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Tutorial 3 solution

Q1.. True or false?With the SR protocol, it is possible for the sender to receive an ACK for a packet that falls outside of its current window.With GBN, it is possible ..(same as above)?The stop-and-wait protocol (e.g., rdt 3.0) is the same as the SR with window size of 1. the same as GBN.

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Tutorial 3 solution

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    1. Tutorial 3 solution

    2. Q1. True or false? With the SR protocol, it is possible for the sender to receive an ACK for a packet that falls outside of its current window. With GBN, it is possible …..(same as above)? The stop-and-wait protocol (e.g., rdt 3.0) is the same as the SR with window size of 1 ……. the same as GBN……. ?

    5. True True True True

    6. Q2.

    7. Assuming TCP Reno is the protocol experiencing the behavior shown above, answer the following questions. In all cases, you should provide a short discussion justifying your answer.

    8. Summary: TCP Congestion Control (Reno) When CongWin is below Threshold, sender in slow-start phase, window grows exponentially. When CongWin is above Threshold, sender is in congestion-avoidance phase, window grows linearly. When a triple duplicate ACK occurs, Threshold set to CongWin/2 and CongWin set to Threshold. When timeout occurs, Threshold set to CongWin/2 and CongWin is set to 1 MSS.

    9. a) Identify the intervals of time when TCP slow start is operating. Answer: TCP slowstart is operating in the intervals [1,6] and [23,26]

    10. b) Identify the intervals of time when TCP congestion avoidance is operating. Answer: TCP congestion advoidance is operating in the intervals [6,16] and [17,22]

    11. c) After the 16th transmission round, is segment loss detected by a triple duplicate ACK or by a timeout? Answer: After the 16th transmission round, packet loss is recognized by a triple duplicate ACK. If there was a timeout, the congestion window size would have dropped to 1.

    12. d) After the 22nd transmission round, is segment loss detected by a triple duplicate ACK or by a timeout? Answer: After the 22nd transmission round, segment loss is detected due to timeout, and hence the congestion window size is set to 1.

    13. e) What is the initial value of Threshold at the first transmission round? Answer: The threshold is initially 32, since it is at this window size that slowstart stops and congestion avoidance begins.

    14. f) What is the value of Threshold at the 18th transmission round? Answer: The threshold is set to half the value of the congestion window when packet loss is detected. When loss is detected during transmission round 16, the congestion windows size is 42. Hence the threshold is 21 during the 18th transmission round.

    15. g) What is the value of Threshold at the 24th transmission round? Answer: The threshold is set to half the value of the congestion window when packet loss is detected. When loss is detected during transmission round 22, the congestion windows size is 26. Hence the threshold is 13 during the 18th transmission round.

    16. h) During what transmission round is the 70th segment sent?

    18. i) Assuming a packet loss is detected after the 26th round by the receipt of a triple duplicate ACK, what will be the values of the congestion-window size and of Threshold? Answer: The congestion window and threshold will be set to half the current value of the congestion window (8) when the loss occurred. Thus the new values of the threshold and window will be 4.

    19. Happy Chinese New Year!

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