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rfc:rfc5162

Network Working Group A. Melnikov Request for Comments: 5162 D. Cridland Category: Standards Track Isode Ltd

                                                             C. Wilson
                                                                 Nokia
                                                            March 2008
        IMAP4 Extensions for Quick Mailbox Resynchronization

Status of This Memo

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

Abstract

 This document defines an IMAP4 extension, which gives an IMAP client
 the ability to quickly resynchronize any previously opened mailbox as
 part of the SELECT command, without the need for server-side state or
 additional client round-trips.  This extension also introduces a new
 response that allows for a more compact representation of a list of
 expunged messages (and always includes the Unique Identifiers (UIDs)
 expunged).

Melnikov, et al. Standards Track [Page 1] RFC 5162 IMAP Quick Mailbox Resync March 2008

Table of Contents

 1.  Introduction and Overview  . . . . . . . . . . . . . . . . . .  2
 2.  Requirements Notation  . . . . . . . . . . . . . . . . . . . .  4
 3.  IMAP Protocol Changes  . . . . . . . . . . . . . . . . . . . .  4
   3.1.  QRESYNC Parameter to SELECT/EXAMINE  . . . . . . . . . . .  4
   3.2.  VANISHED UID FETCH Modifier  . . . . . . . . . . . . . . .  8
   3.3.  EXPUNGE Command  . . . . . . . . . . . . . . . . . . . . . 10
   3.4.  CLOSE Command  . . . . . . . . . . . . . . . . . . . . . . 11
   3.5.  UID EXPUNGE Command  . . . . . . . . . . . . . . . . . . . 11
   3.6.  VANISHED Response  . . . . . . . . . . . . . . . . . . . . 12
   3.7.  CLOSED Response Code . . . . . . . . . . . . . . . . . . . 15
 4.  Server Implementation Considerations . . . . . . . . . . . . . 15
   4.1.  Server Implementations That Don't Store Extra State  . . . 15
   4.2.  Server Implementations Storing Minimal State . . . . . . . 16
   4.3.  Additional State Required on the Server  . . . . . . . . . 16
 5.  Updated Synchronization Sequence . . . . . . . . . . . . . . . 17
 6.  Formal Syntax  . . . . . . . . . . . . . . . . . . . . . . . . 19
 7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 20
 8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 21
 9.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 21
 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 21
   10.1. Normative References . . . . . . . . . . . . . . . . . . . 21
   10.2. Informative References . . . . . . . . . . . . . . . . . . 22

1. Introduction and Overview

 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.  This document defines an extension to [CONDSTORE]
 that allows a reconnecting client to perform full resynchronization,
 including discovery of expunged messages, in a single round-trip.
 This extension also introduces a new response, VANISHED, that allows
 for a more compact representation of a list of expunged messages.
 This extension can be useful for mobile clients that can experience
 frequent disconnects caused by environmental factors (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 (and hence the expense) generated by
 resynchronization.

Melnikov, et al. Standards Track [Page 2] RFC 5162 IMAP Quick Mailbox Resync March 2008

 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 quick resync IMAP extension is present if an IMAP4 server returns
 "QRESYNC" as one of the supported capabilities to the CAPABILITY
 command.
 Servers supporting this extension MUST implement and advertise
 support for the [ENABLE] IMAP extension.  Also, the presence of the
 "QRESYNC" capability implies support for the [CONDSTORE] IMAP
 extension even if the CONDSTORE capability isn't advertised.  A
 server compliant with this specification is REQUIREd to support
 "ENABLE QRESYNC" and "ENABLE QRESYNC CONDSTORE" (which are "CONDSTORE
 enabling commands", as defined in [CONDSTORE], and have identical
 results), but there is no requirement for a compliant server to
 support "ENABLE CONDSTORE" by itself.  The "ENABLE QRESYNC"/"ENABLE
 QRESYNC CONDSTORE" command also tells the server that it SHOULD start
 sending VANISHED responses (see Section 3.6) instead of EXPUNGE
 responses.  This change remains in effect until the connection is
 closed.
 For compatibility with clients that only support the [CONDSTORE] IMAP
 extension, servers SHOULD advertise CONDSTORE in the CAPABILITY
 response as well.
 A client making use of this extension MUST issue "ENABLE QRESYNC"
 once it is authenticated.  A server MUST respond with a tagged BAD
 response if the QRESYNC parameter to the SELECT/EXAMINE command or
 the VANISHED UID FETCH modifier is specified and the client hasn't
 issued "ENABLE QRESYNC" in the current connection.
 This document puts additional requirements on a server implementing
 the [CONDSTORE] extension.  Each mailbox that supports persistent
 storage of mod-sequences, i.e., for which the server has sent a
 HIGHESTMODSEQ untagged OK response code on a successful SELECT/
 EXAMINE, MUST increment the per-mailbox mod-sequence when one or more
 messages are expunged due to EXPUNGE, UID EXPUNGE or CLOSE; the
 server MUST associate the incremented mod-sequence with the UIDs of
 the expunged messages.
 A client that supports CONDSTORE but not this extension might
 resynchronize a mailbox and discover that its HIGHESTMODSEQ has
 increased from the value cached by the client.  If the increase is
 only due to messages having been expunged since the client last
 synchronized, the client is likely to send a FETCH ...  CHANGEDSINCE
 command that returns no data.  Thus, a client that supports CONDSTORE

Melnikov, et al. Standards Track [Page 3] RFC 5162 IMAP Quick Mailbox Resync March 2008

 but not this extension might incur a penalty of an unneeded round-
 trip when resynchronizing some mailboxes (those that have had
 messages expunged but no flag changes since the last
 synchronization).
 This extra round-trip is only incurred by clients that support
 CONDSTORE but not this extension, and only when a mailbox has had
 messages expunged but no flag changes to non-expunged messages.
 Since CONDSTORE is a relatively new extension, it is thought likely
 that clients that support it will also support this extension.

