How to write a server application?

Just because of curiosity, I'd like to get to know better, how the Internet works.

I have already read quite some articles in Wiki开发者_运维百科pedia, but these are very theoretical. I know, which protocols are used where, and what they are good for (IP, TCP, HTTP). But I have no idea how these are actually implemented in Linux or any other OS.

So, for me to understand these things better, I'd like to write a simple implementation of the Echo-Protocol in C, preferrably for Linux, and from ground up. My client-application should be able to send a message to a given port on any other computer, and my server-application should be able to receive this message, and send it back to the client.

Where do I start best? What should I read to get more information on these topics? Has anyone of you already done something like this, and if so, what did you learn?

In addition, it would be great if someone could tell me a bout a program that shows me the raw Internet-traffic that runs trough my PC. I think this would also help me understanding things a bit better.

Thanks in advance for your advice.

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http://www.google.com/search?sourceid=chrome&ie=UTF-8&q=berkley+socket+tutorial

http://beej.us/guide/bgnet/

http://www.tenouk.com/cnlinuxsockettutorials.html

http://www.paulgriffiths.net/program/c/sockets.php

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In addition, it would be great if someone could tell me a bout a program that shows me the raw Internet-traffic that runs trough my PC.

Ethereal is pretty great for this.