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Versions: (draft-melnikov-imap-condstore) 00 01 02 03 04 05 06 07 08 09 RFC 4551

Internet Draft: IMAP Extension for Conditional STORE          A. Melnikov
Document: draft-ietf-imapext-condstore-09.txt                  Isode Ltd.
Expires: August 2006                                              S. Hole
                                            ACI WorldWide/MessagingDirect
                                                            February 2006

               IMAP Extension for Conditional STORE operation
                  or quick flag changes resynchronization

Status of this Memo

     By submitting this Internet-Draft, each author represents that any
     applicable patent or other IPR claims of which he or she is aware
     have been or will be disclosed, and any of which he or she becomes
     aware will be disclosed, in accordance with Section 6 of BCP 79.

     Internet-Drafts are working documents of the Internet Engineering
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Copyright Notice

     Copyright (C) The Internet Society (2006).

Abstract

    Often, multiple IMAP (RFC 3501) clients need to coordinate changes to
    a common IMAP mailbox. Examples include different clients working on
    behalf of 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) 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 writing mail clients.

    The Conditional Store facility also allows a client to quickly
    resynchronize mailbox flag changes.

    This document defines an extension to IMAP (RFC 3501).



                            Table of Contents

   1  Conventions Used in This Document ......................... X
   2  Introduction and Overview ................................. X
   3  IMAP Protocol Changes ..................................... X
   3.1 New OK untagged responses for SELECT and EXAMINE ......... X
   3.1.1 HIGHESTMODSEQ response code ............................ X
   3.1.2 NOMODSEQ response code ................................. X
   3.2 STORE and UID STORE Commands ............................. X
   3.3 FETCH and UID FETCH Commands ............................. X
   3.3.1 CHANGEDSINCE FETCH modifier ............................ X
   3.3.2 MODSEQ message data item in FETCH Command .............. X
   3.4 MODSEQ search criterion in SEARCH ........................ X
   3.5 Modified SEARCH untagged response ........................ X
   3.6 HIGHESTMODSEQ status data items .......................... X
   3.7 CONDSTORE parameter to SELECT and EXAMINE ................ X
   3.8 Additional quality of implementation issues .............. X
   4  Formal Syntax ............................................. X
   5  Server implementation considerations ...................... X
   6  Security Considerations ................................... X
   7  References ................................................ X
   7.1 Normative References ..................................... X
   7.2 Informative References ................................... X
   8  IANA Considerations ....................................... X
   9  Acknowledgments ........................................... X
   10 Author's Addresses ........................................ X
   11 Intellectual Property Rights .............................. X
   12 Full Copyright Statement .................................. X


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 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].

    The term "metadata" or "metadata item" is used throughout this document.
    It refers to any system or user defined keyword. Future documents
    may extend "metadata" to include other dynamic message data.

    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.

    See the next section for the definition of a "CONDSTORE-aware client"
    and a "CONDSTORE enabling command".


2. 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.

    An IMAP server that supports this extension MUST associate a positive
    unsigned 64-bit value called a modification sequence (mod-sequence)
    with every IMAP message. This is an opaque value updated by
    the server whenever a metadata item is modified. 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 successful
    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 where
    previously 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.

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

    The server that supports this extension is not required to be able to store
    mod-sequences for every available mailbox. Section 3.1.2 describes how
    the server may act if a particular mailbox doesn't support the persistent
    storage of mod-sequences.

    This extension makes the following changes to the IMAP4 protocol:

        a) adds UNCHANGEDSINCE STORE modifier

        b) adds the MODIFIED response code which should be used with
           an OK response to the STORE command. (It can also be used
           in a NO response.)

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

        d) adds CHANGEDSINCE FETCH modifier

        e) adds a new MODSEQ search criterion

        f) extends the syntax of untagged SEARCH responses to include mod-sequence

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

        h) defines an additional parameter to SELECT/EXAMINE commands

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

    A client supporting CONDSTORE extension indicates its willingness to receive
    mod-sequence updates in all untagged FETCH responses by issuing a SELECT or
    EXAMINE command with the CONDSTORE parameter, or STATUS (HIGHESTMODSEQ) command,
    or a FETCH, or SEARCH command that includes
    the MODSEQ message data item, a FETCH command with the CHANGEDSINCE modifier,
    or a STORE command with the UNCHANGEDSINCE modifier.
    The server MUST include mod-sequence data in all subsequent untagged FETCH
    responses, whether they were caused by a regular STORE, STORE with
    UNCHANGEDSINCE modifier, or an external agent, until the connection is closed.

