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RFC 7162

Proposed STD
Pages: 52
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IMAP Extensions: Quick Flag Changes Resynchronization (CONDSTORE) and Quick Mailbox Resynchronization (QRESYNC)

Part 1 of 2, p. 1 to 24
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Obsoletes:    4551    5162
Updates:    2683


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Internet Engineering Task Force (IETF)                       A. Melnikov
Request for Comments: 7162                                     Isode Ltd
Obsoletes: 4551, 5162                                        D. Cridland
Updates: 2683                                               Surevine Ltd
Category: Standards Track                                       May 2014
ISSN: 2070-1721


   IMAP Extensions: Quick Flag Changes Resynchronization (CONDSTORE)
             and Quick Mailbox Resynchronization (QRESYNC)

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 efficiently synchronize
   state changes for messages within the mailbox.

   Initially defined in RFC 4551, the Conditional Store facility
   provides a protected update mechanism for message state information
   and a mechanism for requesting only changes to the message state.
   This memo updates that mechanism and obsoletes RFC 4551, based on
   operational experience.

   This document additionally updates another IMAP extension, Quick
   Resynchronization, which builds on the Conditional STORE extension to
   provide an IMAP client the ability to fully resynchronize a mailbox
   as part of the SELECT/EXAMINE command, without the need for
   additional server-side state or client round trips.  Hence, this memo
   obsoletes RFC 5162.

   Finally, this document also updates the line-length recommendation in
   Section 3.2.1.5 of RFC 2683.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7162.

Page 2 
Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Requirements Notation . . . . . . . . . . . . . . . . . . . .   5
   3.  IMAP Protocol Changes . . . . . . . . . . . . . . . . . . . .   5
     3.1.  CONDSTORE Extension . . . . . . . . . . . . . . . . . . .   5
       3.1.1.  Advertising Support for CONDSTORE . . . . . . . . . .   8
       3.1.2.  New OK Untagged Responses for SELECT and EXAMINE  . .   8
       3.1.3.  STORE and UID STORE Commands  . . . . . . . . . . . .  10
       3.1.4.  FETCH and UID FETCH Commands  . . . . . . . . . . . .  16
       3.1.5.  MODSEQ Search Criterion in SEARCH . . . . . . . . . .  19
       3.1.6.  Modified SEARCH Untagged Response . . . . . . . . . .  20
       3.1.7.  HIGHESTMODSEQ Status Data Items . . . . . . . . . . .  21
       3.1.8.  CONDSTORE Parameter to SELECT and EXAMINE . . . . . .  21
       3.1.9.  Interaction with IMAP SORT and THREAD Extensions  . .  22
       3.1.10. Interaction with IMAP ESORT and ESEARCH Extensions  .  22
       3.1.11. Additional Quality-of-Implementation Issues . . . . .  23
       3.1.12. CONDSTORE Server Implementation Considerations  . . .  23
     3.2.  QRESYNC Extension . . . . . . . . . . . . . . . . . . . .  24
       3.2.1.  Impact on CONDSTORE-only Clients  . . . . . . . . . .  25
       3.2.2.  Advertising Support for QRESYNC . . . . . . . . . . .  25
       3.2.3.  Use of ENABLE . . . . . . . . . . . . . . . . . . . .  25
       3.2.4.  Additional Requirements on QRESYNC Servers  . . . . .  26
       3.2.5.  QRESYNC Parameter to SELECT/EXAMINE . . . . . . . . .  26
       3.2.6.  VANISHED UID FETCH Modifier . . . . . . . . . . . . .  31
       3.2.7.  EXPUNGE Command . . . . . . . . . . . . . . . . . . .  32
       3.2.8.  CLOSE Command . . . . . . . . . . . . . . . . . . . .  33
       3.2.9.  UID EXPUNGE Command . . . . . . . . . . . . . . . . .  34
       3.2.10. VANISHED Response . . . . . . . . . . . . . . . . . .  35
       3.2.11. CLOSED Response Code  . . . . . . . . . . . . . . . .  38
   4.  Long Command Lines (Update to RFC 2683) . . . . . . . . . . .  39
   5.  QRESYNC Server Implementation Considerations  . . . . . . . .  39
     5.1.  Server Implementations That Don't Store Extra State . . .  39
     5.2.  Server Implementations Storing Minimal State  . . . . . .  40
     5.3.  Additional State Required on the Server . . . . . . . . .  40
   6.  Updated Synchronization Sequence  . . . . . . . . . . . . . .  41
   7.  Formal Syntax . . . . . . . . . . . . . . . . . . . . . . . .  44
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  48
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  48
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  48
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  48
     10.2.  Informative References . . . . . . . . . . . . . . . . .  49
   Appendix A.  Changes since RFC 4551 . . . . . . . . . . . . . . .  50
   Appendix B.  Changes since RFC 5162 . . . . . . . . . . . . . . .  50
   Appendix C.  Acknowledgements . . . . . . . . . . . . . . . . . .  51

