RFC 7162
IMAP Extensions: Quick Flag Changes Resynchronization (CONDSTORE) and Quick Mailbox Resynchronization (QRESYNC)
Pages: 52
Proposed Standard
→ Errata
Obsoletes: 4551 5162
Updates: 2683
Proposed Standard
→ Errata
Obsoletes: 4551 5162
Updates: 2683
Part 2 of 2 – Pages 24 to 52
3.2. QRESYNC Extension
All protocol changes and requirements specified for the CONDSTORE extension are also a part of the QRESYNC extension. The QRESYNC extension 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 would send 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, CLOSE, or MOVE [RFC6851]; the server MUST associate the incremented mod- sequence with the UIDs of the expunged messages. Additionally, if the server also supports the IMAP METADATA extension [RFC5464], it MUST increment the per-mailbox mod-sequence when SETMETADATA successfully changes an annotation on the corresponding mailbox. A server implementing QRESYNC MUST send untagged events to a client in a way that the client doesn't lose any changes in case of connectivity loss. In particular, this means that if the server
sends MODSEQ FETCH data items while EXPUNGE (or VANISHED) replies with lower mod-sequences being delayed, the server MUST send the HIGHESTMODSEQ response code with a lower value than the EXPUNGE's mod-sequence. See the example in Section 6.3.2.1. Impact on CONDSTORE-only Clients
A client that supports CONDSTORE but not QRESYNC 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 QRESYNC 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 QRESYNC 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 strongly encouraged that clients that support it also support QRESYNC.3.2.2. Advertising Support for QRESYNC
The quick resync IMAP extension is present if an IMAP4 server returns "QRESYNC" as one of the supported capabilities to the CAPABILITY command. For compatibility with clients that only support the CONDSTORE IMAP extension, servers SHOULD also advertise "CONDSTORE" in the CAPABILITY response.3.2.3. Use of ENABLE
Servers supporting QRESYNC MUST implement and advertise support for the ENABLE [RFC5161] 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", see Section 3.1, and have identical results). Note that the order of parameters is not significant, 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 MUST start sending VANISHED responses (see
Section 3.2.10) instead of EXPUNGE responses for all mailboxes for which the server doesn't return the NOMODSEQ response code. This change remains in effect until the connection is closed. A client making use of QRESYNC 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", or the server has not positively responded (in the current connection) to that command with the untagged ENABLED response containing QRESYNC.3.2.4. Additional Requirements on QRESYNC Servers
Once a CONDSTORE enabling command is issued by the client, the server MUST automatically include both UID and mod-sequence data in all subsequent untagged FETCH responses (until the connection is closed), whether they were caused by a regular STORE/UID STORE, a STORE/UID STORE with an UNCHANGEDSINCE modifier, a FETCH/UID FETCH that implicitly set the \Seen flag, or an external agent. Note that this rule doesn't affect untagged FETCH responses caused by a FETCH command that doesn't include UID and/or a MODSEQ FETCH data item (and doesn't implicitly set the \Seen flag) or UID FETCH without the MODSEQ FETCH data item.3.2.5. 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 the SELECT/EXAMINE command is specified and the client hasn't issued "ENABLE QRESYNC" in the current connection, or the server has not positively responded to that command with the untagged ENABLED response containing QRESYNC. 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 the 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] Highest mailbox mod-sequence 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 Remaining parameters are described in the following subsections.3.2.5.1. Modification Sequence and UID Parameters
A server that doesn't support the persistent storage of mod-sequences for the mailbox MUST send an OK untagged response including the NOMODSEQ response code with every successful SELECT or EXAMINE command (see Section 3.1.2.2). 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) In particular, this means that all requirements specified in Section 3.2.6 apply. Example: C: A03 SELECT INBOX (QRESYNC (67890007 90060115194045000 41:211,214:541)) S: * OK [CLOSED] S: * 100 EXISTS S: * 11 RECENT S: * OK [UIDVALIDITY 67890007] UIDVALIDITY S: * OK [UIDNEXT 600] Predicted next UID S: * OK [HIGHESTMODSEQ 90060115205545359] Highest mailbox mod-sequence 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: * 51 FETCH (UID 541 FLAGS (\Seen $Forwarded) MODSEQ (90060115194045001)) S: A03 OK [READ-WRITE] mailbox selected
In the above example, flag information for UID 42 is not returned, presumably because its flags haven't changed since the MODSEQ 90060115194045000.3.2.5.2. 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; thus, it 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 it 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: * 4 RECENT S: * OK [UIDVALIDITY 67890007] UIDVALIDITY S: * OK [UIDNEXT 30013] Predicted next UID S: * OK [HIGHESTMODSEQ 90060115205545359] Highest mailbox mod-sequence 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,[...],
29668:29669,29671:29996
S: * 1 FETCH (UID 3 FLAGS (\Seen \Answered $Important) MODSEQ
(90060115194045001))
S: ...
