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Versions: 00 01 02 03 04 05 06 07 RFC 5182

Document: draft-melnikov-imap-search-res-07.txt           A. Melnikov
Intended status: Proposed Standard                         Isode Ltd.
Updates: RFC 3501 (if approved)                      January 26, 2008
Expires: July 2008
Internet-Draft


         IMAP extension for referencing the last SEARCH result

Status of this Memo

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Copyright Notice

   Copyright (C) The IETF Trust (2008).

Abstract

   Many IMAP clients use the result of a SEARCH command as the input to
   perform another operation, for example fetching the found messages,
   deleting them or copying them to another mailbox.




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   This can be achieved using standard IMAP operations described in RFC
   3501, however this would be suboptimal: the server will send the list
   of found messages to the client, after that the client will have to
   parse the list, reformat it and send it back to the server.  The
   client can't pipeline the SEARCH command with the subsequent command,
   and, as a result, the server might not be able to perform some
   optimizations.

   This document proposes an IMAP extension that allows a client to tell
   a server to use the result of a SEARCH (or UID SEARCH) command as an
   input to any subsequent command.


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 [KEYWORDS].

   Explanatory comments in examples are starting with // and are not
   part of the protocol.

   <<Other editorial comments/questions are shown like this.>>


2.   Introduction and Overview

   Many IMAP clients use the result of a SEARCH command as the input to
   perform another operation, for example fetching the found messages,
   deleting them or copying them to another mailbox.

   This document proposes an IMAP extension that allows a client to tell
   a server to use the result of a SEARCH (or UID SEARCH) command as an
   input to any subsequent command.

   The SEARCH result reference extension defines a new SEARCH result
   option [IMAPABNF] "SAVE" that tells the server to remember the result
   of the SEARCH or UID SEARCH command (as well as any command based on
   SEARCH, e.g. SORT and THREAD [SORT]) and store it in an internal
   variable that we will reference as the "search result variable". The
   client can use the "$" marker to reference the content of this
   internal variable. The "$" marker can be used instead of message
   sequence or UID sequence in order to indicate that the server should
   substitute it with the list of messages from the search result
   variable.  Thus the client can use the result of the latest



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   remembered SEARCH command as a parameter to another command.  The
   search result marker has several advantages:
    * it avoids wasted bandwidth and associated delay;
    * it allows the client to pipeline a SEARCH [IMAP4] command with a
      subsequent FETCH/STORE/COPY/SEARCH [IMAP4] or UID EXPUNGE
      [UIDPLUS] command;
    * the client doesn't need to spend time reformatting the
      result of a SEARCH command into a message set used in
      the subsequent command;
    * it allows the server to perform optimizations. For example if
      the server can execute several pipelined commands in parallel
      (or out of order), presence of the search result marker can
      allow the server to decide which commands may or may not be
      executed out of order.

   In absence of any other SEARCH result option, the SAVE result option
   also suppresses any SEARCH response that would have been otherwise
   returned by the SEARCH command.


 2.1.   Normative description of the SEARCHRES extension

   The SEARCH result reference extension described in this document is
   present in any IMAP4 server implementation which returns "SEARCHRES"
   as one of the supported capabilities in the CAPABILITY command
   response.  Any such server MUST also implement the [ESEARCH]
   extension.

   Upon successful completion of a SELECT or an EXAMINE command (after
   the tagged OK response), the current search result variable is reset
   to the empty sequence.

   A successful SEARCH command with the SAVE result option sets the
   value of the search result variable to the list of messages found in
   the SEARCH command. For example, if no messages were found, the
   search result variable will contain the empty list.

   Any of the following SEARCH command MUST NOT change the search result
   variable:

    o A SEARCH command that caused the server to return the BAD tagged
      response,
    o a SEARCH command with no SAVE result option that caused the server
      to return NO tagged response,
    o a successful SEARCH command with no SAVE result option.

   A SEARCH command with the SAVE result option that caused the server
   to return the NO tagged response sets the value of the search result



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   variable to the empty sequence.

   When a message listed in the search result variable is EXPUNGEd, it
   is automatically removed from the list.  Implementors are reminded
   that if the server stores the list as a list of message numbers, it
   MUST automatically adjust them when notifying the client about
   expunged messages, as described in Section 7.4.1 of [IMAP4].

   If the server decides to send a new UIDVALIDITY value while mailbox
   is opened, this causes resetting of the search variable to the empty
   list.

   Note that even if the "$" marker contains the empty list of messages,
   it must be treated by all commands accepting message sets as
   parameters, as a valid, but non matching list of messages. For
   example, the "FETCH $" command would return tagged OK response and no
   FETCH responses.  See also the Example # 5 below.

