This paper develops the concept of victim-assistance for denial of service (DoS) mitigation. The proposed concept is utilized within a simple, yet effective scheme designed for mitigating TCP-based reflector DoS attacks. The proposed scheme, called SYN number based filtering (SNF), takes into account the TCP's connection establishment behavior and the inherent features of the attack itself. The main idea of the SNF scheme is to restrict the choice of the initial sequence numbers of SYN packets to certain pattern, such that corresponding SYN-ACK packets can be validated at the ISP's perimeter. We evaluate the proposed scheme through analytical studies for classical and advanced attacks using two performance metrics, namely, the false positive and false negative rates. Our analysis shows that the proposed scheme offers low false positive and false negative rates. In addition, we identify several research problems based on the proposed concept.