RFC 1733 (rfc1733) - Page 1 of 3


Distributed Electronic Mail Models in IMAP4



Alternative Format: Original Text Document



Network Working Group                                         M. Crispin
Request for Comments: 1733                      University of Washington
Category: Informational                                    December 1994


              DISTRIBUTED ELECTRONIC MAIL MODELS IN IMAP4


Status of this Memo

   This memo provides information for the Internet community.  This memo
   does not specify an Internet standard of any kind.  Distribution of
   this memo is unlimited.


Distributed Electronic Mail Models

   There are three fundamental models of client/server email: offline,
   online, and disconnected use.  IMAP4 can be used in any one of these
   three models.

   The offline model is the most familiar form of client/server email
   today, and is used by protocols such as POP-3 (RFC 1225) and UUCP.
   In this model, a client application periodically connects to a
   server.  It downloads all the pending messages to the client machine
   and deletes these from the server.  Thereafter, all mail processing
   is local to the client.  This model is store-and-forward; it moves
   mail on demand from an intermediate server (maildrop) to a single
   destination machine.

   The online model is most commonly used with remote filesystem
   protocols such as NFS.  In this model, a client application
   manipulates mailbox data on a server machine.  A connection to the
   server is maintained throughout the session.  No mailbox data are
   kept on the client; the client retrieves data from the server as is
   needed.  IMAP4 introduces a form of the online model that requires
   considerably less network bandwidth than a remote filesystem
   protocol, and provides the opportunity for using the server for CPU
   or I/O intensive functions such as parsing and searching.

   The disconnected use model is a hybrid of the offline and online
   models, and is used by protocols such as PCMAIL (RFC 1056).  In this
   model, a client user downloads some set of messages from the server,
   manipulates them offline, then at some later time uploads the
   changes.  The server remains the authoritative repository of the
   messages.  The problems of synchronization (particularly when
   multiple clients are involved) are handled through the means of
   unique identifiers for each message.



Crispin