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1.  Introduction

   Often, multiple IMAP [RFC3501] clients need to coordinate changes to
   a common IMAP mailbox.  Examples include different clients working on
   behalf of the same user and clients representing multiple users
   accessing shared mailboxes.  These clients need a mechanism to
   synchronize state changes for messages within the mailbox.  The
   Conditional Store ("CONDSTORE") facility allows a client to quickly
   resynchronize mailbox flag changes.

   The Conditional Store facility also provides a protected update
   mechanism for message state information that can detect and resolve
   conflicts between multiple writing mail clients.  The mechanism can
   be used to guarantee that only one client can change the message
   state at any given time.  For example, this can be used by multiple
   clients that treat a mailbox as a message queue.

   The Conditional Store facility is provided by associating a
   modification sequence (mod-sequence) with every IMAP message.  This
   is updated whenever metadata (such as a message flag) is modified.

   The CONDSTORE extension is described in more detail in Section 3.1.

   The CONDSTORE extension gives a disconnected client the ability to
   quickly resynchronize IMAP flag changes for previously seen messages.
   This can be done using the CHANGEDSINCE FETCH modifier once a mailbox
   is opened.  In order for the client to discover which messages have
   been expunged, the client still has to issue a UID FETCH or a UID
   SEARCH command.  The Quick Mailbox Resynchronization (QRESYNC) IMAP
   extension is an extension to CONDSTORE that allows a reconnecting
   client to perform full resynchronization, including discovery of
   expunged messages, in a single round trip.  QRESYNC also introduces a
   new response, VANISHED, that allows for a more compact representation
   of a list of expunged messages.

   QRESYNC can be useful for mobile clients that can experience frequent
   disconnects caused by environmental factors (such as battery life,
   signal strength, etc.).  Such clients need a way to quickly reconnect
   to the IMAP server, while minimizing delay experienced by the user as
   well as the amount of traffic generated by resynchronization.

   By extending the SELECT command to perform the additional
   resynchronization, this also allows clients to reduce concurrent
   connections to the IMAP server held purely for the sake of avoiding
   the resynchronization.

   The QRESYNC extension is described in more detail in Section 3.2.

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2.  Requirements Notation

   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 [RFC2119].

   In the examples that follow, "C:" and "S:" indicate lines sent by the
   client and server, respectively.  If a single "C:" or "S:" label
   applies to multiple lines, then the line breaks between those lines
   are for editorial clarity only and are not part of the actual
   protocol exchange.  The five characters [...] means that something
   has been elided.

   Formal syntax is defined using ABNF [RFC5234].

   The term "metadata" or "metadata item" is used throughout this
   document.  It refers to any system- or user-defined keyword.  If the
   server supports the IMAP ANNOTATE-EXPERIMENT-1 extension [RFC5257],
   then metadata also includes message annotations.  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 [RFC4314] 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 3.1 for the definition of a "CONDSTORE-aware client" and
   a "CONDSTORE enabling command".