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 examples, "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.
 Understanding of the IMAP message sequence numbers and UIDs and the
 EXPUNGE response [RFC3501] is essential when reading this document.

3. IMAP Protocol Changes

3.1. QRESYNC Parameter to SELECT/EXAMINE

 The Quick Resynchronization parameter to SELECT/EXAMINE commands has
 four arguments:
 o  the last known UIDVALIDITY,
 o  the last known modification sequence,
 o  the optional set of known UIDs, and
 o  an optional parenthesized list of known sequence ranges and their
    corresponding UIDs.
 A server MUST respond with a tagged BAD response if the Quick
 Resynchronization parameter to SELECT/EXAMINE command is specified
 and the client hasn't issued "ENABLE QRESYNC" in the current
 connection.

Melnikov, et al. Standards Track [Page 4] RFC 5162 IMAP Quick Mailbox Resync March 2008

 Before opening the specified mailbox, the server verifies all
 arguments for syntactic validity.  If any parameter is not
 syntactically valid, the server returns the tagged BAD response, and
 the mailbox remains unselected.  Once the check is done, the server
 opens the mailbox as if no SELECT/EXAMINE parameters are specified
 (this is subject to processing of other parameters as defined in
 other extensions).  In particular this means that the server MUST
 send all untagged responses as specified in Sections 6.3.1 and 6.3.2
 of [RFC3501].
 After that, the server checks the UIDVALIDITY value provided by the
 client.  If the provided UIDVALIDITY doesn't match the UIDVALIDITY
 for the mailbox being opened, then the server MUST ignore the
 remaining parameters and behave as if no dynamic message data
 changed.  The client can discover this situation by comparing the
 UIDVALIDITY value returned by the server.  This behavior allows the
 client not to synchronize the mailbox or decide on the best
 synchronization strategy.
 Example: Attempting to resynchronize INBOX, but the provided
          UIDVALIDITY parameter doesn't match the current UIDVALIDITY
          value.
 C: A02 SELECT INBOX (QRESYNC (67890007 20050715194045000
          41,43:211,214:541))
          S: * 464 EXISTS
          S: * 3 RECENT
          S: * OK [UIDVALIDITY 3857529045] UIDVALIDITY
          S: * OK [UIDNEXT 550] Predicted next UID
          S: * OK [HIGHESTMODSEQ 90060128194045007]
          S: * OK [UNSEEN 12] Message 12 is first unseen
          S: * FLAGS (\Answered \Flagged \Draft \Deleted \Seen)
          S: * OK [PERMANENTFLAGS (\Answered \Flagged \Draft
          \Deleted \Seen \*)] Permanent flags
          S: A02 OK [READ-WRITE] Sorry, UIDVALIDITY mismatch
 Modification Sequence and UID Parameters:
 A server that doesn't support the persistent storage of mod-sequences
 for the mailbox MUST send the OK untagged response including the
 NOMODSEQ response code with every successful SELECT or EXAMINE
 command, as described in [CONDSTORE].  Such a server doesn't need to
 remember mod-sequences for expunged messages in the mailbox.  It MUST
 ignore the remaining parameters and behave as if no dynamic message
 data changed.
 If the provided UIDVALIDITY matches that of the selected mailbox, the
 server then checks the last known modification sequence.

Melnikov, et al. Standards Track [Page 5] RFC 5162 IMAP Quick Mailbox Resync March 2008

 The server sends the client any pending flag changes (using FETCH
 responses that MUST contain UIDs) and expunges those that have
 occurred in this mailbox since the provided modification sequence.
 If the list of known UIDs was also provided, the server should only
 report flag changes and expunges for the specified messages.  If the
 client did not provide the list of UIDs, the server acts as if the
 client has specified "1:<maxuid>", where <maxuid> is the mailbox's
 UIDNEXT value minus 1.  If the mailbox is empty and never had any
 messages in it, then lack of the list of UIDs is interpreted as an
 empty set of UIDs.
 Thus, the client can process just these pending events and need not
 perform a full resynchronization.  Without the message sequence
 number matching information, the result of this step is semantically
 equivalent to the client issuing:
 tag1 UID FETCH "known-uids" (FLAGS) (CHANGEDSINCE
 "mod-sequence-value" VANISHED)
 Example:
    C: A03 SELECT INBOX (QRESYNC (67890007
       90060115194045000 41,43:211,214:541))
    S: * OK [CLOSED]
    S: * 314 EXISTS
    S: * 15 RECENT
    S: * OK [UIDVALIDITY 67890007] UIDVALIDITY
    S: * OK [UIDNEXT 567] Predicted next UID
    S: * OK [HIGHESTMODSEQ 90060115205545359]
    S: * OK [UNSEEN 7] There are some unseen messages in the mailbox
    S: * FLAGS (\Answered \Flagged \Draft \Deleted \Seen)
    S: * OK [PERMANENTFLAGS (\Answered \Flagged \Draft
       \Deleted \Seen \*)] Permanent flags
    S: * VANISHED (EARLIER) 41,43:116,118,120:211,214:540
    S: * 49 FETCH (UID 117 FLAGS (\Seen \Answered) MODSEQ
       (90060115194045001))
    S: * 50 FETCH (UID 119 FLAGS (\Draft $MDNSent) MODSEQ
       (90060115194045308))
    S: ...
    S: * 100 FETCH (UID 541 FLAGS (\Seen $Forwarded) MODSEQ
       (90060115194045001))
    S: A03 OK [READ-WRITE] mailbox selected
 Message sequence match data:
 A client MAY provide a parenthesized list of a message sequence set
 and the corresponding UID sets.  Both MUST be provided in ascending
 order.  The server uses this data to restrict the range for which it
 provides expunged message information.

