Not all header fields in IP packets are used simultaneously during transmissions. Some fields may provide traceback services, especially for the defense against DDoS attacks. In this paper, we analyze which IP header fields may be used without impacting the other services provided by the IP protocol. We also analyze how unused fields can be filled for the purpose of traceback, and with what probability can a router mark packets. Finally we compare the different existing methods for packet marking based on the fields used and marking information.