs to Indicate
               Requirement Levels", RFC 2119, Harvard University, March
               1997.

    [RFC3501]  Crispin, "Internet Message Access Protocol - Version



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               4rev1", RFC 3501, University of Washington, June 2003.

    [RFC4234]  Crocker, Overell, "Augmented BNF for Syntax
               Specifications: ABNF", RFC 4234, Brandenburg
               Internetworking, Demon Internet Ltd, October 2005.

    [RFC4551]  Melnikov, Hole, "IMAP Extension for Conditional STORE
               Operation or Quick Flag Changes Resynchronization", RFC
               4551, Isode Ltd., June 2006.


8.  Informative References

    [RFC2177]  Leiba, "IMAP4 IDLE Command", RFC 2177, IBM, June 1997.

    [ANNOTATE] Daboo, Gellens, "IMAP ANNOTATE Extension", draft-ietf-
               imapext-annotate, August 2006.


10. Author's Address

    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












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Acknowledgment

    Funding for the RFC Editor function is currently provided by the
    Internet Society.






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          (RFC Editor: Please delete everything after this point)


Changes since -00

    - The IANA asked me to specify the IANA registry exactly

    - Say "clients should only use ENABLE when it's really necessary"

    - Better abstract

    - Wording.

    - Refer to RFCs by number, not by topic.

    - Boilerplate updates - IETF Trust and so on.


Changes since -01

    - Specify that ENABLE ID is BAD, not ignorable.

    - Explicitly allow maximum pipelining.

    - Security implications.


Changes since -02

    - Nits

    - Unique tags in examples

    - Note specifically that a server can reply BAD to ENABLE ID, even
      if it doesn't support ID. All it needs is to know that ID cannot
      be ENABLEd.


Changes since -03

    - Added ENABLED response as per discussion on the mailing list

    - Changed ENABLE to never return BAD

    - Only allow ENABLE in the authenticated state as per consensus in
      Vancouver

    - Clarified [lack of] interaction with the CAPABILITY response



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    - Clarified that the ENABLE command is additive

    - Added more examples


Changes since -04

    - Added rationale for the ENABLE extension

    - Fixed several inconsistencies caused by restring ENABLE to
      authentication state only.








































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