Server emulator for existing software

For example we have chat client application executable (and we can change server Endpoint).

How I can analyze sockets, packets send to server and back? So I can write server emulator for that client?

This is just example. I know this is very general question. But I need general answer.开发者_如何学JAVA What techniques you can suggest? What tools? Any tutorials or books?

This is for educational purpose and I have no intention to violate any law.

Edited: Basically I want to get protocol that client/server communicate.

If you consider about writing a server emulator, you must know what to emulate, and therefore you must know the details of the protocol. So I doubt that approach can help to discover the unkown features.

I think that the Wireshark protocol analyser can help you to see all the network dialog between the server and the client. You do not have to write a custom server, just spy the actual exchanges :o)