Internet Draft: IMAP Extension for Conditional STORE          A. Melnikov
Document: draft-melnikov-imap-condstore-09.txt                    S. Hole
Expires: June 2003                          ACI WorldWide/MessagingDirect
                                                            December 2002

               IMAP Extension for Conditional STORE operation

Status of this Memo

     This document is an Internet-Draft and is in full conformance with
     all provisions of Section 10 of RFC2026.  Internet-Drafts are
     working documents of the Internet Engineering Task Force (IETF), its
     areas, and its working groups.  Note that other groups may also
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     Internet-Drafts are draft documents valid for a maximum of six
     months and may be updated, replaced, or obsoleted by other documents
     at any time.  It is inappropriate to use Internet-Drafts as
     reference material or to cite them other than as "work in progress."

     The list of current Internet-Drafts can be accessed at  The list of Internet-
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Copyright Notice

      Copyright (C) The Internet Society 2001-2002. All Rights Reserved.

0.1. Open issues

     1). Should conditional STORE be atomic accross message set (i.e. either
         all messages in message set weren't changed since and conditional
         STORE succeeds or operation fails for all messages)?
         This can be difficult to implement for some servers.

0.2. Change History

     Changes from -08 to -09:
      1.  Added an extended example about reporting regular (non-conditional) flag
          changes to other sessions.
      2.  Simplified FETCH MODSEQ syntax by removing per-metadata requests and

     Changes from -07 to -08:
      1.  Added note saying the change to UIDVALIDITY also invalidates HIGHESTMODSEQ.
      2.  Fixed several bugs in ABNF for STATUS and STORE commands.

     Changes from -06 to -07:
      1.  Added clarification that when a server does command reordering, the second
          completed operation gets the higher mod sequence.
      2.  Renamed annotation type specifier "both" to "all" as per suggestion
          from Minneapolis meeting.
      3.  Removed PERFLAGMODSEQ capability, as it doesn't buy anything: a client
          has to work with both types of servers (i.e. servers that support per
          message per flag modseqs and servers that support only per message
          modseqs) anyway.
      4.  Per flag modsequences are optional for a server to return. Updated syntax.
      5.  Allow MODSEQ response code only as a result of SEARCH/SORT as suggested
          by John Myers. MODSEQ response code is not allowed after FETCH or STORE.

     Changes from -05 to -06:
      1.  Replaced "/message/flags/system" with "/message/flags" to
          match ANNOTATE draft.
      2.  Extended FETCH/SEARCH/SORT syntax to allow for specifying
          whether an operation should be performed on a shared or a private
          annotation (or both).
      3.  Corrected some examples.

     Changes from -04 to -05:
      1.  Added support for SORT extension.
      2.  Multiple language/spelling fixes by Randall Gellens.

     Changes from -03 to -04:
      1.  Added text saying that MODSEQ fetch data items cause server
          to include MODSEQ data response in all subsuquent unsolicited FETCH
      2.  Added "authors address" section.

     Changes from -02 to -03:
      1.  Changed MODTIME untagged response to MODTIME response code.
      2.  Added MODTIME response code to the tagged OK response for SEARCH.
          Updated examples accordingly.
      3.  Changed rule for sending untagged FETCH response as a result of
          STORE when .SILENT prefix is used. If .SILENT prefix is used,
          server doesn't have to send untagged FETCH response, because
          MODTIME response code already contains modtime.
      4.  Renamed MODTIME to MODSEQ to make sure there is no confusion
          between mod-sequence and ACAP modtime.
      5.  Minor ABNF changes.
      6.  Minor language corrections.

     Changes from -01 to -02:
      1.  Added MODTIME data item to STATUS command.
      2.  Added OK untagged response to SELECT/EXAMINE.
      3.  Clarified that MODIFIED response code contains list of UIDs for
          conditional UID STORE and message set for STORE.
      4.  Added per-message modtime.
      5.  Added PERFLAGMODTIME capability.
      6.  Fixed several bugs in examples.
      7.  Added more comments to ABNF.

