Fast Congestion Notification (FN) one of the proactive queue management mechanisms that practices congestion voidance to help avoid the beginning of congestion by marking/dropping packets before the router’s queue gets full; and exercises congestion control, when congestion avoidance fails, by increasing the rate of packet marking/dropping. Upon arrival of each packet, FN uses the instantaneous queue size and the average arrival rate to calculate the packet marking/dropping probability. This paper presents the Drop/Mark Activation Function, which is an internal (built in) function of FN marking/dropping probably function, and shows the conditions under which the FN will trigger a probabilistic packet marking/dropping. This paper shows that the FN’s drop activation function is given by

L(Ri, Qcur) =(Ri -μ).T-(Qopt -Qcur) which compares the predicted and required/allowed changes in the queue level, over a time period, to decide whether to attempt or not to attempt packet dropping. L(Ri, Qcur) = 0 defines the set of the drop activation threshold , the set of (average rate, current queue size), (Ri, Qcur), points for which the required/allowed and predicted decrease/increase in the queue level exactly equal each other and that identify the boundary between the drop region (L(Ri, Qcur) > 0), the sets of points at which the packet dropping is attempted, and the no-drop region (L(Ri, Qcur) < 0), the set of points at which the packet dropping is not attempted.