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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-02.txt                     S. Hole
Expires: December 2003                      ACI WorldWide/MessagingDirect
                                                                June 2003

               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
     distribute working documents as Internet-Drafts.

     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
     http://www.ietf.org/ietf/1id-abstracts.txt.  The list of Internet-
     Draft Shadow Directories can be accessed at
     http://www.ietf.org/shadow.html.


Copyright Notice

      Copyright (C) The Internet Society 2001-2003. All Rights Reserved.

      Please see the Full Copyright section near the end of this document
      for more information.

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 writing mail clients.


                            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 MODSEQ message data item in FETCH Command ................ X
   3.4 MODSEQ search criterion in SEARCH ........................ X
   3.5 MODSEQ Sort Criterion .................................... X
   3.6 Modified SEARCH and SORT untagged responses .............. X
   3.7 HIGHESTMODSEQ status data items .......................... X
   3.8 CONDSTORE parameter to SELECT and EXAMINE ................ X
   4  Formal Syntax ............................................. X
   5  Security Considerations ................................... X
   6  References ................................................ X
   6.1 Normative References ..................................... X
   6.2 Informative References ................................... X
   7  IANA Considerations ....................................... X
   8  Acknowledgments ........................................... X
   9  Author's Addresses ........................................ X
   10 Intellectual Property Rights .............................. X
   11 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] 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
    [ANNOTATE].

    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.

    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 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 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 mod-sequence is
    the highest mod-sequence of all metadata items for the specified message.

    The server that supports this extention 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) extends the syntax of the STORE command to allow STORE
           modifiers

        b) adds the MODIFIED response code which should be used with
           an OK 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) extends the syntax of untagged SEARCH responses to include mod-sequence

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

        g) defines an additional parameter to SELECT/EXAMINE commands

        h) 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 a FETCH, SEARCH, or SORT
    (if it also supports SORT=MODSEQ extension, see below) command that includes
    the MODSEQ message data item, or a STORE command with the UNCHANGEDSINCE modifier.
    The server will include mod-sequence data in all 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.

    This document also defines a new SORT extension with a capability name
    "SORT=MODSEQ". This extension is upwards compatible with the SORT extension
    defined in [SORT]. Server implementations that support both the CONDSTORE and
    SORT extensions SHOULD also support the SORT=MODSEQ extension.  The
    SORT=MODSEQ extension makes the following additions to the SORT extension:

        a) extends syntax of untagged SORT responses to include mod-sequence
           (see section 3.6)

        b) adds a new MODSEQ sort criterion (see sections 3.4 and 3.5)

    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 and EXAMINE commands.

    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 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 3.4 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


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

    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 [IMAP4]) to include an optional STORE
       modifier.  The document defines the following modifier:

         UNCHANGEDSINCE
            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 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 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

            Example:

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

            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: a105 STORE 7,5,9 (UNCHANGEDSINCE 20000320162338)
                  +FLAGS.SILENT (\Deleted)
              S: * 5 FETCH (MODSEQ (20000320162350))
              S: a105 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: a105 STORE 7,5,9 (UNCHANGEDSINCE 20000320162338)
                  +FLAGS.SILENT (\Deleted)
              S: * 7 FETCH (MODSEQ (20000320162342) FLAGS (\Seen \Deleted))
              S: * 5 FETCH (MODSEQ (20000320162350))
              S: * 9 FETCH (MODSEQ (20000320162349) FLAGS (\Answered))
              S: a105 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

            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.

            Upon the receipt of MODIFIED response code the client SHOULD try to
            figure out if the required metadata items have indeed changed. If they
            haven't the client SHOULD retry the command.

            Example:
              C: a106 STORE 100:150 (UNCHANGEDSINCE 200212030000000)
                  +FLAGS.SILENT ($Processed)
              S: * 100 FETCH (MODSEQ (200303181230852))
              S: * 102 FETCH (MODSEQ (200303181230852))
              ...
              S: * 150 FETCH (MODSEQ (200303181230852))
              S: a106 OK [MODIFIED 101] Conditional STORE failed

             the flag $Processed was set on the message 101 ...
              C: a107 NOOP
              S: * 101 FETCH (MODSEQ (200303011130956) FLAGS ($Processed))
              S: a107 OK

             Or the flag hasn't changed ...
              C: b107 NOOP
              S: * 101 FETCH (MODSEQ (200303011130956) FLAGS (\Deleted \Answered))
              S: b107 OK

             ... and the client retries the operation for the message 100
                 with the updated UNCHANGEDSINCE value

              C: b108 STORE 100 (UNCHANGEDSINCE 200303011130956)
                  +FLAGS.SILENT ($Processed)
              S: * 100 FETCH (MODSEQ (200303181230852))
              S: b108 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 20000320172338) +FLAGS (\SEEN)
              S: * 1 FETCH FLAGS (MODSEQ (20000320172342) \SEEN)
              S: * 3 FETCH FLAGS (MODSEQ (20000320172342) \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 (20000320172340) 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 20000320172340) +FLAGS (\Seen)
              S: * 2 FETCH (MODSEQ (20000320180050))
              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:

               a105 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.


            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.


3.3. 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


    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 (20000624140003))
            S: * 2 FETCH (MODSEQ (20000624140007))
            S: * 3 FETCH (MODSEQ (20000624140005))
            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 (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


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 [ANNOTATE]. 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 3.6.

    Example:
         C: a SEARCH MODSEQ "/flags/\\draft" all 20010320162338
                     ANNOTATION "/comment" "value" "IMAP4"
         S: * SEARCH 2 5 6 7 11 12 18 19 20 23 (MODSEQ 20010917162500)
         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 20010320162338 for the "\Draft" flag are returned in
            the search results.

