Network Working Group A. Melnikov
Request for Comments: 5162 D. Cridland
Category: Standards Track Isode Ltd
C. Wilson
Nokia
March 2008
IMAP4 Extensions for Quick Mailbox Resynchronization
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Abstract
This document defines an IMAP4 extension, which gives an IMAP client
the ability to quickly resynchronize any previously opened mailbox as
part of the SELECT command, without the need for server-side state or
additional client round-trips. This extension also introduces a new
response that allows for a more compact representation of a list of
expunged messages (and always includes the Unique Identifiers (UIDs)
expunged).
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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.
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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
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but not this extension might incur a penalty of an unneeded round-
trip when resynchronizing some mailboxes (those that have had
messages expunged but no flag changes since the last
synchronization).
This extra round-trip is only incurred by clients that support
CONDSTORE but not this extension, and only when a mailbox has had
messages expunged but no flag changes to non-expunged messages.
Since CONDSTORE is a relatively new extension, it is thought likely
that clients that support it will also support this extension.
2. Requirements Notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
In examples, "C:" and "S:" indicate lines sent by the client and
server respectively. If a single "C:" or "S:" label applies to
multiple lines, then the line breaks between those lines are for
editorial clarity only and are not part of the actual protocol
exchange. The five characters [...] means that something has been
elided.
Understanding of the IMAP message sequence numbers and UIDs and the
EXPUNGE response [RFC3501] is essential when reading this document.
3. IMAP Protocol Changes
3.1. QRESYNC Parameter to SELECT/EXAMINE
The Quick Resynchronization parameter to SELECT/EXAMINE commands has
four arguments:
o the last known UIDVALIDITY,
o the last known modification sequence,
o the optional set of known UIDs, and
o an optional parenthesized list of known sequence ranges and their
corresponding UIDs.
A server MUST respond with a tagged BAD response if the Quick
Resynchronization parameter to SELECT/EXAMINE command is specified
and the client hasn't issued "ENABLE QRESYNC" in the current
connection.
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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.
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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.
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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
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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
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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
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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
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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.
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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
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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.
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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
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(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.
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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>.
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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.
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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;
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* 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.
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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].
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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.
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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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Full Copyright Statement
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contained in BCP 78, and except as set forth therein, the authors
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Melnikov, et al. Standards Track [Page 23]