RFC 7162
IMAP Extensions: Quick Flag Changes Resynchronization (CONDSTORE) and Quick Mailbox Resynchronization (QRESYNC)
Internet Engineering Task Force (IETF) A. Melnikov Request for Comments: 7162 Isode Ltd Obsoletes: 4551, 5162 D. Cridland Updates: 2683 Surevine Ltd Category: Standards Track May 2014 ISSN: 2070-1721 IMAP Extensions: Quick Flag Changes Resynchronization (CONDSTORE) and Quick Mailbox Resynchronization (QRESYNC)Abstract
Often, multiple IMAP (RFC 3501) clients need to coordinate changes to a common IMAP mailbox. Examples include different clients working on behalf of the same user and multiple users accessing shared mailboxes. These clients need a mechanism to efficiently synchronize state changes for messages within the mailbox. Initially defined in RFC 4551, the Conditional Store facility provides a protected update mechanism for message state information and a mechanism for requesting only changes to the message state. This memo updates that mechanism and obsoletes RFC 4551, based on operational experience. This document additionally updates another IMAP extension, Quick Resynchronization, which builds on the Conditional STORE extension to provide an IMAP client the ability to fully resynchronize a mailbox as part of the SELECT/EXAMINE command, without the need for additional server-side state or client round trips. Hence, this memo obsoletes RFC 5162. Finally, this document also updates the line-length recommendation in Section 3.2.1.5 of RFC 2683. Status of This Memo This is an Internet Standards Track document. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 5741. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc7162.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Requirements Notation . . . . . . . . . . . . . . . . . . . . 5 3. IMAP Protocol Changes . . . . . . . . . . . . . . . . . . . . 5 3.1. CONDSTORE Extension . . . . . . . . . . . . . . . . . . . 5 3.1.1. Advertising Support for CONDSTORE . . . . . . . . . . 8 3.1.2. New OK Untagged Responses for SELECT and EXAMINE . . 8 3.1.3. STORE and UID STORE Commands . . . . . . . . . . . . 10 3.1.4. FETCH and UID FETCH Commands . . . . . . . . . . . . 16 3.1.5. MODSEQ Search Criterion in SEARCH . . . . . . . . . . 19 3.1.6. Modified SEARCH Untagged Response . . . . . . . . . . 20 3.1.7. HIGHESTMODSEQ Status Data Items . . . . . . . . . . . 21 3.1.8. CONDSTORE Parameter to SELECT and EXAMINE . . . . . . 21 3.1.9. Interaction with IMAP SORT and THREAD Extensions . . 22 3.1.10. Interaction with IMAP ESORT and ESEARCH Extensions . 22 3.1.11. Additional Quality-of-Implementation Issues . . . . . 23 3.1.12. CONDSTORE Server Implementation Considerations . . . 23 3.2. QRESYNC Extension . . . . . . . . . . . . . . . . . . . . 24 3.2.1. Impact on CONDSTORE-only Clients . . . . . . . . . . 25 3.2.2. Advertising Support for QRESYNC . . . . . . . . . . . 25 3.2.3. Use of ENABLE . . . . . . . . . . . . . . . . . . . . 25 3.2.4. Additional Requirements on QRESYNC Servers . . . . . 26 3.2.5. QRESYNC Parameter to SELECT/EXAMINE . . . . . . . . . 26 3.2.6. VANISHED UID FETCH Modifier . . . . . . . . . . . . . 31 3.2.7. EXPUNGE Command . . . . . . . . . . . . . . . . . . . 32 3.2.8. CLOSE Command . . . . . . . . . . . . . . . . . . . . 33 3.2.9. UID EXPUNGE Command . . . . . . . . . . . . . . . . . 34 3.2.10. VANISHED Response . . . . . . . . . . . . . . . . . . 35 3.2.11. CLOSED Response Code . . . . . . . . . . . . . . . . 38 4. Long Command Lines (Update to RFC 2683) . . . . . . . . . . . 39 5. QRESYNC Server Implementation Considerations . . . . . . . . 39 5.1. Server Implementations That Don't Store Extra State . . . 39 5.2. Server Implementations Storing Minimal State . . . . . . 40 5.3. Additional State Required on the Server . . . . . . . . . 40 6. Updated Synchronization Sequence . . . . . . . . . . . . . . 41 7. Formal Syntax . . . . . . . . . . . . . . . . . . . . . . . . 44 8. Security Considerations . . . . . . . . . . . . . . . . . . . 48 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 48 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 48 10.1. Normative References . . . . . . . . . . . . . . . . . . 48 10.2. Informative References . . . . . . . . . . . . . . . . . 49 Appendix A. Changes since RFC 4551 . . . . . . . . . . . . . . . 50 Appendix B. Changes since RFC 5162 . . . . . . . . . . . . . . . 50 Appendix C. Acknowledgements . . . . . . . . . . . . . . . . . . 51
1. Introduction
Often, multiple IMAP [RFC3501] clients need to coordinate changes to a common IMAP mailbox. Examples include different clients working on behalf of the same user and clients representing multiple users accessing shared mailboxes. These clients need a mechanism to synchronize state changes for messages within the mailbox. The Conditional Store ("CONDSTORE") facility allows a client to quickly resynchronize mailbox flag changes. The Conditional Store facility also provides a protected update mechanism for message state information that can detect and resolve conflicts between multiple writing mail clients. The mechanism can be used to guarantee that only one client can change the message state at any given time. For example, this can be used by multiple clients that treat a mailbox as a message queue. The Conditional Store facility is provided by associating a modification sequence (mod-sequence) with every IMAP message. This is updated whenever metadata (such as a message flag) is modified. The CONDSTORE extension is described in more detail in Section 3.1. 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. The Quick Mailbox Resynchronization (QRESYNC) IMAP extension is an extension to CONDSTORE that allows a reconnecting client to perform full resynchronization, including discovery of expunged messages, in a single round trip. QRESYNC also introduces a new response, VANISHED, that allows for a more compact representation of a list of expunged messages. QRESYNC can be useful for mobile clients that can experience frequent disconnects caused by environmental factors (such as 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 generated by resynchronization. By extending the SELECT command to perform the additional resynchronization, this also allows clients to reduce concurrent connections to the IMAP server held purely for the sake of avoiding the resynchronization. The QRESYNC extension is described in more detail in Section 3.2.