   Understanding of the IMAP message sequence numbers and UIDs (see
   Section 2.3.1 of [RFC3501]) and the EXPUNGE response (see
   Section 7.4.1 of [RFC3501]) is essential when reading this document.

3.  IMAP Protocol Changes

3.1.  CONDSTORE Extension

   An IMAP server that supports CONDSTORE MUST associate a positive
   unsigned 63-bit (*) value, called a 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

Top      ToC       Page 6 
   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 operation.  Note that the
   latter rule disallows the direct 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.

   (*) Note: RFC 4551 defined mod-sequences as unsigned 64-bit values.
   In order to make implementations on various platforms (such as Java)
   easier, this version of the document redefines them as unsigned
   63-bit values.

   These rules allow a client to list all metadata changes since a well-
   known point in time, as well as to perform conditional metadata
   modifications based on an assumption that the metadata state hasn't
   changed for a particular message.

   In particular, 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 where previously it was set), the value of the modification
   sequence for the message MUST be updated.  Setting a flag that is
   already set, or clearing a flag that is not set, 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 accessible to the currently logged-in user for the specified
   message.

   The server that supports CONDSTORE is not required to be able to
   store mod-sequences for every available mailbox.  Section 3.1.2.2
   describes how the server may act if a particular mailbox doesn't
   support the persistent storage of mod-sequences.

Top      ToC       Page 7 
   CONDSTORE makes the following changes to the IMAP4 protocol:

   a.  adds the UNCHANGEDSINCE STORE modifier.

   b.  adds the MODIFIED response code that is 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 the CHANGEDSINCE FETCH modifier.

   e.  adds a new MODSEQ search criterion.

   f.  extends the syntax of untagged SEARCH and ESEARCH responses to
       include mod-sequence.

   g.  adds new OK untagged responses (HIGHESTMODSEQ and NOMODSEQ) for
       the SELECT and EXAMINE commands.

   h.  defines an additional CONDSTORE parameter to SELECT/EXAMINE
       commands.

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

   A client supporting the CONDSTORE extension indicates its willingness
   to receive mod-sequence updates in all untagged FETCH responses by
   issuing one of the following, which are called "CONDSTORE enabling
   commands":

   o  a SELECT or EXAMINE command with the CONDSTORE parameter,

   o  a STATUS (HIGHESTMODSEQ) command,

   o  a FETCH or SEARCH command that includes the MODSEQ message data
      item,

   o  a FETCH command with the CHANGEDSINCE modifier,

   o  a STORE command with the UNCHANGEDSINCE modifier, or

   o  an ENABLE command containing "CONDSTORE" as one of the parameters.
      (This option only applies when the client is communicating with a
      server that also implements the ENABLE extension [RFC5161].)

   Once a client issues a CONDSTORE enabling command, it has announced
   itself as a "CONDSTORE-aware client".  The server MUST then include
   mod-sequence data in all subsequent untagged FETCH responses (until

Top      ToC       Page 8 
   the connection is closed), whether they were caused by a regular
   STORE, a STORE with an UNCHANGEDSINCE modifier, or an external agent.

   A future extension to this document may extend the list of CONDSTORE
   enabling commands.  A first CONDSTORE enabling command executed in
   the session with a mailbox selected MUST cause the server to return
   HIGHESTMODSEQ (Section 3.1.2.1) for the mailbox (if any is selected),
   unless the server has sent a NOMODSEQ (Section 3.1.2.2) response code
   when the currently selected mailbox was selected.

3.1.1.  Advertising Support for CONDSTORE

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

3.1.2.  New OK Untagged Responses for SELECT and EXAMINE

   This document adds two new response codes: HIGHESTMODSEQ and
   NOMODSEQ.  One of these two response codes MUST be returned in an OK
   untagged response for any successful SELECT/EXAMINE command issued
   after a CONDSTORE enabling 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 an OK untagged response, including the
   HIGHESTMODSEQ response code.  If the persistent storage for the
   mailbox is not supported, the server MUST send an OK untagged
   response, including the NOMODSEQ response code instead.

