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Network Working Group A. Melnikov, Ed.
Request for Comments: 4549 Isode Ltd.
Category: Informational June 2006
Synchronization Operations for Disconnected IMAP4 Clients
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This document attempts to address some of the issues involved in
building a disconnected IMAP4 client. In particular, it deals with
the issues of what might be called the "driver" portion of the
synchronization tool: the portion of the code responsible for issuing
the correct set of IMAP4 commands to synchronize the disconnected
client in the way that is most likely to make the human who uses the
disconnected client happy.
This note describes different strategies that can be used by
disconnected clients and shows how to use IMAP protocol in order to
minimize the time of the synchronization process.
This note also lists IMAP extensions that a server should implement
in order to provide better synchronization facilities to disconnected
clients.
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RFC 4549 Synch Ops for Disconnected IMAP4 Clients June 2006
Table of Contents
1. Introduction ....................................................3
1.1. Conventions Used in This Document ..........................3
2. Design Principles ...............................................3
3. Overall Picture of Synchronization ..............................4
4. Mailbox Synchronization Steps and Strategies ....................7
4.1. Checking UID Validity ......................................7
4.2. Synchronizing Local Changes with the Server ................8
4.2.1. Uploading Messages to the Mailbox ...................8
4.2.2. Optimizing "move" and "copy" Operations .............9
4.2.3. Replaying Local Flag Changes .......................14
4.2.4. Processing Mailbox Compression (EXPUNGE) Requests ..15
4.2.5. Closing a Mailbox ..................................17
4.3. Details of "Normal" Synchronization of a Single Mailbox ...18
4.3.1. Discovering New Messages and Changes to Old
Messages ...........................................18
4.3.2. Searching for "Interesting" Messages. ..............20
4.3.3. Populating Cache with "Interesting" Messages. ......21
4.3.4. User-Initiated Synchronization .....................22
4.4. Special Case: Descriptor-Only Synchronization .............22
4.5. Special Case: Fast New-Only Synchronization ...............23
4.6. Special Case: Blind FETCH .................................23
5. Implementation Considerations ..................................24
5.1. Error Recovery during Playback ............................26
5.2. Quality of Implementation Issues ..........................28
5.3. Optimizations .............................................28
6. IMAP Extensions That May Help ..................................30
6.1. CONDSTORE Extension .......................................30
7. Security Considerations ........................................33
8. References .....................................................33
8.1. Normative References ......................................33
8.2. Informative References ....................................34
9. Acknowledgements ...............................................34
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1. Introduction
Several recommendations presented in this document are generally
applicable to all types of IMAP clients. However, this document
tries to concentrate on disconnected mail clients [IMAP-MODEL]. It
also suggests some IMAP extensions* that should be implemented by
IMAP servers in order to make the life of disconnected clients
easier. In particular, the [UIDPLUS] extension was specifically
designed to streamline certain disconnected operations, like
expunging, uploading, and copying messages (see Sections 4.2.1,
4.2.2.1, and 4.2.4).
Readers of this document are also strongly advised to read RFC 2683
[RFC2683].
* Note that the functionality provided by the base IMAP protocol
[IMAP4] is sufficient to perform basic synchronization.
1.1. Conventions Used in This Document
In examples, "C:" and "S:" indicate lines sent by the client and
server, respectively. Long lines in examples are broken for
editorial clarity.
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 RFC 2119 [KEYWORDS].
Let's call an IMAP command idempotent if the result of executing the
command twice sequentially is the same as the result of executing the
command just once.
2. Design Principles
All mailbox state or content information stored on the disconnected
client should be viewed strictly as a cache of the state of the
server. The "master" state remains on the server, just as it would
with an interactive IMAP4 client. The one exception to this rule is
that information about the state of the disconnected client's cache
(the state includes flag changes while offline and during scheduled
message uploads) remains on the disconnected client: that is, the
IMAP4 server is not responsible for remembering the state of the
disconnected IMAP4 client.
We assume that a disconnected client is a client that, for whatever
reason, wants to minimize the length of time that it is "on the
phone" to the IMAP4 server. Often this will be because the client is
using a dialup connection, possibly with very low bandwidth, but
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sometimes it might just be that the human is in a hurry to catch an
airplane, or some other event beyond our control. Whatever the
reason, we assume that we must make efficient use of the network
connection, both in the usual sense (not generating spurious traffic)
and in the sense that we would prefer not to have the connection
sitting idle while the client and/or the server is performing
strictly local computation or I/O. Another, perhaps simpler way of
stating this is that we assume that network connections are
"expensive".
Practical experience with disconnected mail systems has shown that
there is no single synchronization strategy that is appropriate for
all cases. Different humans have different preferences, and the same
human's preference will vary depending both on external circumstance
(how much of a hurry the human is in today) and on the value that the
human places on the messages being transferred. The point here is
that there is no way that the synchronization program can guess
exactly what the human wants to do, so the human will have to provide
some guidance.
Taken together, the preceding two principles lead to the conclusion
that the synchronization program must make its decisions based on
some kind of guidance provided by the human, by selecting the
appropriate options in the user interface or through some sort of
configuration file. Almost certainly, it should not pause for I/O
with the human in the middle of the synchronization process. The
human will almost certainly have several different configurations for
the synchronization program, for different circumstances.
Since a disconnected client has no way of knowing what changes might
have occurred to the mailbox while it was disconnected, message
numbers are not useful to a disconnected client. All disconnected
client operations should be performed using UIDs, so that the client
can be sure that it and the server are talking about the same
messages during the synchronization process.
3. Overall Picture of Synchronization
The basic strategy for synchronization is outlined below. Note that
the real strategy may vary from one application to another or may
depend on a synchronization mode.
a) Process any "actions" that were pending on the client that were
not associated with any mailbox. (In particular sending messages
composed offline with SMTP. This is not part of IMAP
synchronization, but it is mentioned here for completeness.)
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b) Fetch the current list of "interesting" mailboxes. (The
disconnected client should allow the user to skip this step
completely.)
c) "Client-to-server synchronization": for each IMAP "action" that
was pending on the client, do the following:
1) If the action implies opening a new mailbox (any operation that
operates on messages), open the mailbox. Check its UID
validity value (see Section 4.1 for more details) returned in
the UIDVALIDITY response code. If the UIDVALIDITY value
returned by the server differs, the client MUST empty the local
cache of the mailbox and remove any pending "actions" that
refer to UIDs in that mailbox (and consider them failed). Note
that this doesn't affect actions performed on client-generated
fake UIDs (see Section 5).
2) Perform the action. If the action is to delete a mailbox
(DELETE), make sure that the mailbox is closed first (see also
Section 3.4.12 of [RFC2683]).
d) "Server-to-client synchronization": for each mailbox that requires
synchronization, do the following:
1) Check the mailbox UIDVALIDITY (see Section 4.1 for more
details) with SELECT/EXAMINE/STATUS.
If UIDVALIDITY value returned by the server differs, the client
MUST
* empty the local cache of that mailbox;
* remove any pending "actions" that refer to UIDs in that
mailbox and consider them failed; and
* skip step 2-II.
2) Fetch the current "descriptors";
I) Discover new messages.
II) Discover changes to old messages.