    This document uses the term "CONSTORE-aware client" to refer to a client
    that announces its willingness to receive mod-sequence updates as described
    above. The term "CONDSTORE enabling command" will refer any of the commands
    listed above. A future extension to this document may extend the list of
    CONDSTORE enabling commands. A first CONDSTORE enabling command executed in
    the session MUST cause the server to return HIGHESTMODSEQ (section 3.1.1) unless
    the server has sent NOMODSEQ (section 3.1.2) response code when the currently
    selected mailbox was selected.

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


3. IMAP Protocol Changes

3.1. New OK untagged responses for SELECT and EXAMINE

    This document adds two new response codes HIGHESTMODSEQ and NOMODSEQ.
    One of those response codes MUST be returned in the OK untagged
    response for a successful SELECT/EXAMINE command.

    When opening a mailbox the server must check if the mailbox supports
    the persistent storage of mod-sequences. If the mailbox supports
    the persistent storage of mod-sequences and mailbox open operation succeeds,
    the server MUST send the OK untagged response including HIGHESTMODSEQ
    response code. If the persistent storage for the mailbox is not supported,
    the server MUST send the OK untagged response including NOMODSEQ response
    code instead.


3.1.1. 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 persistent storage of mod-sequences for the mailbox 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
      mailbox.

    A disconnected client can use the value of HIGHESTMODSEQ to check if
    it has to refetch metadata 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 3.4 to find out exactly
    which metadata items have changed. Alternatively the client MAY issue
    FETCH with CHANGEDSINCE modifier (section 3.3.1) in order to fetch data
    for all messages that have metadata items changed since some known
    modification sequence.

    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 715194045007]
                S: A142 OK [READ-WRITE] SELECT completed


3.1.2. NOMODSEQ response code

    A server that doesn't support the persistent storage of mod-sequences for
    the mailbox MUST send the OK untagged response including NOMODSEQ response
    code with every successful SELECT or EXAMINE command.

    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 [NOMODSEQ] Sorry, this mailbox format doesn't support modsequences
                S: A142 OK [READ-WRITE] SELECT completed


3.2. STORE and UID STORE Commands

       This document defines the following STORE modifier (see section
       2.5 of [IMAPABNF]):

         UNCHANGEDSINCE <mod-sequence>
            For each message specified in the message set the server performs
            the following. If the mod-sequence of any metadata item of the
            message is equal or less than the specified UNCHANGEDSINCE value,
            then the requested operation (as described by the
            message data item) is performed. If the operation is successful
            the server MUST update the mod-sequence attribute of the message.
            An untagged FETCH response MUST be sent, even if the .SILENT suffix
            is specified and the response MUST include the MODSEQ message data
            item. This is required to update the client's cache with the correct
            mod-sequence values. See section 3.3.2 for more details.

            However, if the mod-sequence of any metadata item of the
            message is greater than the specified UNCHANGEDSINCE value, than
            the requested operation MUST NOT be performed. In this case,
            the mod-sequence attribute of the message is not updated, and the
            message number (or unique identifier in the case of the UID STORE
            command) is added to the list of messages that failed the UNCHANGESINCE test.

            When the server finished performing the operation on all the messages
            in the message set, it checks for a non-empty list of messages that
            failed the UNCHANGESINCE test. If this list is non-empty, the server MUST
            return in the tagged response a MODIFIED response code. The MODIFIED
            response code includes the message set (for STORE) or set of UIDs
            (for UID STORE) of all messages that failed the UNCHANGESINCE test.

            Example :

               All messages pass the UNCHANGESINCE test.

              C: a103 UID STORE 6,4,8 (UNCHANGEDSINCE 12121230045)
                  +FLAGS.SILENT (\Deleted)
              S: * 1 FETCH (UID 4 MODSEQ (12121231000))
              S: * 2 FETCH (UID 6 MODSEQ (12121230852))
              S: * 4 FETCH (UID 8 MODSEQ (12121130956))
              S: a103 OK Conditional Store completed

            Example:

              C: a104 STORE * (UNCHANGEDSINCE 12121230045) +FLAGS.SILENT
                 (\Deleted $Processed)
              S: * 50 FETCH (MODSEQ (12111230047))
              S: a104 OK Store (conditional) completed

            Example:
              C: c101 STORE 1 (UNCHANGEDSINCE 12121230045) -FLAGS.SILENT
                 (\Deleted)
              S: * OK [HIGHESTMODSEQ 12111230047]
              S: * 50 FETCH (MODSEQ (12111230048))
              S: c101 OK Store (conditional) completed

               HIGHESTMODSEQ response code was sent by the server
               presumably because this was the first CONDSTORE enabling
               command.