Melnikov, et al. Standards Track [Page 6] RFC 5162 IMAP Quick Mailbox Resync March 2008

 Conceptually, the client provides a small sample of sequence numbers
 for which it knows the corresponding UIDs.  The server then compares
 each sequence number and UID pair the client provides with the
 current state of the mailbox.  If a pair matches, then the client
 knows of any expunges up to, and including, the message, and thus
 will not include that range in the VANISHED response, even if the
 "mod-sequence-value" provided by the client is too old for the server
 to have data of when those messages were expunged.
 Thus, if the Nth message number in the first set in the list is 4,
 and the Nth UID in the second set in the list is 8, and the mailbox's
 fourth message has UID 8, then no UIDs equal to or less than 8 are
 present in the VANISHED response.  If the (N+1)th message number is
 12, and the (N+1)th UID is 24, and the (N+1)th message in the mailbox
 has UID 25, then the lowest UID included in the VANISHED response
 would be 9.
 In the following two examples, the server is unable to remember
 expunges at all, and only UIDs with messages divisible by three are
 present in the mailbox.  In the first example, the client does not
 use the fourth parameter; in the second, it provides it.  This
 example is somewhat extreme, but shows that judicious usage of the
 sequence match data can save a substantial amount of bandwidth.
 Example:
    C: A04 SELECT INBOX (QRESYNC (67890007
       90060115194045000 1:29997))
    S: * 10003 EXISTS
    S: * 5 RECENT
    S: * OK [UIDVALIDITY 67890007] UIDVALIDITY
    S: * OK [UIDNEXT 30013] Predicted next UID
    S: * OK [HIGHESTMODSEQ 90060115205545359]
    S: * OK [UNSEEN 7] There are some unseen messages in the mailbox
    S: * FLAGS (\Answered \Flagged \Draft \Deleted \Seen)
    S: * OK [PERMANENTFLAGS (\Answered \Flagged \Draft
       \Deleted \Seen \*)] Permanent flags
    S: * VANISHED (EARLIER) 1:2,4:5,7:8,10:11,13:14 [...]
       29998:29999,30001:30002,30004:30005,30007:30008
    S: * 9889 FETCH (UID 29667 FLAGS (\Seen \Answered) MODSEQ
       (90060115194045027))
    S: * 9890 FETCH (UID 29670 FLAGS (\Draft $MDNSent) MODSEQ
       (90060115194045028))
    S: ...
    S: * 9999 FETCH (UID 29997 FLAGS (\Seen $Forwarded) MODSEQ
       (90060115194045031))
    S: A04 OK [READ-WRITE] mailbox selected

Melnikov, et al. Standards Track [Page 7] RFC 5162 IMAP Quick Mailbox Resync March 2008

 Example:
    C: B04 SELECT INBOX (QRESYNC (67890007
       90060115194045000 1:29997 (5000,7500,9000,9990:9999 15000,
       22500,27000,29970,29973,29976,29979,29982,29985,29988,29991,
       29994,29997)))
    S: * 10003 EXISTS
    S: * 5 RECENT
    S: * OK [UIDVALIDITY 67890007] UIDVALIDITY
    S: * OK [UIDNEXT 30013] Predicted next UID
    S: * OK [HIGHESTMODSEQ 90060115205545359]
    S: * OK [UNSEEN 7] There are some unseen messages in the mailbox
    S: * FLAGS (\Answered \Flagged \Draft \Deleted \Seen)
    S: * OK [PERMANENTFLAGS (\Answered \Flagged \Draft
       \Deleted \Seen \*)] Permanent flags
    S: * VANISHED (EARLIER) 29998:29999,30001:30002,30004:30005,30007:
       30008
    S: * 9889 FETCH (UID 29667 FLAGS (\Seen \Answered) MODSEQ
       (90060115194045027))
    S: * 9890 FETCH (UID 29670 FLAGS (\Draft $MDNSent) MODSEQ
       (90060115194045028))
    S: ...
    S: * 9999 FETCH (UID 29997 FLAGS (\Seen $Forwarded) MODSEQ
       (90060115194045031))
    S: B04 OK [READ-WRITE] mailbox selected

3.2. VANISHED UID FETCH Modifier

 [IMAPABNF] has extended the syntax of the FETCH and UID FETCH
 commands to include an optional FETCH modifier.  This document
 defines a new UID FETCH modifier: VANISHED.
 Note, that the VANISHED UID FETCH modifier is NOT allowed with a
 FETCH command.  The server MUST return a tagged BAD response if this
 response is specified as a modifier to the FETCH command.
 A server MUST respond with a tagged BAD response if the VANISHED UID
 FETCH modifier is specified and the client hasn't issued "ENABLE
 QRESYNC" in the current connection.
 The VANISHED UID FETCH modifier MUST only be specified together with
 the CHANGEDSINCE UID FETCH modifier.
 The VANISHED UID FETCH modifier instructs the server to report those
 messages from the UID set parameter that have been expunged and whose
 associated mod-sequence is larger than the specified mod-sequence.
 That is, the client requests to be informed of messages from the
 specified set that were expunged since the specified mod-sequence.
 Note that the mod-sequence(s) associated with these messages were

