Java Network Programming, Second Edition

IP, TCP, and UDP

IP, the Internet Protocol, has a number of advantages over other competing protocols such as AppleTalk and IPX, most stemming from its history. It was developed with military sponsorship during the Cold War, and ended up with a lot of features that the military was interested in. First, it had to be robust. The entire network couldn’t stop functioning if the Soviets nuked a router in Cleveland; all messages still had to get through to their intended destinations (except those going to Cleveland, of course). Therefore, IP was designed to allow multiple routes between any two points and to route packets of data around damaged routers.

Second, the military had many different kinds of computers, and they needed all of them to be able to talk to each other. Therefore, the protocol had to be open and platform independent. It wasn’t good enough to have one protocol for IBM mainframes and another for PDP-11s. The IBM mainframes needed to talk to the PDP-11s and any other strange computers that might be around.

Since there are multiple routes between two points and since the quickest path between two points may change over time as a function of network traffic and other factors (for example, the existence of Cleveland), the packets that make up a particular data stream may not all take the same route. Furthermore, they may not arrive in the order they were sent, if they even arrive at all. To improve on the basic scheme, the TCP was layered on top of IP to give each end ...