Before the sample application enqueues packets for the transmit code, it performs RED on the queue. RED is a congestion-avoidance algorithm invented by Sally Floyd and Van Jacobson while at the University of California. It allows packet-processing devices in TCP/IP networks to maintain high utilization of output links without inducing large queuing latencies in the network.

In our device, we are only supporting two ports, so congestion is very unlikely. So, the inclusion of RED in our application is somewhat gratuitous. But, many other devices that can be built with IXP2XXX processors may need congestion avoidance, and implementing RED exposes some exciting features of the IXP2XXX processor, so we included it in our application anyway.

The RED algorithm is fairly simple. When a packet arrives at the system, the RED algorithm decides to either drop the packet or forward it. This decision is based on minimum and maximum thresholds, defined by the user, for a particular queue. If the number of packets in the queue is less than the minimum threshold, the packet is forwarded. If the number of packets in the queue is greater than the maximum threshold, it is dropped. If the number of packets in the queue is between the minimum and maximum thresholds, there is a random probability that the packet will be dropped. This probability increases linearly between the minimum threshold and the maximum threshold. Figure 6.6 shows the probability of a packet being dropped as a function of...