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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 System Overview and Concepts

TCP/IP Electronic Mail System Overview and History 1 TCP/IP Electronic Mail Message Communication Model and Device and Protocol Roles

TCP/IP electronic mail is implemented as a complete system, with a number of different elements that perform different portions of the complete job of electronic mail communication. These included a standard message format, a specific syntax for recipient addressing, and protocols to both deliver mail and allow access to mailboxes from intermittently-connected TCP/IP clients.

To help set the groundwork for examining these components, I want to provide an overview of the complete end-to-end process of e-mail communication, so you can see how everything works. I will show the basic steps in simplified form, and continue the analogy to the regular mail system for comparison.

The modern TCP/IP e-mail communication process consists of the following five basic steps.

A user begins the e-mail journey by creating an electronic mail message. The message contains two sections: the header and the body. The body of the message is the actual information to be communicated; the header contains data that describes the message and controls how it is delivered and processed. The message must be created so that it matches the standard message format for the e-mail system so that it can be processed. It must also specify the e-mail addresses of the intended recipients for the message.

By way of analogy to real mail, the body of the message is like a letter, and the header is like the envelope into which the letter is placed.

As we'll see in greater detail in the next topic, electronic mail is different from many other internetworking applications in that the sender and receiver of a message do not necessarily need to be connected to the network simultaneously, nor even continuously, to use it. The system is designed so that after composing the message, the user decides when to submit it to the electronic mail system so it can be delivered. This is done using the Simple Mail Transfer Protocol (SMTP).

This is analogous to dropping off an envelope at the post office, or to a postal worker picking up an envelope from a mailbox and carrying it to the local post office to insert into the mail delivery stream.

The electronic mail message is accepted by the sender's local SMTP system for delivery through the mail system to the destination user. Today, this is accomplished by performing a Domain Name System (DNS) lookup of the intended recipient's host system and establishing an SMTP connection to that system. SMTP also supports the ability to specify a sequence of SMTP servers through which a message must be passed to reach a destination. One way or the other, eventually the message arrives at the recipient's local SMTP system

This is like the transportation of the envelope through the postal system's internal “internetwork” of trucks, airplanes and other equipment to the intended recipient's local post office.

The local SMTP server accepts the e-mail message and processes it. It places the mail into the intended recipient's mail box, where it waits for the user to retrieve it.

In our physical analogy, this is the step where the recipient's local post office sorts mail coming in from the postal delivery system and puts the mail into individual post office boxes or bins for delivery.

The intended recipient periodically checks with its local SMTP server to see if any mail has arrived. If so, the recipient retrieves the mail, opens it and reads its content. This is done using not SMTP but a special mail access protocol or method. To save time, the access protocol and client e-mail software may allow the user to scan the headers of received mail (such as the subject and sender's identity) to decide which mail messages to download.

This is the step where mail is physically picked up at the post office or delivered to the home.

In some cases not all of these steps are performed. If a user is sending e-mail from a device that is already an SMTP server, then step #2 can be omitted; if the recipient is logged in to a device that is also an SMTP server, step #5 will be skipped, as the user can read mail directly on the server. Thus, in the simplest case all that occurs is composition, delivery and receipt; this occurs when one user of a dial-up UNIX host sends mail to another. In most cases, today, however, all five steps occur.

Again, this is simplified. I have, for example, omitted the step where the recipient notices that the mail is in fact an advertisement from a spammer for how he can “make money fast”, eliminate debt or lose weight overnight, then mutters under his breath and deletes the message. Hey, I never said there were no disadvantages to electronic mail being fast and nearly free. J

TCP/IP Electronic Mail System Overview and History 1 TCP/IP Electronic Mail Message Communication Model and Device and Protocol Roles