   Note that even if the "$" marker contains the empty list of messages,
   it must be treated as a valid but non matching list of messages, by
   all commands that accept message sets as parameters.

   Implementation note: server implementers should note that "$" can
   reference IMAP message sequence or UID sequence, depending on context
   where it is used. For example, the "$" marker can be set as a result
   of a SEARCH (SAVE) command and used as a parameter to a UID FETCH
   command (which accept UID sequence, not message sequence), or the "$"
   marker can be set as a result of a UID SEARCH (SAVE) command and used
   as a parameter to a FETCH command (which accept message sequence, not
   UID sequence).

 2.2.   Examples

   1) The following example demonstrates how the client can use the
      result of a SEARCH command to FETCH headers of interesting
      messages:

   Example 1:
               C: A282 SEARCH RETURN (SAVE) FLAGGED SINCE 1-Feb-1994
                  NOT FROM "Smith"
               S: A282 OK SEARCH completed, result saved
               C: A283 FETCH $ (UID INTERNALDATE FLAGS RFC822.HEADER)
               S: * 2 FETCH (UID 14 ...
               S: * 84 FETCH (UID 100 ...
               S: * 882 FETCH (UID 1115 ...
               S: A283 OK completed

   The client can also pipeline the two commands:



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   Example 2:
               C: A282 SEARCH RETURN (SAVE) FLAGGED SINCE 1-Feb-1994
                  NOT FROM "Smith"
               C: A283 FETCH $ (UID INTERNALDATE FLAGS RFC822.HEADER)
               S: A282 OK SEARCH completed
               S: * 2 FETCH (UID 14 ...
               S: * 84 FETCH (UID 100 ...
               S: * 882 FETCH (UID 1115 ...
               S: A283 OK completed

   2) The following example demonstrates that the result of one SEARCH
      command can be used as input to another SEARCH command:

   Example 3:
               C: A300 SEARCH RETURN (SAVE) SINCE 1-Jan-2004
                   NOT FROM "Smith"
               S: A300 OK SEARCH completed
               C: A301 UID SEARCH UID $ SMALLER 4096
               S: * SEARCH 17 900 901
               S: A301 OK completed

   Note that the second command in Example 3 can be replaced with:
               C: A301 UID SEARCH $ SMALLER 4096
   and the result of the command would be the same.

   3) The following example shows that the "$"
      marker can be combined with other message numbers using the OR
      SEARCH criterion.

   Example 4:
               C: P282 SEARCH RETURN (SAVE) SINCE 1-Feb-1994
                   NOT FROM "Smith"
               S: P282 OK SEARCH completed
               C: P283 SEARCH CHARSET UTF-8 (OR $ 1,3000:3021) TEXT {8}
               C: YYYYYYYY
               S: * SEARCH 882 1102 3003 3005 3006
               S: P283 OK completed

        Note: Since this document format is restricted to 7-bit ASCII
        text, it is not possible to show actual UTF-8 data.  The
        "YYYYYYYY" is a placeholder for what would be 8 octets of 8-bit
        data in an actual transaction.

        4) The following example demonstrates that a failed SEARCH sets
           the search result variable to the empty list.

        Example 5:
                    C: B282 SEARCH RETURN (SAVE) SINCE 1-Feb-1994



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                        NOT FROM "Smith"
                    S: B282 OK SEARCH completed
                    C: B283 SEARCH CHARSET KOI8-R (OR $ 1,3000:3021) TEXT {4}
                    C: XXXX
                    S: B283 NO [BADCHARSET UTF-8] KOI8-R is not supported
                    //After this command the saved result variable contains
                    //no messages. A client that wants to reissue the B283
                    //SEARCH command with another CHARSET would have to reissue
                    //the B282 command as well. One possible workaround for
                    //this is to include the desired CHARSET parameter
                    //in the earliest SEARCH RETURN (SAVE) command in a
                    //sequence of related SEARCH commands.
                    //A better approach might be to always use CHARSET UTF-8
                    //instead.

        Note: Since this document format is restricted to 7-bit ASCII
        text, it is not possible to show actual KOI8-R data.  The "XXXX"
        is a placeholder for what would be 4 octets of 8-bit data in an
        actual transaction.

        5) The following example demonstrates that it is not an error to
        use the "$" marker when it contains no messages.