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 the examples that follow, "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. Formal syntax is defined using ABNF [RFC5234]. The term "metadata" or "metadata item" is used throughout this document. It refers to any system- or user-defined keyword. If the server supports the IMAP ANNOTATE-EXPERIMENT-1 extension [RFC5257], then metadata also includes message annotations. Future documents may extend "metadata" to include other dynamic message data. Some IMAP mailboxes are private, accessible only to the owning user. Other mailboxes are not, either because the owner has set an Access Control List [RFC4314] that permits access by other users or because it is a shared mailbox. Let's call a metadata item "shared" for the mailbox if any changes to the metadata items are persistent and visible to all other users accessing the mailbox. Otherwise, the metadata item is called "private". Note that private metadata items are still visible to all sessions accessing the mailbox as the same user. Also, note that different mailboxes may have different metadata items as shared. See Section 3.1 for the definition of a "CONDSTORE-aware client" and a "CONDSTORE enabling command". Understanding of the IMAP message sequence numbers and UIDs (see Section 2.3.1 of [RFC3501]) and the EXPUNGE response (see Section 7.4.1 of [RFC3501]) is essential when reading this document.3. IMAP Protocol Changes
3.1. CONDSTORE Extension
An IMAP server that supports CONDSTORE MUST associate a positive unsigned 63-bit (*) value, called a mod-sequence, with every IMAP message. This is an opaque value updated by the server whenever a metadata item is modified. The server MUST guarantee that each STORE command performed on the same mailbox (including simultaneous stores
to different metadata items from different connections) will get a different mod-sequence value. Also, for any two successful STORE operations performed in the same session on the same mailbox, the mod-sequence of the second completed operation MUST be greater than the mod-sequence of the first completed operation. Note that the latter rule disallows the direct use of the system clock as a mod- sequence because if system time changes (e.g., an NTP [NTP] client adjusting the time), the next generated value might be less than the previous one. (*) Note: RFC 4551 defined mod-sequences as unsigned 64-bit values. In order to make implementations on various platforms (such as Java) easier, this version of the document redefines them as unsigned 63-bit values. These rules allow a client to list all metadata changes since a well- known point in time, as well as to perform conditional metadata modifications based on an assumption that the metadata state hasn't changed for a particular message. In particular, mod-sequences allow a client that supports the CONDSTORE extension to determine if a message metadata has changed since some known moment. Whenever the state of a flag changes (i.e., the flag is added where previously it wasn't set, or the flag is removed where previously it was set), the value of the modification sequence for the message MUST be updated. Setting a flag that is already set, or clearing a flag that is not set, SHOULD NOT change the mod-sequence. When a message is appended to a mailbox (via the IMAP APPEND command, COPY to the mailbox, or using an external mechanism), the server generates a new modification sequence that is higher than the highest modification sequence of all messages in the mailbox and assigns it to the appended message. The server MAY store separate (per-message) modification sequence values for different metadata items. If the server does so, per- message mod-sequence is the highest mod-sequence of all metadata items accessible to the currently logged-in user for the specified message. The server that supports CONDSTORE is not required to be able to store mod-sequences for every available mailbox. Section 3.1.2.2 describes how the server may act if a particular mailbox doesn't support the persistent storage of mod-sequences.
CONDSTORE makes the following changes to the IMAP4 protocol:
a. adds the UNCHANGEDSINCE STORE modifier.
b. adds the MODIFIED response code that is used with an OK response
to the STORE command. (It can also be used in a NO response.)
c. adds a new MODSEQ message data item for use with the FETCH
command.
d. adds the CHANGEDSINCE FETCH modifier.
e. adds a new MODSEQ search criterion.
f. extends the syntax of untagged SEARCH and ESEARCH responses to
include mod-sequence.
g. adds new OK untagged responses (HIGHESTMODSEQ and NOMODSEQ) for
the SELECT and EXAMINE commands.
h. defines an additional CONDSTORE parameter to SELECT/EXAMINE
commands.
i. adds the HIGHESTMODSEQ status data item to the STATUS command.
