Further Suggestions.

for the project on Design and Implementation of Basic RMI Mechanisms

Here are suggestions if you are really stuck and the time for the project is running out.

1. Try writing a program which only works for ZipCodeServerImpl. This does not involve any remote reference in marshalling, and many aspects would become easier.
2. If you are not sure how a method can be invoked at a remote (server's) site, look among classes in the java.lang package in the JavaAPI specification. It would then jump into your eyes...
3. How to use TCP (or sockets) can be found in the registry program I provided (see here): we can further make this better by using threads etc., but it gives a basic idea.