The space needed at each router is limited and controllable (2n bits). A small "n" makes the probability of collision of packet hashes (and false identification) higher. When a packet is to be traced back, it is forwarded to originating routers where fingerprint matches are checked. As time passes, the fingerprint information is “clobbered” by hashes generated by other packets. Thus, the selectivity of this approach degrades with the time that has passed between the passage of the packet and the traceback interrogation.