If you are feeling keen, here are a few things this simulator would benefit
from:

 - Further abstraction of network building. Some utility classes could be 
   written to allow, for example, a node with an interface and all the  
   routing setup to be built at once rather than specifying all of this.
   You can take the idea further to build whole networks.

 - You might want to put multicasting support in the IP handler so you can
   write higher-level multicasting protocols.

 - There is support for a 'metric' in every routing table entry. You can
   write dynamic routing support by hooking in there. The 'metric' is an 
   Object, so you can put in whatever you want.

 - Complete the go-back-n protocol: Include timeouts to allow for lost
   packets and to allow out of sequence delivery. Complete the multiplexer
   class "SimpleGoBackN" to allow PROPER multiplexing. At the moment, 
   different ports are allowed, but only one connection per port. It should
   be one connection per (source port, source ip, dest port, dest ip
   address).

 - Implement TCP.

 - Implement whatever protocol you want :)