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rfc:rfc4551

Network Working Group A. Melnikov Request for Comments: 4551 Isode Ltd. Updates: 3501 S. Hole Category: Standards Track ACI WorldWide/MessagingDirect

                                                             June 2006
           IMAP Extension for Conditional STORE Operation
              or Quick Flag Changes Resynchronization

Status of This Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

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).

Melnikov & Hole Standards Track [Page 1] RFC 4551 IMAP Extension for Conditional STORE June 2006

Table of Contents

 1.  Introduction and Overview ................................. 3
 2.  Conventions Used in This Document ......................... 5
 3.  IMAP Protocol Changes ..................................... 6
 3.1. New OK untagged responses for SELECT and EXAMINE ......... 6
 3.1.1. HIGHESTMODSEQ response code ............................ 6
 3.1.2. NOMODSEQ response code ................................. 7
 3.2. STORE and UID STORE Commands ............................. 7
 3.3 FETCH and UID FETCH Commands ..............................13
 3.3.1. CHANGEDSINCE FETCH modifier ............................13
 3.3.2. MODSEQ message data item in FETCH Command ..............14
 3.4. MODSEQ search criterion in SEARCH ........................16
 3.5. Modified SEARCH untagged response ........................17
 3.6. HIGHESTMODSEQ status data items ..........................17
 3.7. CONDSTORE parameter to SELECT and EXAMINE ................18
 3.8. Additional quality of implementation issues ..............18
 4.  Formal Syntax .............................................19
 5.  Server implementation considerations ......................21
 6.  Security Considerations ...................................22
 7.  IANA Considerations .......................................22
 8.  References ................................................23
 8.1. Normative References .....................................23
 8.2. Informative References ...................................23
 9.  Acknowledgements ..........................................23

Melnikov & Hole Standards Track [Page 2] RFC 4551 IMAP Extension for Conditional STORE June 2006

1. Introduction and Overview

 The Conditional STORE extension is present in any IMAP4
 implementation that 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., an 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.

Melnikov & Hole Standards Track [Page 3] RFC 4551 IMAP Extension for Conditional STORE June 2006

 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:
  1. a SELECT or EXAMINE command with the CONDSTORE parameter,
  2. a STATUS (HIGHESTMODSEQ) command,
  3. a FETCH or SEARCH command that includes the MODSEQ message data

item,

  1. a FETCH command with the CHANGEDSINCE modifier, or
  2. a STORE command with the UNCHANGEDSINCE modifier.
 The server MUST include mod-sequence data in all subsequent untagged
 FETCH responses (until the connection is closed), whether they were
 caused by a regular STORE, a STORE with UNCHANGEDSINCE modifier, or
 an external agent.
 This document uses the term "CONDSTORE-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

Melnikov & Hole Standards Track [Page 4] RFC 4551 IMAP Extension for Conditional STORE June 2006

 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.

2. 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 Access
 Control List [ACL] that 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 Section 1 for the definition of a "CONDSTORE-aware client" and a
 "CONDSTORE enabling command".

Melnikov & Hole Standards Track [Page 5] RFC 4551 IMAP Extension for Conditional STORE June 2006

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 the 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
 (Section 3.4) to find out exactly which metadata items have changed.
 Alternatively, the client MAY issue FETCH with the 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 1:
    C: A142 SELECT INBOX
    S: * 172 EXISTS

Melnikov & Hole Standards Track [Page 6] RFC 4551 IMAP Extension for Conditional STORE June 2006

    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.  A
 server that returned NOMODSEQ response code for a mailbox, which
 subsequently receives one of the following commands while the mailbox
 is selected:
  1. a FETCH command with the CHANGEDSINCE modifier,
  2. a FETCH or SEARCH command that includes the MODSEQ message data

item, or

  1. a STORE command with the UNCHANGEDSINCE modifier
 MUST reject any such command with the tagged BAD response.
 Example 2:
    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

Melnikov & Hole Standards Track [Page 7] RFC 4551 IMAP Extension for Conditional STORE June 2006