            Example:

               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.

              C: d105 STORE 7,5,9 (UNCHANGEDSINCE 320162338)
                  +FLAGS.SILENT (\Deleted)
              S: * 5 FETCH (MODSEQ (320162350))
              S: d105 OK [MODIFIED 7,9] Conditional STORE failed


            Example:

               Same as above, but the server follows SHOULD recommendation
               in section 6.4.6 of [IMAP4].

              C: d105 STORE 7,5,9 (UNCHANGEDSINCE 320162338)
                  +FLAGS.SILENT (\Deleted)
              S: * 7 FETCH (MODSEQ (320162342) FLAGS (\Seen \Deleted))
              S: * 5 FETCH (MODSEQ (320162350))
              S: * 9 FETCH (MODSEQ (320162349) FLAGS (\Answered))
              S: d105 OK [MODIFIED 7,9] Conditional STORE failed


            Use of UNCHANGEDSINCE with a modification sequence of 0
            always fails if the metadata item exists. A system flag
            MUST always be considered existent, whether it was set or not.

            Example:

              C: a102 STORE 12 (UNCHANGEDSINCE 0)
                  +FLAGS.SILENT ($MDNSent)
              S: a102 OK [MODIFIED 12] Conditional STORE failed

               The client has tested the presence of the $MDNSent user defined
               keyword.

            Note: A client trying to make an atomic change to the state of a particular
            metadata item (or a set of metadata items) should be prepared
            to deal with the case when the server returns MODIFIED response code
            if the state of the metadata item being watched hasn't changed (but
            the state of some other metadata item has). This is necessary, because
            some servers don't store separate mod-sequences for different metadata
            items. However, a server implementation SHOULD avoid generating
            spurious MODIFIED responses for +FLAGS/-FLAGS STORE operations,
            even when the server stores a single mod-sequence per message. Section
            5 describes how this can be achieved.

            Unless the server has included an unsolicited FETCH to update client's
            knowledge about message(s) that has failed UNCHANGEDSINCE test, upon the
            receipt of MODIFIED response code the client SHOULD try to
            figure out if the required metadata items have indeed changed by issuing
            FETCH or NOOP command. It is RECOMMENDED that the server avoids the
            need for the client to do that by sending an unsolicited FETCH response
            (see two following examples).

            If the required metadata items haven't changed the client SHOULD retry
            the command with the new modsequence. The client SHOULD allow for a
            configurable but reasonable number of retries (at least 2).

            Example:
               In the example below the server returns MODIFIED response code
               without sending information describing why the STORE UNCHANGEDSINCE
               operation has failed.

              C: a106 STORE 100:150 (UNCHANGEDSINCE 212030000000)
                  +FLAGS.SILENT ($Processed)
              S: * 100 FETCH (MODSEQ (303181230852))
              S: * 102 FETCH (MODSEQ (303181230852))
              ...
              S: * 150 FETCH (MODSEQ (303181230852))
              S: a106 OK [MODIFIED 101] Conditional STORE failed

             the flag $Processed was set on the message 101 ...
              C: a107 NOOP
              S: * 101 FETCH (MODSEQ (303011130956) FLAGS ($Processed))
              S: a107 OK

             Or the flag hasn't changed, but another has (note, that this
             server behaviour is discouraged. Server implementors should also see
             section 5) ...

              C: b107 NOOP
              S: * 101 FETCH (MODSEQ (303011130956) FLAGS (\Deleted \Answered))
              S: b107 OK

             ... and the client retries the operation for the message 101
                 with the updated UNCHANGEDSINCE value

              C: b108 STORE 101 (UNCHANGEDSINCE 303011130956)
                  +FLAGS.SILENT ($Processed)
              S: * 101 FETCH (MODSEQ (303181230852))
              S: b108 OK Conditional Store completed

            Example:
               Same as above, but the server avoids the need for the client to
               poll for changes.

              the flag $Processed was set on the message 101 by another client ...

