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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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+Melnikov                     Informational                      [Page 1]
+
+RFC 4549        Synch Ops for Disconnected IMAP4 Clients       June 2006
+
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+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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+Melnikov                     Informational                      [Page 2]
+
+RFC 4549        Synch Ops for Disconnected IMAP4 Clients       June 2006
+
+
+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
+
+
+
+Melnikov                     Informational                      [Page 3]
+
+RFC 4549        Synch Ops for Disconnected IMAP4 Clients       June 2006
+
+
+   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.)
+
+
+
+
+
+Melnikov                     Informational                      [Page 4]
+
+RFC 4549        Synch Ops for Disconnected IMAP4 Clients       June 2006
+
+
+   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).
+
+
+
+
+Melnikov                     Informational                      [Page 5]
+
+RFC 4549        Synch Ops for Disconnected IMAP4 Clients       June 2006
+
+
+   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.
+
+
+
+
+
+
+Melnikov                     Informational                      [Page 6]
+
+RFC 4549        Synch Ops for Disconnected IMAP4 Clients       June 2006
+
+
+   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.
+
+
+
+
+
+
+
+
+Melnikov                     Informational                      [Page 7]
+
+RFC 4549        Synch Ops for Disconnected IMAP4 Clients       June 2006
+
+
+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:
+
+
+
+
+Melnikov                     Informational                      [Page 8]
+
+RFC 4549        Synch Ops for Disconnected IMAP4 Clients       June 2006
+
+
+      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,
+
+
+
+Melnikov                     Informational                      [Page 9]
+
+RFC 4549        Synch Ops for Disconnected IMAP4 Clients       June 2006
+
+
+   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
+
+
+
+
+Melnikov                     Informational                     [Page 10]
+
+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*:
+
+
+
+
+Melnikov                     Informational                     [Page 11]
+
+RFC 4549        Synch Ops for Disconnected IMAP4 Clients       June 2006
+
+
+   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.
+
+
+
+
+
+
+Melnikov                     Informational                     [Page 12]
+
+RFC 4549        Synch Ops for Disconnected IMAP4 Clients       June 2006
+
+
+   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:
+
+
+
+Melnikov                     Informational                     [Page 13]
+
+RFC 4549        Synch Ops for Disconnected IMAP4 Clients       June 2006
+
+
+   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
+
+
+
+Melnikov                     Informational                     [Page 14]
+
+RFC 4549        Synch Ops for Disconnected IMAP4 Clients       June 2006
+
+
+   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
+
+
+
+Melnikov                     Informational                     [Page 15]
+
+RFC 4549        Synch Ops for Disconnected IMAP4 Clients       June 2006
+
+
+   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).
+
+
+
+
+Melnikov                     Informational                     [Page 16]
+
+RFC 4549        Synch Ops for Disconnected IMAP4 Clients       June 2006
+
+
+   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.
+
+
+
+Melnikov                     Informational                     [Page 17]
+
+RFC 4549        Synch Ops for Disconnected IMAP4 Clients       June 2006
+
+
+   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
+
+
+
+Melnikov                     Informational                     [Page 18]
+
+RFC 4549        Synch Ops for Disconnected IMAP4 Clients       June 2006
+
+
+   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.
+
+
+
+
+Melnikov                     Informational                     [Page 19]
+
+RFC 4549        Synch Ops for Disconnected IMAP4 Clients       June 2006
+
+
+   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.
+
+
+
+
+Melnikov                     Informational                     [Page 20]
+
+RFC 4549        Synch Ops for Disconnected IMAP4 Clients       June 2006
+
+
+   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.
+
+
+
+
+
+
+
+
+Melnikov                     Informational                     [Page 21]
+
+RFC 4549        Synch Ops for Disconnected IMAP4 Clients       June 2006
+
+
+   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.
+
+
+
+Melnikov                     Informational                     [Page 22]
+
+RFC 4549        Synch Ops for Disconnected IMAP4 Clients       June 2006
+
+
+   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[])".
