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diff docs/rfc/rfc1733.txt @ 0:ada5e610ab86
imap-2007e
author | yuuji@gentei.org |
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date | Mon, 14 Sep 2009 15:17:45 +0900 |
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--- /dev/null Thu Jan 01 00:00:00 1970 +0000 +++ b/docs/rfc/rfc1733.txt Mon Sep 14 15:17:45 2009 +0900 @@ -0,0 +1,171 @@ + + + + + + +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] +