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In opFlow, navigate to menu -> Views -> TopN. The TopN feature can display the top Applications, Top Application Sources, Top Application Conversations, Top Listeners, Top Protocols, Top Talkers, and Top TOS. Select the Top Application Conversations and you will see what device or interface is sending and receiving the most packets. For example, in Figure 2 below flow data captured the Source, Destination, Application, Port, as well as the number of Flows and Packets. Using this information it would be easy to identify a DoS attack on your network by identifying unusually high numbers of Flows/Packets being sent from an untrusted source.
Figure 2 - Top Application Conversations