3) Fetch the bodies of any "interesting" messages that the client
doesn't already have.
e) Close all open mailboxes not required for further operations (if
staying online) or disconnect all open connections (if going
offline).
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Terms used:
"Actions" are queued requests that were made by the human to the
client's Mail User Agent (MUA) software while the client was
disconnected.
We define "descriptors" as a set of IMAP4 FETCH data items.
Conceptually, a message's descriptor is that set of information that
allows the synchronization program to decide what protocol actions
are necessary to bring the local cache to the desired state for this
message; since this decision is really up to the human, this
information probably includes at least a few header fields intended
for human consumption. Exactly what will constitute a descriptor
depends on the client implementation. At a minimum, the descriptor
contains the message's UID and FLAGS. Other likely candidates are
the RFC822.SIZE, RFC822.HEADER, BODYSTRUCTURE, or ENVELOPE data
items.
Comments:
1) The list of actions should be ordered. For example, if the human
deletes message A1 in mailbox A, then expunges mailbox A, and then
deletes message A2 in mailbox A, the human will expect that
message A1 is gone and that message A2 is still present but is now
deleted.
By processing all the actions before proceeding with
synchronization, we avoid having to compensate for the local MUA's
changes to the server's state. That is, once we have processed
all the pending actions, the steps that the client must take to
synchronize itself will be the same no matter where the changes to
the server's state originated.
2) Steps a and b can be performed in parallel. Alternatively, step a
can be performed after d.
3) On step b, the set of "interesting" mailboxes pretty much has to
be determined by the human. What mailboxes belong to this set may
vary between different IMAP4 sessions with the same server,
client, and human. An interesting mailbox can be a mailbox
returned by LSUB command (see Section 6.3.9 of [IMAP4]). The
special mailbox "INBOX" SHOULD be in the default set of mailboxes
that the client considers interesting. However, providing the
ability to ignore INBOX for a particular session or client may be
valuable for some mail filtering strategies.
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4) On step d-2-II, the client also finds out about changes to the
flags of messages that the client already has in its local cache,
and about messages in the local cache that no longer exist on the
server (i.e., messages that have been expunged).
5) "Interesting" messages are those messages that the synchronization
program thinks the human wants to have cached locally, based on
the configuration and the data retrieved in step b.
6) A disconnected IMAP client is a special case of an IMAP client, so
it MUST be able to handle any "unexpected" unsolicited responses,
like EXISTS and EXPUNGE, at any time. The disconnected client MAY
ignore EXPUNGE response during "client-to-server" synchronization
phase (step c).
The rest of this discussion will focus primarily on the
synchronization issues for a single mailbox.
4. Mailbox Synchronization Steps and Strategies
4.1. Checking UID Validity
The "UID validity" of a mailbox is a number returned in an
UIDVALIDITY response code in an OK untagged response at mailbox
selection time. The UID validity value changes between sessions when
UIDs fail to persist between sessions.
Whenever the client selects a mailbox, the client must compare the
returned UID validity value with the value stored in the local cache.
If the UID validity values differ, the UIDs in the client's cache are
no longer valid. The client MUST then empty the local cache of that
mailbox and remove any pending "actions" that refer to UIDs in that
mailbox. The client MAY also issue a warning to the human. The
client MUST NOT cancel any scheduled uploads (i.e., APPENDs) for the
mailbox.
Note that UIDVALIDITY is not only returned on a mailbox selection.
The COPYUID and APPENDUID response codes defined in the [UIDPLUS]
extension (see also 4.2.2) and the UIDVALIDITY STATUS response data
item also contain a UIDVALIDITY value for some other mailbox. The
client SHOULD behave as described in the previous paragraph (but it
should act on the other mailbox's cache), no matter how it obtained
the UIDVALIDITY value.
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4.2. Synchronizing Local Changes with the Server
4.2.1. Uploading Messages to the Mailbox
Two of the most common examples of operations resulting in message
uploads are:
1) Saving a draft message
2) Copying a message between remote mailboxes on two different IMAP
servers or a local mailbox and a remote mailbox.
Message upload is performed with the APPEND command. A message
scheduled to be uploaded has no UID associated with it, as all UIDs
are assigned by the server. The APPEND command will effectively
associate a UID with the uploaded message that can be stored in the
local cache for future reference. However, [IMAP4] doesn't describe
a simple mechanism to discover the message UID by just performing the
APPEND command. In order to discover the UID, the client can do one
of the following:
1) Remove the uploaded message from cache. Then, use the mechanism
described in 4.3 to fetch the information about the uploaded
message as if it had been uploaded by some other client.
2) Try to fetch header information as described in 4.2.2 in order to
find a message that corresponds to the uploaded message. One
strategy for doing this is described in 4.2.2.
Case 1 describes a not particularly smart client.
C: A003 APPEND Drafts (\Seen $MDNSent) {310}
S: + Ready for literal data
C: Date: Mon, 7 Feb 1994 21:52:25 -0800 (PST)
C: From: Fred Foobar <foobar@blt.example.COM>
C: Subject: afternoon meeting
C: To: mooch@owatagu.siam.edu
C: Message-Id: <B27397-0100000@blt.example.COM>
C: MIME-Version: 1.0
C: Content-Type: TEXT/PLAIN; CHARSET=US-ASCII
C:
C: Hello Joe, do you think we can meet at 3:30 tomorrow?
C:
S: A003 OK APPEND Completed
Fortunately, there is a simpler way to discover the message UID in
the presence of the [UIDPLUS] extension:
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C: A003 APPEND Drafts (\Seen $MDNSent) {310}
S: + Ready for literal data
C: Date: Mon, 7 Feb 1994 21:52:25 -0800 (PST)
C: From: Fred Foobar <foobar@blt.example.COM>
C: Subject: afternoon meeting
C: To: mooch@owatagu.siam.edu
C: Message-Id: <B27397-0100000@blt.example.COM>
C: MIME-Version: 1.0
C: Content-Type: TEXT/PLAIN; CHARSET=US-ASCII
C:
C: Hello Joe, do you think we can meet at 3:30 tomorrow?
C:
S: A003 OK [APPENDUID 1022843275 77712] APPEND completed
The UID of the appended message is the second parameter of APPENDUID
response code.
4.2.2. Optimizing "move" and "copy" Operations
Practical experience with IMAP and other mailbox access protocols
that support multiple mailboxes suggests that moving a message from
one mailbox to another is an extremely common operation.
4.2.2.1. Moving a Message between Two Mailboxes on the Same Server
In IMAP4, a "move" operation between two mailboxes on the same server
is really a combination of a COPY operation and a STORE +FLAGS
(\Deleted) operation. This makes good protocol sense for IMAP, but
it leaves a simple-minded disconnected client in the silly position
of deleting and possibly expunging its cached copy of a message, then
fetching an identical copy via the network.
However, the presence of the UIDPLUS extension in the server can
help:
C: A001 UID COPY 567,414 "Interesting Messages"
S: A001 OK [COPYUID 1022843275 414,567 5:6] Completed
This tells the client that the message with UID 414 in the current
mailbox was successfully copied to the mailbox "Interesting Messages"
and was given the UID 5, and that the message with UID 567 was given
the UID 6.
