
306 The Practical OPNET User Guide for Computer Network Simulation
average queue occupancy exceeds the maximum threshold, max
th
, then RED marks
all arriving packets. Marked packets typically have their ECN eld (i.e., two right-
most bits of the ToS byte in the IP header) set to the Congestion Encountered (CE)
value, which noties network edges about congestion in the network. Alternative
RED implementations discard packets instead of marking them, which also effec-
tively serves as a notication to TCP sources about congestion in the network. RED
estimates the average queue occupancy using the exponentially weighted moving
average (EWMA) algorith ...