     Changes from -00 to -01:
      1.  Refreshed the list of Open Issues.
      2.  Changed "attr-name" to "entry-name", because modtime applies to
          entry, not attribute.
      3.  Added MODTIME untagged response.
      4.  Cleaned up ABNF.
      5.  Added "Acknowledgments" section.
      6.  Fixed some spelling mistakes.

                            Table of Contents

   1  Abstract .................................................. X
   2  Conventions Used in This Document ......................... X
   3  Introduction and Overview ................................. X
   4  IMAP Protocol Changes ..................................... X
   4.1 New OK untagged responses for SELECT and EXAMINE ......... X
   4.1.1 HIGHESTMODSEQ response code ............................ X
   4.2 STORE and UID STORE Commands ............................. X
   4.3 MODSEQ message data item in FETCH Command ................ X
   4.4 MODSEQ search criterion in SEARCH ........................ X
   4.5 MODSEQ Sort Criterion .................................... X
   4.6 MODSEQ Response code for successful SEARCH and SORT ...... X
   4.7 HIGHESTMODSEQ status data items .......................... X
   5  Formal Syntax ............................................. X
   6  Security Considerations ................................... X
   7  References ................................................ X
   7.1 Normative References ..................................... X
   7.2 Informative References ................................... X
   8  Acknowledgments ........................................... X
   9  Author's Addresses ........................................ X
   10  Full Copyright Statement ................................. X

1. Abstract

    Often, multiple IMAP clients need to coordinate changes to a common
    IMAP mailbox.  Examples include different clients for the same user,
    and multiple users accessing shared mailboxes. These clients
    need a mechanism to synchronize state changes for messages within the
    mailbox. They must be able to guarantee that only one client can change
    message state (e.g., message flags or annotations) at any time.  An
    example of such an application is use of an IMAP mailbox as a message
    queue with multiple dequeueing clients.

    The Conditional Store facility provides a protected update mechanism for
    message state information that can detect and resolve conflicts between
    multiple writers.

2. Conventions Used in This Document

    The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
    document are to be interpreted as described in RFC 2119 [KEYWORDS].

    In examples, lines beginning with "S:" are sent by the IMAP server, and
    lines beginning with "C:" are sent by the client. Line breaks may appear
    in example commands solely for editorial clarity; when present in
    the actual message they are represented by "CRLF".

    Formal syntax is defined using ABNF [ABNF] as modified by [IMAP4].

    The term "metadata" or "metadata item" is used throughout this document.
    It refers to any system or user defined keyword or an annotation

    Some IMAP mailboxes are private, accessible only to the owning user.
    Other mailboxes are not, either because the owner has set an ACL
    [ACL] which permits access by other users, or because it is a
    shared mailbox. Let's call a metadata item "shared" for the mailbox
    if any changes to the metadata items are persistent and visible to all
    other users accessing the mailbox. Otherwise the metadata item is called
    "private". Note, that private metadata items are still visible to all
    sessions accessing the mailbox as the same user. Also note, that different
    mailboxes may have different metadata items as shared.

3. Introduction and Overview

    The Conditional STORE extension is present in any IMAP4 implementation
    which returns "CONDSTORE" as one of the supported capabilities in the
    CAPABILITY command response.

    Every IMAP message has an associated positive unsigned 64-bit value called a
    modification sequence (mod-sequence). This is an opaque value updated by
    the server whenever a metadata item is modified. The value is intended to
    be used only for comparisons within a server. However, the server MUST
    guarantee that each STORE command performed on the same mailbox, including
    simultaneous stores to different metadata items from different connections,
    will get a different mod-sequence value. Also, for any two successfull
    STORE operations performed in the same session on the same mailbox,
    the mod-sequence of the second completed operation MUST be greater than
    the mod-sequence of the first completed. Note that the latter rule disallows
    the use of the system clock as a mod-sequence, because if system time changes
    (e.g., a NTP [NTP] client adjusting the time), the next generated value might
    be less than the previous one.