A client supporting the CONDSTORE extension indicates its willingness
to receive mod-sequence updates in all untagged FETCH responses by
issuing one of the following, which are called "CONDSTORE enabling
commands":
o a SELECT or EXAMINE command with the CONDSTORE parameter,
o a STATUS (HIGHESTMODSEQ) command,
o a FETCH or SEARCH command that includes the MODSEQ message data
item,
o a FETCH command with the CHANGEDSINCE modifier,
o a STORE command with the UNCHANGEDSINCE modifier, or
o an ENABLE command containing "CONDSTORE" as one of the parameters.
(This option only applies when the client is communicating with a
server that also implements the ENABLE extension [RFC5161].)
Once a client issues a CONDSTORE enabling command, it has announced
itself as a "CONDSTORE-aware client". The server MUST then include
mod-sequence data in all subsequent untagged FETCH responses (until
the connection is closed), whether they were caused by a regular STORE, a STORE with an UNCHANGEDSINCE modifier, or an external agent. A future extension to this document may extend the list of CONDSTORE enabling commands. A first CONDSTORE enabling command executed in the session with a mailbox selected MUST cause the server to return HIGHESTMODSEQ (Section 3.1.2.1) for the mailbox (if any is selected), unless the server has sent a NOMODSEQ (Section 3.1.2.2) response code when the currently selected mailbox was selected.3.1.1. Advertising Support for CONDSTORE
The Conditional STORE extension is present in any IMAP4 implementation that returns "CONDSTORE" as one of the supported capabilities in the CAPABILITY command response.3.1.2. New OK Untagged Responses for SELECT and EXAMINE
This document adds two new response codes: HIGHESTMODSEQ and NOMODSEQ. One of these two response codes MUST be returned in an OK untagged response for any successful SELECT/EXAMINE command issued after a CONDSTORE enabling command. When opening a mailbox, the server must check if the mailbox supports the persistent storage of mod-sequences. If the mailbox supports the persistent storage of mod-sequences and the mailbox open operation succeeds, the server MUST send an OK untagged response, including the HIGHESTMODSEQ response code. If the persistent storage for the mailbox is not supported, the server MUST send an OK untagged response, including the NOMODSEQ response code instead.3.1.2.1. HIGHESTMODSEQ Response Code
This document adds a new response code that is returned in an OK untagged response for the SELECT and EXAMINE commands. Once a CONDSTORE enabling command is issued, a server supporting the persistent storage of mod-sequences for the mailbox MUST send an OK untagged response, including the HIGHESTMODSEQ response code with every successful SELECT or EXAMINE command: OK [HIGHESTMODSEQ <mod-sequence-value>] where <mod-sequence-value> is the highest mod-sequence value of all messages in the mailbox. When the server changes UIDVALIDITY for a mailbox, it doesn't have to keep the same HIGHESTMODSEQ for the mailbox.
Note that some existing CONDSTORE servers don't start tracking mod- sequences or don't report them until after a CONDSTORE enabling command is issued. Because of that, a client wishing to receive HIGHESTMODSEQ/NOMODSEQ information must first send a CONDSTORE enabling command, for example, by using SELECT/EXAMINE with the CONDSTORE parameter (see Section 3.1.8). A disconnected client can use the value of HIGHESTMODSEQ to check if it has to refetch metadata from the server. If the UIDVALIDITY value has changed for the selected mailbox, the client MUST delete the cached value of HIGHESTMODSEQ. If UIDVALIDITY for the mailbox is the same, and if the HIGHESTMODSEQ value stored in the client's cache is less than the value returned by the server, then some metadata items on the server have changed since the last synchronization, and the client needs to update its cache. The client MAY use SEARCH MODSEQ (Section 3.1.5) to find out exactly which metadata items have changed. Alternatively, the client MAY issue FETCH with the CHANGEDSINCE modifier (Section 3.1.4.1) in order to fetch data for all messages that have metadata items changed since some known modification sequence. C: A142 SELECT INBOX S: * 172 EXISTS S: * 1 RECENT S: * OK [UNSEEN 12] Message 12 is first unseen S: * OK [UIDVALIDITY 3857529045] UIDs valid S: * OK [UIDNEXT 4392] Predicted next UID S: * FLAGS (\Answered \Flagged \Deleted \Seen \Draft) S: * OK [PERMANENTFLAGS (\Deleted \Seen \*)] Limited S: * OK [HIGHESTMODSEQ 715194045007] S: A142 OK [READ-WRITE] SELECT completed Example 13.1.2.2. NOMODSEQ Response Code
Once a CONDSTORE enabling command is issued, 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. Note that some existing CONDSTORE servers don't return NOMODSEQ until after a CONDSTORE enabling command is issued. Because of that, a client wishing to receive HIGHESTMODSEQ/NOMODSEQ information must first send a CONDSTORE enabling command, for example, by using SELECT/EXAMINE with the CONDSTORE parameter (see Section 3.1.8).
