TCP/IP Electronic Mail Message Communication Model and Device and Protocol Roles
(Page 1 of 3)
The purpose of the electronic mail system as a whole is to accomplish the transmission of electronic mail messages from a user of a TCP/IP internetwork to one or more recipients. In order to accomplish this, a special method of communication is required that makes the electronic mail system quite different from that used by most other protocols. To understand what I mean by this, you need only recognize the difference in communication between sending a letter and making a phone call.
Most TCP/IP protocols are analogous to making a phone call in this respect: the sender and the receiver must both be on the network at the same time. You can't call someone and talk to them if they aren't around to answer the phone. (Yeah, yeah, answering machines and voice mail. Stop interrupting, will you? J) Most TCP/IP protocols are like this: to send a file using FTP, for example, you must make a direct connection from the sender's machine to the recipient's machine. If the recipient's machine is not on the network at the exact time that sender's is, no communication is possible.
For electronic mail, this type of communication is simply unacceptable. Electronic mail is like its real-world counterpart; Joe wants to be able to put a message into the system at a time that is convenient for him, and Ellen wants to be able to receive Joe's mail at a time that works for her. For this to work, e-mail must use a send and forget model, just like real mail, where Joe drops the envelope into the e-mail system and it eventually gets to its destination.
This decoupling of the sender and receiver is critical to the design of the electronic mail system. This is especially true because many of the users of Internet electronic mail are not on the Internet all the time. Just as you wouldn't want real mail to be rejected if it arrived when you are not home, you don't want electronic mail to not be delivered if you are not on the Internet when it arrives. Similarly, you may not want to be connected to the Internet for the entire time it takes to write a message, especially if you have access to the Internet for only a limited amount of time each day.
Also critical to the entire electronic mail system is that idea that communication is between specific users, and not between particular machines. This makes e-mail inherently different than many other types of communication on TCP/IP internetworks. We'll see more of why this is important when we look at e-mail addressing.
Home - Table Of Contents - Contact Us
The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
© Copyright 2001-2005 Charles M. Kozierok. All Rights Reserved.
Not responsible for any loss resulting from the use of this site.