A DoS (Denial of Service) attack is a generic description of the process of preventing the target of the attack carrying out its normal role. DoS attacks can be directed at network entities such as hosts and servers or alternatively, the network infrastructure itself could be the target. Attack mechanisms are extremely varied, largely due to the fact that the variety of potential targets is also hugely varied. Flooding type DoS attacks are commonplace; for example the well known TCP SYN Flood attack sees the target device swamped with TCP SYN messages until it’s kernel resources are eventually exhausted. Alternatively, a SIP server may succumb to a SIP INVITE flood, which involves large amounts of INVITEs to be directed to the target, ultimately causing the target to fail. Nowadays, Distributed DoS attacks are used as a much more affective DoS attack mechanism.