A server that returned the NOMODSEQ response code for a mailbox MUST
reject (with a tagged BAD response) any of the following commands
while the mailbox remains selected:
o a FETCH command with the CHANGEDSINCE modifier,
o a FETCH or SEARCH command that includes the MODSEQ message data
item, or
o a STORE command with the UNCHANGEDSINCE modifier.
C: A142 SELECT INBOX
S: * 172 EXISTS
S: * 1 RECENT
S: * OK [UNSEEN 12] Message 12 is first unseen
S: * OK [UIDVALIDITY 3857529045] UIDs valid
S: * OK [UIDNEXT 4392] Predicted next UID
S: * FLAGS (\Answered \Flagged \Deleted \Seen \Draft)
S: * OK [PERMANENTFLAGS (\Deleted \Seen \*)] Limited
S: * OK [NOMODSEQ] Sorry, this mailbox format doesn't support
modsequences
S: A142 OK [READ-WRITE] SELECT completed
Example 2
3.1.3. STORE and UID STORE Commands
This document defines the following STORE modifier (see Section 2.5
of [RFC4466]):
UNCHANGEDSINCE <mod-sequence>
For each message specified in the message set, the server performs
the following. If the mod-sequence of every metadata item of the
message affected by the STORE/UID STORE is equal to or less than the
specified UNCHANGEDSINCE value, then the requested operation (as
described by the message data item) is performed. If the operation
is successful, the server MUST update the mod-sequence attribute of
the message. An untagged FETCH response MUST be sent, even if the
.SILENT suffix is specified, and the response MUST include the MODSEQ
message data item. This is required to update the client's cache
with the correct mod-sequence values. See Section 3.1.4.2 for more
details.
However, if the mod-sequence of any metadata item of the message is
greater than the specified UNCHANGEDSINCE value, then the requested
operation MUST NOT be performed. In this case, the mod-sequence
attribute of the message is not updated, and the message number (or
unique identifier in the case of the UID STORE command) is added to
the list of messages that failed the UNCHANGEDSINCE test.
When the server finishes performing the operation on all the messages
in the message set, it checks for a non-empty list of messages that
failed the UNCHANGEDSINCE test. If this list is non-empty, the
server MUST return in the tagged response a MODIFIED response code.
The MODIFIED response code includes the message set (for STORE) or
set of UIDs (for UID STORE) of all messages that failed the
UNCHANGEDSINCE test.
All messages pass the UNCHANGEDSINCE test.
C: a103 UID STORE 6,4,8 (UNCHANGEDSINCE 12121230045)
+FLAGS.SILENT (\Deleted)
S: * 1 FETCH (UID 4 MODSEQ (12121231000))
S: * 2 FETCH (UID 6 MODSEQ (12121230852))
S: * 4 FETCH (UID 8 MODSEQ (12121230956))
S: a103 OK Conditional Store completed
Example 3
C: a104 STORE * (UNCHANGEDSINCE 12121230045) +FLAGS.SILENT
(\Deleted $Processed)
S: * 50 FETCH (MODSEQ (12111230047))
S: a104 OK Store (conditional) completed
Example 4
C: c101 STORE 50 (UNCHANGEDSINCE 12121230045) -FLAGS.SILENT
(\Deleted)
S: * OK [HIGHESTMODSEQ 12111230047]
S: * 50 FETCH (MODSEQ (12111230048))
S: c101 OK Store (conditional) completed
The HIGHESTMODSEQ response code was sent by the server presumably
because this was the first CONDSTORE enabling command.
Example 5
The failure of the conditional STORE operation for any particular
message or messages (7 in this example) does not stop the server from
finding all messages that fail the UNCHANGEDSINCE test. All such
messages are returned in the MODIFIED response code.
C: d105 STORE 7,5,9 (UNCHANGEDSINCE 320162338)
+FLAGS.SILENT (\Deleted)
S: * 5 FETCH (MODSEQ (320162350))
S: d105 OK [MODIFIED 7,9] Conditional STORE failed
Example 6
Same as above, but the server follows the SHOULD recommendation in
Section 6.4.6 of [RFC3501].
C: d105 STORE 7,5,9 (UNCHANGEDSINCE 320162338)
+FLAGS.SILENT (\Deleted)
S: * 7 FETCH (MODSEQ (320162342) FLAGS (\Seen \Deleted))
S: * 5 FETCH (MODSEQ (320162350))
S: * 9 FETCH (MODSEQ (320162349) FLAGS (\Answered))
S: d105 OK [MODIFIED 7,9] Conditional STORE failed
Use of UNCHANGEDSINCE with a modification sequence of 0 always fails
if the metadata item exists. A system flag MUST always be considered
existent, whether it was set or not.
Example 7
C: a102 STORE 12 (UNCHANGEDSINCE 0)
+FLAGS.SILENT ($MDNSent)
S: a102 OK [MODIFIED 12] Conditional STORE failed
The client has tested the presence of the $MDNSent user-defined
keyword.
