Network based intrusions have become a serious threat to Internet users. Despite many sophisticated defense techniques, attacks continue to increase. At present, in order to hide the source of the attack, many attackers prefer a stepping stone to launch their attack due to the anonymous nature of the Internet. The size, header and content of an IP packet will be changed because of the stepping stone and all these changes make it more difficult to trace the source of attacks. Currently, researchers study the time interval between IP packets and embed the watermark into the packet stream by adjusting the time interval between IP packets to trace the source of attacks. In this paper we study the previous watermarking schemes based on inter packet delay and propose a novel watermark scheme based on the rate of packets. For the first time we used weak signal detection model and cluster technology to resume the watermark, so as to avoid the present schemes which are based on precision time synchronization or packet number. Simulation tests show that the novel watermark is robust and can countermine the time perturbation, packet losing perturbation and packet padding perturbation caused by an attacker on purpose.