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: * 4 RECENT
S: * OK [UIDVALIDITY 67890007] UIDVALIDITY
S: * OK [UIDNEXT 30013] Predicted next UID
S: * OK [HIGHESTMODSEQ 90060115205545359] Highest mailbox mod-
sequence
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: * 1 FETCH (UID 3 FLAGS (\Seen \Answered $Important) MODSEQ
(90060115194045001))
S: ...
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
3.2.6. VANISHED UID FETCH Modifier
[RFC4466] 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. If the VANISHED UID FETCH modifier is used without the CHANGEDSINCE UID FETCH modifier, the server MUST respond with a tagged BAD response. 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 was updated when the messages were expunged (as described above). The expunged messages are reported using the VANISHED (EARLIER) response as described in Section 3.2.10.1. Any VANISHED (EARLIER) responses MUST be returned before any FETCH responses, 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 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 $MDNSent))
S: s100 OK FETCH completed
C: s101 UID SEARCH 300:500
S: * SEARCH 404 406 407 408 410 412
S: s101 OK search completed
Where 300 and 500 are the lowest and highest UIDs from the client's
cache. The second SEARCH response tells the client that the messages
with UIDs 407, 410, and 412 are still present, but their flags
haven't changed since the specified modification sequence.
Using the VANISHED UID FETCH modifier, it is sufficient to issue only
a single command:
C: s100 UID FETCH 300:500 (FLAGS) (CHANGEDSINCE 12345
VANISHED)
S: * VANISHED (EARLIER) 300:310,405,411
S: * 1 FETCH (UID 404 MODSEQ (65402) FLAGS (\Seen))
S: * 2 FETCH (UID 406 MODSEQ (75403) FLAGS (\Deleted))
S: * 4 FETCH (UID 408 MODSEQ (29738) FLAGS ($NoJunk
$AutoJunk $MDNSent))
S: s100 OK FETCH completed
3.2.7. 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 and QRESYNC was enabled, the server MUST send the updated per-mailbox modification sequence using the HIGHESTMODSEQ response code (see Section 3.1.2.1) 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, the client hasn't enabled QRESYNC, so the server is still using untagged EXPUNGE responses. Note that the presence of the HIGHESTMODSEQ response code is optional in this case. If the selected mailbox returned NOMODSEQ, the HIGHESTMODSEQ response code will be absent. 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 was decremented due to the previous EXPUNGE response). See the description of the EXPUNGE response in [RFC3501] for further explanation. Once the client enables QRESYNC, the server will always send VANISHED responses instead of EXPUNGE responses for mailboxes that support the storing of modification sequences, so 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 425.3.2.8. 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. The server MUST NOT send the updated per-mailbox modification sequence using the HIGHESTMODSEQ response code (see Section 3.1.2.1) in the tagged OK response, as this might cause loss of synchronization on the client. Example: C: A202 CLOSE S: A202 OK done3.2.9. 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], in the presence of QRESYNC. 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 have both the \Deleted flag set and 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 and QRESYNC was enabled, the server MUST send the updated per-mailbox modification sequence using the HIGHESTMODSEQ response code (see Section 3.1.2.1) 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, the client hasn't enabled QRESYNC, so the server is still using untagged EXPUNGE responses instead of VANISHED responses. Note that the presence of the HIGHESTMODSEQ response code is optional. If the selected mailbox returned NOMODSEQ, the HIGHESTMODSEQ response code will be absent. 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 was decremented due to the previous EXPUNGE response). See the description of the EXPUNGE response in [RFC3501] for further explanation.3.2.10. VANISHED Response
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). Once a client has issued "ENABLE QRESYNC" (and the server has positively responded to that command with the untagged ENABLED response containing QRESYNC), the server MUST use the VANISHED response without the EARLIER tag instead of the EXPUNGE response for all mailboxes that don't return NOMODSEQ when selected. The server continues 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.
