172 lines
6.1 KiB
Plaintext
172 lines
6.1 KiB
Plaintext
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Network Working Group M. Crispin
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Request for Comments: 1733 University of Washington
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Category: Informational December 1994
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DISTRIBUTED ELECTRONIC MAIL MODELS IN IMAP4
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Status of this Memo
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This memo provides information for the Internet community. This memo
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does not specify an Internet standard of any kind. Distribution of
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this memo is unlimited.
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Distributed Electronic Mail Models
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There are three fundamental models of client/server email: offline,
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online, and disconnected use. IMAP4 can be used in any one of these
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three models.
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The offline model is the most familiar form of client/server email
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today, and is used by protocols such as POP-3 (RFC 1225) and UUCP.
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In this model, a client application periodically connects to a
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server. It downloads all the pending messages to the client machine
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and deletes these from the server. Thereafter, all mail processing
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is local to the client. This model is store-and-forward; it moves
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mail on demand from an intermediate server (maildrop) to a single
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destination machine.
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The online model is most commonly used with remote filesystem
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protocols such as NFS. In this model, a client application
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manipulates mailbox data on a server machine. A connection to the
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server is maintained throughout the session. No mailbox data are
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kept on the client; the client retrieves data from the server as is
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needed. IMAP4 introduces a form of the online model that requires
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considerably less network bandwidth than a remote filesystem
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protocol, and provides the opportunity for using the server for CPU
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or I/O intensive functions such as parsing and searching.
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The disconnected use model is a hybrid of the offline and online
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models, and is used by protocols such as PCMAIL (RFC 1056). In this
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model, a client user downloads some set of messages from the server,
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manipulates them offline, then at some later time uploads the
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changes. The server remains the authoritative repository of the
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messages. The problems of synchronization (particularly when
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multiple clients are involved) are handled through the means of
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unique identifiers for each message.
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Crispin [Page 1]
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RFC 1733 IMAP4 - Model December 1994
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Each of these models have their own strengths and weaknesses:
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Feature Offline Online Disc
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------- ------- ------ ----
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Can use multiple clients NO YES YES
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Minimum use of server connect time YES NO YES
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Minimum use of server resources YES NO NO
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Minimum use of client disk resources NO YES NO
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Multiple remote mailboxes NO YES YES
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Fast startup NO YES NO
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Mail processing when not online YES NO YES
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Although IMAP4 has its origins as a protocol designed to accommodate
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the online model, it can support the other two models as well. This
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makes possible the creation of clients that can be used in any of the
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three models. For example, a user may wish to switch between the
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online and disconnected models on a regular basis (e.g. owing to
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travel).
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IMAP4 is designed to transmit message data on demand, and to provide
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the facilities necessary for a client to decide what data it needs at
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any particular time. There is generally no need to do a wholesale
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transfer of an entire mailbox or even of the complete text of a
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message. This makes a difference in situations where the mailbox is
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large, or when the link to the server is slow.
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More specifically, IMAP4 supports server-based RFC 822 and MIME
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processing. With this information, it is possible for a client to
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determine in advance whether it wishes to retrieve a particular
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message or part of a message. For example, a user connected to an
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IMAP4 server via a dialup link can determine that a message has a
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2000 byte text segment and a 40 megabyte video segment, and elect to
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fetch only the text segment.
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In IMAP4, the client/server relationship lasts only for the duration
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of the TCP connection. There is no registration of clients. Except
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for any unique identifiers used in disconnected use operation, the
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client initially has no knowledge of mailbox state and learns it from
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the IMAP4 server when a mailbox is selected. This initial transfer
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is minimal; the client requests additional state data as it needs.
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As noted above, the choice for the location of mailbox data depends
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upon the model chosen. The location of message state (e.g. whether
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or not a message has been read or answered) is also determined by the
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model, and is not necessarily the same as the location of the mailbox
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data. For example, in the online model message state can be co-
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located with mailbox data; it can also be located elsewhere (on the
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client or on a third agent) using unique identifiers to achieve
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Crispin [Page 2]
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RFC 1733 IMAP4 - Model December 1994
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common reference across sessions. The latter is particularly useful
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with a server that exports public data such as netnews and does not
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maintain per-user state.
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The IMAP4 protocol provides the generality to implement these
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different models. This is done by means of server and (especially)
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client configuration, and not by requiring changes to the protocol or
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the implementation of the protocol.
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Security Considerations
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Security issues are not discussed in this memo.
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Author's Address:
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Mark R. Crispin
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Networks and Distributed Computing, JE-30
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University of Washington
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Seattle, WA 98195
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Phone: (206) 543-5762
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EMail: MRC@CAC.Washington.EDU
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Crispin [Page 3]
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