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opFlow can detect anomalies by determining an average network usage baseline and comparing it with traffic of a suspected anomaly event. Using a threshold system, you could create a rule to notify you of network behavior which current value exceeds the mean by two or three times the standard deviation. DoS attacks flood the network with packets from an untrusted source and usually it is a rather large packet size. NetFlow collects: Packet source, Port number, Destination Packet size, and Protocol number.