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|| The TCP/IP Guide|
9 TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)
9 TCP/IP Key Applications and Application Protocols
9 TCP/IP File and Message Transfer Applications and Protocols (FTP, TFTP, Electronic Mail, USENET, HTTP/WWW, Gopher)
9 TCP/IP Electronic Mail System: Concepts and Protocols (RFC 822, MIME, SMTP, POP3, IMAP)
9 TCP/IP Electronic Mail Access and Retrieval Protocols and Methods
TCP/IP Electronic Mail Mailbox Access Model, Method and Protocol Overview
(Page 3 of 3)
Comparing E-Mail Access and Retrieval Models
Of the three, which is best? You
should know better than to ask me that question. J
Each has advantages and disadvantages, which is why it is good that
we have these options rather than the single SMTP protocol for mail
server access has the main benefits of
instant speed and universal access from any location. It has the disadvantage
that you must be online to read mail, and that it usually requires you
to use UNIX e-mail clients that most people are not familiar with. However,
Message Access Protocol (IMAP) can also
be used for online access.
Offline access has the main advantages
of simplicity and short connection time requirements; you can easily
connect to the mailbox, download messages and then read them locally.
But that makes this method somewhat inflexible, and poorly-suited to
access from different machines. Still, it is right now the most popular
access method, because simplicity is important; it is best typified
by the popular Post
Office Protocol (POP).
Disconnected access attempts to combine
the advantages of offline and online access without combining their
disadvantages, and does a pretty good job. The advantages are significant:
the ability to quickly access mail and use it offline, while retaining
and updating the mailbox on the server to allow access from different
client machines. IMAP
is the protocol popularly used for disconnected access. In the IMAP
overview I explore its advantages over
offline access, as well as its main disadvantages: complexity and far
less universal support than POP (though acceptance of IMAP is slowly
Finally, in recent years, a somewhat
new mailbox access method has become popular: e-mail
access using the World Wide Web. This
technique allows a user to access his or her mailbox from any computer
with an Internet connection and a Web browser. It is a good example
of line blurring, not only between the access models discussed
here, but between TCP/IP applications, in this case the
Web and e-mail.
Key Concept: For flexibility, TCP/IP uses a variety of mailbox access and retrieval protocols and methods to allow users to read e-mail. Three different models describe how these different methods work: the online model, in which e-mail is accessed and read on the server; the offline model, in which mail is transferred to the client device and used there; and the disconnected model, where mail is retrieved and read offline but remains on the server with changes synchronized for consistency.
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The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
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