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. The second form doesn't contain the EARLIER tag and is used for announcing message removals within an already selected mailbox. Because clients handle the two different forms of the VANISHED response differently, servers MUST NOT combine them. Messages are reported in VANISHED responses with or without the EARLIER tag, as appropriate to the cause, and, if necessary, two VANISHED responses are sent (one with EARLIER and one without).3.2.10.1. VANISHED (EARLIER) Response
Contents: an EARLIER tag list of UIDs The VANISHED (EARLIER) response is 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.3.2.10.2. VANISHED Response without the (EARLIER) Tag
Contents: list of UIDs Once a client has issued "ENABLE QRESYNC" (and the server has positively responded to that command with the untagged ENABLED response containing QRESYNC), the server MUST use the VANISHED response without the EARLIER tag instead of the EXPUNGE response for all mailboxes that don't return NOMODSEQ when selected. The server continues 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. Unlike VANISHED (EARLIER), this response also decrements the number of messages in the mailbox and adjusts the message sequence numbers for the messages remaining in the mailbox to account for the expunged messages. Because of this housekeeping, it is not necessary for the server to send an EXISTS response to report the new message count. See the example at the end of this section.
A VANISHED response without the EARLIER tag MUST refer only to messages that are visible to the client in the current session at the time the VANISHED response is sent. That is, servers MUST NOT send UIDs for previously expunged messages or messages that were not announced to the client via EXISTS. This means that each UID listed in a VANISHED response results in the client decrementing the message count by one. This is required to prevent a possible race condition where new arrivals for which the UID is not yet known by the client are immediately expunged. 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 the 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 "D" 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: D D D D 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 from the 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 For Example, the last VANISHED response only contains UIDs of messages expunged since the previous VANISHED response. To illustrate the difference between VANISHED and VANISHED (EARLIER), suppose the mailbox contains UIDs 2 and 4. Any of the following responses would constitute a broken server implementation: S: * VANISHED 1 S: * VANISHED 3 S: * VANISHED 5 However, any of these UIDs can easily be referenced by the VANISHED (EARLIER) response.3.2.11. 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. A server that advertises "QRESYNC" or "CONDSTORE" in the capability string must return the CLOSED response code in this case, whether or not a CONDSTORE enabling command was issued.
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. Long Command Lines (Update to RFC 2683)
While [RFC3501] doesn't specify a specific line-length limit, several server implementations chose to implement the recommended line-length limit suggested in Section 3.2.1.5 of [RFC2683] in order to protect from Denial-of-Service attacks. When the line-length limit is exceeded, such servers return a BAD response (as required by [RFC3501] in case of a syntactic error) and may even close the connection. Clients that support CONDSTORE/QRESYNC extensions can trigger this limit by sending a long UID sequence (previously returned by the server) in an extended SELECT or FETCH command. This document updates recommended line-length limits specified in Section 3.2.1.5 of [RFC2683]. While the advice in the first paragraph of that section still applies (use compact message/UID set representations), the 1000-octet limit suggested in the second paragraph turns out to be quite problematic when the CONDSTORE and/or QRESYNC extension is used. The updated recommendation is as follows: a client should limit the length of the command lines it generates to approximately 8192 octets (including all quoted strings but not including literals). If the client is unable to group things into ranges so that the command line is within that length, it should split the request into multiple commands. The client should use literals instead of long quoted strings in order to keep the command length down.5. QRESYNC Server Implementation Considerations
This section describes a minimalist implementation, a moderate implementation, and an example of a full implementation.5.1. 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.