Melnikov, et al. Standards Track [Page 8] RFC 5162 IMAP Quick Mailbox Resync March 2008

 updated when the messages were expunged (as described above).  The
 expunged messages are reported using the VANISHED response as
 described in Section 3.6, which MUST contain the EARLIER tag.  Any
 VANISHED (EARLIER) responses MUST be returned before any FETCH
 responses, as otherwise the client might get confused about how
 message numbers map to UIDs.
 Note: A server that receives a mod-sequence smaller than <minmodseq>,
 where <minmodseq> is the value of the smallest expunged mod-sequence
 it remembers minus one, MUST behave as if it was requested to report
 all expunged messages from the provided UID set parameter.
 Example 1: Without the VANISHED UID FETCH modifier, a CONDSTORE-aware
 client [CONDSTORE] needs to issue separate commands to learn of flag
 changes and expunged messages since the last synchronization:
 C: s100 UID FETCH 300:500 (FLAGS) (CHANGEDSINCE 12345)
 S: * 1 FETCH (UID 404 MODSEQ (65402) FLAGS (\Seen))
 S: * 2 FETCH (UID 406 MODSEQ (75403) FLAGS (\Deleted))
 S: * 4 FETCH (UID 408 MODSEQ (29738) FLAGS ($NoJunk
     $AutoJunk $MDNSent))
 S: s100 OK FETCH completed
 C: s101 UID SEARCH 300:500
 S: * SEARCH 404 406 407 408 410 412
 S: s101 OK search completed
 Where 300 and 500 are the lowest and highest UIDs from client's
 cache.  The second SEARCH response tells the client that the messages
 with UIDs 407, 410, and 412 are still present, but their flags
 haven't changed since the specified modification sequence.
 Using the VANISHED UID FETCH modifier, it is sufficient to issue only
 a single command:
 C: s100 UID FETCH 300:500 (FLAGS) (CHANGEDSINCE 12345
     VANISHED)
 S: * VANISHED (EARLIER) 300:310,405,411
 S: * 1 FETCH (UID 404 MODSEQ (65402) FLAGS (\Seen))
 S: * 2 FETCH (UID 406 MODSEQ (75403) FLAGS (\Deleted))
 S: * 4 FETCH (UID 408 MODSEQ (29738) FLAGS ($NoJunk
     $AutoJunk $MDNSent))
 S: s100 OK FETCH completed

Melnikov, et al. Standards Track [Page 9] RFC 5162 IMAP Quick Mailbox Resync March 2008

3.3. EXPUNGE Command

 Arguments: none
 Responses: untagged responses: EXPUNGE or VANISHED
 Result: OK - expunge completed
         NO - expunge failure: can't expunge (e.g., permission denied)
         BAD - command unknown or arguments invalid
 This section updates the definition of the EXPUNGE command described
 in Section 6.4.3 of [RFC3501].
 The EXPUNGE command permanently removes all messages that have the
 \Deleted flag set from the currently selected mailbox.  Before
 returning an OK to the client, those messages that are removed are
 reported using a VANISHED response or EXPUNGE responses.
 If the server is capable of storing modification sequences for the
 selected mailbox, it MUST increment the per-mailbox mod-sequence if
 at least one message was permanently removed due to the execution of
 the EXPUNGE command.  For each permanently removed message, the
 server MUST remember the incremented mod-sequence and corresponding
 UID.  If at least one message got expunged, the server MUST send the
 updated per-mailbox modification sequence using the HIGHESTMODSEQ
 response code (defined in [CONDSTORE]) in the tagged OK response.
    Example:    C: A202 EXPUNGE
                S: * 3 EXPUNGE
                S: * 3 EXPUNGE
                S: * 5 EXPUNGE
                S: * 8 EXPUNGE
                S: A202 OK [HIGHESTMODSEQ 20010715194045319] expunged
 Note: In this example, messages 3, 4, 7, and 11 had the \Deleted flag
 set.  The first "* 3 EXPUNGE" reports message # 3 as expunged.  The
 second "* 3 EXPUNGE" reports message # 4 as expunged (the message
 number got decremented due to the previous EXPUNGE response).  See
 the description of the EXPUNGE response in [RFC3501] for further
 explanation.
 Note that if the server chooses to always send VANISHED responses
 instead of EXPUNGE responses, the previous example might look like
 this:
    Example:    C: B202 EXPUNGE
                S: * VANISHED 405,407,410,425
                S: B202 OK [HIGHESTMODSEQ 20010715194045319] expunged

Melnikov, et al. Standards Track [Page 10] RFC 5162 IMAP Quick Mailbox Resync March 2008

 Here messages with message numbers 3, 4, 7, and 11 have respective
 UIDs 405, 407, 410, and 425.

3.4. CLOSE Command

 Arguments: none
 Responses: no specific responses for this command
 Result: OK - close completed, now in authenticated state
         BAD - command unknown or arguments invalid
 This section updates the definition of the CLOSE command described in
 Section 6.4.2 of [RFC3501].
 The CLOSE command permanently removes all messages that have the
 \Deleted flag set from the currently selected mailbox, and returns to
 the authenticated state from the selected state.  No untagged EXPUNGE
 (or VANISHED) responses are sent.
 If the server is capable of storing modification sequences for the
 selected mailbox, it MUST increment the per-mailbox mod-sequence if
 at least one message was permanently removed due to the execution of
 the CLOSE command.  For each permanently removed message, the server
 MUST remember the incremented mod-sequence and corresponding UID.  If
 at least one message got expunged, the server MUST send the updated
 per-mailbox modification sequence using the HIGHESTMODSEQ response
 code (defined in [CONDSTORE]) in the tagged OK response.
    Example:    C: A202 CLOSE
                S: A202 OK [HIGHESTMODSEQ 20010715194045319] done

3.5. UID EXPUNGE Command

 Arguments: message set
 Responses: untagged responses: EXPUNGE or VANISHED
 Result: OK - expunge completed
         NO - expunge failure: can't expunge (e.g., permission denied)
         BAD - command unknown or arguments invalid
 This section updates the definition of the UID EXPUNGE command
 described in Section 2.1 of [UIDPLUS].  Servers that implement both
 [UIDPLUS] and QRESYNC extensions must implement UID EXPUNGE as
 described in this section.

