When a denial of service attack happens, a user or organization is deprived of service of a resource they would normally expected to have. Or it may result in a degradation of Qos, which is hard to detect. Normal commercial activities will be disrupted by delay of communication on purpose. In the worst cases, a website can be forced to cease operating. Attacker may use one or some of the following methods: flooding packets until a computer or the whole network cease operating; preventing legitimate network traffic, which will cause lost of network resource. In the cases of web corpse, results were paralysis of services provided or degradation of Qos.