    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, then 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 3:
    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 4:
    C: a104 STORE * (UNCHANGEDSINCE 12121230045) +FLAGS.SILENT
       (\Deleted $Processed)
    S: * 50 FETCH (MODSEQ (12111230047))
    S: a104 OK Store (conditional) completed
 Example 5:
    C: c101 STORE 1 (UNCHANGEDSINCE 12121230045) -FLAGS.SILENT
       (\Deleted)
    S: * OK [HIGHESTMODSEQ 12111230047]

Melnikov & Hole Standards Track [Page 8] RFC 4551 IMAP Extension for Conditional STORE June 2006

    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 6:
    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 that 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 7:
    Same as above, but the server follows the 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 8:
    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 the 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

Melnikov & Hole Standards Track [Page 9] RFC 4551 IMAP Extension for Conditional STORE June 2006

 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 messages that have failed the UNCHANGEDSINCE
 test, upon receipt of the 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 (Examples 9 and 10).
 If the required metadata items haven't changed, the client SHOULD
 retry the command with the new mod-sequence.  The client SHOULD allow
 for a configurable but reasonable number of retries (at least 2).
 Example 9:
    In the example below, the server returns the 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 implementers 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

Melnikov & Hole Standards Track [Page 10] RFC 4551 IMAP Extension for Conditional STORE June 2006

    C: b108 STORE 101 (UNCHANGEDSINCE 303011130956)
        +FLAGS.SILENT ($Processed)
    S: * 101 FETCH (MODSEQ (303181230852))
    S: b108 OK Conditional Store completed
 Example 10:
    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 ($Processed))
    S: * 102 FETCH (MODSEQ (303181230852))
    ...
    S: * 150 FETCH (MODSEQ (303181230852))
    S: a106 OK [MODIFIED 101] Conditional STORE failed
    Or the flag hasn't changed, but another has (note that this server
    behaviour is discouraged.  Server implementers should also see
    Section 5)...
    C: a106 STORE 100:150 (UNCHANGEDSINCE 212030000000)
        +FLAGS.SILENT ($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 ($Processed)
    S: * 101 FETCH (MODSEQ (303181230852))
    S: b108 OK Conditional Store completed
    Or the flag hasn't changed, but another has (nice server
    behaviour.  Server implementers should also see Section 5)...
    C: a106 STORE 100:150 (UNCHANGEDSINCE 212030000000)
        +FLAGS.SILENT ($Processed)

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    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 11:
    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 through 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:

Melnikov & Hole Standards Track [Page 12] RFC 4551 IMAP Extension for Conditional STORE June 2006

       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 implementers 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 create a further subset of
    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 12:
    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

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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 13:
    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 14:
    (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

Melnikov & Hole Standards Track [Page 14] RFC 4551 IMAP Extension for Conditional STORE June 2006

       ...
       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 session 1 are also reported in 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 session 1...
       C: A240 STORE 7 +FLAGS.SILENT (\Seen)
       S: * 7 FETCH (MODSEQ (12121231777))
       S: A240 OK Store completed
    ...which is only reported in session 2...
       C: C270 NOOP
       S: * 7 FETCH (FLAGS (\Deleted \Answered \Seen) MODSEQ
            (12121231777))
       S: C270 OK Noop completed
    ...but not in session 3.
       C: D300 NOOP
       S: D300 OK Noop completed
    And finally, the user removes flags \Answered (shared) and \Seen
    (private) in session 1.
       C: A330 STORE 7 -FLAGS.SILENT (\Answered \Seen)
       S: * 7 FETCH (MODSEQ (12121245160))
       S: A330 OK Store completed

Melnikov & Hole Standards Track [Page 15] RFC 4551 IMAP Extension for Conditional STORE June 2006

    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 implementers 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 that 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.

Melnikov & Hole Standards Track [Page 16] RFC 4551 IMAP Extension for Conditional STORE June 2006

 Example 15:
    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 16:
    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 of all messages in the mailbox.
    This is the same value that is returned by the server in the
    HIGHESTMODSEQ response code in an OK untagged response (see
    Section 3.1.1).  If the server doesn't support the persistent
    storage of mod-sequences for the mailbox (see Section 3.1.2), the
    server MUST return 0 as the value of HIGHESTMODSEQ status data
    item.

Melnikov & Hole Standards Track [Page 17] RFC 4551 IMAP Extension for Conditional STORE June 2006

 Example 17:
    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 avoid a race
 condition that might arise when one or more metadata items 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 18:
    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 to a FETCH command that
 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.