    Mod-sequences allow a client that supports the CONDSTORE extension to
    determine if a message metadata has changed since some known
    moment.  Whenever the state of a flag changes (i.e., the flag is added and
    before it wasn't set, or the flag is removed and before it was set) the
    value of the modification sequence for the message MUST be updated.
    Adding the flag when it is already present or removing when it is not
    present SHOULD NOT change the mod-sequence.

    When a message is appended to a mailbox (via the IMAP APPEND command,
    COPY to the mailbox or using an external mechanism) the server
    generates a new modification sequence that is higher than the highest
    modification sequence of all messages in the mailbox and assigns it to
    the appended message.

    When an annotation is added, modified or removed the corresponding message
    mod-sequence MUST be updated.

    The server MAY store separate (per message) modification sequence values for
    different metadata items. If the server does so, per message modsequence is
    the highest modsequence of all metadata items for the specified message.

    This extension makes the following changes to the IMAP4 protocol:

        a) extends the syntax of the STORE command to allow STORE

        b) adds the MODIFIED response code which should be used with
           a NO response to the STORE command

        c) adds a new MODSEQ message data item for use with the FETCH command

        d) adds a new MODSEQ search criterion

        e) adds a new MODSEQ response code

        f) adds a new OK untagged responses for the SELECT and EXAMINE commands

        g) adds the HIGHESTMODSEQ status data item to the STATUS command

        h) adds a new MODSEQ sort criterion

    The rest of this document describes the protocol changes more rigorously.

4. IMAP Protocol Changes

4.1. New OK untagged responses for SELECT and EXAMINE

4.1.1. HIGHESTMODSEQ response code

    This document adds a new response code that is returned in the OK
    untagged response for the SELECT and EXAMINE commands. A server
    supporting the CONDSTORE extension MUST send the OK untagged
    response including HIGHESTMODSEQ response code with every successful
    SELECT or EXAMINE command:

      OK [HIGHESTMODSEQ <mod-sequence-value>]

      Where <mod-sequence-value> is the highest mod-sequence value of all
      messages in the mailbox. When the server changes UIDVALIDITY for a
      mailbox, it doesn't have to keep the same HIGHESTMODSEQ for the

    A disconnected client can use the value of HIGHESTMODSEQ to check if
    it has to refetch flags and/or annotations from the server. If the
    UIDVALIDITY value has changed for the selected mailbox, the client
    MUST delete the cached value of HIGHESTMODSEQ. If UIDVALIDITY for
    the mailbox is the same and if the HIGHESTMODSEQ value stored in
    the client's cache is less than the value returned by the server,
    then some metadata items on the server have changed since the last
    synchronization, and the client needs to update its cache. The client
    MAY use SEARCH MODSEQ as described in section 4.4 to find out exactly
    which metadata items have changed.

    Example:    C: A142 SELECT INBOX
                S: * 172 EXISTS
                S: * 1 RECENT
                S: * OK [UNSEEN 12] Message 12 is first unseen
                S: * OK [UIDVALIDITY 3857529045] UIDs valid
                S: * OK [UIDNEXT 4392] Predicted next UID
                S: * FLAGS (\Answered \Flagged \Deleted \Seen \Draft)
                S: * OK [PERMANENTFLAGS (\Deleted \Seen \*)] Limited
                S: * OK [HIGHESTMODSEQ 20010715194045007]
                S: A142 OK [READ-WRITE] SELECT completed

4.2. STORE and UID STORE Commands

    Arguments:  message set
                OPTIONAL store modifiers
                message data item name
                value for message data item

    Responses:  untagged responses: FETCH

    Result:     OK - store completed
                NO - store error: can't store that data
                BAD - command unknown or arguments invalid

       This document extends the syntax of the STORE and UID STORE
       commands (see section 6.4.6 of [IMAP]) to include an optional STORE
       modifier.  The document defines the following modifier:

            If the mod-sequence of any metadata item specified in the STORE
            operation for any message in the message set is greater than the
            specified unchangedsince value, then the command fails.
            On failure, a MODIFIED response code is returned which includes
            the message set (for STORE) or set of UIDs (for UID STORE)
            of all messages that failed the UNCHANGESINCE test.


