GATE CS 2024 Set 2 Q44 — Transport Layer: Flow Control, Congestion Control, UDP, TCP, Sockets

When a timeout occurs in a TCP connection, the congestion control state is updated as follows:

1. Slow Start Threshold ($ssthresh$) is set to half of the current congestion window size ($cwnd$): $$ssthresh = \frac{\text{Current } cwnd}{2} = \frac{12}{2} = 6 \text{ MSS}$$
2. Congestion Window ($cwnd$) is reset to $1 \text{ MSS}$: $$cwnd = 1 \text{ MSS}$$

Now, we track the behavior over the subsequent Round Trip Times (RTTs) during the Slow Start phase (since $cwnd < ssthresh$):

• During RTT 1:
  • The congestion window size is $cwnd = 1 \text{ MSS}$.
  • $1$ segment is transmitted and successfully acknowledged.
  • At the end of RTT 1, $cwnd$ doubles: $cwnd = 1 \times 2 = 2 \text{ MSS}$.
• During RTT 2:
  • The congestion window size is $cwnd = 2 \text{ MSS}$.
  • $2$ segments are transmitted and successfully acknowledged.
  • At the end of RTT 2, $cwnd$ doubles: $cwnd = 2 \times 2 = 4 \text{ MSS}$.
• During RTT 3:
  • The congestion window size is $cwnd = 4 \text{ MSS}$.

Thus, the congestion window size during the third RTT is 4.