Linux Protocol Implementation

For my final school project our team has been trying to implement an EPON topology on 5 Linux computers running red hat 6.2 for solaris. Ok, we are trying to actually access the packets on the IP layer or before the IP layer so that we can alter the code an input data into it. We are currently trying to understand ethertap cause it hopefully will do what we want it to do. Which is enable us to read the packets and alter or add code so that we can label each packet. Has anyone ever used Ethertap or anything of the sort?