IMAP Extensions: Quick Flag Changes Resynchronization (CONDSTORE) and Quick Mailbox Resynchronization (QRESYNC)
RFC 7162
| Document | Type |
RFC
- Proposed Standard
(May 2014)
Errata
Updates RFC 2683
|
|
|---|---|---|---|
| Authors | Alexey Melnikov , Dave Cridland | ||
| Last updated | 2020-02-22 | ||
| Replaces | draft-melnikov-5162bis, draft-ietf-qresync-rfc4551bis, draft-ietf-qresync-rfc4551bis | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Formats | |||
| Reviews |
GENART Telechat review
Ready
GENART Last Call review
(of
-09)
Ready with Issues
SECDIR Last Call review
(of
-09)
Has Issues
|
||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | Submitted to IESG for Publication | |
| Document shepherd | Eliot Lear | ||
| Shepherd write-up | Show Last changed 2014-01-09 | ||
| IESG | IESG state | RFC 7162 (Proposed Standard) | |
| Action Holders |
(None)
|
||
| Consensus boilerplate | Yes | ||
| Telechat date | (None) | ||
| Responsible AD | Barry Leiba | ||
| Send notices to | (None) | ||
| IANA | IANA review state | IANA OK - Actions Needed | |
| IANA action state | RFC-Ed-Ack |
RFC 7162
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.
Melnikov & Cridland Standards Track [Page 1]
RFC 7162 IMAP CONDSTORE & QRESYNC May 2014
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
Melnikov & Cridland Standards Track [Page 2]
RFC 7162 IMAP CONDSTORE & QRESYNC May 2014
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
Melnikov & Cridland Standards Track [Page 3]
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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.
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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
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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.
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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
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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.
Melnikov & Cridland Standards Track [Page 8]
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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 1
3.1.2.2. 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).
Melnikov & Cridland Standards Track [Page 9]
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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
Melnikov & Cridland Standards Track [Page 10]
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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 $Processed)
S: * 50 FETCH (MODSEQ (12111230047))
S: a104 OK Store (conditional) completed
Example 4
C: c101 STORE 50 (UNCHANGEDSINCE 12121230045) -FLAGS.SILENT
(\Deleted)
S: * OK [HIGHESTMODSEQ 12111230047]
S: * 50 FETCH (MODSEQ (12111230048))
S: c101 OK Store (conditional) completed
The HIGHESTMODSEQ response code was sent by the server presumably
because this was the first CONDSTORE enabling command.
Example 5
The failure of the conditional STORE operation for any particular
message or messages (7 in this example) does not stop the server from
finding all messages that fail the UNCHANGEDSINCE test. All such
messages are returned in the MODIFIED response code.
Melnikov & Cridland Standards Track [Page 11]
RFC 7162 IMAP CONDSTORE & QRESYNC May 2014
C: d105 STORE 7,5,9 (UNCHANGEDSINCE 320162338)
+FLAGS.SILENT (\Deleted)
S: * 5 FETCH (MODSEQ (320162350))
S: d105 OK [MODIFIED 7,9] Conditional STORE failed
Example 6
Same as above, but the server follows the SHOULD recommendation in
Section 6.4.6 of [RFC3501].
C: d105 STORE 7,5,9 (UNCHANGEDSINCE 320162338)
+FLAGS.SILENT (\Deleted)
S: * 7 FETCH (MODSEQ (320162342) FLAGS (\Seen \Deleted))
S: * 5 FETCH (MODSEQ (320162350))
S: * 9 FETCH (MODSEQ (320162349) FLAGS (\Answered))
S: d105 OK [MODIFIED 7,9] Conditional STORE failed
Use of UNCHANGEDSINCE with a modification sequence of 0 always fails
if the metadata item exists. A system flag MUST always be considered
existent, whether it was set or not.
Example 7
C: a102 STORE 12 (UNCHANGEDSINCE 0)
+FLAGS.SILENT ($MDNSent)
S: a102 OK [MODIFIED 12] Conditional STORE failed
The client has tested the presence of the $MDNSent user-defined
keyword.
Example 8
Note: A client trying to make an atomic change to the state of a
particular metadata item (or a set of metadata items) MUST be
prepared to deal with the case when the server returns the MODIFIED
response code if the state of the metadata item being watched hasn't
changed (but the state of some other metadata item has). This is
necessary because some servers don't store separate mod-sequences for
different metadata items. However, a server implementation SHOULD
avoid generating spurious MODIFIED responses for +FLAGS/-FLAGS STORE
operations, even when the server stores a single mod-sequence per
message. Section 3.1.12 describes how this can be achieved.
Unless the server has included an unsolicited FETCH to update the
client's knowledge about messages that have failed the UNCHANGEDSINCE
test, upon receipt of the MODIFIED response code, the client SHOULD
try to figure out if the required metadata items have indeed changed
Melnikov & Cridland Standards Track [Page 12]
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by issuing the FETCH or NOOP command. It is RECOMMENDED that the
server avoids the need for the client to do that by sending an
unsolicited FETCH response (see Examples 9 and 10).
If the required metadata items haven't changed, the client SHOULD
retry the command with the new mod-sequence. The client needs to
allow for a reasonable number of retries (at least 2).
In the example below, the server returns the MODIFIED response code
without sending information describing why the STORE UNCHANGEDSINCE
operation has failed.
C: a106 STORE 100:150 (UNCHANGEDSINCE 212030000000)
+FLAGS.SILENT ($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.
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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
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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.
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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
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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.
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(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
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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.
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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.
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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 17
3.1.8. 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
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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
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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.
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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.
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
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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
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RFC 7162 IMAP CONDSTORE & QRESYNC May 2014
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
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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.
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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)
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
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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)
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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
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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:
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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
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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].
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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 done
3.2.9. 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.
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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.
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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.
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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
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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.
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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.
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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
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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
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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.
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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.
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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.
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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
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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.
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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
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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.
Melnikov & Cridland Standards Track [Page 48]
RFC 7162 IMAP CONDSTORE & QRESYNC May 2014
[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.
Melnikov & Cridland Standards Track [Page 49]
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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.
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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.
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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
Melnikov & Cridland Standards Track [Page 52]