        Example 6:
                    C: E282 SEARCH RETURN (SAVE) SINCE 28-Oct-2006
                        NOT FROM "Eric"
                    C: E283 COPY $ "Other Messages"
                    //The "$" contains no messages
                    S: E282 OK SEARCH completed
                    S: E283 OK COPY completed, nothing copied


 2.3.   Multiple Commands in Progress

        Use of a SEARCH RETURN (SAVE) command followed by a command
        using the "$" marker creates direct dependency between the two
        commands. As directed by Section 5.5 of [IMAP4], a server MUST
        execute the two commands in the order they were received.  (A
        server capable of out-of-order execution can in some cases
        execute the two commands in parallel, for example if a SEARCH
        RETURN (SAVE) is followed by SEARCH $, the search criteria from
        the first command can be directly substituted into the second
        command.)

        A client supporting this extension MAY pipeline a SEARCH RETURN
        (SAVE) command with one or more command using the "$" marker, as
        long as this doesn't create an ambiguity described in Section
        5.5 of [IMAP4].



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        Example 7:
                    C: F282 SEARCH RETURN (SAVE) KEYWORD $Junk
                    C: F283 COPY $ "Junk"
                    C: F284 STORE $ +FLAGS.Silent (\Deleted)
                    S: F282 OK SEARCH completed
                    S: F283 OK COPY completed
                    S: F284 OK STORE completed

        Example 8:
                    C: G282 SEARCH RETURN (SAVE) KEYWORD $Junk
                    C: G283 SEARCH RETURN (ALL) SINCE 28-Oct-2006
                        FROM "Eric"
                    //The server can execute the two SEARCH commands
                    //in any order, as they don't have any dependency.
                    //Note that the second command is making use of
                    //the [ESEARCH] extension.
                    S: * ESEARCH (TAG "G283") ALL 3:15,27,29:103
                    S: G283 OK SEARCH completed
                    S: G282 OK SEARCH completed

        The following example demonstrates that the result of the second
        SEARCH always overrides the result of the first.

        Example 9:
                    C: H282 SEARCH RETURN (SAVE) KEYWORD $Junk
                    C: H283 SEARCH RETURN (SAVE) SINCE 28-Oct-2006
                        FROM "Eric"
                    S: H283 OK SEARCH completed
                    S: H282 OK SEARCH completed


 2.4.   Interaction with ESEARCH extension

        Servers that implement the extension defined in this document
        are required to implement [ESEARCH] and to conform to additional
        requirements listed in this section.

        The SAVE result option doesn't change whether the server would
        return items corresponding to MIN, MAX, ALL or COUNT [ESEARCH]
        result options.

        When the SAVE result option is combined with the MIN or MAX
        [ESEARCH] result option, and none of the other ESEARCH result
        options are present, the corresponding MIN/MAX is returned (if
        the search result is not empty) but the "$" marker would contain
        a single message as returned in the MIN/MAX return item.

        If the SAVE result option is combined with both MIN and MAX



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        result options, and none of the other ESEARCH result options are
        present, the "$" marker would contain one or two messages as
        returned in the MIN/MAX return items.

        If the SAVE result option is combined with ALL and/or COUNT
        result option, the "$" marker would always contain all messages
        found by the SEARCH or UID SEARCH command.  (Note that the last
        rule might affect ESEARCH implementations that optimize how
        COUNT result is constructed.)

        The following table summarizes the additional requirement on
        ESEARCH server implementations described in this section.

        +----------------+-------------------+
        | Combination of | "$" marker value  |
        |  Result option |                   |
        +----------------+-------------------+
        |   SAVE MIN     |        MIN        |
        +----------------+-------------------+
        |   SAVE MAX     |        MAX        |
        +----------------+-------------------+
        |   SAVE MIN MAX |     MIN & MAX     |
        +----------------+-------------------+
        |   SAVE * [m]   | all found messages|
        +----------------+-------------------+

        where  '*'  means "ALL" and/or "COUNT"
              '[m]' means optional "MIN" and/or "MAX"


        The following example demonstrates behavioural difference
        for different combinations of ESEARCH result options.
        Explanatory comments are starting with // and are not
        part of the protocol:

        Example 10:
                    C: C282 SEARCH RETURN (ALL) SINCE 12-Feb-2006
                        NOT FROM "Smith"
                    S: * ESEARCH (TAG "C283") ALL 2,10:15,21
                  //$ value hasn't changed
                    S: C282 OK SEARCH completed

                    C: C283 SEARCH RETURN (ALL SAVE) SINCE 12-Feb-2006
                        NOT FROM "Smith"
                    S: * ESEARCH (TAG "C283") ALL 2,10:15,21
                  //$ value is 2,10:15,21
                    S: C283 OK SEARCH completed




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                    C: C284 SEARCH RETURN (SAVE MIN) SINCE 12-Feb-2006
                        NOT FROM "Smith"
                    S: * ESEARCH (TAG "C284") MIN 2
                  //$ value is 2
                    S: C284 OK SEARCH completed

