DDoS attacks are either of volumetric or application type. We often face a combination of both these types, or several kinds of application-type DDoS attacks. Volumetric DDoS attacks are trying to flood Internet connectivity in order to prevent any further connections. These attacks are mostly massive ones and can even reach dozens or hundreds Gbps. With increasingly available computing technologies and the number of attacking devices, these attacks are becoming ever more frequent. Application-type attacks are more sophisticated than volumetric ones. As an example, we can name 'SYN flood' that generates relatively little traffic in terms of volume, while opening many server connections that remain incomplete, thereby prohibiting additional connections. The server is no longer able to handle new rightful users and the service becomes unavailable.