Synchronization Operations for Disconnected IMAP4 Clients
RFC 4549

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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.

Melnikov                     Informational                      [Page 1]
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

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.