Example 8
Note: A client trying to make an atomic change to the state of a
particular metadata item (or a set of metadata items) MUST be
prepared to deal with the case when the server returns the MODIFIED
response code if the state of the metadata item being watched hasn't
changed (but the state of some other metadata item has). This is
necessary because some servers don't store separate mod-sequences for
different metadata items. However, a server implementation SHOULD
avoid generating spurious MODIFIED responses for +FLAGS/-FLAGS STORE
operations, even when the server stores a single mod-sequence per
message. Section 3.1.12 describes how this can be achieved.
Unless the server has included an unsolicited FETCH to update the
client's knowledge about messages that have failed the UNCHANGEDSINCE
test, upon receipt of the MODIFIED response code, the client SHOULD
try to figure out if the required metadata items have indeed changed
by issuing the FETCH or NOOP command. It is RECOMMENDED that the
server avoids the need for the client to do that by sending an
unsolicited FETCH response (see Examples 9 and 10).
If the required metadata items haven't changed, the client SHOULD
retry the command with the new mod-sequence. The client needs to
allow for a reasonable number of retries (at least 2).
In the example below, the server returns the MODIFIED response code
without sending information describing why the STORE UNCHANGEDSINCE
operation has failed.
C: a106 STORE 100:150 (UNCHANGEDSINCE 212030000000)
+FLAGS.SILENT ($Processed)
S: * 100 FETCH (MODSEQ (303181230852))
S: * 102 FETCH (MODSEQ (303181230852))
...
S: * 150 FETCH (MODSEQ (303181230852))
S: a106 OK [MODIFIED 101] Conditional STORE failed
The flag $Processed was set on the message 101...
C: a107 NOOP
S: * 101 FETCH (MODSEQ (303011130956) FLAGS ($Processed))
S: a107 OK
Example 9
Or, the flag hasn't changed, but another has (note that this server
behavior is discouraged. Server implementers should also see
Section 3.1.12)...
C: b107 NOOP
S: * 101 FETCH (MODSEQ (303011130956) FLAGS (\Deleted \Answered))
S: b107 OK
...and the client retries the operation for the message 101 with
the updated UNCHANGEDSINCE value.
C: b108 STORE 101 (UNCHANGEDSINCE 303011130956)
+FLAGS.SILENT ($Processed)
S: * 101 FETCH (MODSEQ (303181230852))
S: b108 OK Conditional Store completed
Same as above, but the server avoids the need for the client to poll
for changes.
The flag $Processed was set on the message 101 by another
client...
C: a106 STORE 100:150 (UNCHANGEDSINCE 212030000000)
+FLAGS.SILENT ($Processed)
S: * 100 FETCH (MODSEQ (303181230852))
S: * 101 FETCH (MODSEQ (303011130956) FLAGS ($Processed))
S: * 102 FETCH (MODSEQ (303181230852))
...
S: * 150 FETCH (MODSEQ (303181230852))
S: a106 OK [MODIFIED 101] Conditional STORE failed
Example 10
Or, the flag hasn't changed, but another has (note that this server
behavior is discouraged. Server implementers should also see
Section 3.1.12)...
C: a106 STORE 100:150 (UNCHANGEDSINCE 212030000000)
+FLAGS.SILENT ($Processed)
S: * 100 FETCH (MODSEQ (303181230852))
S: * 101 FETCH (MODSEQ (303011130956) FLAGS (\Deleted \Answered))
S: * 102 FETCH (MODSEQ (303181230852))
...
S: * 150 FETCH (MODSEQ (303181230852))
S: a106 OK [MODIFIED 101] Conditional STORE failed
...and the client retries the operation for the message 101 with
the updated UNCHANGEDSINCE value.
C: b108 STORE 101 (UNCHANGEDSINCE 303011130956)
+FLAGS.SILENT ($Processed)
S: * 101 FETCH (MODSEQ (303181230852))
S: b108 OK Conditional Store completed
Or, the flag hasn't changed, but another has (nice server behavior.
Server implementers should also see Section 3.1.12)...
C: a106 STORE 100:150 (UNCHANGEDSINCE 212030000000)
+FLAGS.SILENT ($Processed)
S: * 100 FETCH (MODSEQ (303181230852))
S: * 101 FETCH (MODSEQ (303011130956) FLAGS ($Processed \Deleted
\Answered))
S: * 102 FETCH (MODSEQ (303181230852))
...
