 Welcome back to this course. The topic of this week will be denial of service attacks. We talk about denial of service attacks when an online system, the target, is made unable to deliver service to users due to unwanted malicious traffic from an attacker. This happens usually because the attack depletes the resources of the target system, for example bandwidth or processing power. A denial of service attack can, in principle, be carried on by a single attacking host, but this can require that the attacker host has itself enough resources available. Clearly, a large number of attacking hosts can easily boost the denial of service attack, providing a quick time, a higher traffic load, or a larger number of requests, for example. We call this a distributed denial of service attacks, or DDoS. This week, we will explore the following topics. After this introduction, we will learn about DDoS attacks, exploring various flavors of existing attacks and analyzing some real example. We will then discuss about mitigation. After that, we will look in depth into two topics, reflecting DDoS attacks and booters, or DDoS as a service. Finally, we will summarize.