Q3- Consider Figure 1 shown below. Assuming TCP Reno is the protocol experiencin
ID: 3765055 • Letter: Q
Question
Q3- Consider Figure 1 shown below. Assuming TCP Reno is the protocol experiencing the behav-ior shown below; answer the following questions. (2 marks)
a. Identify the intervals of time when TCP slow start (SS) is operating. (0.4 marks)
b. Identify the intervals of time when TCP congestion avoidance (CA) is operating. (0.4 marks)
c. After the 16th transmission round (RTT), is segment loss detected by a triple duplicate ACK
(3dupACKs) or by a timeout? (0.2 mark)
d. After the 22
nd
transmission round (RTT), is segment loss detected by a triple duplicate ACK
(3dupACKs) or by a timeout? (0.2 mark)
e. What is the initial value of ssthresh (or threshold) at the first transmission round (RTT)? (0.2 mark)
f. What is the value of ssthresh (or threshold) at the 18th transmission round (RTT)? (0.2 mark)
g. Assuming a packet loss is detected after the 26th transmission round (RTT) by the receipt of a
triple duplicate ACK (3dupACKs), what will be the values of the congestion window (cwnd) and
ssthresh (or threshold)? (0.4 marks)
Note: When asked about intervals and transmission rounds consider answering using ‘x-axis’ and
when asked about ssthresh and congestion window try answering using ‘y-axis
Explanation / Answer
a)
TCP slowstart is operating in the intervals [1,6] and [23,26]
b)
TCP congestion avoidance is operating in the intervals [6,16]and [17,22] (assuming that the ACK of the lost segement arrives directly after the three duplicate ACKs).
c)
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.
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 initially 32, since it is at this window size that slow start stops and congestion avoidance begins
f)
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 in round 17 is 42/2 + 3 = 24 and in round 18 it is 24+1=25
g)
The threshold will be set to half the current value of the congestion window (8) when the loss occurred and congestion window will be set to the new threshold value + 3 MSS. Thus the new values of the threshold and window will be 4 and 7 respectively
Related Questions
Navigate
Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.