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Versions: (draft-melnikov-rfc4551-bis) 00 01 02 03 04 draft-ietf-qresync-rfc5162bis

Network Working Group                                        A. Melnikov
Internet-Draft                                                Isode Ltd.
Obsoletes: 4551 (if approved)                         September 11, 2013
Updates: 3501, 2683 (if approved)
Intended status: Standards Track
Expires: March 15, 2014


  IMAP Extension for Conditional STORE Operation or Quick Flag Changes
                           Resynchronization
                  draft-ietf-qresync-rfc4551bis-04.txt

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).

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."




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Copyright Notice

   Copyright (c) 2013 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.

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  . . . . . . . . . . . . . .   8
     3.3.  FETCH and UID FETCH Commands  . . . . . . . . . . . . . .  14
       3.3.1.  CHANGEDSINCE FETCH Modifier . . . . . . . . . . . . .  14
       3.3.2.  MODSEQ Message Data Item in FETCH Command . . . . . .  14
     3.4.  MODSEQ Search Criterion in SEARCH . . . . . . . . . . . .  17
     3.5.  Modified SEARCH Untagged Response . . . . . . . . . . . .  18
     3.6.  HIGHESTMODSEQ Status Data Items . . . . . . . . . . . . .  18
     3.7.  CONDSTORE Parameter to SELECT and EXAMINE . . . . . . . .  19
     3.8.  Interaction with IMAP SORT and THREAD extensions  . . . .  19
     3.9.  Interaction with IMAP ESORT and ESEARCH extensions  . . .  20
     3.10. Additional Quality-of-Implementation Issues . . . . . . .  20
   4.  Formal Syntax . . . . . . . . . . . . . . . . . . . . . . . .  20
   5.  Server Implementation Considerations  . . . . . . . . . . . .  23
   6.  Long Command Lines  . . . . . . . . . . . . . . . . . . . . .  24
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  24
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  25
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  25
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  25
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  25
     10.2.  Informative References . . . . . . . . . . . . . . . . .  26
   Appendix A.  Changes since RFC 4551 . . . . . . . . . . . . . . .  26
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  27




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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
   [RFC1305] client adjusting the time), the next generated value might
   be less than the previous one.

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

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

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

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

   This extension makes the following changes to the IMAP4 protocol:

   a.  adds UNCHANGEDSINCE STORE modifier.



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   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:

   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 requirement only applies to servers that also implement the
      ENABLE extension [RFC5161].)

   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



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   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 with a mailbox
   selected MUST cause the server to return HIGHESTMODSEQ
   (Section 3.1.1) for the mailbox, 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 [RFC2119].

   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 [RFC5234].

   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 [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 1 for the definition of a "CONDSTORE-aware client" and a
   "CONDSTORE enabling command".









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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 these 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.

   Note that this requirement applies whether or not a CONDSTORE
   enabling command was issued in the session.

   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.





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   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.  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.  Note
   that this requirement applies whether or not a CONDSTORE enabling
   command was issued in the session.

   A server that returned NOMODSEQ response code for a mailbox, which
   subsequently receives one of the following commands while the mailbox
   is 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

   MUST reject any such command with a tagged BAD response.

   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




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3.2.  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 any metadata item of the message 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 3.3.2 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 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 (12121130956))
   S: a103 OK Conditional Store completed

                                 Example 3









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   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

   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.

   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)



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   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) 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
   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).






















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   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
   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

   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.












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   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
   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 ($Processed)
   S: * 100 FETCH (MODSEQ (303181230852))
   S: * 101 FETCH (MODSEQ (303011130956) FLAGS ($Processed \Deleted
      \Answered))
   S: * 102 FETCH (MODSEQ (303181230852))
   ...
   S: * 150 FETCH (MODSEQ (303181230852))



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   S: a106 OK Conditional STORE completed


   The following example is based on the example from the Section 4.2.3
   of [RFC2180] 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) 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.



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   As specified in Section 1, once the client specified 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 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 3.10 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 [RFC4466]):

   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).

   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

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.




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   As specified in Section 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 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.

   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

   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.

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

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

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

       C: C180 NOOP



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       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

   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

                                Example 14



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   Server implementers should also see Section 3.10 for additional
   quality of implementation issues related to the FETCH command.

3.4.  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 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 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 [RFC4466].  Note that the leading
      "\" character that denotes a system flag has to be escaped as per
      Section 4.3 of [RFC3501], as the <entry-name> uses syntax for
      quoted strings.

   If 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 3.5.  Note that other IMAP extensions such as ESEARCH
   [RFC4731] can override this requirement (see Section 3.9 for more
   details.)













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   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 15

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

                                Example 16

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.

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



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                                Example 17

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.

   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

3.8.  Interaction with IMAP SORT and THREAD extensions

   MODSEQ Search Criterion (see Section 3.4) causes modifications to
   SORT [RFC5256] responses similar to modifications to SEARCH responses
   defined in Section 3.5:

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


   This document extends 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 3.9 for more details.)




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   THREAD commands which include a MODSEQ Search Criterion return THREAD
   responses as specified in [RFC5256].