                    C: C285 SEARCH RETURN (MAX SAVE MIN) SINCE
                        12-Feb-2006 NOT FROM "Smith"
                    S: * ESEARCH (TAG "C285") MIN 2 MAX 21
                  //$ value is 2,21
                    S: C285 OK SEARCH completed

                    C: C286 SEARCH RETURN (MAX SAVE MIN COUNT)
                        SINCE 12-Feb-2006 NOT FROM "Smith"
                    S: * ESEARCH (TAG "C286") MIN 2 MAX 21 COUNT 8
                  //$ value is 2,10:15,21
                    S: C286 OK SEARCH completed

                    C: C286 SEARCH RETURN (ALL SAVE MIN) SINCE
                        12-Feb-2006 NOT FROM "Smith"
                    S: * ESEARCH (TAG "C286") MIN 2 ALL 2,10:15,21
                  //$ value is 2,10:15,21
                    S: C286 OK SEARCH completed


 2.5.   Refusing to save search results

        In some cases, the server MAY refuse to save a SEARCH (SAVE)
        result, for example if an internal limit on the number of saved
        results is reached.

        In this case, the server MUST return a tagged NO response
        containing the NOTSAVED response code, and set the search result
        variable to the empty sequence, as described in Section 2.1.


   3.   Formal Syntax

   The following syntax specification uses the Augmented Backus-Naur
   Form (ABNF) notation as specified in [ABNF]. Non-terminals referenced
   but not defined below are as defined in [IMAP4] or [IMAPABNF].

   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         =/ "SEARCHRES"



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                        ;; capability is defined in [IMAP4]

   sequence-set       =/ seq-last-command
                        ;; extends sequence-set to allow for
                        ;; "result of the last command" indicator.

   seq-last-command   = "$"

   search-return-opt  = "SAVE"
                        ;; conforms to generic search-return-opt
                        ;; syntax defined in [IMAPABNF]

   resp-text-code     =/ "NOTSAVED"
                        ;; <resp-text-code> from [IMAP4]


4.   Security Considerations

   This extension requires the server to keep additional state, that may
   be used to simplify Deny of Service attacks. In order to minimize
   damage from such attacks server implementations MAY limit the number
   of saved searches they allow across all connections at any given time
   and return the tagged NO response containing the NOTSAVED response
   code (see Section 2.5) to a SEARCH RETURN (SAVE) command when this
   limit is exceeded.

   Apart from that it is believed that this extension doesn't raise any
   additional security concerns not already discussed in [IMAP4].


5.  IANA Considerations

   This document defines the "SEARCHRES" IMAP capability. IANA is
   requested to add it to the IMAP4 Capabilities Registry, which is
   currently located at:

      http://www.iana.org/assignments/imap4-capabilities


6.   Acknowledgments

   The author would like to thank Mark Crispin, Cyrus Daboo and Curtis
   King for reminding that this document has to be written, as well as
   for comments and corrections received.

   The author would also like to thank Dave Cridland, Mark Crispin,
   Chris Newman, Dan Karp and Spencer Dawkins for comments and
   corrections received.



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   Valuable comments, both in agreement and in dissent, were received
   from Arnt Gulbrandsen.

7.   References

7.1. Normative references

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

   [ABNF] Crocker, D., and Overell, P. "Augmented BNF for Syntax
   Specifications: ABNF", RFC 4234, October 2005.

   [IMAP4] Crispin, M., "Internet Message Access Protocol - Version
   4rev1", RFC 3501, University of Washington, March 2003.

   [IMAPABNF] Melnikov, A., and C. Daboo, "Collected extensions to IMAP4
   ABNF", RFC 4466, April 2006.

   [ESEARCH] Melnikov, A., and D. Cridland, "IMAP4 Extension to SEARCH
   Command for Controlling What Kind of Information Is Returned", RFC
   4731, November 2006.


7.2. Informative references

   [UIDPLUS] Crispin, M., "Internet Message Access Protocol (IMAP) -
   UIDPLUS extension", RFC 4315, December 2005.

   [SORT] Crispin, M. and  K. Murchison, "INTERNET MESSAGE ACCESS
   PROTOCOL - SORT AND THREAD EXTENSIONS", work in progress, draft-ietf-
   imapext-sort-XX.txt

8.   Author's Addresses

   Alexey Melnikov
   Isode Ltd.
   5 Castle Business Village,
   36 Station Road,
   Hampton, Middlesex,
   TW12 2BX, United Kingdom

   Email: Alexey.Melnikov@isode.com


9.   Full Copyright Statement

   Copyright (C) The IETF Trust (2008).



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   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.

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
   THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
   OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
   THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

   Funding for the RFC Editor function is provided by the IETF
   Administrative Support Activity (IASA).


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