About TCP-IP

R. Craig Collins > Web Page Design > TCP-IP

TCP/IP is the basis for Internet Communications. When participating on the Internet, you have many options: any number of pieces of hardware such as a computer or cell phone with a physical or wireless connection to the Internet, at least one of a variety of software tools, such as telnet, ftp, or a browser, but you must have TCP/IP. So how does TCP/IP work?

Many people believe since a lot of the Internet traffic flows over phone lines that all the information follows one path straight from the sender to the receiver immediately, just as a phone call works. That is called circuit switched delivery. But in fact, the Internet works more like the post office, moving information by different routes when a carrier is available to pick up and forward. This is called packet switched delivery. The only difference between the post office and the Internet is that the whole process may take less than a second to break, distribute, collect and reassemble a message whose parts may have gone through Texas and Japan on the way. Why do you suppose a straight connection from sender to receiver is not used on the Internet?

The answer is: much of the time when you are on the Internet, nothing is being transferred, such as when you are reading a page. It would be wasteful to tie up a line. You may also recall that one of the major reasons to build the Internet was to get away from centralized communications. Any number of routes can be used, even if a particular station is not working.

TCP/IP first takes the data you are ready to move across the internet, and breaks it into smaller pieces, called packets. These packets are each numbered, and are stamped with information concerning who the packet is from, where it is going, and how many packets there are total.

Then TCP/IP places the information on the Internet, starting with the local host computer. This first computer examines each packet, finds the 'TO' address, and polls nearby computer on the Internet to see which one a) is not busy, and b) if that computer can forward a packet.

When a computer responds, the packet is sent to that machine, where the process is repeated.

It is important to note that the same computer(s) probably will not handle all of the out-going packets. Some of the computers that 'volunteer' may actually be out of the way, depending on how busy the Internet is at that time.

Therefore it is possible that many of the last packets sent will actually arrive at the destination before some of the first packets, depending on which computers handled what packets. However, the difference is often measured in milliseconds, and is not noticed.

At the receiving host computer, the message is reassembled. TCP/IP uses error-correcting 'checksums' to validate the data received, and requests replacement packets from the sending machine. Once the packets are all accounted for, they are re-assembled and held, until the recipient requests that host deliver their mail.

To begin a (very large) demo, click on the link below. It will show a message being composed near Redmond Washington, broken into packets, placed upon the Internet where they find their way to Central Texas. The packets are sequenced, checked for errors, and finally the mail is made available for reading...

300 KB Demo