              C: a106 STORE 100:150 (UNCHANGEDSINCE 212030000000)
                  +FLAGS.SILENT ($Processed)
              S: * 100 FETCH (MODSEQ (303181230852))
              S: * 101 FETCH (MODSEQ (303011130956) FLAGS ($Processed))
              S: * 102 FETCH (MODSEQ (303181230852))
              ...
              S: * 150 FETCH (MODSEQ (303181230852))
              S: a106 OK [MODIFIED 101] Conditional STORE failed

             Or the flag hasn't changed, but another has (note, that this
             server behaviour is discouraged. Server implementors should also see
             section 5) ...

              C: a106 STORE 100:150 (UNCHANGEDSINCE 212030000000)
                  +FLAGS.SILENT ($Processed)
              S: * 100 FETCH (MODSEQ (303181230852))
              S: * 101 FETCH (MODSEQ (303011130956) FLAGS (\Deleted \Answered))
              S: * 102 FETCH (MODSEQ (303181230852))
              ...
              S: * 150 FETCH (MODSEQ (303181230852))
              S: a106 OK [MODIFIED 101] Conditional STORE failed

             ... and the client retries the operation for the message 101
                 with the updated UNCHANGEDSINCE value

              C: b108 STORE 101 (UNCHANGEDSINCE 303011130956)
                  +FLAGS.SILENT ($Processed)
              S: * 101 FETCH (MODSEQ (303181230852))
              S: b108 OK Conditional Store completed

             Or the flag hasn't changed, but another has (nice server behaviour.
             Server implementors should also see section 5) ...

              C: a106 STORE 100:150 (UNCHANGEDSINCE 212030000000)
                  +FLAGS.SILENT ($Processed)
              S: * 100 FETCH (MODSEQ (303181230852))
              S: * 101 FETCH (MODSEQ (303011130956) FLAGS ($Processed \Deleted \Answered))
              S: * 102 FETCH (MODSEQ (303181230852))
              ...
              S: * 150 FETCH (MODSEQ (303181230852))
              S: a106 OK Conditional STORE completed

            Example:

               The following example is based on the example from the section 4.2.3 of
               [RFC-2180] and demonstrates that the MODIFIED response code may be also
               returned in the tagged NO response.

               Client tries to conditionally STORE flags on a mixture of expunged
               and non-expunged messages, one message fails the UNCHANGEDSINCE test.

              C: B001 STORE 1:7 (UNCHANGEDSINCE 320172338) +FLAGS (\SEEN)
              S: * 1 FETCH (MODSEQ (320172342) FLAGS (\SEEN))
              S: * 3 FETCH (MODSEQ (320172342) FLAGS (\SEEN))
              S: B001 NO [MODIFIED 2] Some of the messages no longer exist.

              C: B002 NOOP
              S: * 4 EXPUNGE
              S: * 4 EXPUNGE
              S: * 4 EXPUNGE
              S: * 4 EXPUNGE
              S: * 2 FETCH (MODSEQ (320172340) FLAGS (\Deleted \Answered))
              S: B002 OK NOOP Completed.

               By receiving FETCH responses for messages 1 and 3, and EXPUNGE
               responses that indicate that messages 4:7 have been expunged,
               the client retries the operation only for the message 2. The
               updated UNCHANGEDSINCE value is used.

              C: b003 STORE 2 (UNCHANGEDSINCE 320172340) +FLAGS (\Seen)
              S: * 2 FETCH (MODSEQ (320180050))
              S: b003 OK Conditional Store 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:

               e105 STORE 7,3:9 (UNCHANGEDSINCE 12121230045)
                +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.


            Once the client specified the UNCHANGEDSINCE modifier in a STORE command,
            the server MUST include the MODSEQ fetch response data items in all
            subsequent unsolicited FETCH responses.

       This document also changes the behaviour of the server when it has performed
       a STORE or UID STORE command and the UNCHANGEDSINCE modifier is not specified.
       If the operation is successful for a message, the server MUST update
       the mod-sequence attribute of the message. The server is REQUIRED to
       include the mod-sequence value whenever it decides to send the
       unsolicited FETCH response to all CONDSTORE-aware clients that have opened
       the mailbox containing the message.

    Server implementors should also see section 3.8 for additional quality of
    implementation issues related to the STORE command.


