TCP Congestion Control Consider the figure below. Assume TCP Reno is the protoco
ID: 3834225 • Letter: T
Question
TCP Congestion Control
Consider the figure below. Assume TCP Reno is the protocol experiencing the behavior, and answer the following questions. Justify your answer briefly.
a) Identify the intervals of time when slow start is operating
b) Identify the intervals of time when TCP congestion avoidance is operating
c) After the 16th transmission round, is segment loss detected by a triple duplicate ack or a timeout?
d) After the 22nd transmission round, is segment loss detected by a triple duplicate ack or a timeout?
e) What is the initial value of ssthresh at the first transmission round?
f) What is the value of ssthresh at the 18th transmission round?
g) What is the value of ssthresh at the 24th transmission round?
h) Suppose TCP Tahoe is used (instead of TCP Reno) and assume that triple duplicate acks are received at the 16th round. What are the ssthresh and the congestion window size at the 19th round?
45 40 30 25 20 10 TCP window size as function of time 1 2 3 4 5 6 7 8 9 1011 1213 14 15 16 1718 19 20 21 22 23 24 25 26 Transmission RoundExplanation / Answer
a) TCP slowstart is operating in the intervals [1,6] and [23,26]
b) TCP congestion advoidance is operating in the intervals [6,16] and [17,22] c)
After the 16th transmission round, packet loss is recognized by a triple duplicate ACK. If there were a timeout, the congestion window size would have dropped to 1.
d) After the 22nd transmission round, segment loss is detected due to timeout, and hence the congestion window size is set to 1.
e) 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.
f) During the 1st transmission round, packet 1 is sent; packet 2-3 are sent in the 2nd transmission round; packets 4-7 are sent in the 3rd transmission round; packets 8- 15 are sent in the 4th transmission round; packets 16-31 are sent in the 5th transmission round; packets 32-63 are sent in the 6th transmission round; packets 64 – 96 are sent in the 7th transmission round. Thus packet 70 is sent in the 7th transmission round.
g) 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.
h) Threshold is 21, and congestion window size is 1.
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