              C: a101 STORE 7,5,9 (UNCHANGEDSINCE 20000320162338)
                  +FLAGS.SILENT (\Deleted)
              S: a101 NO [MODIFIED 7,9] Conditional STORE failed

                   In spite of the failure of the conditional STORE operation
               for message 7, the server continues to process the conditional
               STORE in order to find all messages which fail the test.

                   Use of UNCHANGEDSINCE with a modification sequence of 0
                   always fails if the metadata item exists.


              C: a102 STORE 12 (UNCHANGEDSINCE 0)
                  +FLAGS.SILENT ($MDNSent)
              S: a102 NO [MODIFIED 12] Conditional STORE failed

            If the operation is successful the server MUST update the
            mod-sequence attribute for every message that was changed.
            Untagged FETCH responses MUST be sent (even if .SILENT is
            specified) and each response MUST include MODSEQ message data
            item if its mod-sequence has changed. This is required to
            update clients cache with the correct mod-sequence values.
            See section 4.3 for more details.


              C: a103 UID STORE 6,4,8 (UNCHANGEDSINCE 200012121230045)
                  +FLAGS.SILENT (\Deleted)
              S: * 1 FETCH (UID 4 MODSEQ (200012121231000))
              S: * 2 FETCH (UID 6 MODSEQ (200012101230852))
              S: * 4 FETCH (UID 8 MODSEQ (200012121130956))
              S: a103 OK Conditional Store completed


              C: a104 STORE * (UNCHANGEDSINCE 200012121230045) +FLAGS.SILENT
                 (\Deleted $Processed)
              S: * 50 FETCH (MODSEQ (200012111230045))
              S: a104 OK Store (conditional) completed

            Note: If a message is specified multiple times in the message
            set, and the server doesn't internally eliminate duplicates from
            the message set, it MUST NOT fail the conditional STORE
            operation for the second (or subsequent) occurrence of the message
            if the operation completed successfully for the first occurrence.
            For example, if the client specifies:

               a100 STORE 7,3:9 (UNCHANGEDSINCE 200012121230045)
                +FLAGS.SILENT (\Deleted)

            the server must not fail the operation for message 7 as part of
            processing "3:9" if it succeeded when message 7 was processed
            the first time.

4.3. MODSEQ message data item in FETCH Command

    This extension adds a MODSEQ message data item to the FETCH command.
    The MODSEQ message data item allows clients to retrieve mod-sequence
    values for a range of messages in the currently selected mailbox.

    Once the client specified the MODSEQ message data item in a FETCH request,
    the server MUST include the MODSEQ fetch response data items in all
    subsequent unsolicited FETCH responses.

    Syntax:  MODSEQ [<entry-names>]

    The MODSEQ message data item causes the server to return MODSEQ fetch
    response data items.

    Syntax:  MODSEQ ( <permsg-modsequence> )

        MODSEQ response data items contain per-message mod-sequences.

        The MODSEQ response data item is returned if the client issued FETCH with
        MODSEQ message data item. It also allows the server to notify the client
        about mod-sequence changes caused by conditional STOREs (section 4.2) and/or
        changes caused by external sources.


            C: a FETCH 1:3 (MODSEQ)
            S: * 1 FETCH (MODSEQ (20000624140003))
            S: * 2 FETCH (MODSEQ (20000624140007))
            S: * 3 FETCH (MODSEQ (20000624140005))
            S: a OK Fetch complete

          In this example the client requests per message modsequences for a
          set of messages.

