Consider Figure 1 shown below. Assuming TCP Reno is the protocol experiencing th
ID: 3766051 • Letter: C
Question
Consider Figure 1 shown below. Assuming TCP Reno is the protocol experiencing the behavior shown below; answer the following questions.
a. Identify the intervals of time when TCP slow start (SS) is operating.
b. Identify the intervals of time when TCP congestion avoidance (CA) is operating.
c. After the 16th transmission round (RTT), is segment loss detected by a triple duplicate ACK (3dupACKs) or by a timeout?
d. After the 22nd transmission round (RTT), is segment loss detected by a triple duplicate ACK (3dupACKs) or by a timeout?
e. What is the initial value of ssthresh (or threshold) at the first transmission round (RTT)?
f. What is the value of ssthresh (or threshold) at the 18th transmission round (RTT)?
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)?
Explanation / Answer
From the given graph:
RTT 1 - 1 (Exponential increase) slow start start
RTT 2 - 2
RTT 3 - 4
RTT 4 - 8
RTT 5 - 16
RTT 6 - 32 slow start end
RTT 7 - 33 (Linear additive increase) - fast recovery – congetion avoidance start
RTT 8 - 34
RTT 9 - 35
RTT 10 -36
RTT 11 -37
RTT 12 -38
RTT 13 -39
RTT 14 -40
RTT 15 -41
RTT 16 -42
RTT 17 -24 (Multiplicative decrease, i.e cwnd= cwnd/2) congestion avoidance start
RTT 18 -25
RTT 19 -26
RTT 20 -27
RTT 21 -28
RTT 22 -29 (time out)
RTT 23 -01 (restart slow start) slow start
RTT 24 -02
RTT 25 -04
Solution:
[1.6] and [23-]
[6-16] and [17-22]
If the packet loss is recognized by time out, size of the congestion window (cndw) would be reduced to 1. But the window size is not reduced in the 17th transmission round. But the cwnd is reduced to half. Thus, the packet loss is identified by triple duplicate ACK.
In the 23 transmission round, cwnd is 1, thus the packet loss is recognized by time out.
The initial congestion window size is 32, because after segment size reached to 32, linear additive increase is started.
In the transmission round 16, congestion window size is 42. After transmission round 16, packet loss is identified. When the packet loss is identified, the threshold value is set to half of the size of the congestion window. Thus, the threshold value is 21.
At transmission round 26, cwnd is 8. Hence, if packet loss is identified by triple duplicate ACK, the window size is reduced to half, i.e cwnd=4 , and cwnd is set as new threshold , i.e threshold = 4.
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