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 240 msec and 25 msec, respectively (see Section 3.5.3 for a discussion of these variables). Suppose that the next three measured values of the RTT are 240 msec, 390 msec, and 290 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 240

2. The DevRTT for RTT1 is 18.75

3. The timeout for RTT1 is 315

4. The estimatedRTT for RTT2 is 258.75

5. The DevRTT for RTT2 is 51.56

6. The timeout for RTT2 is 465

7. The estimatedRTT for RTT3 is 262.66

8. The DevRTT for RTT3 is 46.48

9. The timeout for RTT3 is 448.59



That's incorrect

That's correct

The answer was: 240

Question 1 of 9

The answer was: 18.75

Question 2 of 9

The answer was: 315

Question 3 of 9

The answer was: 258.75

Question 4 of 9

The answer was: 51.56

Question 5 of 9

The answer was: 465

Question 6 of 9

The answer was: 262.66

Question 7 of 9

The answer was: 46.48

Question 8 of 9

The answer was: 448.59

Question 9 of 9

Try Another Problem

We gratefully acknowledge the programming and problem design work of John Broderick (UMass '21), which has really helped to substantially improve this site.

Copyright © 2010-2022 J.F. Kurose, K.W. Ross
Comments welcome and appreciated: kurose@cs.umass.edu