Interactive end-of-chapter exercises


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 400 msec and 12 msec, respectively (see Section 3.5.3 for a discussion of these variables). Suppose that the next three measured values of the RTT are 280 msec, 340 msec, and 360 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 385

2. The DevRTT for RTT1 is 39

3. The timeout for RTT1 is 541

4. The estimatedRTT for RTT2 is 379.38

5. The DevRTT for RTT2 is 40.5

6. The timeout for RTT2 is 541.38

7. The estimatedRTT for RTT3 is 376.95

8. The DevRTT for RTT3 is 35.22

9. The timeout for RTT3 is 517.83



That's incorrect

That's correct

The answer was: 385

Question 1 of 9

The answer was: 39

Question 2 of 9

The answer was: 541

Question 3 of 9

The answer was: 379.38

Question 4 of 9

The answer was: 40.5

Question 5 of 9

The answer was: 541.38

Question 6 of 9

The answer was: 376.95

Question 7 of 9

The answer was: 35.22

Question 8 of 9

The answer was: 517.83

Question 9 of 9

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We greatly appreciate the work of John Broderick (UMass '21) in helping to develop these interactive problems.

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