3.1.2.1.  HIGHESTMODSEQ Response Code

   This document adds a new response code that is returned in an OK
   untagged response for the SELECT and EXAMINE commands.  Once a
   CONDSTORE enabling command is issued, a server supporting the
   persistent storage of mod-sequences for the mailbox MUST send an OK
   untagged response, including the 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.

Top      ToC       Page 9 
   Note that some existing CONDSTORE servers don't start tracking mod-
   sequences or don't report them until after a CONDSTORE enabling
   command is issued.  Because of that, a client wishing to receive
   HIGHESTMODSEQ/NOMODSEQ information must first send a CONDSTORE
   enabling command, for example, by using SELECT/EXAMINE with the
   CONDSTORE parameter (see Section 3.1.8).

   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 3.1.5) to find out exactly which metadata items have
   changed.  Alternatively, the client MAY issue FETCH with the
   CHANGEDSINCE modifier (Section 3.1.4.1) in order to fetch data for
   all messages that have metadata items changed since some known
   modification sequence.

   C: A142 SELECT INBOX
   S: * 172 EXISTS
   S: * 1 RECENT
   S: * OK [UNSEEN 12] Message 12 is first unseen
   S: * OK [UIDVALIDITY 3857529045] UIDs valid
   S: * OK [UIDNEXT 4392] Predicted next UID
   S: * FLAGS (\Answered \Flagged \Deleted \Seen \Draft)
   S: * OK [PERMANENTFLAGS (\Deleted \Seen \*)] Limited
   S: * OK [HIGHESTMODSEQ 715194045007]
   S: A142 OK [READ-WRITE] SELECT completed

                                 Example 1

3.1.2.2.  NOMODSEQ Response Code

   Once a CONDSTORE enabling command is issued, a server that doesn't
   support the persistent storage of mod-sequences for the mailbox MUST
   send an OK untagged response, including the NOMODSEQ response code
   with every successful SELECT or EXAMINE command.  Note that some
   existing CONDSTORE servers don't return NOMODSEQ until after a
   CONDSTORE enabling command is issued.  Because of that, a client
   wishing to receive HIGHESTMODSEQ/NOMODSEQ information must first send
   a CONDSTORE enabling command, for example, by using SELECT/EXAMINE
   with the CONDSTORE parameter (see Section 3.1.8).

Top      ToC       Page 10 
   A server that returned the NOMODSEQ response code for a mailbox MUST
   reject (with a tagged BAD response) any of the following commands
   while the mailbox remains selected:

   o  a FETCH command with the CHANGEDSINCE modifier,

   o  a FETCH or SEARCH command that includes the MODSEQ message data
      item, or

   o  a STORE command with the UNCHANGEDSINCE modifier.

   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

                                 Example 2

3.1.3.  STORE and UID STORE Commands

   This document defines the following STORE modifier (see Section 2.5
   of [RFC4466]):

   UNCHANGEDSINCE <mod-sequence>

   For each message specified in the message set, the server performs
   the following.  If the mod-sequence of every metadata item of the
   message affected by the STORE/UID STORE is equal to 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 3.1.4.2 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

Top      ToC       Page 11 
   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 UNCHANGEDSINCE test.

   When the server finishes performing the operation on all the messages
   in the message set, it checks for a non-empty list of messages that
   failed the UNCHANGEDSINCE 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
   UNCHANGEDSINCE test.

   All messages pass the UNCHANGEDSINCE 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 (12121230956))
   S: a103 OK Conditional Store completed

                                 Example 3

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

                                 Example 4

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

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

                                 Example 5

   The failure of the conditional STORE operation for any particular
   message or messages (7 in this example) does not stop the server from
   finding all messages that fail the UNCHANGEDSINCE test.  All such
   messages are returned in the MODIFIED response code.

