How TCP/IP Works
The Internet is a packet-switched network. This means that when you send information from one computer to another, the data is broken into small packets. “Routers” choose which path each packet will take to arrive at its destination, and then they send the packets individually. Once all of the packets arrive at the destination, they are reassembled into their original form. There are two protocols that do this work. They are:
When data is sent across the Internet, it must be broken into packets of fewer than about 500 characters each. Each packet receives a header with a great deal of information, such as the order in which packets should be reassembled. TCP adds a “checksum” to the header of each packet as it is being constructed. The checksum is a number used by TCP on the receiving end to determine whether any errors have been made during transmission. The checksum is based on the amount of data in the packet. Each packet is then put into a separate IP “envelope” with identical addressing information so that they can all arrive at the same destination to be reassembled.
During the packets’ journeys, routers look at the IP envelopes addresses, and send them on the most efficient path to the next router, and eventually to the destination. Internet traffic constantly changes, so packets may take different routes and arrive out of order, thus the reason for the headers.
When each packet arrives at its destination, TCP calculates a new checksum for each packet. If the new checksum does not match the old one, this means an error has occurred and the packet is discarded. TCP then asks for the original packet to be sent again. Then, when all of the packets arrive error-free, TCP recombines them into their original form.