3.3. FETCH and UID FETCH Commands

3.3.1. CHANGEDSINCE FETCH modifier

       This document defines the following FETCH modifier (see section
       2.4 of [IMAPABNF]):

         CHANGEDSINCE <mod-sequence>

            CHANGEDSINCE FETCH modifier allows to further subset the list of
            messages described by sequence set. The information described by
            message data items is only returned for messages that have
            mod-sequence bigger than <mod-sequence>.

            When CHANGEDSINCE FETCH modifier is specified, it implicitly adds
            MODSEQ FETCH message data item (section 3.3.2).

            Example:

              C: s100 UID FETCH 1:* (FLAGS) (CHANGEDSINCE 12345)
              S: * 1 FETCH (UID 4 MODSEQ (65402) FLAGS (\Seen))
              S: * 2 FETCH (UID 6 MODSEQ (75403) FLAGS (\Deleted))
              S: * 4 FETCH (UID 8 MODSEQ (29738) FLAGS ($NoJunk $AutoJunk $MDNSent))
              S: s100 OK FETCH completed


3.3.2. 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


    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 3.2) and/or
        changes caused by external sources.

        Example:

            C: a FETCH 1:3 (MODSEQ)
            S: * 1 FETCH (MODSEQ (624140003))
            S: * 2 FETCH (MODSEQ (624140007))
            S: * 3 FETCH (MODSEQ (624140005))
            S: a OK Fetch complete

          In this example the client requests per message mod-sequences 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 mod-sequence for the
        message.

        Example:

         (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 (2121231000))
            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 (12121231000))
            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 (12121231000))
            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 (12121231777))
            S: A240 OK Store completed

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

            C: C270 NOOP
            S: * 7 FETCH (FLAGS (\Deleted \Answered \Seen) MODSEQ (12121231777))
            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 (12121245160))
            S: A330 OK Store completed

        Both changes are reported in the session 2 ...

            C: C360 NOOP
            S: * 7 FETCH (FLAGS (\Deleted) MODSEQ (12121245160))
            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 (12121245160))
            S: D390 OK Noop completed

    Server implementors should also see section 3.8 for additional quality of
    implementation issues related to the FETCH command.


3.4. 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-valzer>

             Messages that have modification values which are equal to or
             greater than <mod-sequence-valzer>.  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 may also specify <entry-name> (name of metadata
             item) and <entry-type-req> (type of metadata item) before
             <mod-sequence-valzer>. <entry-type-req> can be one of "shared",
             "priv" (private) or "all". The latter means that the server should use
             the biggest value among "priv" and "shared" mod-sequences for the
             metadata item. If the server doesn't store internally separate
             mod-sequences for different metadata items, it MUST ignore
             <entry-name> and <entry-type-req>. Otherwise the server should
             use them to narrow down the search.

             For a flag <flagname> the corresponding <entry-name> has a form
             "/flags/<flagname>" as defined in [IMAPABNF]. Note, that
             the leading "\" character that denotes a system flag has to be
             escaped as per Section 4.3 of [IMAP4], as the <entry-name> uses
             syntax for quoted strings.

    If client specifies a MODSEQ criterion in a SEARCH command and
    the server returns a non-empty SEARCH result, the server MUST also
    append (to the end of the untagged SEARCH response) the highest
    mod-sequence for all messages being returned. See also section 3.5.

    Example:
         C: a SEARCH MODSEQ "/flags/\\draft" all 620162338
         S: * SEARCH 2 5 6 7 11 12 18 19 20 23 (MODSEQ 917162500)
         S: a OK Search complete

            In the above example, the message numbers of any messages
            containing the string "IMAP4" in the "value" attribute of the
            "/comment" entry and having a mod-sequence equal to or
            greater than 620162338 for the "\Draft" flag are returned in
            the search results.

    Example:
         C: t SEARCH OR NOT MODSEQ 720162338 LARGER 50000
         S: * SEARCH
         S: t OK Search complete, nothing found


3.5. Modified SEARCH untagged response

    Data:       zero or more numbers
                mod-sequence value (omitted if no match)

    This document extends syntax of the untagged SEARCH response
    to include the highest mod-sequence for all messages being returned.

    If a client specifies a MODSEQ criterion in a SEARCH (or UID SEARCH)
    command and the server returns a non-empty SEARCH result, the server
    MUST also append (to the end of the untagged SEARCH response) the
    highest mod-sequence for all messages being returned. See section
    3.4 for examples.


