The Drop Activation Function of the Fast Congestion Notification (FN) Mechanism

Fast Congestion Notification (FN) one of the proactive queue management mechanisms that practices congestion avoidance 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) = ( ). ( ) R T Q Q i opt cur μ − − − 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.