A DDOS(Distributed Denial of Service) attack is one of the major problem, that organizations are dealing with today.
Such a kind of attack is very difficult to mitigate, especially for small organizations with small infrastructure. The main difficulty in dealing with DDOS attack is the fact that, traditional firewall filtering rules does not play well. The main reason behind this problem is that, most of the time the attacking machines(machine's that take part in a DDOS attack, and becomes part of a bot-net) are large in number and are from diverse geographical locations.