3.6. HIGHESTMODSEQ status data items

    This document defines a new status data item:

      HIGHESTMODSEQ
         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 3.1.1).

    Example:    C: A042 STATUS blurdybloop (UIDNEXT MESSAGES HIGHESTMODSEQ)
                S: * STATUS blurdybloop (MESSAGES 231 UIDNEXT 44292
                     HIGHESTMODSEQ 7011231777)
                S: A042 OK STATUS completed


3.7. CONDSTORE parameter to SELECT and EXAMINE

    The CONDSTORE extension defines a single optional select parameter
    "CONDSTORE", which tells the server that it MUST include the MODSEQ
    fetch response data items in all subsequent unsolicited FETCH responses.

    The CONDSTORE parameter to SELECT/EXAMINE helps to avoid a race condition
    that might arise when a metadata item(s) is(are) modified in another session
    after the server has sent the HIGHESTMODSEQ response code and before the
    client was able to issue a CONDSTORE enabling command.

    Example:    C: A142 SELECT INBOX (CONDSTORE)
                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 715194045007]
                S: A142 OK [READ-WRITE] SELECT completed, CONDSTORE is now enabled


3.8. Additional quality of implementation issues.

    Server implementations should follow the following rule, which applies
    to any successfully completed STORE/UID STORE (with and without UNCHANGEDSINCE
    modifier), as well as FETCH command which implicitly sets \Seen flag:

      Adding the flag when it is already present or removing when it is not
      present SHOULD NOT change the mod-sequence.

    This will prevent spurious client synchronization requests.

    However note that client implementors MUST NOT rely on this server behaviour.
    Client can't distinguish between the case when a server has violated the SHOULD
    mentioned above, or one or more client(s) setting and unsetting (or unsetting and
    setting) the flag in another session(s).


4. Formal Syntax

    The following syntax specification uses the Augmented Backus-Naur
    Form (ABNF) [ABNF] notation.  Elements not defined here can be found in
    the formal syntax of the ABNF [ABNF], IMAP [IMAP4], and IMAP ABNF extensions
    [IMAPABNF] specifications.

    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-att          =/ "HIGHESTMODSEQ"
                           ;; extends non-terminal defined in RFC 3501.

    status-att-val      =/ "HIGHESTMODSEQ" SP mod-sequence-value
                           ;; extends non-terminal defined in [IMAPABNF].

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

    fetch-modifier      =/ chgsince-fetch-mod
                           ;; conforms to the generic "fetch-modifier" syntax
                           ;; defined in [IMAPABNF].

    chgsince-fetch-mod  = "CHANGEDSINCE" SP mod-sequence-value
                           ;; CHANGEDSINCE FETCH modifier conforms to
                           ;; the fetch-modifier syntax

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

    fetch-mod-sequence  = "MODSEQ"

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

    msg-att-dynamic     =/ fetch-mod-resp

    search-key          =/ search-modsequence
                           ;; modifies original IMAP4 search-key
                           ;;
                           ;; This change applies to all command referencing this
                           ;; non-terminal, in particular SEARCH.

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

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

    resp-text-code      =/ "HIGHESTMODSEQ" SP mod-sequence-value /
                           "NOMODSEQ" /
                           "MODIFIED" SP set

    entry-name          = entry-flag-name

    entry-flag-name     = DQUOTE "/flags/" attr-flag DQUOTE
                           ;; each system or user defined flag <flag>
                           ;; is mapped to "/flags/<flag>".
                           ;;
                           ;; <entry-flag-name> follows the escape rules used
                           ;; by "quoted" string as described in Section
                           ;; 4.3 of [IMAP4], e.g. for the flag \Seen
                           ;; the corresponding <entry-name> is
                           ;; "/flags/\\seen", and for the flag
                           ;; $MDNSent, the corresponding <entry-name>
                           ;; is "/flags/$mdnsent".

    entry-type-resp     = "priv" / "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
                           ;; Positive unsigned 64-bit integer (mod-sequence)
                           ;; (1 <= n < 18,446,744,073,709,551,615)

    mod-sequence-valzer = "0" / mod-sequence-value

    search-sort-mod-seq = "(" "MODSEQ" SP mod-sequence-value ")"

    select-param        =/ condstore-param
                           ;; conforms to the generic "select-param" non-terminal
                           ;; syntax defined in [IMAPABNF].

