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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 [Page 1]  RFC 1733 IMAP4 - Model December 1994

 Each of these models have their own strengths and weaknesses:

    Feature                               Offline Online  Disc
    -------                               ------- ------  ----
    Can use multiple clients               NO      YES     YES
    Minimum use of server connect time     YES     NO      YES
    Minimum use of server resources        YES     NO      NO
    Minimum use of client disk resources   NO      YES     NO
    Multiple remote mailboxes              NO      YES     YES
    Fast startup                           NO      YES     NO
    Mail processing when not online        YES     NO      YES

 Although IMAP4 has its origins as a protocol designed to accommodate
 the online model, it can support the other two models as well.  This
 makes possible the creation of clients that can be used in any of the
 three models.  For example, a user may wish to switch between the
 online and disconnected models on a regular basis (e.g. owing to
 travel).

 IMAP4 is designed to transmit message data on demand, and to provide
 the facilities necessary for a client to decide what data it needs at
 any particular time.  There is generally no need to do a wholesale
 transfer of an entire mailbox or even of the complete text of a
 message.  This makes a difference in situations where the mailbox is
 large, or when the link to the server is slow.

 More specifically, IMAP4 supports server-based RFC 822 and MIME
 processing.  With this information, it is possible for a client to
 determine in advance whether it wishes to retrieve a particular
 message or part of a message.  For example, a user connected to an
 IMAP4 server via a dialup link can determine that a message has a
 2000 byte text segment and a 40 megabyte video segment, and elect to
 fetch only the text segment.

 In IMAP4, the client/server relationship lasts only for the duration
 of the TCP connection.  There is no registration of clients.  Except
 for any unique identifiers used in disconnected use operation, the
 client initially has no knowledge of mailbox state and learns it from
 the IMAP4 server when a mailbox is selected.  This initial transfer
 is minimal; the client requests additional state data as it needs.

 As noted above, the choice for the location of mailbox data depends
 upon the model chosen.  The location of message state (e.g. whether
 or not a message has been read or answered) is also determined by the
 model, and is not necessarily the same as the location of the mailbox
 data.  For example, in the online model message state can be co-
 located with mailbox data; it can also be located elsewhere (on the
 client or on a third agent) using unique identifiers to achieve

Crispin [Page 2]  RFC 1733 IMAP4 - Model December 1994

 common reference across sessions.  The latter is particularly useful
 with a server that exports public data such as netnews and does not
 maintain per-user state.

 The IMAP4 protocol provides the generality to implement these
 different models.  This is done by means of server and (especially)
 client configuration, and not by requiring changes to the protocol or
 the implementation of the protocol.

Security Considerations

 Security issues are not discussed in this memo.

Author's Address:

 Mark R. Crispin
 Networks and Distributed Computing, JE-30
 University of Washington
 Seattle, WA  98195

 Phone: (206) 543-5762

 EMail: MRC@CAC.Washington.EDU

Crispin [Page 3]