A client can substantially reduce the size of VANISHED responses by providing the server with message sequence match data (see Section 3.2.5.2). This is especially effective in the typical case where no messages have been expunged, or all expunges were toward the end of the mailbox.5.2. 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 that HIGHESTMODSEQ value because there have been no messages expunged during the time period the client is concerned about. 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.5.3. Additional State Required on the Server
When compared to the CONDSTORE extension, QRESYNC requires servers to store an 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 QRESYNC 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 64 GB 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) is 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. 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, it reduces 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.6. Updated Synchronization Sequence
This section updates the description of optimized synchronization in Section 6.1 of [IMAP-DISC], in the presence of QRESYNC. An advanced disconnected mail client SHOULD use the QRESYNC extension when it is supported by the server and SHOULD use CONDSTORE if it is supported and QRESYNC is not. The client uses the value from the HIGHESTMODSEQ OK response code received on the 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 and HIGHESTMODSEQ response codes received from the server. Whenever the client receives a tagged response to a command, it checks the received unsolicited responses to calculate the new HIGHESTMODSEQ value. If the HIGHESTMODSEQ response code is received, the client MUST use it even if it has seen higher mod-sequences. Otherwise, the client 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. Example: C: A150 STORE 1:2 (UNCHANGEDSINCE 96) +FLAGS.SILENT \Seen S: * 1 FETCH (UID 6 MODSEQ (103)) S: * 2 FETCH (UID 7 MODSEQ (101)) S: * OK [HIGHESTMODSEQ 99] VANISHED reply with MODSEQ 100 is delayed S: A150 OK [MODIFIED 3] done C: A151 STORE 3 +FLAGS.SILENT \Seen S: * 3 FETCH (UID 8 MODSEQ (104)) S: A151 OK [HIGHESTMODSEQ 99] Still delaying VANISHED C: A152 NOOP S: * VANISHED 8 S: A153 OK [HIGHESTMODSEQ 104] done 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 mod-sequence order. Some commands may also delay EXPUNGE (or VANISHED) replies with smaller mod-sequences. These 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 6.1 of [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 [IMAP-DISC] for more details) after issuing the 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; and * remove any pending "actions" that 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]). 1b) This step is no longer required. 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] Highest
mailbox mod-sequence
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
7. Formal Syntax
The following syntax specification uses the Augmented Backus-Naur
Form (ABNF) notation as specified in [RFC5234].
Non-terminals referenced but not defined below are as defined by
[RFC5234], [RFC3501], or [RFC4466].
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 =/ "CONDSTORE" / "QRESYNC"
status-att =/ "HIGHESTMODSEQ"
;; Extends non-terminal defined in [RFC3501].
status-att-val =/ "HIGHESTMODSEQ" SP mod-sequence-valzer
;; Extends non-terminal defined in [RFC4466].
;; Value 0 denotes that the mailbox doesn't
;; support persistent mod-sequences
;; as described in Section 3.1.2.2.
store-modifier =/ "UNCHANGEDSINCE" SP mod-sequence-valzer
;; Only a single "UNCHANGEDSINCE" may be
;; specified in a STORE operation.
fetch-modifier =/ chgsince-fetch-mod
;; Conforms to the generic "fetch-modifier"
;; syntax defined in [RFC4466].
chgsince-fetch-mod = "CHANGEDSINCE" SP mod-sequence-value
;; CHANGEDSINCE FETCH modifier conforms to
;; the fetch-modifier syntax.
fetch-att =/ fetch-mod-sequence
;; Modifies original IMAP4 fetch-att.
fetch-mod-sequence = "MODSEQ"
fetch-mod-resp = "MODSEQ" SP "(" permsg-modsequence ")"
msg-att-dynamic =/ fetch-mod-resp
search-key =/ search-modsequence
;; Modifies original IMAP4 search-key.
