Perfection Vs. Effectiveness

In fact a system too rigid and centralized, though highly reliable and strong-knotted, would represent a much higher risk of total disruption of the network, whereas the occasional flaw in the delivery of single messages, gives an overall better performance of the system, especially if the network is likely to be severely battered in some of its parts.

In other words, Paul Baran's priority was the stability of the network as a whole, and the paramount concepts were decentralization and redundancy.

Packet switching & the TCP/IP protocol

In the packet switching system, messages are not sent as a whole but they are divided into smaller pieces, called packets, that can be individuallyforwarded to the receiver. At the receiver's end these packets are put back together and the whole message reconstructed. However, you need specific procedures (protocols) to make the network function, instructions computers have to follow in order to exchange data.

So, a number of protocols were devised in the 1970s to let computers communicate. The one that eventually became a standard was the TCP (Transmission Control Protocol), which converts messages into streams of packets at the source, then reassembles them back into messages once they reach their destination.
The TCP is associated with the IP (Internet Protocol), which handles the addressing: its task is to make sure that packets are routed across multiple nodes and even across multiple networks with different standards, till they reach their destination.

This is how Vinton Cerf (1) (who invented the general architecture of the Internet together with Bob Kahn) describes how the Internet works:

... Internet packets are just like postcards, except they run about 100 million times faster. A postcard has a "from" address and a "to" address, and there is a limited amount of room to write things on. An Internet packet has a "from" address and a "to" address, and there is a limited amount of space to carry information. Now, we know some other things about postcards. When you put a postcard into the post-box, it's not always sure that it will arrive on the other side. Sometimes postcards get lost. This is true of INternet packets also, they can laso sometimes get lost. If you put three postcards into the postalsystem, they don't necessarily come out in the same order, they might not even come out on the same day. In the Internet we have the same situation: the packets can get out of order, they can get lost, and in a very strange circumstance, sometimes on the Internet an Internet packet can get copied, and so you may get two copies instead of one. So we have to make this rather unreliable communication network - the basic Internet packet or postcard - reliable for computers to talk to each other. The way we do that is to build another layer of protocol on top of the basic postcard and we call this layer TCP, or transmission control protocol. 

There is a very easy way to understand how that works too. Suppose that you wanted to send a book to a friend and the only way you could send it was with postcards. You would cut the book's pages out and glue them on th epostcard. Then you'd remember that not every postcard would have the page number on it because you have the pages, so your friend may get these postcards out ot order. so it would be a good idea if you numbered each postcard so your friend can put them back together in order. Then you remember that some of the postcards are going to be lost, so you keep a copy for yourself and you can send copies to your friend if he loses any. How do you know which postcards your friend has got? Good solution. Your friend can send you a postcard every once in a while and say, I got every postcard, except postcard number 102, except then you remember that the postcard that tells you how many your friend got might get lost too. So now you have to decide, if I haven't heard anythingfrom my friend about how many postcards he got, then I should start sending him copies fo postcards that he has not acknowledged yet. And when he finally sends me an acknowledgement, you can throw away the postcards copies that you kept because he got all that he needed. That's all that goes on in the TCP protocol. ...

 

(1) Excerpt taken from "The History of the Internet", a 1997 interview with Vinton Cerf appeared in www.mediamente.rai.it