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).

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.

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 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:

• the last known UIDVALIDITY,

• the last known modification sequence,

• the optional set of known UIDs, and

• 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.

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.

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.

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

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 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

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

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.

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 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.

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

(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.

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>.

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.

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;
       *   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.

   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].

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.

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

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