        When a flag for a message is modified in a different session, the server
        sends an unsolicited FETCH response containing the modsequence for the


         (Session 1, authenticated as a user "alex"). The user adds a shared
         flag \Deleted:

            C: A142 SELECT INBOX
            S: * FLAGS (\Answered \Flagged \Deleted \Seen \Draft)
            S: * OK [PERMANENTFLAGS (\Answered \Deleted \Seen \*)] Limited

            C: A160 STORE 7 +FLAGS.SILENT (\Deleted)
            S: * 7 FETCH (MODSEQ (200012121231000))
            S: A160 OK Store completed

        (Session 2, also authenticated as the user "alex"). Any changes to flags
         are always reported to all sessions authenticated as the same user as in
         the session 1.

            C: C180 NOOP
            S: * 7 FETCH (FLAGS (\Deleted \Answered) MODSEQ (200012121231000))
            S: C180 OK Noop completed

        (Session 3, authenticated as a user "andrew"). As \Deleted is a shared
         flag, changes in the session 1 are also reported in the session 3:

            C: D210 NOOP
            S: * 7 FETCH (FLAGS (\Deleted \Answered) MODSEQ (200012121231000))
            S: D210 OK Noop completed

        The user modifies a private flag \Seen in the session 1 ...

            C: A240 STORE 7 +FLAGS.SILENT (\Seen)
            S: * 7 FETCH (MODSEQ (200012121231777))
            S: A240 OK Store completed

        ... which is only reported in the session 2 ...

            C: C270 NOOP
            S: * 7 FETCH (FLAGS (\Deleted \Answered \Seen) MODSEQ (200012121231777))
            S: C270 OK Noop completed

        ... but not in the session 3.

            C: D300 NOOP
            S: D300 OK Noop completed

        And finally the user removes flags \Answered (shared) and \Seen (private)
        in the session 1.

            C: A330 STORE 7 -FLAGS.SILENT (\Answered \Seen)
            S: * 7 FETCH (MODSEQ (200012121245160))
            S: A330 OK Store completed

        Both changes are reported in the session 2 ...

            C: C360 NOOP
            S: * 7 FETCH (FLAGS (\Deleted) MODSEQ (200012121245160))
            S: C360 OK Noop completed

        ... and only changes to shared flags are reported in session 3.

            C: D390 NOOP
            S: * 7 FETCH (FLAGS (\Deleted) MODSEQ (200012121245160))
            S: D390 OK Noop completed

4.4. MODSEQ search criterion in SEARCH

    The MODSEQ criterion for the SEARCH command allows a client to search
    for the metadata items that were modified since a specified moment.

    Syntax:  MODSEQ [<entry-name> <entry-type-req>] <mod-sequence-value>

             Messages that have modification values which are equal to or
             greater than <mod-sequence-value>.  This allows a client, for example,
             to find out which messages contain metadata items that have changed
             since the last time it updated its disconnected cache.
             The client can also specify <entry-name> and entry type (one of
             "shared", "private" or "all") before <mod-sequence-value>.
             If the server doesn't store internally separate mod-sequences
             for different flags and annotations, it MUST ignore
             <entry-name> and <entry-type-req>. Otherwise the server should
             use them to narrow down the search.

    If client specifies a MODSEQ criterion in a SEARCH command and
    the server returns a non-empty SEARCH result, the server MUST also
    return a MODSEQ response code in the tagged OK response. The MODSEQ
    response code covers all messages returned in the untagged SEARCH results.
    See also section 4.6.

         C: a SEARCH MODSEQ "/message/flags/draft" all 20010320162338
                     ANNOTATION "/message/comment" "value" "IMAP4"
         S: * SEARCH 2 5 6 7 11 12 18 19 20 23
         S: a OK [MODSEQ 2,5:7,11:12,18:20,23 20010917162500] Search complete

            In the above example, the message numbers of any messages
            containing the string "IMAP4" in the "value" attribute of the
            "/message/comment" entry and having a mod-sequence equal to or
            greater than 20010320162338 for the "\Draft" flag are returned in
            the search results.