Melnikov, et al. Standards Track [Page 11] RFC 5162 IMAP Quick Mailbox Resync March 2008

 The UID EXPUNGE command permanently removes from the currently
 selected mailbox all messages that both have the \Deleted flag set
 and have a UID that is included in the specified message set.  If a
 message either does not have the \Deleted flag set or has a UID that
 is not included in the specified message set, it is not affected.
 This command is particularly useful for disconnected mode clients.
 By using UID EXPUNGE instead of EXPUNGE when resynchronizing with the
 server, the client can avoid inadvertently removing any messages that
 have been marked as \Deleted by other clients between the time that
 the client was last connected and the time the client resynchronizes.
 Before returning an OK to the client, those messages that are removed
 are reported using a VANISHED response or EXPUNGE responses.
 If the server is capable of storing modification sequences for the
 selected mailbox, it MUST increment the per-mailbox mod-sequence if
 at least one message was permanently removed due to the execution of
 the UID EXPUNGE command.  For each permanently removed message, the
 server MUST remember the incremented mod-sequence and corresponding
 UID.  If at least one message got expunged, the server MUST send the
 updated per-mailbox modification sequence using the HIGHESTMODSEQ
 response code (defined in [CONDSTORE]) in the tagged OK response.
 Example:    C: . UID EXPUNGE 3000:3002
             S: * 3 EXPUNGE
             S: * 3 EXPUNGE
             S: * 3 EXPUNGE
             S: . OK [HIGHESTMODSEQ 20010715194045319] Ok
 Note: In this example, at least messages with message numbers 3, 4,
 and 5 (UIDs 3000 to 3002) had the \Deleted flag set.  The first "* 3
 EXPUNGE" reports message # 3 as expunged.  The second "* 3 EXPUNGE"
 reports message # 4 as expunged (the message number got decremented
 due to the previous EXPUNGE response).  See the description of the
 EXPUNGE response in [RFC3501] for further explanation.

3.6. VANISHED Response

 Contents:  an optional EARLIER tag
 list of UIDs
 The VANISHED response reports that the specified UIDs have been
 permanently removed from the mailbox.  This response is similar to
 the EXPUNGE response [RFC3501]; however, it can return information
 about multiple messages, and it returns UIDs instead of message

Melnikov, et al. Standards Track [Page 12] RFC 5162 IMAP Quick Mailbox Resync March 2008

 numbers.  The first benefit saves bandwidth, while the second is more
 convenient for clients that only use UIDs to access the IMAP server.
 The VANISHED response has the same restrictions on when it can be
 sent as does the EXPUNGE response (see below).
 The VANISHED response has two forms.  The first form contains the
 EARLIER tag, which signifies that the response was caused by a UID
 FETCH (VANISHED) or a SELECT/EXAMINE (QRESYNC) command.  This
 response is sent if the UID set parameter to the UID FETCH (VANISHED)
 command includes UIDs of messages that are no longer in the mailbox.
 When the client sees a VANISHED EARLIER response, it MUST NOT
 decrement message sequence numbers for each successive message in the
 mailbox.
 The second form doesn't contain the EARLIER tag and is described
 below.  Once a client has issued "ENABLE QRESYNC", the server SHOULD
 use the VANISHED response without the EARLIER tag instead of the
 EXPUNGE response.  The server SHOULD continue using VANISHED in lieu
 of EXPUNGE for the duration of the connection.  In particular, this
 affects the EXPUNGE [RFC3501] and UID EXPUNGE [UIDPLUS] commands, as
 well as messages expunged in other connections.  Such a VANISHED
 response MUST NOT contain the EARLIER tag.
 A VANISHED response sent because of an EXPUNGE or UID EXPUNGE command
 or because messages were expunged in other connections (i.e., the
 VANISHED response without the EARLIER tag) also decrements the number
 of messages in the mailbox; it is not necessary for the server to
 send an EXISTS response with the new value.  It also decrements
 message sequence numbers for each successive message in the mailbox
 (see the example at the end of this section).  Note that a VANISHED
 response caused by EXPUNGE, UID EXPUNGE, or messages expunged in
 other connections SHOULD only contain UIDs for messages expunged
 since the last VANISHED/EXPUNGE response sent for the currently
 opened mailbox or since the mailbox was opened.  That is, servers
 SHOULD NOT send UIDs for previously expunged messages, unless
 explicitly requested to do so by the UID FETCH (VANISHED) command.
 Note that client implementors must take care to properly decrement
 the number of messages in the mailbox even if a server violates this
 last SHOULD or repeats the same UID multiple times in the returned
 UID set.  In general, this means that a client using this extension
 should either avoid using message numbers entirely, or have a
 complete mapping of UIDs to message sequence numbers for the selected
 mailbox.

Melnikov, et al. Standards Track [Page 13] RFC 5162 IMAP Quick Mailbox Resync March 2008