    condstore-param     = "CONDSTORE"

    mailbox-data        =/ "SEARCH" [1*(SP nz-number) SP search-sort-mod-seq]

    attr-flag           = "\\Answered" / "\\Flagged" / "\\Deleted" /
                          "\\Seen" / "\\Draft" / attr-flag-keyword /
                          attr-flag-extension
                           ;; 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 [IMAP4].

    attr-flag-keyword   = atom


5. Server implementation considerations

    This section describes how a server implementation that
    doesn't store separate per-metadata modsequences for different metadata
    items can avoid sending MODIFIED response to any of the following
    conditional STORE operations:
      +FLAGS
      -FLAGS
      +FLAGS.SILENT
      -FLAGS.SILENT

    Note, that the optimization described in this section can't be performed
    in case of a conditional STORE FLAGS operation.

    Let's use the following example. The client has issued

      C: a106 STORE 100:150 (UNCHANGEDSINCE 212030000000)
         +FLAGS.SILENT ($Processed)

    When the server receives the command and parses it successfully it
    iterates through the message set and tries to execute the conditional
    STORE command for each message.

    Each server internally works as a client, i.e. it has to cache the
    current state of all IMAP flags as it is known to the client.
    In order to report flag changes to the client the server compares the
    cached values with the values in its database for IMAP flags.

    Imagine that another client has changed the state of a flag \Deleted on
    message 101 and the change updated the modsequence for the message.
    The server knows that the modsequence for the mailbox has changed, however
    it also knows that

     a) The client is not interested in \Deleted flag, as it hasn't included
        it in +FLAGS.SILENT operation.
     b) The state of the flag $Processed hasn't changed (server can determine
        this by comparing cached flag state with the state of the flag in the
        database),
    so the server doesn't have to report MODIFIED to the client. Instead the
    server may set $Processed flag, update the modsequence for the message 101
    once again and send an untagged FETCH response with new modsequence and
    flags:

      S: * 101 FETCH (MODSEQ (303011130956)
         FLAGS ($Processed \Deleted \Answered))


    See also section 3.8 for additional quality of implementation issues.



6. Security Considerations

    It is believed that the Conditional STORE extension doesn't raise
    any new security concerns that are not already discussed in [IMAP4].
    However, the availability of this extension may make it possible
    for IMAP4 to be used in critical applications it could not be used
    for previously, making correct IMAP server implementation and
    operation even more important.


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 4234, October 2005.

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

    [IMAPABNF]  Melnikov, A., "Collected extensions to IMAP4 ABNF",
    work in progress.
    <http://www.ietf.org/internet-drafts/draft-melnikov-imap-ext-abnf-xx.txt>


7.2. Informative References

    [ACAP] Newman, Myers, "ACAP -- Application Configuration Access
    Protocol", RFC 2244, Innosoft, Netscape, November 1997.
    <ftp://ftp.isi.edu/in-notes/rfc2244.txt>

    [ACL] Melnikov, A., "IMAP4 Access Control List (ACL) Extension",
    RFC 4314, December 2005.
    <ftp://ftp.isi.edu/in-notes/rfc4314.txt>

    [NTP] Mills, D, "Network Time Protocol (Version 3) Specification,
    Implementation and Analysis", RFC 1305, March 1992.
    <ftp://ftp.isi.edu/in-notes/rfc1305.txt>

    [RFC-2180] Gahrns, M., "IMAP4 Multi-Accessed Mailbox Practice",
    RFC 2180, July 1997.
    <ftp://ftp.isi.edu/in-notes/rfc2180.txt>


8. IANA Considerations

   IMAP4 capabilities are registered by publishing a standards track or
   IESG approved experimental RFC.  The registry is currently located
   at:

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

   This document defines the CONDSTORE IMAP capability.
   IANA should add them to the registry accordingly.


9. 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 thorough review of the document.

    The authors also acknowledge the feedback provided by Cyrus Daboo, Larry
    Greenfield, Chris Newman, Harrie Hazewinkel, Arnt Gulbrandsen, Timo
    Sirainen, Mark Crispin, Ned Freed, Ken Murchison and Dave Cridland.


10. Author's Addresses

    Alexey Melnikov
    mailto: Alexey.Melnikov@isode.com

    Isode Limited
    5 Castle Business Village, 36 Station Road,
    Hampton, Middlesex, TW12 2BX, United Kingdom

    Steve Hole
    mailto: Steve.Hole@messagingdirect.com

    ACI WorldWide/MessagingDirect


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Copyright Statement

   Copyright (C) The Internet Society (2006).  This document is subject
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   except as set forth therein, the authors retain all their rights.

