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.
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