S: * 150 FETCH (MODSEQ (303181230852))
S: a106 OK Conditional STORE completed
The following example is based on the example from Section 4.2.3 of [RFC2180] and demonstrates that the MODIFIED response code MAY also be returned in the tagged NO response. The client tries to conditionally STORE flags on a mixture of expunged and non-expunged messages; one message fails the UNCHANGEDSINCE test. C: B001 STORE 1:7 (UNCHANGEDSINCE 320172338) +FLAGS (\SEEN) S: * 1 FETCH (MODSEQ (320172342) FLAGS (\SEEN)) S: * 3 FETCH (MODSEQ (320172342) FLAGS (\SEEN)) S: B001 NO [MODIFIED 2] Some of the messages no longer exist. C: B002 NOOP S: * 4 EXPUNGE S: * 4 EXPUNGE S: * 4 EXPUNGE S: * 4 EXPUNGE S: * 2 FETCH (MODSEQ (320172340) FLAGS (\Deleted \Answered)) S: B002 OK NOOP Completed. By receiving FETCH responses for messages 1 and 3, and EXPUNGE responses that indicate that messages 4 through 7 have been expunged, the client retries the operation only for message 2. The updated UNCHANGEDSINCE value is used. C: b003 STORE 2 (UNCHANGEDSINCE 320172340) +FLAGS (\Seen) S: * 2 FETCH (MODSEQ (320180050) FLAGS (\SEEN \Flagged)) S: b003 OK Conditional Store completed Example 11 Note: If a message is specified multiple times in the message set, and the server doesn't internally eliminate duplicates from the message set, it MUST NOT fail the conditional STORE operation for the second (or subsequent) occurrence of the message if the operation completed successfully for the first occurrence. For example, if the client specifies: e105 STORE 7,3:9 (UNCHANGEDSINCE 12121230045) +FLAGS.SILENT (\Deleted) the server must not fail the operation for message 7 as part of processing "3:9" if it succeeded when message 7 was processed the first time.
As specified in Section 3.1, once the client specifies the UNCHANGEDSINCE modifier in a STORE command, the server starts including the MODSEQ FETCH response data items in all subsequent unsolicited FETCH responses. This document also changes the behavior of the server when it has performed a STORE or UID STORE command and the UNCHANGEDSINCE modifier is not specified. If the operation is successful for a message, the server MUST update the mod-sequence attribute of the message. The server is REQUIRED to include the mod-sequence value whenever it decides to send the unsolicited FETCH response to all CONDSTORE-aware clients that have opened the mailbox containing the message. Server implementers should also see Section 3.1.11 for additional quality of implementation issues related to the STORE command.3.1.4. FETCH and UID FETCH Commands
3.1.4.1. CHANGEDSINCE FETCH Modifier
This document defines the following FETCH modifier (see Section 2.4 of [RFC4466]): CHANGEDSINCE <mod-sequence>: The CHANGEDSINCE FETCH modifier allows the client to further subset the list of messages described by the sequence set. The information described by message data items is only returned for messages that have a mod-sequence bigger than <mod-sequence>. When the CHANGEDSINCE FETCH modifier is specified, it implicitly adds the MODSEQ FETCH message data item (Section 3.1.4.2). C: s100 UID FETCH 1:* (FLAGS) (CHANGEDSINCE 12345) S: * 1 FETCH (UID 4 MODSEQ (65402) FLAGS (\Seen)) S: * 2 FETCH (UID 6 MODSEQ (75403) FLAGS (\Deleted)) S: * 4 FETCH (UID 8 MODSEQ (29738) FLAGS ($NoJunk $AutoJunk $MDNSent)) S: s100 OK FETCH completed Example 12
3.1.4.2. MODSEQ Message Data Item in FETCH Command
CONDSTORE adds a MODSEQ message data item to the FETCH command. The MODSEQ message data item allows clients to retrieve mod-sequence values for a range of messages in the currently selected mailbox. As specified in Section 3.1, once the client has specified the MODSEQ message data item in a FETCH request, the server starts including the MODSEQ FETCH response data items in all subsequent unsolicited FETCH responses. Syntax: MODSEQ The MODSEQ message data item causes the server to return MODSEQ FETCH response data items. Syntax: MODSEQ ( <permsg-modsequence> ) MODSEQ response data items contain per-message mod-sequences. The MODSEQ response data item is returned if the client issued FETCH with the MODSEQ message data item. It also allows the server to notify the client about mod-sequence changes caused by conditional STOREs (Section 3.1.3) and/or changes caused by external sources. C: a FETCH 1:3 (MODSEQ) S: * 1 FETCH (MODSEQ (624140003)) S: * 2 FETCH (MODSEQ (624140007)) S: * 3 FETCH (MODSEQ (624140005)) S: a OK Fetch complete In this example, the client requests per-message mod-sequences for a set of messages. Example 13 Servers that only support the CONDSTORE extension (and not QRESYNC) SHOULD comply with requirements from Section 3.2.4. When a flag for a message is modified in a different session, the server sends an unsolicited FETCH response containing the mod- sequence for the message, as demonstrated in Example 14. Note that when the server also supports the QRESYNC extension (Section 3.2.3) and a CONDSTORE enabling command has been issued, all FETCH responses in Example 14 must also include UID FETCH items as prescribed by Section 3.2.4.
(Session 1, authenticated as the user "alex".) The user adds a
shared flag \Deleted:
C: A142 SELECT INBOX
...
S: * FLAGS (\Answered \Flagged \Deleted \Seen \Draft)
S: * OK [PERMANENTFLAGS (\Answered \Deleted \Seen \*)] Limited
...
C: A160 STORE 7 +FLAGS.SILENT (\Deleted)
S: * 7 FETCH (MODSEQ (2121231000))
S: A160 OK Store completed
(Session 2, also authenticated as the user "alex".) Any changes
to flags are always reported to all sessions authenticated as the
same user as in session 1.