         C: a SEARCH OR NOT MODSEQ 20010320162338 LARGER 50000
         S: * SEARCH
         S: a OK Search complete, nothing found

4.5. MODSEQ Sort Criterion

    If a server implementing CONDSTORE also implements the SORT
    extension as defined by [SORT], it MUST also support sorting on
    per-message mod-sequence.

    Syntax:  MODSEQ

    If client specifies a MODSEQ search (as per section 4.4) or sort
    criterion in the SORT command and the server returns a non-empty
    SORT result, the server MUST also return a MODSEQ response
    code in the tagged OK response which covers all messages returned
    in untagged SORT responses. See also section 4.6.

        C: A282 SORT (SUBJECT MODSEQ) UTF-8 SINCE 1-Feb-2001
        S: * SORT 2 81 83 84 82 882
        S: A282 OK [MODSEQ 2,81:84,882 117] SORT completed

        S: * SORT 6 3 4 5 2
        S: A283 OK [MODSEQ 2:6 125] SORT completed

        C: A284 SORT (MODSEQ) KOI8-R OR NOT MODSEQ 20010320162338
                SUBJECT "Privet"
        S: * SORT
        S: A284 OK Sort complete, nothing found

4.6. MODSEQ Response code for successful SEARCH and SORT

    Data:       message set
                mod-sequence value

    The MODSEQ response code is sent in the following two cases:

     1) If a client specifies a MODSEQ criterion in a SEARCH command
        and the server returns a non-empty SEARCH result, the server MUST
        also return a MODSEQ response code in the tagged OK response.
        The MODSEQ response code MUST be for all messages which were returned
        in the untagged SEARCH response.

        The MODSEQ response code contains the message set to which
        the mod-sequence applies if it is in response to a SEARCH command;
        or the UID set if it is caused by a UID SEARCH command.

     2) If client specifies a MODSEQ search or sort criterion in a
        SORT command and the server returns a non-empty SORT result,
        the server MUST also return a MODSEQ response code in the tagged
        OK response for all messages returned in the untagged SORT response.

        The MODSEQ response code contains the message set to which
        the mod-sequence applies if it is sent in response to a SORT command,
        or the UID set if it is caused by UID SORT.

4.7. HIGHESTMODSEQ status data items

    This document defines a new status data item:

         The highest mod-sequence value all messages
         in the mailbox. This is the same value that is returned by the server
         in the HIGHESTMODSEQ response code in OK untagged response
         (see section 4.1.1).

    Example:    C: A042 STATUS blurdybloop (UIDNEXT MESSAGES HIGHESTMODSEQ)
                S: * STATUS blurdybloop (MESSAGES 231 UIDNEXT 44292
                     HIGHESTMODSEQ 200201011231777)
                S: A042 OK STATUS completed

5. 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 by

    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          =/ "CONDSTORE"

    status              = "STATUS" SP mailbox SP
                          "(" status-att-req *(SP status-att-req) ")"
                          ;; redefine STATUS command syntax defined in [IMAP4]

    status-att-req      = status-att / "HIGHESTMODSEQ"

    mailbox-data        =/ "STATUS" SP mailbox SP "("
                           [status-rsp-info *(SP status-rsp-info)] ")"

    status-rsp-info     = status-att SP number /
                          "HIGHESTMODSEQ" SP mod-sequence-value

    store               = "STORE" SP set store-modifiers SP store-att-flags

    store-modifiers     = [ SP "(" store-modifier *(SP store-modifier) ")" ]

    store-modifier      = "UNCHANGEDSINCE" SP mod-sequence-value
                          ;; Only single "UNCHANGEDSINCE" may be specified
                          ;; in a STORE operation

    fetch-att           =/ fetch-mod-sequence
                           ;; modifies original IMAP4 fetch-att

    fetch-mod-sequence  = "MODSEQ"

    fetch-mod-resp      = "MODSEQ" SP "(" permsg-modsequence ")"

    search-key          =/ search-modsequence
                           ;; modifies original IMAP4 search-key