Top      ToC       Page 12 
   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 6

   Same as above, but the server follows the SHOULD recommendation in
   Section 6.4.6 of [RFC3501].

   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 7

   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.

                                 Example 8

   Note: A client trying to make an atomic change to the state of a
   particular metadata item (or a set of metadata items) MUST 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 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 3.1.12 describes how this can be achieved.

   Unless the server has included an unsolicited FETCH to update the
   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

Top      ToC       Page 13 
   by issuing the FETCH or NOOP command.  It is RECOMMENDED that the
   server avoids the need for the client to do that by sending an
   unsolicited FETCH response (see 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 needs to
   allow for a reasonable number of retries (at least 2).

   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

                                 Example 9

   Or, the flag hasn't changed, but another has (note that this server
   behavior is discouraged.  Server implementers should also see
   Section 3.1.12)...

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

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

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

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

Top      ToC       Page 14 
   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

                                Example 10

   Or, the flag hasn't changed, but another has (note that this server
   behavior is discouraged.  Server implementers should also see
   Section 3.1.12)...


   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 behavior.
   Server implementers should also see Section 3.1.12)...

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

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   The following example is based on the example from Section 4.2.3 of
   [RFC2180] and demonstrates that the MODIFIED response code MAY also
   be returned in the tagged NO response.

   The 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 message 2.
   The updated UNCHANGEDSINCE value is used.

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

                                Example 11

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

      e105 STORE 7,3:9 (UNCHANGEDSINCE 12121230045) +FLAGS.SILENT
      (\Deleted)

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

Top      ToC       Page 16 
   As specified in Section 3.1, once the client specifies the
   UNCHANGEDSINCE modifier in a STORE command, the server starts
   including the MODSEQ FETCH response data items in all subsequent
   unsolicited FETCH responses.

   This document also changes the behavior 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 3.1.11 for additional
   quality of implementation issues related to the STORE command.

3.1.4.  FETCH and UID FETCH Commands

3.1.4.1.  CHANGEDSINCE FETCH Modifier

   This document defines the following FETCH modifier (see Section 2.4
   of [RFC4466]):

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

      When the CHANGEDSINCE FETCH modifier is specified, it implicitly
      adds the MODSEQ FETCH message data item (Section 3.1.4.2).

   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

                                Example 12

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3.1.4.2.  MODSEQ Message Data Item in FETCH Command

   CONDSTORE 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.

   As specified in Section 3.1, once the client has specified the MODSEQ
   message data item in a FETCH request, the server starts including 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 the MODSEQ message data item.  It also allows the
      server to notify the client about mod-sequence changes caused by
      conditional STOREs (Section 3.1.3) and/or changes caused by
      external sources.

   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.

                                Example 13

   Servers that only support the CONDSTORE extension (and not QRESYNC)
   SHOULD comply with requirements from Section 3.2.4.

   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, as demonstrated in Example 14.  Note that
   when the server also supports the QRESYNC extension (Section 3.2.3)
   and a CONDSTORE enabling command has been issued, all FETCH responses
   in Example 14 must also include UID FETCH items as prescribed by
   Section 3.2.4.

Top      ToC       Page 18 
   (Session 1, authenticated as the user "alex".)  The user adds a
   shared flag \Deleted:

       C: A142 SELECT INBOX
       ...
       S: * FLAGS (\Answered \Flagged \Deleted \Seen \Draft)
       S: * OK [PERMANENTFLAGS (\Answered \Deleted \Seen \*)] Limited
       ...
       C: A160 STORE 7 +FLAGS.SILENT (\Deleted)
       S: * 7 FETCH (MODSEQ (2121231000))
       S: A160 OK Store completed

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

       C: C180 NOOP
       S: * 7 FETCH (FLAGS (\Deleted \Answered) MODSEQ (12121231000))
       S: C180 OK Noop completed

   (Session 3, authenticated as the 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

Top      ToC       Page 19 
   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

   Both changes are reported in 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

                                Example 14

   Server implementers should also see Section 3.1.11 for additional
   quality of implementation issues related to the FETCH command.