C: C180 NOOP
S: * 7 FETCH (FLAGS (\Deleted \Answered) MODSEQ (12121231000))
S: C180 OK Noop completed
(Session 3, authenticated as the user "andrew".) As \Deleted is a
shared flag, changes in session 1 are also reported in session 3:
C: D210 NOOP
S: * 7 FETCH (FLAGS (\Deleted \Answered) MODSEQ (12121231000))
S: D210 OK Noop completed
The user modifies a private flag, \Seen, in session 1...
C: A240 STORE 7 +FLAGS.SILENT (\Seen)
S: * 7 FETCH (MODSEQ (12121231777))
S: A240 OK Store completed
...which is only reported in session 2...
C: C270 NOOP
S: * 7 FETCH (FLAGS (\Deleted \Answered \Seen) MODSEQ
(12121231777))
S: C270 OK Noop completed
...but not in session 3.
C: D300 NOOP
S: D300 OK Noop completed
And, finally, the user removes flags \Answered (shared) and \Seen
(private) in session 1.
C: A330 STORE 7 -FLAGS.SILENT (\Answered \Seen)
S: * 7 FETCH (MODSEQ (12121245160))
S: A330 OK Store completed
Both changes are reported in session 2...
C: C360 NOOP
S: * 7 FETCH (FLAGS (\Deleted) MODSEQ (12121245160))
S: C360 OK Noop completed
...and only changes to shared flags are reported in session 3.
C: D390 NOOP
S: * 7 FETCH (FLAGS (\Deleted) MODSEQ (12121245160))
S: D390 OK Noop completed
Example 14
Server implementers should also see Section 3.1.11 for additional
quality of implementation issues related to the FETCH command.
3.1.5. MODSEQ Search Criterion in SEARCH
The MODSEQ criterion for the SEARCH (or UID SEARCH) command allows a
client to search for the metadata items that were modified since a
specified moment.
Syntax: MODSEQ [<entry-name> <entry-type-req>] <mod-sequence-valzer>
Messages that have modification values that are equal to or
greater than <mod-sequence-valzer>. This allows a client, for
example, to find out which messages contain metadata items that
have changed since the last time it updated its disconnected
cache. The client may also specify <entry-name> (name of the
metadata item) and <entry-type-req> (type of metadata item) before
<mod-sequence-valzer>. <entry-type-req> can be one of "shared",
"priv" (private), or "all". The last means that the server MUST
use the biggest value among "priv" and "shared" mod-sequences for
the metadata item. If the server doesn't store separate mod-
sequences for different metadata items, it MUST ignore <entry-
name> and <entry-type-req>. Otherwise, the server should use them
to narrow down the search.
For a flag <flagname>, the corresponding <entry-name> has the form
"/flags/<flagname>". Note that the leading "\" character that
denotes a system flag has to be escaped as per Section 4.3 of
[RFC3501], as <entry-name> uses the syntax for quoted strings (see
the examples below).
If the client specifies a MODSEQ criterion in a SEARCH (or UID
SEARCH) command and the server returns a non-empty SEARCH result, the
server MUST also append (to the end of the untagged SEARCH response)
the highest mod-sequence for all messages being returned. See also
Section 3.1.6. Note that other IMAP extensions such as ESEARCH
[RFC4731] can override this requirement (see Section 3.1.10 for more
details.)
C: a SEARCH MODSEQ "/flags/\\draft" all 620162338
S: * SEARCH 2 5 6 7 11 12 18 19 20 23 (MODSEQ 917162500)
S: a OK Search complete
In the above example, the message numbers of any messages having a
mod-sequence equal to or greater than 620162338 for the "\Draft" flag
are returned in the search results.
Example 15
C: t SEARCH OR NOT MODSEQ 720162338 LARGER 50000
S: * SEARCH
S: t OK Search complete, nothing found
Example 16
3.1.6. Modified SEARCH Untagged Response
Data: zero or more numbers
mod-sequence value (omitted if no match)
This document extends the syntax of the untagged SEARCH response to
include the highest mod-sequence for all messages being returned.
If a client specifies a MODSEQ criterion in a SEARCH (or UID SEARCH)
command and the server returns a non-empty SEARCH result, the server
MUST also append (to the end of the untagged SEARCH response) the
highest mod-sequence for all messages being returned. See
Section 3.1.5 for examples.
