The modern Internet is at rather dangerous stage of its life cycle. Taking into account a today's level of computer network security, the Internet can simply cease to work, if the current tendency of growth of number and capacity of distributed denial-of-service (DDoS) attacks to root servers will proceed. In the paper we discuss that in order to combat DDoS, the computer community needs to develop a strong theoretical basis upon which to harden information systems and infrastructures so they can survive such attacks. We introduce an agent-based formal framework for modeling and simulation of DDoS attacks. The framework and software tool developed can be used for conducting experiments to analyze computer network vulnerabilities and evaluate efficiency and effectiveness of security policy.