In the absence of UIDPLUS extension support in the server, the
following trick can be used. By including the Message-ID: header and
the INTERNALDATE data item as part of the descriptor, the client can
check the descriptor of a "new" message against messages that are
already in its cache and avoid fetching the extra copy. Of course,
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it's possible that the cost of checking to see if the message is
already in the local cache may exceed the cost of just fetching it,
so this technique should not be used blindly. If the MUA implements
a "move" command, it makes special provisions to use this technique
when it knows that a copy/delete sequence is the result of a "move"
command.
Note that servers are not required (although they are strongly
encouraged with "SHOULD language") to preserve INTERNALDATE when
copying messages.
Also note that since it's theoretically possible for this algorithm
to find the wrong message (given sufficiently malignant Message-ID
headers), implementers should provide a way to disable this
optimization, both permanently and on a message-by-message basis.
Example 1: Copying a message in the absence of UIDPLUS extension.
At some point in time the client has fetched the source message and
some information was cached:
C: C021 UID FETCH <uids> (BODY.PEEK[] INTERNALDATE FLAGS)
...
S: * 27 FETCH (UID 123 INTERNALDATE "31-May-2002 05:26:59 -0600"
FLAGS (\Draft $MDNSent) BODY[] {1036}
S: ...
S: Message-Id: <20040903110856.22a127cd@chardonnay>
S: ...
S: ...message body...
S: )
...
S: C021 OK fetch completed
Later on, the client decides to copy the message:
C: C035 UID COPY 123 "Interesting Messages"
S: C035 OK Completed
As the server hasn't provided the COPYUID response code, the client
tries the optimization described above:
C: C036 SELECT "Interesting Messages"
...
C: C037 UID SEARCH ON 31-May-2002 HEADER
"Message-Id" "20040903110856.22a127cd@chardonnay"
S: SEARCH 12368
S: C037 OK completed
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RFC 4549 Synch Ops for Disconnected IMAP4 Clients June 2006
Note that if the server has returned multiple UIDs in the SEARCH
response, the client MUST NOT use any of the returned UID.
4.2.2.2. Moving a Message from a Remote Mailbox to a Local
Moving a message from a remote mailbox to a local is done with FETCH
(that includes FLAGS and INTERNALDATE) followed by UID STORE <uid>
+FLAGS.SILENT (\Deleted):
C: A003 UID FETCH 123 (BODY.PEEK[] INTERNALDATE FLAGS)
S: * 27 FETCH (UID 123 INTERNALDATE "31-May-2002 05:26:59 -0600"
FLAGS (\Seen $MDNSent) BODY[]
S: ...message body...
S: )
S: A003 OK UID FETCH completed
C: A004 UID STORE <uid> +FLAGS.SILENT (\Deleted)
S: A004 STORE completed
Note that there is no reason to fetch the message during
synchronization if it's already in the client's cache. Also, the
client SHOULD preserve delivery date in the local cache.
4.2.2.3. Moving a Message from a Local Mailbox to a Remote
Moving a message from a local mailbox to a remote is done with
APPEND:
C: A003 APPEND Drafts (\Seen $MDNSent) "31-May-2002 05:26:59 -0600"
{310}
S: + Ready for literal data
C: Date: Mon, 7 Feb 1994 21:52:25 -0800 (PST)
C: From: Fred Foobar <foobar@blt.example.COM>
C: Subject: afternoon meeting
C: To: mooch@owatagu.siam.edu
C: Message-Id: <B27397-0100000@blt.example.COM>
C: MIME-Version: 1.0
C: Content-Type: TEXT/PLAIN; CHARSET=US-ASCII
C:
C: Hello Joe, do you think we can meet at 3:30 tomorrow?
C:
S: A003 OK [APPENDUID 1022843275 77712] completed
The client SHOULD specify the delivery date from the local cache in
the APPEND.
If the [LITERAL+] extension is available, the client can save a
round-trip*:
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C: A003 APPEND Drafts (\Seen $MDNSent) "31-May-2002 05:26:59 -0600"
{310+}
C: Date: Mon, 7 Feb 1994 21:52:25 -0800 (PST)
C: From: Fred Foobar <foobar@blt.example.COM>
C: Subject: afternoon meeting
C: To: mooch@owatagu.siam.edu
C: Message-Id: <B27397-0100000@blt.example.COM>
C: MIME-Version: 1.0
C: Content-Type: TEXT/PLAIN; CHARSET=US-ASCII
C:
C: Hello Joe, do you think we can meet at 3:30 tomorrow?
C:
S: A003 OK [APPENDUID 1022843275 77712] completed
* Note that there is a risk that the server will reject the message
due to its size. If this happens, the client will waste bandwidth
transferring the whole message. If the client wouldn't have used
the LITERAL+, this could have been avoided:
C: A003 APPEND Drafts (\Seen $MDNSent) "31-May-2004 05:26:59 -0600"
{16777215}
S: A003 NO Sorry, message is too big
4.2.2.4. Moving a Message between Two Mailboxes on Different Servers
Moving a message between two mailbox on two different servers is a
combination of the operations described in 4.2.2.2 followed by the
operations described in 4.2.2.3.
4.2.2.5. Uploading Multiple Messages to a Remote Mailbox with
MULTIAPPEND
When there is a need to upload multiple messages to a remote mailbox
(e.g., as per 4.2.2.3), the presence of certain IMAP extensions may
significantly improve performance. One of them is [MULTIAPPEND].
For some mail stores, opening a mailbox for appending might be
expensive. [MULTIAPPEND] tells the server to open the mailbox once
(instead of opening and closing it "n" times per "n" messages to be
uploaded) and to keep it open while a group of messages is being
uploaded to the server.
Also, if the server supports both [MULTIAPPEND] and [LITERAL+]
extensions, the entire upload is accomplished in a single
command/response round-trip.
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Note: Client implementers should be aware that [MULTIAPPEND] performs
append of multiple messages atomically. This means, for example, if
there is not enough space to save "n"-th message (or the message has
invalid structure and is rejected by the server) after successful
upload of "n-1" messages, the whole upload operation fails, and no
message will be saved in the mailbox. Although this behavior might
be desirable in certain situations, it might not be what you want.
Otherwise, the client should use the regular APPEND command (Section
4.2.2.3), possibly utilizing the [LITERAL+] extension. See also
Section 5.1 for discussions about error recovery.
Note: MULTIAPPEND can be used together with the UIDPLUS extension in
a way similar to what was described in Section 4.2.1. [MULTIAPPEND]
extends the syntax of the APPENDUID response code to allow for
multiple message UIDs in the second parameter.
Example 2:
This example demonstrates the use of MULTIAPPEND together with
UIDPLUS (synchronization points where the client waits for
confirmations from the server are marked with "<--->"):
C: A003 APPEND Jan-2002 (\Seen $MDNSent) "31-May-2002 05:26:59 -0600"
{310}
<--->
S: + Ready for literal data
C: Date: Mon, 7 Feb 1994 21:52:25 -0800 (PST)
C: From: Fred Foobar <foobar@blt.example.COM>
C: Subject: afternoon meeting
C: To: mooch@owatagu.siam.edu
C: Message-Id: <B27397-0100000@blt.example.COM>
C: MIME-Version: 1.0
C: Content-Type: TEXT/PLAIN; CHARSET=US-ASCII
C:
C: Hello Joe, do you think we can meet at 3:30 tomorrow?