 Because clients handle the two different forms of the VANISHED
 response differently, servers MUST NOT report UIDs resulting from a
 UID FETCH (VANISHED) or a SELECT/EXAMINE (QRESYNC) in the same
 VANISHED response as UIDs of messages expunged now (i.e., messages
 expunged in other connections).  Instead, the server MUST send
 separate VANISHED responses: one with the EARLIER tag and one
 without.
 A VANISHED response MUST NOT be sent when no command is in progress,
 nor while responding to a FETCH, STORE, or SEARCH command.  This rule
 is necessary to prevent a loss of synchronization of message sequence
 numbers between client and server.  A command is not "in progress"
 until the complete command has been received; in particular, a
 command is not "in progress" during the negotiation of command
 continuation.
 Note: UID FETCH, UID STORE, and UID SEARCH are different commands
 from FETCH, STORE, and SEARCH.  A VANISHED response MAY be sent
 during a UID command.  However, the VANISHED response MUST NOT be
 sent during a UID SEARCH command that contains message numbers in the
 search criteria.
 The update from the VANISHED response MUST be recorded by the client.
 Example: Let's assume that there is the following mapping between
 message numbers and UIDs in the currently selected mailbox (here "X"
 marks messages with the \Deleted flag set, and "x" represents UIDs
 which are not relevant for the example):
 Message numbers:   1    2    3    4    5  6   7  8  9 10  11
 UIDs:              x  504  505  507  508  x 510  x  x  x 625
 \Deleted messages:           X    X           X            X
 In the presence of the extension defined in this document:
 C: A202 EXPUNGE
 S: * VANISHED 505,507,510,625
 S: A202 OK EXPUNGE completed
 Without the QRESYNC extension, the same example might look like:
 C: A202 EXPUNGE
 S: * 3 EXPUNGE
 S: * 3 EXPUNGE
 S: * 5 EXPUNGE
 S: * 8 EXPUNGE
 S: A202 OK EXPUNGE completed

Melnikov, et al. Standards Track [Page 14] RFC 5162 IMAP Quick Mailbox Resync March 2008

 (Continuing previous example) If subsequently messages with UIDs 504
 and 508 got marked as \Deleted:
 C: A210 EXPUNGE
 S: * VANISHED 504,508
 S: A210 OK EXPUNGE completed
 i.e., the last VANISHED response only contains UIDs of messages
 expunged since the previous VANISHED response.

3.7. CLOSED Response Code

 The CLOSED response code has no parameters.  A server implementing
 the extension defined in this document MUST return the CLOSED
 response code when the currently selected mailbox is closed
 implicitly using the SELECT/EXAMINE command on another mailbox.  The
 CLOSED response code serves as a boundary between responses for the
 previously opened mailbox (which was closed) and the newly selected
 mailbox: all responses before the CLOSED response code relate to the
 mailbox that was closed, and all subsequent responses relate to the
 newly opened mailbox.
 There is no need to return the CLOSED response code on completion of
 the CLOSE or the UNSELECT [UNSELECT] command (or similar) whose
 purpose is to close the currently selected mailbox without opening a
 new one.

4. Server Implementation Considerations

 This section describes a minimalist implementation, a moderate
 implementation, and an example of a full implementation.

4.1. Server Implementations That Don't Store Extra State

 Strictly speaking, a server implementation that doesn't remember mod-
 sequences associated with expunged messages can be considered
 compliant with this specification.  Such implementations return all
 expunged messages specified in the UID set of the UID FETCH
 (VANISHED) command every time, without paying attention to the
 specified CHANGEDSINCE mod-sequence.  Such implementations are
 discouraged, as they can end up returning VANISHED responses that are
 bigger than the result of a UID SEARCH command for the same UID set.
 Clients that use the message sequence match data can reduce the scope
 of this VANISHED response substantially in the typical case where
 expunges have not happened, or happen only toward the end of the
 mailbox.

Melnikov, et al. Standards Track [Page 15] RFC 5162 IMAP Quick Mailbox Resync March 2008

4.2. Server Implementations Storing Minimal State

 A server that stores the HIGHESTMODSEQ value at the time of the last
 EXPUNGE can omit the VANISHED response when a client provides a
 MODSEQ value that is equal to, or higher than, the current value of
 this datum, that is, when there have been no EXPUNGEs.
 A client providing message sequence match data can reduce the scope
 as above.  In the case where there have been no expunges, the server
 can ignore this data.

4.3. Additional State Required on the Server

 When compared to the [CONDSTORE] extension, this extension requires
 servers to store additional state associated with expunged messages.
 Note that implementations are not required to store this state in
 persistent storage; however, use of persistent storage is advisable.
 One possible way to correctly implement the extension described in
 this document is to store a queue of <UID set, mod-sequence> pairs.
 <UID set> can be represented as a sequence of <min UID, max UID>
 pairs.
 When messages are expunged, one or more entries are added to the
 queue tail.
 When the server receives a request to return messages expunged since
 a given mod-sequence, it will search the queue from the tail (i.e.,
 going from the highest expunged mod-sequence to the lowest) until it
 sees the first record with a mod-sequence less than or equal to the
 given mod-sequence or it reaches the head of the queue.
 Note that indefinitely storing information about expunged messages
 can cause storage and related problems for an implementation.  In the
 worst case, this could result in almost 64Gb of storage for each IMAP
 mailbox.  For example, consider an implementation that stores <min
 UID, max UID, mod-sequence> triples for each range of messages
 expunged at the same time.  Each triple requires 16 octets: 4 octets
 for each of the two UIDs, and 8 octets for the mod-sequence.  Assume
 that there is a mailbox containing a single message with a UID of
 2**32-1 (the maximum possible UID value), where messages had
 previously existed with UIDs starting at 1, and have been expunged
 one at a time.  For this mailbox alone, storage is required for the
 triples <1, 1, modseq1>, <2, 2, modseq2>, ..., <2**32-2, 2**32-2,
 modseq4294967294>.

Melnikov, et al. Standards Track [Page 16] RFC 5162 IMAP Quick Mailbox Resync March 2008

 Hence, implementations are encouraged to adopt strategies to protect
 against such storage problems, such as limiting the size of the queue
 used to store mod-sequences for expunged messages and "expiring"
 older records when this limit is reached.  When the selected
 implementation-specific queue limit is reached, the oldest record(s)
 are deleted from the queue (note that such records are located at the
 queue head).  For all such "expired" records, the server needs to
 store a single mod-sequence, which is the highest mod-sequence for
 all "expired" expunged messages.
 Note that if the client provides the message sequence match data,
 this can heavily reduce the data cost of sending a complete set of
 missing UIDs; thus, reducing the problems for clients if a server is
 unable to persist much of this queue.  If the queue contains data
 back to the requested mod-sequence, this data can be ignored.
 Also, note that if the UIDVALIDITY of the mailbox changes or if the
 mailbox is deleted, then any state associated with expunged messages
 doesn't need to be preserved and SHOULD be deleted.

