Mitigating Denial of Service Attacks: A Tutorial

This paper describes what Denial of Service (DoS) attacks are, how they can be carried out in IP networks, and how one can defend against them. Distributed DoS (DDoS) attacks are included here as a subset of DoS attacks. A DoS attack has two phases: a deployment and an attack phase. A DoS program must first be deployed on one or more compromised hosts before an attack is possible. Mitigation of DoS attacks requires thus defense mechanisms for both phases. Completely reliable protection against DoS attacks is, however, not possible. There will always be vulnerable hosts in the Internet, and many attack mechanisms are based on ordinary use of protocols. Defense in depth is thus needed to mitigate the effect of DoS attacks.