For each metered flow in a network unit there is a memory entry which defines a time stamp and a rate parameter. When a packet in the respective flow arrives, the time of arrival is compared with the time stamp. If the time of arrival is before the time stamp the packet is discarded. If the packet arrives after the date stamp, it is allowed to proceed and a new time stamp is computed in accordance with the rate parameter. One exemplary computation adds to the time stamp a time interval obtained by dividing the size of the packet by the rate parameter and preferably subtracting from the quotient the time interval between the arrival time and the previous time stamp.