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Chapter 6
“Do I Know This Already?” Quiz
Congestion-Avoidance Concepts and RED
1Describe the process of TCP slow start and discuss when it occurs.
TCP slow start governs the growth of the TCP congestion window after the window has been lowered in reaction to a packet drop. Slow start increases the window by one for each positively acknowledged packet received.
2Describe the meaning of the term “global synchronization,” and discuss what causes it.
Global synchronization describes a condition in which many TCP connections have their congestion windows lowered due to unacknowledged or lost segments at around the same instant in time. The connections all grow CWND at about the same rate, re-creating the same congestion levels again, causing more drops, which in turn reduces again the TCP congestion windows. Global synchronization is caused by a large number of packet drops in a very short period, typically caused by tail drops.
3Define the meaning of the term “tail drop.”
When a queue fills, and a new packet must be placed into the queue, the packet is dropped. Because the packet would be placed into the end, or tail, of the queue, it is called tail drop.
4Does RED compare the actual queue depth or the average queue depth to queue thresholds when deciding whether it should discard a packet? Why this one, and not the other?
RED uses average queue depth. By using the average, rather than the actual queue depth, RED behaves more consistently, rather than more erratically, which helps prevent synchronization of TCP flows.
WRED
5List the queuing tools that can enable WRED for use with some or all of their queues, effectively enabling WRED concurrently with the queuing tool.
CBWFQ and LLQ.
6Describe how WRED “weights” packets.
WRED weights packets based on precedence or DSCP by assigning different minimum threshold, maximum threshold, and mark probability denominator values for each precedence or DSCP.
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Q&A
Congestion-Avoidance Concepts and Random Early Detection (RED)
1Describe the function of the congestion window in TCP, and how it is changed as a result of packet loss.
The TCP congestion window, or CWND, is one of two windowing mechanisms that limit TCP senders. CWND can be split in half as a result of packet loss, slowing the sending rate. CWND can also be slammed shut to the size of a single segment in some cases.
2Identify the two TCP windowing mechanisms, and describe when each is used.
The TCP congestion window, or CWND, and the TCP receiver window, are the two windowing mechanisms. The lower of the two values is used at all times.
3Describe the process of TCP slow start, and when it occurs.
TCP slow start governs the growth of the TCP congestion window after the window has been lowered in reaction to a packet drop. Slow start increases the window by one for each positively acknowledged packet received.
4Describe the process of TCP congestion avoidance, and when it occurs.
TCP congestion avoidance then governs the growth of the TCP congestion window after the slow start phase has been completed.
5Describe the meaning of the term “global synchronization,” and discuss what causes it.
Global synchronization describes a condition in which many TCP connections have their congestion windows lowered due to unacknowledged or lost segments at around the same instant in time. The connections all grow CWND at about the same rate, re-creating the same congestion levels again, causing more drops, which in turn reduces again the TCP congestion windows. Global synchronization is caused by a large number of packet drops in a very short period, typically the result of tail drops.
6Define the meaning of the term “tail drop.”
When a queue fills, and a new packet must be placed into the queue, the packet is dropped. Because the packet would be placed into the end, or tail, of the queue, it is called tail drop.
7Define the meaning of the term “TCP starvation.”
When packets are dropped, TCP connections slow down, but UDP flows do not slow down. UDP packets can consume a disproportionate amount of queue space as a result, which could get to the point that the TCP connections simply get little or no queue space; this is called TCP starvation.
804 Appendix A: Answers to the “Do I Know This Already?” Quizzes and Q&A Sections
8Does RED compare the actual queue depth or the average queue depth to queue thresholds when deciding whether it should discard a packet? Why this one, and not the other?
RED uses average queue depth. By using the average, rather than the actual queue depth, RED behaves more consistently, rather than more erratically, which helps prevent synchronization of TCP flows.
9Describe how RED uses actual queue depth to calculate average queue depth. Do not list the formula, but just describe the general idea.
RED calculates the average by adjusting the previously calculated average a small amount based on the current actual queue depth. By default, the current queue depth is weighted at about .2 percent in the formula.
10Assume the RED minimum threshold is 20, the maximum threshold is 40, and the mark probability denominator is 10. What must be true for RED to discard all new packets?
The average queue depth must be above 40.
11Assume the RED minimum threshold is 20, the maximum threshold is 40, and the mark probability denominator is 10. What must be true for RED to discard 5 percent of all new packets?
The average queue depth must be at 30. Because the discard percentage grows linearly from 0 percent to 10 percent (in this case), between average queue depth of 20 through 40, average queue depth of 30 would mean that the discard percentage had grown to 5 percent.
12Define how RED uses the mark probability denominator. Give one example.
RED calculates the discard percentage based on the formula 1/MPD. For instance, with an MPD of 20, the discard percentage is 1/20, or 5 percent.
13Define the term “exponential weighting constant.” If the value is lowered compared to the default setting of 9, how does RED behave differently?
The exponential weighting constant defines how quickly the average queue depth changes, by determining how much the actual queue depth affects the rolling average queue depth. If EWC is lowered, the average changes more quickly, because the formula weights the current actual queue depth more than before.
Weighted RED (WRED)
14Spell out the words represented by the initialisms RED, WRED, and FRED.
Random Early Detection (RED), Weighted Random Early Detection (WRED), FlowBased Weighted Random Early Detection (FRED).
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15List the queuing tools that can be concurrently supported on an interface when WRED has been enabled directly on a serial interface.
FIFO Queuing only.
16Identify the most important difference between RED operation and WRED operation.
WRED weights its discard decisions based on precedence or DSCP, whereas RED ignores precedence and DSCP.
17Describe how WRED “weights” packets.
WRED weights packets based on precedence or DSCP by assigning different minimum threshold, maximum threshold, and mark probability denominator values for each precedence or DSCP.
18List the queuing tools that can enable WRED for use with some or all of their queues, effectively enabling WRED concurrently with the queuing tool.
CBWFQ and LLQ.
19What command enables you to look at WRED drop statistics when WRED is configured inside an MQC class?
show policy-map interface
20Taking as many defaults as possible, list the configuration commands needed to configure precedence-based WRED on interface S1/1.
interface serial 1/1
random-detect
21Taking as many defaults as possible, list the configuration commands needed to configure DSCP-based WRED on interface S1/1.
interface serial 1/1
random-detect dscp-based
22Taking as many defaults as possible, list the configuration commands needed to configure DSCP-based WRED inside class class1, inside policy map my-policy. (You can assume that the CBWFQ configuration has already been completed, and you just entered global configuration mode. Assume that you need just to enable WRED in class class1.)
policy-map my-policy
class class1
random-detect dscp-based