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An attack is defined as an excessive packet discard rate (of various packet types), when the rate of incoming packets exceeds the Fill Rate. Fill Rate is measured in "packets per second" or "pps". The policing is done based on fill rate and token buckets. Fill rate determines the rate in which credits are applied to the bucket.  So a 20 pps Fill Rate means one credit every in 50 millisecond.   If you have a Bucket Size of 50 packets and Fill Rate of 20 pps, the policer can handle a burst of 50 packets but if the 51st packet arrives 49 millisecond later, that packet will be dropped. This is because the Fill Rate applies credit every 50 millisecond so a packet arriving before that will get dropped. The Bucket Size allows room for sudden bursts of traffic, whereas the Fill Rate indicates the expected steady state flow of the traffic.  For more information on Token Buckets and Fill Rates, please consult Token Bucket Policers.

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