WRED Summary

Keep in mind that FRED specifically wants to defeat the UDP flows that consume too many queue entries. FRED still drops some packets from UDP flows when congestion occurs, based on normal WRED logic. For UDP flows, FRED simply decides which flows are taking too much of the queue, classifies these flows as nonadaptive, and discards packets in those flows more aggressively. For other UDP flows that FRED believes are not sending too many packets (fragile flows) FRED discards their packets less frequently.

NOTE UDP does not adapt to packet loss, so by definition, all UDP flows are nonadaptive. However, FRED uses the term "nonadaptive" to categorize those flows that are currently using too much of the queue.

The key to understanding how FRED works is to understand how FRED determines whether a particular UDP flow is fragile or nonadaptive. A couple of simple examples make the logic much more obvious. First, suppose FRED is enabled on a router's S0/0 interface, and the interface FIFO output queue has 40 queue entries maximum. At a particular point in time, 10 flows exist, called Flow 1, Flow 2, and so on. (Keep in mind that FRED only supports FIFO Queuing.) With a maximum queue depth of 40, and 10 flows, you can think of each flow's fair share of the queue space to be 40/10, or 4, in this case. FRED multiplies this fair share by a scaling factor, which defaults to 4. The scaling factor is used so that each flow has some capability to burst. This final number, 16 in this case, is the dividing line between fragile flows and nonadaptive flows.

Suppose, for instance, that Flow 1 and Flow 2 both use UDP. Flow 1 has 3 packets in the FIFO output queue, and Flow 2 has 20 packets in the queue. At first glance, it seems that Flow 2 should be considered a nonadaptive flow, because it has many packets in the queue. The calculated value of 4 * 40/10 (scaling factor * maximum queue length/number of flows), or 16, is greater than the number of packets in the queue that are part of Flow 1 (3 packets in the queue).

Therefore, FRED considers Flow 1 to be a fragile flow. Similarly, the calculated value of 16 is smaller than the number of packets in the queue that are part of Flow 2 (20 packets in the queue), so FRED considers Flow 2 to be nonadaptive.

The first example hides one subtlety in how FRED decides which UDP flows are fragile, and which are nonadaptive. Consider the same example, but suppose now that only five flows exist. The formula works out to be 4 * 40/5 = 32. (Notice that the only part of the formula that changes over time is the number of flows; the scaling factor remains static, as does the maximum queue length.) As the number of flows decreases, the threshold that determines whether a flow is nonadaptive increases. If Flow 2 still has 20 packets in the queue, Flow 2 would now be considered to be a fragile flow, with packets discarded less aggressively.

After FRED decides which flows are robust, fragile, and nonadaptive, FRED can apply WRED-like logic to decide whether to discard any packets at all, and if so, what percentage of the packets. Frankly, the published details on how FRED determines the flows are sketchy at best. For you QoS exam takers, the following details about how FRED discards packets for the three types of flows is unlikely to be covered, because the coverage in the corresponding courses is also sketchy, or not even mentioned.

For robust and fragile flows, FRED acts just like WRED in terms of packet discard. FRED still uses the per-precedence or per-DSCP minimum threshold, maximum threshold, and MPD to determine when to discard packets, and how many. You can override the default values for each precedence or DSCP value through configuration, just like with WRED.

For nonadaptive flows, FRED lowers the maximum threshold. By doing so, FRED increases the packet discard rate more quickly. More importantly, lowering the maximum threshold for packets in the flow causes FRED to discard all packets for these nonadaptive flows more quickly than for fragile and robust flows, essentially preventing these flows from consuming the entire queue.

To get a full sense of what happens to nonadaptive flows, consider the following example. For nonadaptive flows, FRED reduces the maximum threshold used for the flow by half of the difference between the maximum and minimum thresholds. FRED and WRED use defaults for precedence 0 as follows: minimum threshold 20, maximum threshold 40, drop percentage 10 percent (MPD=10). FRED reduces the maximum threshold for a precedence 0 nonadaptive flow to 30, because the difference between the maximum and minimum is 20, and half of that is 10. By reducing the maximum threshold by half of the difference between the minimum and maximum, FRED increases the packet discard rate more quickly as the average queue depth nears 30. Most importantly, FRED discards all packets for these nonadaptive precedence 0 flows when the average queue depth reaches 30, preventing these flows from taking over the entire queue.

Table 6-10 lists the terms used by FRED to describe these processes.