IMAP SEARCHRES: IMAP Extension for Referencing the Last SEARCH Result

Network Working Group                                     A. Melnikov
Request for Comments: 5182                                 Isode Ltd.
Updates: 3501                                              March 2008
Category: Standards Track


         IMAP Extension for Referencing the Last SEARCH Result

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

   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 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 Unique Identifier (UID)
   SEARCH) command as an input to any subsequent command.

1.   Introduction

   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
   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.

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 [KEYWORDS].

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

2.  Overview

2.1.   Normative Description of the SEARCHRES Extension

   The SEARCH result reference extension described in this document is
   present in any IMAP4 server implementation that 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 commands 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
   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 the
   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 a 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 implementors should note that "$" can
   reference IMAP message sequences or UID sequences, depending on the
   context where it is used.  For example, the "$" marker can be set as
   a result of a SEARCH (SAVE) command and used as a parameter to a UID
   FETCH command (which accepts a UID sequence, not a 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 accepts a
   message sequence, not a 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:

   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
                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, as described in Section 5.5 of
   [IMAP4].

   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: H282 OK SEARCH completed
               S: H283 OK SEARCH completed

2.4.   Interaction with ESEARCH Extension

   Servers that implement the extension defined in this document MUST
   implement [ESEARCH] and 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 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 the ALL and/or COUNT
   result option(s), 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 the 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 behavioral difference for
   different combinations of ESEARCH result options.  Explanatory
   comments start 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

              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"
                              ;; 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 Denial 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 has
   added 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 remembering that this document had 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.

   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., Ed., and P. Overell, "Augmented BNF for
              Syntax Specifications: ABNF", STD 68, RFC 5234, January
              2008.

   [IMAP4]    Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
              4rev1", RFC 3501, 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,
              Septemeber 2007.

Author's Address

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

   EMail: Alexey.Melnikov@isode.com

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