5. Updated Synchronization Sequence

 This section updates the description of optimized synchronization in
 Section 6.1 of the [IMAP-DISC].
 An advanced disconnected mail client should use the QRESYNC and
 [CONDSTORE] extensions when they are supported by the server.  The
 client uses the value from the HIGHESTMODSEQ OK response code
 received on mailbox opening to determine if it needs to
 resynchronize.  Once the synchronization is complete, it MUST cache
 the received value (unless the mailbox UIDVALIDITY value has changed;
 see below).  The client MUST update its copy of the HIGHESTMODSEQ
 value whenever the server sends a subsequent HIGHESTMODSEQ OK
 response code.
 After completing a full synchronization, the client MUST also take
 note of any unsolicited MODSEQ FETCH data items received from the
 server.  Whenever the client receives a tagged response to a command,
 it calculates the highest value among all MODSEQ FETCH data items
 received since the last tagged response.  If this value is bigger
 than the client's copy of the HIGHESTMODSEQ value, then the client
 MUST use this value as its new HIGHESTMODSEQ value.
 Note: It is not safe to update the client's copy of the HIGHESTMODSEQ
 value with a MODSEQ FETCH data item value as soon as it is received
 because servers are not required to send MODSEQ FETCH data items in
 increasing modseqence order.  This can lead to the client missing
 some changes in case of connectivity loss.

Melnikov, et al. Standards Track [Page 17] RFC 5162 IMAP Quick Mailbox Resync March 2008

 When opening the mailbox for synchronization, the client uses the
 QRESYNC parameter to the SELECT/EXAMINE command.  The QRESYNC
 parameter is followed by the UIDVALIDITY and mailbox HIGHESTMODSEQ
 values, as known to the client.  It can be optionally followed by the
 set of UIDs, for example, if the client is only interested in partial
 synchronization of the mailbox.  The client may also transmit a list
 containing its knowledge of message numbers.
 If the SELECT/EXAMINE command is successful, the client compares
 UIDVALIDITY as described in step d)1) in Section 3 of the
 [IMAP-DISC].  If the cached UIDVALIDITY value matches the one
 returned by the server and the server also returns the HIGHESTMODSEQ
 response code, then the server reports expunged messages and returns
 flag changes for all messages specified by the client in the UID set
 parameter (or for all messages in the mailbox, if the client omitted
 the UID set parameter).  At this point, the client is synchronized,
 except for maybe the new messages.
 If upon a successful SELECT/EXAMINE (QRESYNC) command the client
 receives a NOMODSEQ OK untagged response (instead of the
 HIGHESTMODSEQ response code), it MUST remove the last known
 HIGHESTMODSEQ value from its cache and follow the more general
 instructions in Section 3 of the [IMAP-DISC].
 At this point, the client is in sync with the server regarding old
 messages.  This client can now fetch information about new messages
 (if requested by the user).
 Step d) ("Server-to-client synchronization") in Section 4 of the
 [IMAP-DISC] in the presence of the QRESYNC & CONDSTORE extensions is
 amended as follows:
 d) "Server-to-client synchronization" -- for each mailbox that
    requires synchronization, do the following:
 1a) Check the mailbox UIDVALIDITY (see Section 4.1 of the [IMAP-DISC]
     for more details) after issuing SELECT/EXAMINE (QRESYNC) command.
     If the UIDVALIDITY value returned by the server differs, the
     client MUST
  • empty the local cache of that mailbox;
  • "forget" the cached HIGHESTMODSEQ value for the mailbox;

Melnikov, et al. Standards Track [Page 18] RFC 5162 IMAP Quick Mailbox Resync March 2008

  • remove any pending "actions" which refer to UIDs in that

mailbox. Note, this doesn't affect actions performed on

         client generated fake UIDs (see Section 5 of the
         [IMAP-DISC]);
 2)  Fetch the current "descriptors";
     I) Discover new messages.
 3)  Fetch the bodies of any "interesting" messages that the client
     doesn't already have.
 Example: The UIDVALIDITY value is the same, but the HIGHESTMODSEQ
          value has changed on the server while the client was
          offline:
  C: A142 SELECT INBOX (QRESYNC (3857529045 20010715194032001 1:198))
  S: * 172 EXISTS
  S: * 1 RECENT
  S: * OK [UNSEEN 12] Message 12 is first unseen
  S: * OK [UIDVALIDITY 3857529045] UIDs valid
  S: * OK [UIDNEXT 201] Predicted next UID
  S: * FLAGS (\Answered \Flagged \Deleted \Seen \Draft)
  S: * OK [PERMANENTFLAGS (\Deleted \Seen \*)] Limited
  S: * OK [HIGHESTMODSEQ 20010715194045007]
  S: * VANISHED (EARLIER) 1:5,7:8,10:15
  S: * 2 FETCH (UID 6 MODSEQ (20010715205008000)
      FLAGS (\Deleted))
  S: * 5 FETCH (UID 9 MODSEQ (20010715195517000)
      FLAGS ($NoJunk $AutoJunk $MDNSent))
     ...
  S: A142 OK [READ-WRITE] SELECT completed

6. Formal Syntax

 The following syntax specification uses the Augmented Backus-Naur
 Form (ABNF) notation as specified in [ABNF].
 Non-terminals referenced but not defined below are as defined by
 [RFC3501], [CONDSTORE], or [IMAPABNF].
 Except as noted otherwise, all alphabetic characters are case-
 insensitive.  The use of upper or lower case characters to define
 token strings is for editorial clarity only.  Implementations MUST
 accept these strings in a case-insensitive fashion.