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


3.5. MODSEQ Sort Criterion

    If a server implementing CONDSTORE also implements the SORT
    extension as defined by [SORT], it SHOULD implement the
    SORT=MODSEQ extension that allows for sorting on per-message
    mod-sequence. SORT=MODSEQ extension adds MODSEQ sort criterion
    that allows to sort the matching messages based on their mod-sequence.

    If client specifies a MODSEQ search (as per section 3.4) or sort
    criterion in the SORT command and the server returns a non-empty
    SORT result, the server MUST also append (to the end of the untagged
    SORT response) the highest mod-sequence for all messages being returned.
    See also section 3.6.

    Example (MODSEQ search criterion):

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

    Example (MODSEQ sort criterion):

        C: A283 SORT (SUBJECT REVERSE DATE) UTF-8 MODSEQ 21
        S: * SORT 6 3 4 5 2 (MODSEQ 125)
        S: A283 OK SORT completed

    Example (MODSEQ search criterion and MODSEQ SORT criterion,
             but no messages matching the search criteria):

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


3.6. Modified SEARCH and SORT untagged responses

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

    This document extends syntax of the untagged SEARCH and SORT responses
    to include 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.

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


3.7. 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 200201011231777)
                S: A042 OK STATUS completed


3.8. 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 20010715194045007]
                S: A142 OK [READ-WRITE] SELECT completed, CONDSTORE is now enabled


4. 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
    [IMAP4].

    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" / "SORT=MODSEQ"

    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"

    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-valzer
                          ;; 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
                           ;;
                           ;; This change applies to all command referencing this
                           ;; non-terminal, in particular SEARCH and SORT.

    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-name-flag / entry-annotate-name
                          ;; The server MUST understand entry-name-flag.
                          ;; If the server also supports [ANNOTATE], it MUST
                          ;; also support entry-annotate-name.

    entry-flag-name     = '"' "/flags/" attr-flag '"'
                          ;; 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-annotate-name = entry
                          ;; <entry> is defined in [ANNOTATE]

    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 ")"

    sort-key            =/ "MODSEQ"

    condstore-param     = "CONDSTORE"
                          ;; defines the select parameter used with
                          ;; CONDSTORE extension


;;Borrowed from IMAP4rev1 and modified accordingly:

    mailbox-data        =/ "STATUS" SP mailbox SP "("
                           [status-rsp-info *(SP status-rsp-info)] ")" /
                           "SEARCH" [1*(SP nz-number) SP search_sort_mod_seq] /
                           "SORT" [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. Security Considerations

    It is believed that the Conditional STORE extension doesn't raise
    any new security concerns that are not already discussed in [IMAP4].


6. References

6.1. 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 3501, University of Washington, March 2003.

    [ANNOTATE] Gellens, R., Daboo, C., "IMAP ANNOTATE Extension",
    work in progress.
    <http://www.ietf.org/internet-drafts/draft-ietf-imapext-annotate-xx.txt>

    [SORT] Crispin, M., Murchison, K., "Internet Message Access Protocol --
    SORT AND THREAD EXTENSIONS", work in progress.
    <http://www.ietf.org/internet-drafts/draft-ietf-imapext-sort-xx.txt>


6.2. 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] Myers, "IMAP4 ACL extension", RFC 2086, Carnegie Mellon,
    January 1997.
    <ftp://ftp.isi.edu/in-notes/rfc2086.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>


7. 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 consitutes registration of the CONDSTORE and SORT=MODSEQ
   IMAP capabilities.


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, Harrie Hazewinkel, Arnt Gulbrandsen, Timo
    Sirainen and Mark Crispin.


9. Author's Addresses

    Alexey Melnikov
    mailto: mel@isode.com

    Isode Limited

    Steve Hole
    mailto: Steve.Hole@messagingdirect.com

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


10. Intellectual Property Rights

    The IETF takes no position regarding the validity or scope of any
    intellectual property or other rights that might be claimed to pertain
    to the implementation or use of the technology described in this
    document or the extent to which any license under such rights might or
    might not be available; neither does it represent that it has made any
    effort to identify any such rights.  Information on the IETF's
    procedures with respect to rights in standards-track and
    standards-related documentation can be found in BCP-11.  Copies of
    claims of rights made available for publication and any assurances of
    licenses to be made available, or the result of an attempt made to
    obtain a general license or permission for the use of such proprietary
    rights by implementors or users of this specification can be obtained
    from the IETF Secretariat.

    The IETF invites any interested party to bring to its attention any
    copyrights, patents or patent applications, or other proprietary
    rights which may cover technology that may be required to practice
    this standard.  Please address the information to the IETF Executive
    Director.


11. Full Copyright Statement

    Copyright (C) The Internet Society 2001-2003.  All Rights Reserved.

    This document and translations of it may be copied and furnished to
    others, and derivative works that comment on or otherwise explain it
    or assist in its implementation may be prepared, copied, published
    and distributed, in whole or in part, without restriction of any
    kind, provided that the above copyright notice and this paragraph
    are included on all such copies and derivative works.  However, this
    document itself may not be modified in any way, such as by removing
    the copyright notice or references to the Internet Society or other
    Internet organizations, except as needed for the purpose of
    developing Internet standards in which case the procedures for
    copyrights defined in the Internet Standards process must be
    followed, or as required to translate it into languages other than
    English.

    The limited permissions granted above are perpetual and will not be
    revoked by the Internet Society or its successors or assigns.

    This document and the information contained herein is provided on an
    "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
    TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
    BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
    HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
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Acknowledgement

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


Appendix A. Open Issues and Change History

    Note that this appendix will be removed before publication.

0.1. Change History

     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 extention.
      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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