Show all your work. No credit will be given if work is not shown!*** Problem # 1
ID: 3736990 • Letter: S
Question
Show all your work. No credit will be given if work is not shown!*** Problem # 1 (14 points): Consider the following plot of TCP window size as a function of time 12 10 10 0 1 2 3 45 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 Transmission round Assuming TCP Reno is the protocol experiencing the behavior shown above, answer the following questions. a) (3 points) Identify the intervals of time when TCP slow start is operating. b) (3 points) Identify the intervals of time when TCP congestion avoidance is operating. c ( point) After the 16h transmission round, is segment loss detected by a triple duplicate ACK or by timeout? d) (1 point) After the 25th transmission round, is segment loss detected by a triple duplicate ACK or by timcout? e) (1 point) What is the initial value of Threshold at the first transmission round? f) (1 point) What is the value of Threshold at the 12th transmission round? g) (1 point) What is the value of Threshold at the 23d transmission round? h) (2 points) During what transmission round is the 23rd segment sent? i) (1 point) Assuming a packet loss is detected after the 30th round by a triple duplicate ACK, what will be the values of the congestion-window size and of Threshold?Explanation / Answer
Answer: See the answers below:
a) TCP slow start is the algorithm used in congestion control process that slowly increases the window size (amount of data being transmitted). For the given graph, TCP slow start is operating at intervals [1,3], [10,12] and [26,28].
b) Congestion avoidance is the period when data is transmitted at a rate very close to the rate of data congestion. For the given graph, congestion avoidance is operating at intervals [3,9], [12,16] and [20,25];
c) After the 16th transmission round, the segment loss is detected by triple duplicate ACK as window size has not dropped to 1.
d) After the 25th transmission round, the segment loss is detected by timeout as window size has been dropped to 1.
e) Initial size of threshold is 4 because at this value slow starts stops and congestion avoidance begins.
f) At 12th transmission round, the threshold size will be 5. This is due to the fact that threshold is half of the size of window when packet lost was last detected. For the given graph, window size was 10 when packet loss was detected in 16th transmission round.
g) Threshold size at 23rd transmission round is 3 as window size at the time of last packet loss was 6.
h) 23rd segment is sent during 23 transmission round.
i) Window size at 30th transmission round is 6. So if triple duplicate ACK is detected at this point, threshold size will be 3 and congestion window size will be 3 + 3 MSS i.e. 6.
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