C: (\Seen) " 1-Jun-2002 22:43:04 -0800" {286}
<--->
S: + Ready for literal data
C: Date: Mon, 7 Feb 1994 22:43:04 -0800 (PST)
C: From: Joe Mooch <mooch@OWaTaGu.siam.EDU>
C: Subject: Re: afternoon meeting
C: To: foobar@blt.example.com
C: Message-Id: <a0434793874930@OWaTaGu.siam.EDU>
C: MIME-Version: 1.0
C: Content-Type: TEXT/PLAIN; CHARSET=US-ASCII
C:
C: 3:30 is fine with me.
C:
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S: A003 OK [APPENDUID 1022843275 77712,77713] completed
The upload takes 3 round-trips.
Example 3:
In this example, Example 2 was modified for the case when the server
supports MULTIAPPEND, LITERAL+, and UIDPLUS. The upload takes only 1
round-trip.
C: A003 APPEND Jan-2002 (\Seen $MDNSent) "31-May-2002 05:26:59 -0600"
{310+}
C: Date: Mon, 7 Feb 1994 21:52:25 -0800 (PST)
C: From: Fred Foobar <foobar@blt.example.COM>
C: Subject: afternoon meeting
C: To: mooch@owatagu.siam.edu
C: Message-Id: <B27397-0100000@blt.example.COM>
C: MIME-Version: 1.0
C: Content-Type: TEXT/PLAIN; CHARSET=US-ASCII
C:
C: Hello Joe, do you think we can meet at 3:30 tomorrow?
C: (\Seen) " 1-Jun-2002 22:43:04 -0800" {286+}
C: Date: Mon, 7 Feb 1994 22:43:04 -0800 (PST)
C: From: Joe Mooch <mooch@OWaTaGu.siam.EDU>
C: Subject: Re: afternoon meeting
C: To: foobar@blt.example.com
C: Message-Id: <a0434793874930@OWaTaGu.siam.EDU>
C: MIME-Version: 1.0
C: Content-Type: TEXT/PLAIN; CHARSET=US-ASCII
C:
C: 3:30 is fine with me.
C:
S: A003 OK [APPENDUID 1022843275 77712,77713] completed
4.2.3. Replaying Local Flag Changes
The disconnected client uses the STORE command to synchronize local
flag state with the server. The disconnected client SHOULD use
+FLAGS.SILENT or -FLAGS.SILENT in order to set or unset flags
modified by the user while offline. The FLAGS form MUST NOT be used,
as there is a risk that this will overwrite flags on the server that
have been changed by some other client.
Example 4:
For the message with UID 15, the disconnected client stores the
following flags \Seen and $Highest. The flags were modified on the
server by some other client: \Seen, \Answered, and $Highest. While
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offline, the user requested that the $Highest flags be removed and
that the \Deleted flag be added. The flag synchronization sequence
for the message should look like:
C: A001 UID STORE 15 +FLAGS.SILENT (\Deleted)
S: A001 STORE completed
C: A002 UID STORE 15 -FLAGS.SILENT ($Highest)
S: A002 STORE completed
If the disconnected client is able to store an additional binary
state information (or a piece of information that can take a value
from a predefined set of values) in the local cache of an IMAP
mailbox or in a local mailbox (e.g., message priority), and if the
server supports storing of arbitrary keywords, the client MUST use
keywords to store this state on the server.
Example 5:
Imagine a speculative mail client that can mark a message as one of
work-related ($Work), personal ($Personal), or spam ($Spam). In
order to mark a message as personal, the client issues:
C: A001 UID STORE 15 +FLAGS.SILENT ($Personal)
S: A001 STORE completed
C: A002 UID STORE 15 -FLAGS.SILENT ($Work $Spam)
S: A002 STORE completed
In order to mark the message as not work, not personal and not spam,
the client issues:
C: A003 UID STORE 15 -FLAGS.SILENT ($Personal $Work $Spam)
S: A003 STORE completed
4.2.4. Processing Mailbox Compression (EXPUNGE) Requests
A naive disconnected client implementation that supports compressing
a mailbox while offline may decide to issue an EXPUNGE command to the
server in order to expunge messages marked \Deleted. The problem
with this command during synchronization is that it permanently
erases all messages with the \Deleted flag set, i.e., even those
messages that were marked as \Deleted on the server while the user
was offline. Doing this might result in an unpleasant surprise for
the user.
Fortunately the [UIDPLUS] extension can help in this case as well.
The extension introduces UID EXPUNGE command, that, unlike EXPUNGE,
takes a UID set parameter, that lists UIDs of all messages that can
be expunged. When processing this command the server erases only
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messages with \Deleted flag listed in the UID list. Thus, messages
not listed in the UID set will not be expunged even if they have the
\Deleted flag set.
Example 6:
While the user was offline, 3 messages with UIDs 7, 27, and 65 were
marked \Deleted when the user requested to compress the open mailbox.
Another client marked a message \Deleted on the server (UID 34).
During synchronization, the disconnected client issues:
C: A001 UID EXPUNGE 7,27,65
S: * ... EXPUNGE
S: * ... EXPUNGE
S: * ... EXPUNGE
S: A001 UID EXPUNGE completed
If another client issues UID SEARCH DELETED command (to find all
messages with the \Deleted flag) before and after the UID EXPUNGE, it
will get:
Before:
C: B001 UID SEARCH DELETED
S: * SEARCH 65 34 27 7
S: B001 UID SEARCH completed
After:
C: B002 UID SEARCH DELETED
S: * SEARCH 34
S: B002 UID SEARCH completed
In the absence of the [UIDPLUS] extension, the following sequence of
commands can be used as an approximation. Note: It's possible for
another client to mark additional messages as deleted while this
sequence is being performed. In this case, these additional messages
will be expunged as well.
1) Find all messages marked \Deleted on the server.
C: A001 UID SEARCH DELETED
S: * SEARCH 65 34 27 7
S: A001 UID SEARCH completed
2) Find all messages that must not be erased (for the previous
example the list will consist of the message with UID 34).
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3) Temporarily remove \Deleted flag on all messages found in step 2.
C: A002 UID STORE 34 -FLAGS.SILENT (\Deleted)
S: A002 UID STORE completed
4) Expunge the mailbox.
C: A003 EXPUNGE
S: * 20 EXPUNGE
S: * 7 EXPUNGE
S: * 1 EXPUNGE
S: A003 EXPUNGE completed
Here, the message with UID 7 has message number 1, with UID 27 has
message number 7, and with UID 65 has message number 20.
5) Restore \Deleted flag on all messages found when performing step
2.
C: A004 UID STORE 34 +FLAGS.SILENT (\Deleted)
S: A004 UID STORE completed
4.2.5. Closing a Mailbox
When the disconnected client has to close a mailbox, it should not
use the CLOSE command, because CLOSE does a silent EXPUNGE. (Section
4.2.4 explains why EXPUNGE should not be used by a disconnected
client.) It is safe to use CLOSE only if the mailbox was opened with
EXAMINE.
If the mailbox was opened with SELECT, the client can use one of the
following commands to implicitly close the mailbox and prevent the
silent expunge:
1) UNSELECT - This is a command described in [UNSELECT] that works as
CLOSE, but doesn't cause the silent EXPUNGE. This command is
supported by the server if it reports UNSELECT in its CAPABILITY
list.