    search-modsequence  = "MODSEQ" [search-modseq-ext] SP mod-sequence-value

    search-modseq-ext   = SP entry-name SP entry-type-req

    resp-text-code      =/ "HIGHESTMODSEQ" SP mod-sequence-value /
                           "MODIFIED" SP set /
                           "MODSEQ" SP set SP mod-sequence-value
                           ;; set of message numbers for STORE/FETCH or
                           ;; set of UIDs for UID STORE/UID FECTH

    entry-name          = '"' "/message/flags/" attr-flag '"'
                          ;; each system or user defined flag <flag>
                          ;; is mapped to "/message/flags/<flag>",
                          ;; where <flag> has no leading "\" for system
                          ;; flags and has a leading "-" for all user
                          ;; defined flags.

    entry-type-resp     = "private" | "shared"
                          ;; metadata item type

    entry-type-req      = entry-type-resp | "all"
                          ;; perform SEARCH operation on private
                          ;; metadata item, shared metadata item or both

    permsg-modsequence  = mod-sequence-value
                          ;; per message mod-sequence

    mod-sequence-value  = 1*DIGIT
                          ;; Unsigned 64-bit integer (mod-sequence)
                          ;; (0 <= n < 18,446,744,073,709,551,615)

;;Borrowed from IMAP4rev1 and modified accordingly:

    attr-flag           = "Answered" / "Flagged" / "Deleted" /
                          "Seen" / "Draft" / "-" attr-flag-keyword /
                          ;; Does not include "\Recent"

    attr-flag-extension = atom
                          ;; Future expansion.  Client implementations
                          ;; MUST accept flag-extension flags.  Server
                          ;; implementations MUST NOT generate
                          ;; flag-extension flags except as defined by
                          ;; future standard or standards-track
                          ;; revisions of this specification.

    attr-flag-keyword   = atom

;;Extension to SORT

    sort-key            =/ "MODSEQ"

6. Security Considerations

    There are no known security issues with this extension, not already
    found in [IMAP4].

7. References

7.1. Normative References

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

    [ABNF] Crocker, Overell, "Augmented BNF for Syntax Specifications:
    ABNF", RFC 2234, Internet Mail Consortium, Demon Internet Ltd,
    November 1997.

    [IMAP4] Crispin, M., "Internet Message Access Protocol - Version
    4rev1", RFC 2060, University of Washington, December 1996.

    [ANNOTATE] Gellens, R., Daboo, C., "IMAP ANNOTATE Extension",
    work in progress.

    [SORT] Crispin, M., "Internet Message Access Protocol -- SORT
    Extension", work in progress.

7.2. Informative References

    [ACAP] Newman, Myers, "ACAP -- Application Configuration Access
    Protocol", RFC 2244, Innosoft, Netscape, November 1997.

    [ACL] Myers, "IMAP4 ACL extension", RFC 2086, Carnegie Mellon,
    January 1997.

    [NTP] Mills, D, "Network Time Protocol (Version 3) Specification,
    Implementation and Analysis", RFC 1305, March 1992.

8. Acknowledgments

    Some text was borrowed from "IMAP ANNOTATE Extension" by Randall Gellens
    and Cyrus Daboo, and "ACAP -- Application Configuration Access Protocol"
    by Chris Newman and John Myers.

    Many thanks to Randall Gellens for his comments on how CONDSTORE should
    interact with ANNOTATE extension and for thorough review of the document.

    Authors also acknowledge the feedback provided by Cyrus Daboo, Larry
    Greenfield, Chris Newman and Arnt Gulbrandsen.

9. Author's Addresses

    Alexey Melnikov

    ACI WorldWide/MessagingDirect
    59 Clarendon Road, Watford, Hertfordshire,
    WD17 1FQ, United Kingdom

    Steve Hole

    ACI WorldWide/MessagingDirect
    #900, 10117 Jasper Avenue,
    Edmonton, Alberta, T5J 1W8, CANADA

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