;;
;; This change applies to all commands
;; referencing this non-terminal -- in
;; particular, SEARCH, SORT, and THREAD.
search-modsequence = "MODSEQ" [search-modseq-ext] SP
mod-sequence-valzer
search-modseq-ext = SP entry-name SP entry-type-req
resp-text-code =/ "HIGHESTMODSEQ" SP mod-sequence-value /
"NOMODSEQ" /
"MODIFIED" SP sequence-set
entry-name = entry-flag-name
entry-flag-name = DQUOTE "/flags/" attr-flag DQUOTE
;; Each system or user-defined flag <flag>
;; is mapped to "/flags/<flag>".
;;
;; <entry-flag-name> follows the escape rules
;; used by "quoted" string as described in
;; Section 4.3 of [RFC3501]; e.g., for the
;; flag \Seen, the corresponding <entry-name>
;; is "/flags/\\seen", and for the flag
;; $MDNSent, the corresponding <entry-name>
;; is "/flags/$mdnsent".
entry-type-resp = "priv" / "shared"
;; Metadata item type.
entry-type-req = entry-type-resp / "all"
;; Perform SEARCH operation on a private
;; metadata item, shared metadata item,
;; or both.
permsg-modsequence = mod-sequence-value
;; Per-message mod-sequence.
mod-sequence-value = 1*DIGIT
;; Positive unsigned 63-bit integer
;; (mod-sequence)
;; (1 <= n <= 9,223,372,036,854,775,807).
mod-sequence-valzer = "0" / mod-sequence-value
search-sort-mod-seq = "(" "MODSEQ" SP mod-sequence-value ")"
select-param =/ condstore-param
;; Conforms to the generic "select-param"
;; non-terminal syntax defined in [RFC4466].
condstore-param = "CONDSTORE"
mailbox-data =/ "SEARCH" [1*(SP nz-number) SP
search-sort-mod-seq]
sort-data = "SORT" [1*(SP nz-number) SP
search-sort-mod-seq]
; Updates the SORT response from RFC 5256.
attr-flag = "\\Answered" / "\\Flagged" / "\\Deleted" /
"\\Seen" / "\\Draft" / attr-flag-keyword /
attr-flag-extension
;; Does not include "\\Recent".
attr-flag-extension = "\\" atom
;; Future expansion. Client implementations
;; MUST accept flag-extension flags. Server
;; implementations MUST NOT generate
;; flag-extension flags, except as defined by
;; future standards or Standards Track
;; revisions of [RFC3501].
attr-flag-keyword = atom
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 [RFC4466].
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 [RFC4466]. It is only
;; allowed in the UID FETCH command.
resp-text-code =/ "CLOSED"
8. Security Considerations
As always, it is important to thoroughly test clients and servers
implementing QRESYNC, as it changes how the server reports expunged
messages to the client.
It is believed that the CONDSTORE or the QRESYNC extensions don't
raise any new security concerns that are not already discussed in
[RFC3501]. However, the availability of CONDSTORE may make it
possible for IMAP4 to be used in critical applications it could not
be used for previously, making correct IMAP server implementation and
operation even more important.
9. 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/imap-capabilities
This document defines the CONDSTORE and QRESYNC IMAP capabilities.
IANA has updated references for both extensions to point to this
document.
10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2683] Leiba, B., "IMAP4 Implementation Recommendations", RFC
2683, September 1999.
[RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
4rev1", RFC 3501, March 2003.
[RFC4466] Melnikov, A. and C. Daboo, "Collected Extensions to IMAP4
ABNF", RFC 4466, April 2006.