2) SELECT <another_mailbox> - SELECT causes implicit CLOSE without
EXPUNGE.
3) If the client intends to issue LOGOUT after closing the mailbox,
it may just issue LOGOUT, because LOGOUT causes implicit CLOSE
without EXPUNGE as well.
4) SELECT <non_existing_mailbox> - If the client knows a mailbox that
doesn't exist or can't be selected, it MAY SELECT it.
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If the client opened the mailbox with SELECT and just wants to avoid
implicit EXPUNGE without closing the mailbox, it may also use the
following:
5) EXAMINE <mailbox> - Reselect the same mailbox in read-only mode.
4.3. Details of "Normal" Synchronization of a Single Mailbox
The most common form of synchronization is where the human trusts the
integrity of the client's copy of the state of a particular mailbox
and simply wants to bring the client's cache up to date so that it
accurately reflects the mailbox's current state on the server.
4.3.1. Discovering New Messages and Changes to Old Messages
Let <lastseenuid> represent the highest UID that the client knows
about in this mailbox. Since UIDs are allocated in strictly
ascending order, this is simply the UID of the last message in the
mailbox that the client knows about. Let <lastseenuid+1> represent
<lastseenuid>'s UID plus one. Let <descriptors> represent a list
consisting of all the FETCH data item items that the implementation
considers part of the descriptor; at a minimum this is just the FLAGS
data item, but it usually also includes BODYSTRUCTURE and
RFC822.SIZE. At this step, <descriptors> SHOULD NOT include RFC822.
With no further information, the client can issue the following two
commands:
tag1 UID FETCH <lastseenuid+1>:* <descriptors>
tag2 UID FETCH 1:<lastseenuid> FLAGS
The first command will request some information about "new" messages
(i.e., messages received by the server since the last
synchronization). It will also allow the client to build a message
number to UID map (only for new messages). The second command allows
the client to
1) update cached flags for old messages;
2) find out which old messages got expunged; and
3) build a mapping between message numbers and UIDs (for old
messages).
The order here is significant. We want the server to start returning
the list of new message descriptors as fast as it can, so that the
client can start issuing more FETCH commands, so we start out by
asking for the descriptors of all the messages we know the client
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cannot possibly have cached yet. The second command fetches the
information we need to determine what changes may have occurred to
messages that the client already has cached. Note that the former
command should only be issued if the UIDNEXT value cached by the
client differs from the one returned by the server. Once the client
has issued these two commands, there's nothing more the client can do
with this mailbox until the responses to the first command start
arriving. A clever synchronization program might use this time to
fetch its local cache state from disk or to start the process of
synchronizing another mailbox.
The following is an example of the first FETCH:
C: A011 UID fetch 131:* (FLAGS BODYSTRUCTURE INTERNALDATE
RFC822.SIZE)
Note 1: The first FETCH may result in the server's sending a huge
volume of data. A smart disconnected client should use message
ranges (see also Section 3.2.1.2 of [RFC2683]), so that the user is
able to execute a different operation between fetching information
for a group of new messages.
Example 7:
Knowing the new UIDNEXT returned by the server on SELECT or EXAMINE
(<uidnext>), the client can split the UID range
<lastseenuid+1>:<uidnext> into groups, e.g., 100 messages. After
that, the client can issue:
C: A011 UID fetch <lastseenuid+1>:<lastseenuid+100>
(FLAGS BODYSTRUCTURE INTERNALDATE RFC822.SIZE)
...
C: A012 UID fetch <lastseenuid+101>:<lastseenuid+200>
(FLAGS BODYSTRUCTURE INTERNALDATE RFC822.SIZE)
...
...
C: A0FF UID fetch <lastseenuid+901>:<uidnext>
(FLAGS BODYSTRUCTURE INTERNALDATE RFC822.SIZE)
Note that unless a SEARCH command is issued, it is impossible to
determine how many messages will fall into a subrange, as UIDs are
not necessarily contiguous.
Note 2: The client SHOULD ignore any unsolicited EXPUNGE responses
received during the first FETCH command. EXPUNGE responses contain
message numbers that are useless to a client that doesn't have the
message-number-to-UID translation table.
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The second FETCH command will result in zero or more untagged fetch
responses. Each response will have a corresponding UID FETCH data
item. All messages that didn't have a matching untagged FETCH
response MUST be removed from the local cache.
For example, if the <lastseenuid> had a value 15000 and the local
cache contained 3 messages with the UIDs 12, 777, and 14999,
respectively, then after receiving the following responses from the
server, the client must remove the message with UID 14999 from its
local cache.
S: * 1 FETCH (UID 12 FLAGS (\Seen))
S: * 2 FETCH (UID 777 FLAGS (\Answered \Deleted))
Note 3: If the client is not interested in flag changes (i.e., the
client only wants to know which old messages are still on the
server), the second FETCH command can be substituted with:
tag2 UID SEARCH UID 1:<lastseenuid>
This command will generate less traffic. However, an implementor
should be aware that in order to build the mapping table from message
numbers to UIDs, the output of the SEARCH command MUST be sorted
first, because there is no requirement for a server to return UIDs in
SEARCH response in any particular order.
4.3.2. Searching for "Interesting" Messages.
This step is performed entirely on the client (from the information
received in the step described in 4.3.1), entirely on the server, or
on some combination of both. The decision on what is an
"interesting" message is up to the client software and the human.
One easy criterion that should probably be implemented in any client
is whether the message is "too big" for automatic retrieval, where
"too big" is a parameter defined in the client's configuration.
Another commonly used criterion is the age of a message. For
example, the client may choose to download only messages received in
the last week (in this case, <date> would be today's date minus 7
days):
tag3 UID SEARCH UID <uidset> SINCE <date>
Keep in mind that a date search disregards time and time zone. The
client can avoid doing this search if it specified INTERNALDATE in
<descriptors> on the step described in 4.3.1. If the client did, it
can perform the local search on its message cache.
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At this step, the client also decides what kind of information about
a particular message to fetch from the server. In particular, even
for a message that is considered "too big", the client MAY choose to
fetch some part(s) of it. For example, if the message is a
multipart/mixed containing a text part and a MPEG attachment, there
is no reason for the client not to fetch the text part. The decision
of which part should or should not be fetched can be based on the
information received in the BODYSTRUCTURE FETCH response data item
(i.e., if BODYSTRUCTURE was included in <descriptors> on the step
described in 4.3.1).
4.3.3. Populating Cache with "Interesting" Messages.
Once the client has found out which messages are "interesting", it
can start issuing appropriate FETCH commands for "interesting"
messages or parts thereof.
Note that fetching a message into the disconnected client's local
cache does NOT imply that the human has (or even will) read the
message. Thus, the synchronization program for a disconnected client
should always be careful to use the .PEEK variants of the FETCH data
items that implicitly set the \Seen flag.
Once the last descriptor has arrived and the last FETCH command has
been issued, the client simply needs to process the incoming fetch
items and use them to update the local message cache.
In order to avoid deadlock problems, the client must give processing
of received messages priority over issuing new FETCH commands during
this synchronization process. This may necessitate temporary local
queuing of FETCH requests that cannot be issued without causing a
deadlock. In order to achieve the best use of the "expensive"
network connection, the client will almost certainly need to pay
careful attention to any flow-control information that it can obtain
from the underlying transport connection (usually a TCP connection).