3.1.5.  MODSEQ Search Criterion in SEARCH

   The MODSEQ criterion for the SEARCH (or UID 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 the
      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 last means that the server MUST
      use the biggest value among "priv" and "shared" mod-sequences for
      the metadata item.  If the server doesn't store 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.

Top      ToC       Page 20 
      For a flag <flagname>, the corresponding <entry-name> has the form
      "/flags/<flagname>".  Note that the leading "\" character that
      denotes a system flag has to be escaped as per Section 4.3 of
      [RFC3501], as <entry-name> uses the syntax for quoted strings (see
      the examples below).

   If the 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 also
   Section 3.1.6.  Note that other IMAP extensions such as ESEARCH
   [RFC4731] can override this requirement (see Section 3.1.10 for more
   details.)

   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 having a
   mod-sequence equal to or greater than 620162338 for the "\Draft" flag
   are returned in the search results.

                                Example 15

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

                                Example 16

3.1.6.  Modified SEARCH Untagged Response

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

   This document extends the 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 3.1.5 for examples.

Top      ToC       Page 21 
3.1.7.  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 3.1.2.1).  If the server doesn't support the persistent
      storage of mod-sequences for the mailbox (see Section 3.1.2.2),
      the server MUST return 0 as the value of the HIGHESTMODSEQ status
      data item.

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

                                Example 17

3.1.8.  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 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.

   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

                                Example 18

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3.1.9.  Interaction with IMAP SORT and THREAD Extensions

   The MODSEQ Search Criterion (see Section 3.1.5) causes modifications
   to SORT [RFC5256] responses similar to modifications to SEARCH
   responses defined in Section 3.1.6:

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

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

   If a client specifies a MODSEQ 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.  Note that
   other IMAP extensions such as ESORT [RFC5267] can override this
   requirement (see Section 3.1.10 for more details.)

   THREAD commands that include a MODSEQ Search Criterion return THREAD
   responses as specified in [RFC5256], i.e., THREAD responses are
   unchanged by the CONDSTORE extension.

3.1.10.  Interaction with IMAP ESORT and ESEARCH Extensions

   If a client specifies a MODSEQ criterion in an extended SEARCH (or
   extended UID SEARCH) [RFC4731] command and the server returns a non-
   empty SEARCH result, the server MUST return the ESEARCH response
   containing the MODSEQ result option as defined in Section 3.2 of
   [RFC4731].

   C: a SEARCH RETURN (ALL) MODSEQ 1234
   S: * ESEARCH (TAG "a") ALL 1:3,5 MODSEQ 1236
   S: a OK Extended SEARCH completed

                                Example 19

   If a client specifies a MODSEQ criterion in an extended SORT (or
   extended UID SORT) [RFC5267] command and the server returns a non-
   empty SORT result, the server MUST return the ESEARCH response
   containing the MODSEQ result option defined in Section 3.2 of
   [RFC4731].

   C: a SORT RETURN (ALL) (DATE) UTF-8 MODSEQ 1234
   S: * ESEARCH (TAG "a") ALL 5,3,2,1 MODSEQ 1236
   S: a OK Extended SORT completed

                                Example 20

Top      ToC       Page 23 
3.1.11.  Additional Quality-of-Implementation Issues

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

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

   This will prevent spurious client synchronization requests.

   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 when one or more clients
   set and unset (or unset and set) the flag in another session.

3.1.12.  CONDSTORE 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 (without "+" or "-")
   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.

Top      ToC       Page 24 
   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 the \Deleted flag, as it hasn't
       included it in the +FLAGS.SILENT operation and

   b.  the state of the flag $Processed hasn't changed (the server can
       determine this by comparing the 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 the $Processed flag, update the mod-
   sequence for the message 101 once again, and send an untagged FETCH
   response with a new mod-sequence and flags:

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

   See also Section 3.1.11 for additional quality-of-implementation
   issues.



(page 24 continued on part 2)

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