Melnikov & Hole Standards Track [Page 18] RFC 4551 IMAP Extension for Conditional STORE June 2006

 However, note that client implementers MUST NOT rely on this server
 behavior.  A client can't distinguish between the case when a server
 has violated the SHOULD mentioned above, and that when one or more
 clients set and unset (or unset and set) the flag in another session.

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 lowercase 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-valzer
                        ;; extends non-terminal defined in [IMAPABNF].
                        ;; Value 0 denotes that the mailbox doesn't
                        ;; support persistent mod-sequences
                        ;; as described in Section 3.1.2
 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

Melnikov & Hole Standards Track [Page 19] RFC 4551 IMAP Extension for Conditional STORE June 2006

 search-key          =/ search-modsequence
                        ;; modifies original IMAP4 search-key
                        ;;
                        ;; This change applies to all commands
                        ;; 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 ")"

Melnikov & Hole Standards Track [Page 20] RFC 4551 IMAP Extension for Conditional STORE June 2006

 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 mod-sequences for different metadata items can
 avoid sending the 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.

Melnikov & Hole Standards Track [Page 21] RFC 4551 IMAP Extension for Conditional STORE June 2006

 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 the message 101 and that the change updated the mod-sequence for
 the message.  The server knows that the mod-sequence 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; and
 b) the state of the flag $Processed hasn't changed (the server can
    determine this by comparing cached flag state with the state of
    the flag in the database).
 Therefore, the server doesn't have to report MODIFIED to the client.
 Instead, the server may set $Processed flag, update the mod-sequence
 for the message 101 once again and send an untagged FETCH response
 with new mod-sequence and flags:
    S: * 101 FETCH (MODSEQ (303011130956)
       FLAGS ($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. 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 has added
 it to the registry accordingly.

Melnikov & Hole Standards Track [Page 22] RFC 4551 IMAP Extension for Conditional STORE June 2006

8. References

8.1. Normative References

 [KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997.
 [ABNF]     Crocker, D. and P. Overell, "Augmented BNF for Syntax
            Specifications: ABNF", RFC 4234, October 2005.
 [IMAP4]    Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
            4rev1", RFC 3501, March 2003.
 [IMAPABNF] Melnikov, A. and C. Daboo, "Collected Extensions to IMAP4
            ABNF", RFC 4466, April 2006.

8.2. Informative References

 [ACAP]     Newman, C. and J. Myers, "ACAP -- Application
            Configuration Access Protocol", RFC 2244, November 1997.
 [ACL]      Melnikov, A., "IMAP4 Access Control List (ACL) Extension",
            RFC 4314, December 2005.
 [ANN]      Daboo, C. and R. Gellens, "IMAP ANNOTATE Extension", Work
            in Progress, March 2006.
 [NTP]      Mills, D., "Network Time Protocol (Version 3)
            Specification, Implementation and Analysis", RFC 1305,
            March 1992.
 [RFC-2180] Gahrns, M., "IMAP4 Multi-Accessed Mailbox Practice", RFC
            2180, July 1997.

9. Acknowledgements

 Some text was borrowed from "IMAP ANNOTATE Extension" [ANN] by
 Randall Gellens and Cyrus Daboo and from "ACAP -- Application
 Configuration Access Protocol" [ACAP] 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.

Melnikov & Hole Standards Track [Page 23] RFC 4551 IMAP Extension for Conditional STORE June 2006

Authors' Addresses

 Alexey Melnikov
 Isode Limited
 5 Castle Business Village
 36 Station Road
 Hampton, Middlesex
 TW12 2BX,
 United Kingdom
 EMail: Alexey.Melnikov@isode.com
 Steve Hole
 ACI WorldWide/MessagingDirect
 #1807, 10088 102 Ave
 Edmonton, AB
 T5J 2Z1
 Canada
 EMail: Steve.Hole@messagingdirect.com

Melnikov & Hole Standards Track [Page 24] RFC 4551 IMAP Extension for Conditional STORE June 2006

Full Copyright Statement

 Copyright (C) The Internet Society (2006).
 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
 This document and the information contained herein are provided on an
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 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
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 INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
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Acknowledgement

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Melnikov & Hole Standards Track [Page 25]

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