Note: The requirement stated in the previous paragraph might result
in an unpleasant user experience, if followed blindly. For example,
the user might be unwilling to wait for the client to finish
synchronization before starting to process the user's requests. A
smart disconnected client should allow the user to perform requested
operations in between IMAP commands that are part of the
synchronization process. See also Note 1 in Section 4.3.1.
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Example 8:
After fetching a message BODYSTRUCTURE, the client discovers a
complex MIME message. Then, it decides to fetch MIME headers of the
nested MIME messages and some body parts.
C: A011 UID fetch 11 (BODYSTRUCTURE)
S: ...
C: A012 UID fetch 11 (BODY[HEADER] BODY[1.MIME] BODY[1.1.MIME]
BODY[1.2.MIME] BODY[2.MIME] BODY[3.MIME] BODY[4.MIME]
BODY[5.MIME] BODY[6.MIME] BODY[7.MIME] BODY[8.MIME] BODY[9.MIME]
BODY[10.MIME] BODY[11.MIME] BODY[12.MIME] BODY[13.MIME]
BODY[14.MIME] BODY[15.MIME] BODY[16.MIME] BODY[17.MIME]
BODY[18.MIME] BODY[19.MIME] BODY[20.MIME] BODY[21.MIME])
S: ...
C: A013 UID fetch 11 (BODY[1.1] BODY[1.2])
S: ...
C: A014 UID fetch 11 (BODY[3] BODY[4] BODY[5] BODY[6] BODY[7] BODY[8]
BODY[9] BODY[10] BODY[11] BODY[13] BODY[14] BODY[15] BODY[16]
BODY[21])
S: ...
4.3.4. User-Initiated Synchronization
After the client has finished the main synchronization process as
described in Sections 4.3.1-4.3.3, the user may optionally request
additional synchronization steps while the client is still online.
This is not any different from the process described in Sections
4.3.2 and 4.3.3.
Typical examples are:
1) fetch all messages selected in UI.
2) fetch all messages marked as \Flagged on the server.
4.4. Special Case: Descriptor-Only Synchronization
For some mailboxes, fetching the descriptors might be the entire
synchronization step. Practical experience with IMAP has shown that
a certain class of mailboxes (e.g., "archival" mailboxes) are used
primarily for long-term storage of important messages that the human
wants to have instantly available on demand but does not want
cluttering up the disconnected client's cache at any other time.
Messages in this kind of mailbox would be fetched exclusively by
explicit actions queued by the local MUA. Thus, the only
synchronization desirable on this kind of mailbox is fetching enough
descriptor information for the user to be able to identify messages
for subsequent download.
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Special mailboxes that receive messages from a high volume, low
priority mailing list might also be in this category, at least when
the human is in a hurry.
4.5. Special Case: Fast New-Only Synchronization
In some cases, the human might be in such a hurry that he or she
doesn't care about changes to old messages, just about new messages.
In this case, the client can skip the UID FETCH command that obtains
the flags and UIDs for old messages (1:<lastseenuid>).
4.6. Special Case: Blind FETCH
In some cases, the human may know (for whatever reason) that he or
she always wants to fetch any new messages in a particular mailbox,
unconditionally. In this case, the client can just fetch the
messages themselves, rather than just the descriptors, by using a
command like:
tag1 UID FETCH <lastseenuid+1>:* (FLAGS BODY.PEEK[])
Note that this example ignores the fact that the messages can be
arbitrary long. The disconnected client MUST always check for
message size before downloading, unless explicitly told otherwise. A
well-behaved client should instead use something like the following:
1) Issue "tag1 UID FETCH <lastseenuid+1>:* (FLAGS RFC822.SIZE)".
2) From the message sizes returned in step 1, construct UID set
<required_messages>.
3) Issue "tag2 UID FETCH <required_messages> (BODY.PEEK[])".
or
1) Issue "tag1 UID FETCH <lastseenuid+1>:* (FLAGS)".
2) Construct UID set <old_uids> from the responses of step 1.
3) Issue "tag2 SEARCH UID <old_uids> SMALLER <message_limit>".
Construct UID set <required_messages> from the result of the
SEARCH command.
4) Issue "tag3 UID FETCH <required_messages> (BODY.PEEK[])".
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or
1) Issue "tag1 UID FETCH <lastseenuid+1>:* (FLAGS
BODY.PEEK[]<0.<length>>)", where <length> should be replaced with
the maximal message size the client is willing to download.
Note: In response to such a command, the server will only return
partial data if the message is longer than <length>. It will
return the full message data for any message whose size is smaller
than or equal to <length>. In the former case, the client will
not be able to extract the full MIME structure of the message from
the truncated data, so the client should include BODYSTRUCTURE in
the UID FETCH command as well.
5. Implementation Considerations
Below are listed some common implementation pitfalls that should be
considered when implementing a disconnected client.
1) Implementing fake UIDs on the client.
A message scheduled to be uploaded has no UID, as UIDs are
selected by the server. The client may implement fake UIDs
internally in order to reference not-yet-uploaded messages in
further operations. (For example, a message could be scheduled to
be uploaded, but subsequently marked as deleted or copied to
another mailbox). Here, the client MUST NOT under any
circumstances send these fake UIDs to the server. Also, client
implementers should be reminded that according to [IMAP4] a UID is
a 32-bit unsigned integer excluding 0. So, both 4294967295 and
2147483648 are valid UIDs, and 0 and -1 are both invalid. Some
disconnected mail clients have been known to send negative numbers
(e.g., "-1") as message UIDs to servers during synchronization.
Situation 1: The user starts composing a new message, edits it,
saves it, continues to edit it, and saves it again.
A disconnected client may record in its replay log (log of
operations to be replayed on the server during synchronization)
the sequence of operations as shown below. For the purpose of
this situation, we assume that all draft messages are stored in
the mailbox called Drafts on an IMAP server. We will also use the
following conventions: <old_uid> is the UID of the intermediate
version of the draft when it was saved for the first time. This
is a fake UID generated on the client. <new_uid> is the UID of
the final version of the draft. This is another fake UID
generated on the client.
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1) APPEND Drafts (\Seen $MDNSent \Drafts) {<nnn>}
...first version of the message follows...
2) APPEND Drafts (\Seen $MDNSent \Drafts) {<mmm>}
...final version of the message follows...
3) STORE <old_uid> +FLAGS (\Deleted)
Step 1 corresponds to the first attempt to save the draft message,
step 2 corresponds to the second attempt to save the draft
message, and step 3 deletes the first version of the draft message
saved in step 1.
A naive disconnected client may send the command in step 3 without
replacing the fake client generated <old_uid> with the value
returned by the server in step 1. A server will probably reject
this command, which will make the client believe that the
synchronization sequence has failed.
2) Section 5.1 discusses common implementation errors related to
error recovery during playback.
3) Don't assume that the disconnected client is the only client used
by the user.
Situation 2: Some clients may use the \Deleted flag as an
indicator that the message should not appear in the user's view.
Usage of the \Deleted flag for this purpose is not safe, as other
clients (e.g., online clients) might EXPUNGE the mailbox at any
time.
4) Beware of data dependencies between synchronization operations.