+
+
+
+
+
+
+
+Melnikov                     Informational                     [Page 23]
+
+RFC 4549        Synch Ops for Disconnected IMAP4 Clients       June 2006
+
+
+   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.
+
+
+
+
+Melnikov                     Informational                     [Page 24]
+
+RFC 4549        Synch Ops for Disconnected IMAP4 Clients       June 2006
+
+
+      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.
+
+
+
+
+
+
+Melnikov                     Informational                     [Page 25]
+
+RFC 4549        Synch Ops for Disconnected IMAP4 Clients       June 2006
+
+
+         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.)
+
+
+
+
+
+Melnikov                     Informational                     [Page 26]
+
+RFC 4549        Synch Ops for Disconnected IMAP4 Clients       June 2006
+
+
+      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).
+
+
+
+
+Melnikov                     Informational                     [Page 27]
+
+RFC 4549        Synch Ops for Disconnected IMAP4 Clients       June 2006
+
+
+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.
+
+
+
+
+
+Melnikov                     Informational                     [Page 28]
+
+RFC 4549        Synch Ops for Disconnected IMAP4 Clients       June 2006
+
+
+      * 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.
+
+
+
+
+Melnikov                     Informational                     [Page 29]
+
+RFC 4549        Synch Ops for Disconnected IMAP4 Clients       June 2006
+
+
+   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.
+
+
+
+
+Melnikov                     Informational                     [Page 30]
+
+RFC 4549        Synch Ops for Disconnected IMAP4 Clients       June 2006
+
+
+   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.
+
+
+
+
+
+
+Melnikov                     Informational                     [Page 31]
+
+RFC 4549        Synch Ops for Disconnected IMAP4 Clients       June 2006
+
+
+      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:
+
+
+
+
+
+
+
+
+
+Melnikov                     Informational                     [Page 32]
+
+RFC 4549        Synch Ops for Disconnected IMAP4 Clients       June 2006
+
+
+      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.
+
+
+
+
+Melnikov                     Informational                     [Page 33]
+
+RFC 4549        Synch Ops for Disconnected IMAP4 Clients       June 2006
+
+
+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
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Melnikov                     Informational                     [Page 34]
+
+RFC 4549        Synch Ops for Disconnected IMAP4 Clients       June 2006
+
+
+Full Copyright Statement
+
+   Copyright (C) The Internet Society (2006).
+
+   This document is subject to the rights, licenses and restrictions
+   contained in BCP 78, and except as set forth therein, the authors
+   retain all their rights.
+
+   This document and the information contained herein are provided on an
+   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
+   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
+   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
+   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
+   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
+   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
+
+Intellectual Property
+
+   The IETF takes no position regarding the validity or scope of any
+   Intellectual Property Rights or other rights that might be claimed to
+   pertain to the implementation or use of the technology described in
+   this document or the extent to which any license under such rights
+   might or might not be available; nor does it represent that it has
+   made any independent effort to identify any such rights.  Information
+   on the procedures with respect to rights in RFC documents can be
+   found in BCP 78 and BCP 79.
+
+   Copies of IPR disclosures made to the IETF Secretariat and any
+   assurances of licenses to be made available, or the result of an
+   attempt made to obtain a general license or permission for the use of
+   such proprietary rights by implementers or users of this
+   specification can be obtained from the IETF on-line IPR repository at
+   http://www.ietf.org/ipr.
+
+   The IETF invites any interested party to bring to its attention any
+   copyrights, patents or patent applications, or other proprietary
+   rights that may cover technology that may be required to implement
+   this standard.  Please address the information to the IETF at
+   ietf-ipr@ietf.org.
+
+Acknowledgement
+
+   Funding for the RFC Editor function is provided by the IETF
+   Administrative Support Activity (IASA).
+
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+Melnikov                     Informational                     [Page 35]
+