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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 System Overview and Concepts
TCP/IP Electronic Mail System Overview and History
(Page 3 of 3)
Overview of the TCP/IP Electronic Mail System
One of the most important general
concepts in the modern electronic mail system is that a distinction
is made between protocols that deliver electronic mail between SMTP
hosts on the internetwork, and those that let users access received
mail on their local hosts. To continue our analogy, different protocols
are used for sending mail between post offices, and for home delivery.
As we'll see, this was done intentionally, to make it possible to send
mail to users even if they are not connected to the Internet at the
time mail was sent. This decoupling is critical as it enables delayed
communication, where mail can be sent when the sender wants to transmit
it, and received when the recipient wants to read it.
Over the years, the basic components
defined in the early 1980s have not changed substantially, but how they
are used has evolved and been improved. Early electronic mail delivery
involved the use of route specifications by one SMTP host to dictate
how mail was to be delivered through intermediate systems; today, the
Name System makes much of that obsolete,
facilitating nearly immediate direct mail delivery in most cases. Early
electronic mail supported only simple text, where we can now send graphical
images, programs and other files in e-mail. Modern high-speed Internet
connections and updated access protocols allow modern e-mail to be the
realization of the ultimate goal of nearly instantaneous communication
even across continents.
Key Concept: One of the most important TCP/IP applications is the internetworking equivalent of the real-world postal delivery system, commonly called electronic mail or e-mail. The history of e-mail goes back to the very earliest days of TCP/IPs development; today it is used by millions of people every day to send both simple and complex messages around the world. TCP/IP e-mail is not a single application, but rather a complete system that includes several protocols, software elements and components.
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The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
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