Interactive end-of-chapter exercises


Supplement to Computer Networking: A Top Down Approach 8th Edition


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Computing TCP's RTT and timeout values

Suppose that TCP's current estimated values for the round trip time (estimatedRTT) and deviation in the RTT (DevRTT) are 360 msec and 18 msec, respectively (see Section 3.5.3 for a discussion of these variables). Suppose that the next three measured values of the RTT are 380 msec, 330 msec, and 400 msec respectively.

RTT Diagram

Compute TCP's new value of DevRTT, estimatedRTT, and the TCP timeout value after each of these three measured RTT values is obtained. Use the values of α = 0.125, and β = 0.25. Round your answers to two decimal places after leading zeros.



Question List


1. What is the estimatedRTT after the first RTT?

2. What is the RTT Deviation for the the first RTT?

3. What is the TCP timeout for the first RTT?

4. What is the estimatedRTT after the second RTT?

5. What is the RTT Deviation for the the second RTT?

6. What is the TCP timeout for the second RTT?

7. What is the estimatedRTT after the third RTT?

8. What is the RTT Deviation for the the third RTT?

9. What is the TCP timeout for the third RTT?




Solution


DevRTT is calculated with the following equation: (1-beta)*DevRTT + beta * |estimatedRTT - sampleRTT|
estimatedRTT is calculated with the following equation: (1-alpha)*estimatedRTT + alpha*sampleRTT
TCP timeout is calculated with the following equation: estimatedRTT + (4*DevRTT)

1. The estimatedRTT for RTT1 is 362.5

2. The DevRTT for RTT1 is 18.5

3. The timeout for RTT1 is 436.5

4. The estimatedRTT for RTT2 is 358.44

5. The DevRTT for RTT2 is 22

6. The timeout for RTT2 is 446.44

7. The estimatedRTT for RTT3 is 363.63

8. The DevRTT for RTT3 is 26.89

9. The timeout for RTT3 is 471.2



That's incorrect

That's correct

The answer was: 362.5

Question 1 of 9

The answer was: 18.5

Question 2 of 9

The answer was: 436.5

Question 3 of 9

The answer was: 358.44

Question 4 of 9

The answer was: 22

Question 5 of 9

The answer was: 446.44

Question 6 of 9

The answer was: 363.63

Question 7 of 9

The answer was: 26.89

Question 8 of 9

The answer was: 471.2

Question 9 of 9

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