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