Melnikov, et al. Standards Track [Page 19] RFC 5162 IMAP Quick Mailbox Resync March 2008

 capability          =/ "QRESYNC"
 select-param        =  "QRESYNC" SP "(" uidvalidity SP
                     mod-sequence-value [SP known-uids]
                     [SP seq-match-data] ")"
                     ;; conforms to the generic select-param
                     ;; syntax defined in [IMAPABNF]
 seq-match-data      =  "(" known-sequence-set SP known-uid-set ")"
 uidvalidity         =  nz-number
 known-uids          =  sequence-set
                     ;; sequence of UIDs, "*" is not allowed
 known-sequence-set  =  sequence-set
                     ;; set of message numbers corresponding to
                     ;; the UIDs in known-uid-set, in ascending order.
                     ;; * is not allowed.
 known-uid-set       =  sequence-set
                     ;; set of UIDs corresponding to the messages in
                     ;; known-sequence-set, in ascending order.
                     ;; * is not allowed.
 message-data        =/ expunged-resp
 expunged-resp       =  "VANISHED" [SP "(EARLIER)"] SP known-uids
 rexpunges-fetch-mod =  "VANISHED"
                     ;; VANISHED UID FETCH modifier conforms
                     ;; to the fetch-modifier syntax
                     ;; defined in [IMAPABNF].  It is only
                     ;; allowed in the UID FETCH command.
 resp-text-code      =/ "CLOSED"

7. Security Considerations

 As always, it is important to thoroughly test clients and servers
 implementing this extension, as it changes how the server reports
 expunged messages to the client.
 Security considerations relevant to [CONDSTORE] are relevant to this
 extension.
 This document doesn't raise any new security concerns not already
 raised by [CONDSTORE] or [RFC3501].

Melnikov, et al. Standards Track [Page 20] RFC 5162 IMAP Quick Mailbox Resync March 2008

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 QRESYNC IMAP capability.  IANA has added
 this capability to the registry.

9. Acknowledgments

 Thanks to Steve Hole, Cyrus Daboo, and Michael Wener for encouraging
 creation of this document.
 Valuable comments, both in agreement and in dissent, were received
 from Timo Sirainen, Michael Wener, Randall Gellens, Arnt Gulbrandsen,
 Chris Newman, Peter Coates, Mark Crispin, Elwyn Davies, Dan Karp,
 Eric Rescorla, and Mike Zraly.
 This document takes substantial text from [RFC3501] by Mark Crispin.

10. References

10.1. Normative References

 [ABNF]       Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.
 [CONDSTORE]  Melnikov, A. and S. Hole, "IMAP Extension for
              Conditional STORE Operation or Quick Flag Changes
              Resynchronization", RFC 4551, June 2006.
 [ENABLE]     Gulbrandsen, A., Ed. and A. Melnikov, Ed., "The IMAP
              ENABLE Extension", RFC 5161, March 2008.
 [IMAPABNF]   Melnikov, A. and C. Daboo, "Collected Extensions to
              IMAP4 ABNF", RFC 4466, April 2006.
 [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.
 [UIDPLUS]    Crispin, M., "Internet Message Access Protocol (IMAP) -
              UIDPLUS extension", RFC 4315, December 2005.

Melnikov, et al. Standards Track [Page 21] RFC 5162 IMAP Quick Mailbox Resync March 2008

10.2. Informative References

 [IMAP-DISC]  Melnikov, A., Ed., "Synchronization Operations For
              Disconnected Imap4 Clients", RFC 4549, June 2006.
 [UNSELECT]   Melnikov, A., "Internet Message Access Protocol (IMAP)
              UNSELECT command", RFC 3691, February 2004.

Authors' Addresses

 Alexey Melnikov
 Isode Ltd
 5 Castle Business Village
 36 Station Road
 Hampton, Middlesex  TW12 2BX
 UK
 EMail: Alexey.Melnikov@isode.com
 Dave Cridland
 Isode Ltd
 5 Castle Business Village
 36 Station Road
 Hampton, Middlesex  TW12 2BX
 UK
 EMail: dave.cridland@isode.com
 Corby Wilson
 Nokia
 5 Wayside Rd.
 Burlington, MA  01803
 USA
 EMail: corby@computer.org

Melnikov, et al. Standards Track [Page 22] RFC 5162 IMAP Quick Mailbox Resync March 2008

Full Copyright Statement

 Copyright (C) The IETF Trust (2008).
 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
 This document and the information contained herein are provided on an
 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
 THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
 OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
 THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Intellectual Property

 The IETF takes no position regarding the validity or scope of any
 Intellectual Property Rights or other rights that might be claimed to
 pertain to the implementation or use of the technology described in
 this document or the extent to which any license under such rights
 might or might not be available; nor does it represent that it has
 made any independent effort to identify any such rights.  Information
 on the procedures with respect to rights in RFC documents can be
 found in BCP 78 and BCP 79.
 Copies of IPR disclosures made to the IETF Secretariat and any
 assurances of licenses to be made available, or the result of an
 attempt made to obtain a general license or permission for the use of
 such proprietary rights by implementers or users of this
 specification can be obtained from the IETF on-line IPR repository at
 http://www.ietf.org/ipr.
 The IETF invites any interested party to bring to its attention any
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 rights that may cover technology that may be required to implement
 this standard.  Please address the information to the IETF at
 ietf-ipr@ietf.org.

Melnikov, et al. Standards Track [Page 23]

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