Foundation Summary

The "Foundation Summary" is a collection of tables and figures that provide a convenient review of many key concepts in this chapter. For those of you already comfortable with the topics in this chapter, this summary could help you recall a few details. For those of you who just read this chapter, this review should help solidify some key facts. For any of you doing your final prep before the exam, these tables and figures are a convenient way to review the day before the exam.

Figure 7-14 shows a graph of CWND after packet loss just using slow start, and another with slow start plus congestion avoidance.

The key information about TCP and UDP operation when packets are dropped is summarized in the following list:

• UDP senders do not reduce or increase sending rates as a result of lost packets.

• TCP senders do reduce their sending rates as a result of lost packets.

• TCP senders decide to use either the receiver window or the CWND, based on whichever is lower at the time.

• TCP slow start and congestion avoidance dictate how fast the CWND rises after the window was lowered due to packet loss.

Table 7-9 describes the overall logic of when RED discards packets, with the same ideas outlined in Figure 7-15.

Table 7-9. Three Categories of When RED Will Discard Packets, and How Many

Average Queue Depth Versus Thresholds	Action	Name
Average < minimum threshold	No packets dropped.	No Drop
Minimum threshold < average depth < maximum threshold	A percentage of packets dropped. Drop percentage increases from 0 to a maximum percent as the average depth moves from the minimum threshold to the maximum.	Random Drop
Average depth > maximum threshold	All new packets discarded similar to tail dropping.	Full Drop

Table 7-10 summarizes some of the key terms related to RED.

Table 7-10. RED Terminology

Term	Meaning
Actual queue depth	The actual number of packets in a queue at a particular point in time.
Average queue depth	Calculated measurement based on the actual queue depth and the previous average. Designed to adjust slowly to the rapid changes of the actual queue depth.
Minimum threshold	Compares this setting to the average queue depth to decide whether packets should be discarded. No packets are discarded if the average queue depth falls below this minimum threshold.
Maximum threshold	Compares this setting to the average queue depth to decide whether packets should be discarded. All packets are discarded if the average queue depth falls above this maximum threshold.
Mark probability denominator	Used to calculate the maximum percentage of packets discarded when the average queue depth falls between the minimum and maximum thresholds.
Exponential weighting constant	Used to calculate the rate at which the average queue depth changes as compared with the current queue depth. The larger the number, the slower the change in the average queue depth.
No Drop	State in which RED's average queue depth falls below the minimum threshold. No packets are discarded.
Random Drop	State in which RED's average queue depth falls between the minimum and maximum thresholds. A percentage of randomly-selected packets are discarded.
Full Drop	State in which RED's average queue depth exceeds the maximum threshold. All packets are dropped

Figure 7-16 shows the default WRED settings for precedence 0, with some nondefault settings for precedence 3 traffic.

WRED measures the average queue depth of the FIFO queue on an interface, as shown in Figure 7-17.

ECN allows WRED to signal a TCP sender to slow down, instead of discarding a packet sent by that TCP sender. Figure 7-18 shows the entire process.