3.1.7. HIGHESTMODSEQ Status Data Items
This document defines a new status data item: HIGHESTMODSEQ: The highest mod-sequence value of all messages in the mailbox. This is the same value that is returned by the server in the HIGHESTMODSEQ response code in an OK untagged response (see Section 3.1.2.1). If the server doesn't support the persistent storage of mod-sequences for the mailbox (see Section 3.1.2.2), the server MUST return 0 as the value of the HIGHESTMODSEQ status data item. C: A042 STATUS blurdybloop (UIDNEXT MESSAGES HIGHESTMODSEQ) S: * STATUS blurdybloop (MESSAGES 231 UIDNEXT 44292 HIGHESTMODSEQ 7011231777) S: A042 OK STATUS completed Example 173.1.8. CONDSTORE Parameter to SELECT and EXAMINE
The CONDSTORE extension defines a single optional select parameter, "CONDSTORE", which tells the server that it MUST include the MODSEQ FETCH response data items in all subsequent unsolicited FETCH responses. The CONDSTORE parameter to SELECT/EXAMINE helps avoid a race condition that might arise when one or more metadata items are modified in another session after the server has sent the HIGHESTMODSEQ response code and before the client was able to issue a CONDSTORE enabling command. C: A142 SELECT INBOX (CONDSTORE) S: * 172 EXISTS S: * 1 RECENT S: * OK [UNSEEN 12] Message 12 is first unseen S: * OK [UIDVALIDITY 3857529045] UIDs valid S: * OK [UIDNEXT 4392] Predicted next UID S: * FLAGS (\Answered \Flagged \Deleted \Seen \Draft) S: * OK [PERMANENTFLAGS (\Deleted \Seen \*)] Limited S: * OK [HIGHESTMODSEQ 715194045007] S: A142 OK [READ-WRITE] SELECT completed, CONDSTORE is now enabled Example 18
3.1.9. Interaction with IMAP SORT and THREAD Extensions
The MODSEQ Search Criterion (see Section 3.1.5) causes modifications to SORT [RFC5256] responses similar to modifications to SEARCH responses defined in Section 3.1.6: SORT Response Data: zero or more numbers mod-sequence value (omitted if no match) This document extends the syntax of the untagged SORT response to include the highest mod-sequence for all messages being returned. If a client specifies a MODSEQ criterion in a SORT (or UID SORT) command and the server returns a non-empty SORT result, the server MUST also append (to the end of the untagged SORT response) the highest mod-sequence for all messages being returned. Note that other IMAP extensions such as ESORT [RFC5267] can override this requirement (see Section 3.1.10 for more details.) THREAD commands that include a MODSEQ Search Criterion return THREAD responses as specified in [RFC5256], i.e., THREAD responses are unchanged by the CONDSTORE extension.3.1.10. Interaction with IMAP ESORT and ESEARCH Extensions
If a client specifies a MODSEQ criterion in an extended SEARCH (or extended UID SEARCH) [RFC4731] command and the server returns a non- empty SEARCH result, the server MUST return the ESEARCH response containing the MODSEQ result option as defined in Section 3.2 of [RFC4731]. C: a SEARCH RETURN (ALL) MODSEQ 1234 S: * ESEARCH (TAG "a") ALL 1:3,5 MODSEQ 1236 S: a OK Extended SEARCH completed Example 19 If a client specifies a MODSEQ criterion in an extended SORT (or extended UID SORT) [RFC5267] command and the server returns a non- empty SORT result, the server MUST return the ESEARCH response containing the MODSEQ result option defined in Section 3.2 of [RFC4731]. C: a SORT RETURN (ALL) (DATE) UTF-8 MODSEQ 1234 S: * ESEARCH (TAG "a") ALL 5,3,2,1 MODSEQ 1236 S: a OK Extended SORT completed Example 20
3.1.11. Additional Quality-of-Implementation Issues
Server implementations should follow the following rule, which applies to any successfully completed STORE/UID STORE (with and without an UNCHANGEDSINCE modifier), as well as to a FETCH command that implicitly sets the \Seen flag: Adding the flag when it is already present or removing it when it is not present SHOULD NOT change the mod-sequence. This will prevent spurious client synchronization requests. However, note that client implementers MUST NOT rely on this server behavior. A client can't distinguish between the case when a server has violated the SHOULD mentioned above and when one or more clients set and unset (or unset and set) the flag in another session.3.1.12. CONDSTORE Server Implementation Considerations
This section describes how a server implementation that doesn't store separate per-metadata mod-sequences for different metadata items can avoid sending the MODIFIED response to any of the following conditional STORE operations: +FLAGS -FLAGS +FLAGS.SILENT -FLAGS.SILENT Note that the optimization described in this section can't be performed in case of a conditional STORE FLAGS (without "+" or "-") operation. Let's use the following example. The client has issued: C: a106 STORE 100:150 (UNCHANGEDSINCE 212030000000) +FLAGS.SILENT ($Processed) When the server receives the command and parses it successfully, it iterates through the message set and tries to execute the conditional STORE command for each message.
Each server internally works as a client, i.e., it has to cache the
current state of all IMAP flags as it is known to the client. In
order to report flag changes to the client, the server compares the
cached values with the values in its database for IMAP flags.
Imagine that another client has changed the state of a flag \Deleted
on the message 101 and that the change updated the mod-sequence for
the message. The server knows that the mod-sequence for the mailbox
has changed; however, it also knows that:
a. the client is not interested in the \Deleted flag, as it hasn't
included it in the +FLAGS.SILENT operation and
b. the state of the flag $Processed hasn't changed (the server can
determine this by comparing the cached flag state with the state
of the flag in the database).
Therefore, the server doesn't have to report MODIFIED to the client.
Instead, the server may set the $Processed flag, update the mod-
sequence for the message 101 once again, and send an untagged FETCH
response with a new mod-sequence and flags:
S: * 101 FETCH (MODSEQ (303011130956) FLAGS ($Processed \Deleted
\Answered))
See also Section 3.1.11 for additional quality-of-implementation
issues.
(page 24 continued on part 2)