[RFC5161] Gulbrandsen, A. and A. Melnikov, "The IMAP ENABLE Extension", RFC 5161, March 2008. [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, January 2008. [RFC5256] Crispin, M. and K. Murchison, "Internet Message Access Protocol - SORT and THREAD Extensions", RFC 5256, June 2008. [RFC5464] Daboo, C., "The IMAP METADATA Extension", RFC 5464, February 2009. [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. [NTP] Mills, D., Martin, J., Burbank, J., and W. Kasch, "Network Time Protocol Version 4: Protocol and Algorithms Specification", RFC 5905, June 2010. [RFC2180] Gahrns, M., "IMAP4 Multi-Accessed Mailbox Practice", RFC 2180, July 1997. [RFC4314] Melnikov, A., "IMAP4 Access Control List (ACL) Extension", RFC 4314, December 2005. [RFC4731] Melnikov, A. and D. Cridland, "IMAP4 Extension to SEARCH Command for Controlling What Kind of Information Is Returned", RFC 4731, November 2006. [RFC5257] Daboo, C. and R. Gellens, "Internet Message Access Protocol - ANNOTATE Extension", RFC 5257, June 2008. [RFC5267] Cridland, D. and C. King, "Contexts for IMAP4", RFC 5267, July 2008. [RFC6851] Gulbrandsen, A. and N. Freed, "Internet Message Access Protocol (IMAP) - MOVE Extension", RFC 6851, January 2013. [UNSELECT] Melnikov, A., "Internet Message Access Protocol (IMAP) UNSELECT command", RFC 3691, February 2004.
Appendix A. Changes since RFC 4551
Changed mod-sequences to be unsigned 63-bit values (instead of unsigned 64-bit values). Fixed the following errata, as posted on <http://www.rfc-editor.org>: o Errata ID 3401 ("several typos in UNCHANGEDSINCE spelling") o Errata ID 3506 ("invalid ABNF for the MODIFIED response code") o Errata ID 3509 ("correction to an example") Clarified that the returning of HIGHESTMODSEQ/NOMODSEQ response codes is only required once a CONDSTORE enabling command is issued. Clarified that if multiple mod-sequences (for different metadata items) are associated with a message, then all of them affecting a particular STORE UNCHANGEDSINCE must be checked. Updated references. Made editorial corrections.Appendix B. Changes since RFC 5162
Changed mod-sequences to be unsigned 63-bit values (instead of unsigned 64-bit values). Addressed the following errata, as posted on <http://www.rfc-editor.org>: o Errata ID 1365 ("clarified that QRESYNC is only enabled when ENABLED QRESYNC is returned") o Errata ID 1807 ("unsolicited FETCH responses must include UID fetch response item") o Errata ID 1808 ("HIGHESTMODSEQ response code must not be returned for CLOSE") o Errata ID 1809 ("clarify how updated mailbox mod-sequence is calculated") o Errata ID 1810 ("server must send untagged events to client in a way that client doesn't lose any changes in case of connectivity loss") o Errata ID 3322 ("VANISHED responses must not reference non- existing UIDs") Clarified that ENABLE QRESYNC CONDSTORE and ENABLE CONDSTORE QRESYNC are equivalent.
Changed the requirement to return VANISHED from SHOULD to MUST as per the mailing list discussion. The only exception is for mailboxes that return the NOMODSEQ response code when they are selected. Specified that IMAP SETMETADATA changes update per-mailbox HIGHESTMODSEQ. Clarified that per-message annotations are also considered "metadata". Fixed some examples to report data that match requirements specified in the document. Clarified some text and made some requirements normative. Also, corrected a couple of SHOULDs to be MUSTs. Updated references. Made editorial corrections.Appendix C. Acknowledgements
Thank you to Steve Hole for co-editing RFC 4551. In this revision of the document, the authors also acknowledge the feedback provided by Timo Sirainen, Jan Kundrat, Pete Maclean, Barry Leiba, Eliot Lear, Chris Newman, Claudio Allocchio, Michael Slusarz, Bron Gondwana, Arnt Gulbrandsen, David Black, Hoa V. DINH, and Nick Hudson. Mark Crispin contributed to RFCs 4551 and 5162 that this document is replacing, and much of his contribution remains in this merged document. See also the list of people who contributed to RFC 4551, which this document obsoletes.
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 Surevine Ltd PO Box 1136 Guildford, Surrey GU1 9ND UK EMail: dave.cridland@surevine.com