Source: 


  http://download.oracle.com/javase/tutorial/networking/sockets

   http://download.oracle.com/javase/tutorial/networking/sockets/clientServer.html

Modified by P. Conrad to add command line parameter for host in client program.


To run: 

  On server side: java KnockKnockServer
  On client side: java KnockKnockClient [serverhostname]

The [serverhostname] is optional--it defaults to csil.cs.ucsb.edu