It might be very tempting for a client writer to perform some
optimizations on the playback log. Such optimizations might
include removing redundant operations (for example, see
optimization 2 in Section 5.3), or their reordering.
It is not always safe to reorder or remove redundant operations
during synchronization because some operations may have
dependencies (as Situation 3 demonstrates). So, if in doubt,
don't do this.
Situation 3: The user copies a message out of a mailbox and then
deletes the mailbox.
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C: A001 SELECT Old-Mail
S: ...
C: A002 UID COPY 111 ToDo
S: A002 OK [COPYUID 1022843345 111 94] Copy completed
...
C: A015 CLOSE
S: A015 OK Completed
C: A016 DELETE Old-Mail
S: A016 OK Mailbox deletion completed successfully
If the client performs DELETE (tag A016) first and COPY (tag A002)
second, then the COPY fails. Also, the message that the user so
carefully copied into another mailbox has been lost.
5.1. Error Recovery during Playback
Error recovery during synchronization is one of the trickiest parts
to get right. Below, we will discuss certain error conditions and
suggest possible choices for handling them.
1) Lost connection to the server.
The client MUST remember the current position in the playback
(replay) log and replay it starting from the interrupted operation
(the last command issued by the client, but not acknowledged by
the server) the next time it successfully connects to the same
server. If the connection was lost while executing a non-
idempotent IMAP command (see the definition in Section 1), then
when the client is reconnected, it MUST make sure that the
interrupted command was indeed not executed. If it wasn't
executed, the client must restart playback from the interrupted
command, otherwise from the following command.
Upon reconnect, care must be taken in order to properly reapply
logical operations that are represented by multiple IMAP commands,
e.g., UID EXPUNGE emulation when UID EXPUNGE is not supported by
the server (see Section 4.2.4).
Once the client detects that the connection to the server was
lost, it MUST stop replaying its log. There are existing
disconnected clients that, to the great annoyance of users, pop up
an error dialog for each and every playback operation that fails.
2) Copying/appending messages to a mailbox that doesn't exist. (The
server advertises this condition by sending the TRYCREATE response
code in the tagged NO response to the APPEND or COPY command.)
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The user should be advised about the situation and be given one of
the following choices:
a) Try to recreate a mailbox.
b) Copy/upload messages to another mailbox.
c) Skip copy/upload.
d) Abort replay.
3) Copying messages from a mailbox that doesn't exist, or renaming or
getting/changing ACLs [ACL] on a mailbox that doesn't exist:
a) Skip operation.
b) Abort replay.
4) Deleting mailboxes or deleting/expunging messages that no longer
exist.
This is actually is not an error and should be ignored by the
client.
5) Performing operations on messages that no longer exist.
a) Skip operation.
b) Abort replay.
In the case of changing flags on an expunged message, the client
should silently ignore the error.
Note 1: Several synchronization operations map to multiple IMAP
commands (for example, "move" described in 4.2.2). The client must
guarantee atomicity of each such multistep operation. For example,
when performing a "move" between two mailboxes on the same server, if
the server is unable to copy messages, the client MUST NOT attempt to
set the \Deleted flag on the messages being copied, let alone expunge
them. However, the client MAY consider that move operation to have
succeeded even if the server was unable to set the \Deleted flag on
copied messages.
Note 2: Many synchronization operations have data dependencies. A
failed operation must cause all dependent operations to fail as well.
The client should check this and MUST NOT try to perform all
dependent operations blindly (unless the user corrected the original
problem). For example, a message may be scheduled to be appended to
a mailbox on the server and later on the appended message may be
copied to another mailbox. If the APPEND operation fails, the client
must not attempt to COPY the failed message later on. (See also
Section 5, Situation 3).
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5.2. Quality of Implementation Issues
Below, some quality of implementation issues are listed for
disconnected clients. They will help to write a disconnected client
that works correctly, performs synchronization as quickly as possible
(and thus can make the user happier as well as save her some money),
and minimizes the server load:
1) Don't lose information.
No matter how smart your client is in other areas, if it loses
information, users will get very upset.
2) Don't do work unless explicitly asked. Be flexible. Ask all
questions BEFORE starting synchronization, if possible.
3) Minimize traffic.
The client MUST NOT issue a command if the client already received
the required information from the server.
The client MUST make use of UIDPLUS extension if it is supported
by the server.
See also optimization 1 in Section 5.3.
4) Minimize the number of round-trips.
Round-trips kill performance, especially on links with high
latency. Sections 4.2.2.5 and 5.2 give some advice on how to
minimize the number of round-trips.
See also optimization 1 in Section 5.3.
5.3. Optimizations
Some useful optimizations are described in this section. A
disconnected client that supports the recommendations listed below
will give the user a more pleasant experience.
1) The initial OK or PREAUTH responses may contain the CAPABILITY
response code as described in Section 7.1 of [IMAP4]. This
response code gives the same information as returned by the
CAPABILITY command*. A disconnected client that pays attention to
this response code can avoid sending CAPABILITY command and will
save a round-trip.
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* Note: Some servers report in the CAPABILITY response code
extensions that are only relevant in unauthenticated state or in
all states. Such servers usually send another CAPABILITY
response code upon successful authentication using LOGIN or
AUTHENTICATE command (that negotiates no security layer; see
Section 6.2.2 of [IMAP4]). The CAPABILITY response code sent
upon successful LOGIN/AUTHENTICATE might be different from the
CAPABILITY response code in the initial OK response, as
extensions only relevant for unauthenticated state will not be
advertised, and some additional extensions available only in
authenticated and/or selected state will be.
Example 9:
S: * OK [CAPABILITY IMAP4REV1 LOGIN-REFERRALS STARTTLS
AUTH=DIGEST-MD5 AUTH=SRP] imap.example.com ready
C: 2 authenticate DIGEST-MD5
S: 2 OK [CAPABILITY IMAP4REV1 IDLE NAMESPACE MAILBOX-REFERRALS SCAN
SORT THREAD=REFERENCES THREAD=ORDEREDSUBJECT MULTIAPPEND]
User authenticated (no layer)
2) An advanced disconnected client may choose to optimize its replay
log. For example, there might be some operations that are
redundant (the list is not complete):
a) an EXPUNGE followed by another EXPUNGE or CLOSE;
b) changing flags (other than the \Deleted flag) on a message that
gets immediately expunged;
c) opening and closing the same mailbox.
When optimizing, be careful about data dependencies between commands.
For example, if the client is wishing to optimize (see case b, above)
tag1 UID STORE <uid1> +FLAGS (\Deleted)
...
tag2 UID STORE <uid1> +FLAGS (\Flagged)
...
tag3 UID COPY <uid1> "Backup"
...
tag4 UID EXPUNGE <uid1>
it can't remove the second UID STORE command because the message is
being copied before it gets expunged.
In general, it might be a good idea to keep mailboxes open during
synchronization (see case c above), if possible. This can be more
easily achieved in conjunction with optimization 3 described below.
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3) Perform some synchronization steps in parallel, if possible.
Several synchronization steps don't depend on each other and thus
can be performed in parallel. Because the server machine is
usually more powerful than the client machine and can perform some
operations in parallel, this may speed up the total time of
synchronization.
In order to achieve such parallelization, the client will have to
open more than one connection to the same server. Client writers
should not forget about non-trivial cost associated with
establishing a TCP connection and performing an authentication.