3.9.  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

3.10.  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.

   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




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   The following syntax specification uses the Augmented Backus-Naur
   Form (ABNF) [RFC5234] notation.  Elements not defined here can be
   found in the formal syntax of the ABNF [RFC5234], IMAP [RFC3501], and
   IMAP ABNF extensions [RFC4466] 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 [RFC4466].
                             ;; Value 0 denotes that the mailbox doesn't
                             ;; support persistent mod-sequences
                             ;; as described in Section 3.1.2 [[Check the ref]]

      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 [RFC4466].

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

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

      fetch-mod-sequence  = "MODSEQ"

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

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

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



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      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 sequence-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 [RFC3501], e.g., for the flag
                             ;; \Seen the corresponding <entry-name> is
                             ;; "/flags/\\seen", and for the flag
                             ;; $MDNSent, the corresponding <entry-name>
                             ;; is "/flags/$mdnsent".

      entry-type-resp     = "priv" / "shared"
                             ;; metadata item type

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

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

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

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

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

      select-param        =/ condstore-param
                             ;; conforms to the generic "select-param"
                             ;; non-terminal syntax defined in [RFC4466].

      condstore-param     = "CONDSTORE"




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

      sort-data           = "SORT" [1*(SP nz-number) SP
                             search-sort-mod-seq]
                             ; Updates SORT response from RFC 5256

      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 [RFC3501].

      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)






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

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

   Imagine that another client has changed the state of a flag \Deleted
   on 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 3.10 for additional quality-of-implementation
   issues.

6.  Long Command Lines

   This document updates recommended line length limits specified in
   Section 3.2.1.5 of [RFC2683].  While the advice in the first
   paragraph of that section still applies ("use compact message/UID set
   representations"), the 1000 octet limit suggested in the second
   paragraph turned out to be quite problematic when the CONDSTORE
   extension is used.  The updated recommendation is as follows: a
   client should limit the length of the command lines it generates to
   approximately 8192 octets (including all quoted strings but not
   including literals).

7.  Security Considerations





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   It is believed that the Conditional STORE extension doesn't raise any
   new security concerns that are not already discussed in [RFC3501].
   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.

8.  IANA Considerations

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

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

   This document defines the CONDSTORE IMAP capability.  IANA has added
   it to the registry accordingly.

9.  Acknowledgements

   Thank you to Steve Hole for co-editing RFC 4551.

   Thank you to Dave Cridland for helping to convert the original text
   RFC to xml2rfc format.

   Some text was borrowed from "IMAP ANNOTATE Extension" [RFC5257] by
   Randall Gellens and Cyrus Daboo and from "ACAP -- Application
   Configuration Access Protocol" [RFC2244] 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 Pete Maclean,
   Cyrus Daboo, Larry Greenfield, Chris Newman, Harrie Hazewinkel, Arnt
   Gulbrandsen, Timo Sirainen, Mark Crispin, Ned Freed, Ken Murchison,
   and Dave Cridland.

10.  References

10.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3501]  Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
              4rev1", RFC 3501, March 2003.




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   [RFC4466]  Melnikov, A. and C. Daboo, "Collected Extensions to IMAP4
              ABNF", RFC 4466, April 2006.

   [RFC5161]  Gulbrandsen, A. and A. Melnikov, "The IMAP ENABLE
              Extension", RFC 5161, March 2008.

   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

   [RFC5256]  Crispin, M. and K. Murchison, "Internet Message Access
              Protocol - SORT and THREAD Extensions", RFC 5256, June
              2008.

10.2.  Informative References

   [RFC1305]  Mills, D., "Network Time Protocol (Version 3)
              Specification, Implementation", RFC 1305, March 1992.

   [RFC2180]  Gahrns, M., "IMAP4 Multi-Accessed Mailbox Practice", RFC
              2180, July 1997.

   [RFC2244]  Newman, C. and J. Myers, "ACAP -- Application
              Configuration Access Protocol", RFC 2244, November 1997.

   [RFC2683]  Leiba, B., "IMAP4 Implementation Recommendations", RFC
              2683, September 1999.

   [RFC4314]  Melnikov, A., "IMAP4 Access Control List (ACL) Extension",
              RFC 4314, December 2005.

   [RFC4731]  Melnikov, A. and D. Cridland, "IMAP4 Extension to SEARCH
              Command for Controlling What Kind of Information Is
              Returned", RFC 4731, November 2006.

   [RFC5257]  Daboo, C. and R. Gellens, "Internet Message Access
              Protocol - ANNOTATE Extension", RFC 5257, June 2008.

   [RFC5267]  Cridland, D. and C. King, "Contexts for IMAP4", RFC 5267,
              July 2008.

Appendix A.  Changes since RFC 4551

   Fixed errata 3401, 3506 and 3509.

   Updated references.

   Incorporated some text from RFC 5161 (no semantic change.)




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   Editorial corrections.

Author's Address

   Alexey Melnikov
   Isode Ltd.
   5 Castle Business Village
   36 Station Road
   Hampton, Middlesex  TW12 2BX
   UK

   Email: Alexey.Melnikov@isode.com







































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