The very first time I tried to setup a server at home I discovered something unexpected: What I thought was going to be simple and easy turned out to be more complicated than expected because - in spite of many years of working with networks - it turned out that there was still so much more that I needed to know.

The next shock came soon after: virtually every book or document that I found on the subject attacked it in a way that made it seem terribly complicated. I wanted something simple and easy to read. I wanted something that was organized in such a way as to allow me to jump in and quickly start getting things done.

So here it is.

You can find a rather large number of Introductions to Networking on the Internet today. This one is intended to be different. In fact, let's go ahead and jump right in; let's start looking seriously at what we need to know to get simple things done.

TCP / IP

Virtually all the communications that we see these days take place using something called the Internet Protocol Suite. We call it TCP/IP - Transmission Control Protocol / Internet Protocol - the acronyms for two of the protocols in the suite.

The overall suite of protocols handle all the issues related to communications between computers. However, there are three main protocols that stand out: ICMP, UDP and TCP. These three protocols stand front and center in our lives when we are building, maintaining and using servers. So, let's get a basic picture of how they work into our minds:

Postcards vs. Operator Assisted Calls

The two main paradigms in communications are the post card (delivered by a mail man,) and the operator-assisted long-distance call. We will make regular use of these paradigms as we discuss networking, so let's start with a short review:

Post Cards

A Post Card is a small card - often a photo. You write, on the back of the card, the address that you want the card to be sent to and a short note for the person who receives it. The post man will carry the card to the address you specified in exchange for the value of the postage stamp - but that's all! The postage does not include any kind of reply service - so you don't know if your friend got the card or not.

Operator Assisted Calls

Many years ago, before the advent of the telephone exchange, there was a person who worked as a long distance telephone operator. In those days you would dial zero on your rotary phone and the operator would join the call. You would then speak to the operator about the long distance call that you wanted to make. You would tell the operator what country you wanted to call and who you wanted to speak with. The operator would then stay on the line with you until you were connected to the person you wanted to speak with. If there was any problem and the call did not go through - you would not be charged.

The difference here is a huge one: the post card is sort of a hit-and-miss proposition - it might work or it might not and you don't really know either way. The operator assisted call, though, is guaranteed service - you don't pay a cent unless the call goes through. Of course, operator assisted calls used to cost a fortune.

UDP & TCP

Not surprisingly, the two main protocols in the TCP/IP protocol suite are UDP and TCP:

UDP, the User Datagram Protocol, works like a post card carried by a post man. You send it out and the system will try to deliver it. It's surprising to find out about all the things that can go wrong:

• A computer in the network can be busy. UDP messages sent in a sequence can arrive at their destination in a completely different sequence because some of the messages can be held-up by busy computers.

• A computer can be really busy - so it might not pass along the UDP message at all. This is called a dropped packet in network parlance.

TCP, the Transmission Control Protocol, works like an operator-assisted long-distance call. It establishes a connection and then works hard to get around the various problems that come up during the conversation:

• When a TCP connection receives a message it sends a reply.

• When a TCP connection does not get a reply within a certain delay period, it resends the message.

• When a TCP connection receives parts of a long message in the wrong sequence, it sorts the parts back into the proper sequence.

• When a TCP connection is sending lots of data, the receiver will send rate control information back to the sender. This ensures that the data is sent as fast as possible without flooding the receiver.

TCP works by sending UDP packets. As you might imagine, there are lots of UDP messages that go back and forth during a TCP connection. TCP, therefore, is more expensive than UDP in terms of overhead.

Control Messages

There is one more protocol that you will really need to know about: ICMP. The Internet Control Message Protocol is used to send important information. For example:

• If a connection is attempted but there is no software listening to the specified port, a Connection Refused reply is sent via ICMP.

• If a connection is attempted to a computer that is not available for some reason, a Destination Unreachable ICMP message is returned.

• Probably the most popular message is called an Echo Request It is more popularly known as a ping. When a ping request goes out, the connection that receives it simply sends back a reply. Pinging is a really easy way of checking out a network.

The three ICMP messages above are probably the most common ones but, of course, there are many more.