The disconnected client MUST NOT use one connection per mailbox.
In most cases, it is sufficient to have two connections. The
disconnected client SHOULD avoid selecting the same mailbox in
more than one connection; see Section 3.1.1 of [RFC2683] for more
details.
Any mailbox synchronization MUST start with checking the
UIDVALIDITY as described in Section 4.1 of this document. The
client MAY use STATUS command to check UID Validity of a non-
selected mailbox. This is preferable to opening many connections
to the same server to perform synchronization of multiple
mailboxes simultaneously. As described in Section 5.3.10 of
[IMAP4], this SHOULD NOT be used on the selected mailbox.
6. IMAP Extensions That May Help
The following extensions can save traffic and/or the number of
round-trips:
1) The use of [UIDPLUS] is discussed in Sections 4.1, 4.2.1, 4.2.2.1
and 4.2.4.
2) The use of the MULTIAPPEND and LITERAL+ extensions for uploading
messages is discussed in Section 4.2.2.5.
3) Use the CONDSTORE extension (see Section 6.1) for quick flag
resynchronization.
6.1. CONDSTORE Extension
An advanced disconnected mail client should use the [CONDSTORE]
extension when it is supported by the server. The client must cache
the value from HIGHESTMODSEQ OK response code received on mailbox
opening and update it whenever the server sends MODSEQ FETCH data
items.
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If the client receives NOMODSEQ OK untagged response instead of
HIGHESTMODSEQ, it MUST remove the last known HIGHESTMODSEQ value from
its cache and follow the more general instructions in Section 3.
When the client opens the mailbox for synchronization, it first
compares UIDVALIDITY as described in step d-1 in Section 3. If the
cached UIDVALIDITY value matches the one returned by the server, the
client MUST compare the cached value of HIGHESTMODSEQ with the one
returned by the server. If the cached HIGHESTMODSEQ value also
matches the one returned by the server, then the client MUST NOT
fetch flags for cached messages, as they hasn't changed. If the
value on the server is higher than the cached one, the client MAY use
"SEARCH MODSEQ <cached-value>" to find all messages with flags
changed since the last time the client was online and had the mailbox
opened. Alternatively, the client MAY use "FETCH 1:* (FLAGS)
(CHANGEDSINCE <cached-value>)". The latter operation combines
searching for changed messages and fetching new information.
In all cases, the client still needs to fetch information about new
messages (if requested by the user) as well as discover which
messages have been expunged.
Step d ("Server-to-client synchronization") in Section 4 in the
presence of the CONDSTORE extension 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 for more
details) with SELECT/EXAMINE/STATUS.
If the UIDVALIDITY value returned by the server differs, the
client MUST
* empty the local cache of that mailbox;
* "forget" the cached HIGHESTMODSEQ value for the mailbox;
* remove any pending "actions" that refer to UIDs in that
mailbox (note that this doesn't affect actions performed on
client-generated fake UIDs; see Section 5); and
* skip steps 1b and 2-II;
1b) Check the mailbox HIGHESTMODSEQ. If the cached value is the
same as the one returned by the server, skip fetching message
flags on step 2-II, i.e., the client only has to find out
which messages got expunged.
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2) Fetch the current "descriptors".
I) Discover new messages.
II) Discover changes to old messages and flags for new messages
using
"FETCH 1:* (FLAGS) (CHANGEDSINCE <cached-value>)" or
"SEARCH MODSEQ <cached-value>".
Discover expunged messages; for example, using
"UID SEARCH 1:<lastseenuid>". (All messages not returned
in this command are expunged.)
3) Fetch the bodies of any "interesting" messages that the client
doesn't already have.
Example 10:
The UIDVALIDITY value is the same, but the HIGHESTMODSEQ value
has changed on the server while the client was offline.
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 201] Predicted next UID
S: * FLAGS (\Answered \Flagged \Deleted \Seen \Draft)
S: * OK [PERMANENTFLAGS (\Deleted \Seen \*)] Limited
S: * OK [HIGHESTMODSEQ 20010715194045007]
S: A142 OK [READ-WRITE] SELECT completed
After that, either:
C: A143 UID FETCH 1:* (FLAGS) (CHANGEDSINCE 20010715194032001)
S: * 2 FETCH (UID 6 MODSEQ (20010715205008000) FLAGS (\Deleted))
S: * 5 FETCH (UID 9 MODSEQ (20010715195517000) FLAGS ($NoJunk
$AutoJunk $MDNSent))
...
S: A143 OK FETCH completed
or:
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C: A143 UID SEARCH MODSEQ 20010715194032001 UID 1:20
S: * SEARCH 6 9 11 12 18 19 20 23 (MODSEQ 20010917162500)
S: A143 OK Search complete
C: A144 UID SEARCH 1:20
S: * SEARCH 6 9 ...
S: A144 OK FETCH completed
7. Security Considerations
It is believed that this document does not raise any new security
concerns that are not already present in the base [IMAP4] protocol,
and these issues are discussed in [IMAP4]. Additional security
considerations may be found in different extensions mentioned in this
document; in particular, in [UIDPLUS], [LITERAL+], [CONDSTORE],
[MULTIAPPEND], and [UNSELECT].
Implementers are also reminded about the importance of thorough
testing.
8. References
8.1. Normative References
[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[IMAP4] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL -
VERSION 4rev1", RFC 3501, March 2003.
[UIDPLUS] Crispin, M., "Internet Message Access Protocol (IMAP) -
UIDPLUS extension", RFC 4315, December 2005.
[LITERAL+] Myers, J., "IMAP4 non-synchronizing literals", RFC
2088, January 1997.
[CONDSTORE] Melnikov, A. and S. Hole, "IMAP Extension for
Conditional STORE Operation or Quick Flag Changes
Resynchronization", RFC 4551, June 2006.
[MULTIAPPEND] Crispin, M., "Internet Message Access Protocol (IMAP) -
MULTIAPPEND Extension", RFC 3502, March 2003.
[UNSELECT] Melnikov, A., "Internet Message Access Protocol (IMAP)
UNSELECT command", RFC 3691, February 2004.
[RFC2683] Leiba, B., "IMAP4 Implementation Recommendations", RFC
2683, September 1999.
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8.2. Informative References
[ACL] Melnikov, A., "IMAP4 Access Control List (ACL)
Extension", RFC 4314, December 2005.
[IMAP-MODEL] Crispin, M., "Distributed Electronic Mail Models in
IMAP4", RFC 1733, December 1994.
9. Acknowledgements
This document is based on version 01 of the text written by Rob
Austein in November 1994.
The editor appreciates comments posted by Mark Crispin to the IMAP
mailing list and the comments/corrections/ideas received from Grant
Baillie, Cyrus Daboo, John G. Myers, Chris Newman, and Timo Sirainen.
The editor would also like to thank the developers of Netscape
Messenger and Mozilla mail clients for providing examples of
disconnected mail clients that served as a base for many
recommendations in this document.
Editor's Address
Alexey Melnikov
Isode Limited
5 Castle Business Village
36 Station Road
Hampton, Middlesex
TW12 2BX
United Kingdom
Phone: +44 77 53759732
EMail: alexey.melnikov@isode.com
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Full Copyright Statement
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