Question

In which state does the cwnd move to congestion avoidance, but the size of the cwnd is reduced to the ssthresh value?

a) Slow start
b) Fast retransmit
c) Congestion avoidance
d) Congestion collapse

Answer 1

The state where cwnd moves to congestion avoidance and reduces its size to ssthresh is fast retransmit. It happens after receiving duplicate ACKs and indicates a need for segment retransmission, leading to a linear increase in cwnd.

Step-by-step explanation:

The state in which the congestion window (cwnd) moves to congestion avoidance and the size of the cwnd is reduced to the slow start threshold (ssthresh) is known as fast retransmit. This occurs when a TCP sender receives a series of duplicate ACKs indicating that a segment has been lost and needs to be retransmitted. Instead of waiting for a retransmission timer to expire, the sender immediately retransmits the missing segment and then enters congestion avoidance, slowing down the sending rate to prevent further congestion. However, it does not enter slow start mode unless another timeout occurs, hence the cwnd is reduced to the ssthresh value, and then increases linearly instead of exponentially as in the slow start phase.