Acknowledgment

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


Appendix A. Change History

    [[Note to RFC editor: please remove this appendix before publication.]]

0.1. Change History

     Changes from draft-ietf-imapext-condstore-08
      1. Updated examples not to use time-like modification sequences,
         in order not to confuse implementors. (As per comment from
         Mark Crispin)
      2. Clarified beginning of the second paragraph of section 2,
         as per GenArt comment.
      3. Updated (informative) reference to ACL RFC.
      4. Some other editorial changes to reference [IMAPABNF] content.
      5. Repeated and explained the requirement on server implementations
         not to bump modification sequence when a flag change operation
         results in no flag changes.
      6. Explained in Abstract that the CONDSTORE extension can also
         be used for quick mailbox resynchronization.

     Changes from draft-ietf-imapext-condstore-07
      1. Reworded not to have any normative references to SORT.
         SORT=MODSEQ has been moved to a separate draft.

     Changes from draft-ietf-imapext-condstore-06
      1. Minor ABNF update to reference IMAP ABNF properly.

     Changes from draft-ietf-imapext-condstore-05
      1. Reworded not to have a normative reference to ANNOTATE.
      2. Updated ABNF to reference IMAP ABNF.
      3. Clarified that STATUS (HIGHESTMODSEQ) also enables
         CONDSTORE notifications.
      4. Fixed few typos in examples or example titles.
      5. Updated boilerplate, references.

     Changes from draft-ietf-imapext-condstore-04
      1.  Fixed typo in an example, added more examples.
      2.  Clarified client behavior regarding retrying the request
          when the server returns MODIFIED (IESG comment)
      3.  Added new section describing how a CONDSTORE server implementation
          should avoid sending MODIFIED when the client has requested
          a conditional store on a flag A and a flag B was modified
          by another client. (IESG comment)

     Changes from draft-ietf-imapext-condstore-03
      1.  ABNF corrections from Ned Freed.
      2.  Minor spelling/wording corrections from Ned Freed.

     Changes from draft-ietf-imapext-condstore-02
      1.  Added FETCH modifiers.
      2.  Added example for using ANNOTATE with UNCHANGEDSINCE STORE
          modifier.
      3.  Added a new requirement to send HIGHESTMODSEQ response code
          when implicit enabling is used.
      4.  Fixed syntax in an example in section 3.2.

     Changes from draft-ietf-imapext-condstore-01
      1.  Fixed missing \\ in one example.
      2.  Added explanatory comment that search-key modifications apply at
          least to SEARCH and SORT command.
      3.  Don't require from a conditional store operation to be atomic accross
          message set, updated text and examples.
      4.  Added SORT=MODSEQ extension and reworked text in the Introduction section.
      5.  Added Conditional STORE example based on suggestions from RFC 2180.
      6.  Removed the paragraph about DOS attack from the Security considerations
          section, as it doesn't apply anymore.
      7.  Updated entry-name ABNF.
      8.  Added an optional CONDSTORE parameter to SELECT/EXAMINE.

     Changes from draft-ietf-imapext-condstore-00
      1.  Dropped "/message" prefix in entry names as per decision in San Francisco.
      2.  Fixed ABNF for SEARCH and SORT untagged responses.
      3.  Changed "private" to "priv" to be consistent with ANNOTATE.
      4.  MODIFIED response code is now returned in OK response, not NO.
      5.  Added NOMODSEQ response code.

     Changes from draft-melnikov-imap-condstore-09:
      1.  Some text clarifications based on suggestions by Harrie Hazewinkel
      2.  Added paragraph about mailbox locking and DOS when conditional STORE
          operation is performed on a large mailbox.
      3.  Fixed syntax of <entry-name> to match the ANNOTATE extension.
      4.  Added sentence that a system flag MUST always be considered existent,
          when UNCHANGEDSINCE 0 is used. Is this a good idea?
      5.  Clarified client behavior upon receipt of MODIFIED response code.
      6.  Updated ABNF to clarify where 0 is allowed as mod-sequence and where
          it is not.
      7.  Got rid of MODSEQ response code and return this data in the untagged
          SEARCH/SORT responses.
      8.  Updated RFC number for the IMAP4rev1 document.

     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
          responses.

     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 mod-sequences 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
          responses.
      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.


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