Consider a TCP connection operating at a point of time with the congestion window of size 12 MSS (Maximum Segment Size), when a timeout occurs due to packet loss. Assuming that all the segments transmitted in the next two RTTs (Round Trip Time) are acknowledged correctly, the congestion window size ($\textit{in MSS}$) during the third RTT will be ________

When a TCP timeout occurs due to packet loss:

1. Threshold Update:

   $$\text{ssthresh} = \max\left(2, \left\lfloor \frac{\text{cwnd}}{2} \right\rfloor\right) = \frac{12}{2} = 6\text{ MSS}$$

2. Slow Start Initialization:

   $$\text{cwnd} = 1\text{ MSS}$$

3. Window Growth across RTTs:

   • 1st RTT: $\text{cwnd} = 1\text{ MSS}$ is transmitted. Upon receiving the ACK, $\text{cwnd}$ doubles: $\text{cwnd} = 2\text{ MSS}$.

   • 2nd RTT: $\text{cwnd} = 2\text{ MSS}$ are transmitted. Upon receiving both ACKs, $\text{cwnd}$ doubles: $\text{cwnd} = 4\text{ MSS}$.

   • 3rd RTT: Since $\text{cwnd} = 4\text{ MSS} < \text{ssthresh} = 6\text{ MSS}$, the transmission during the 3rd RTT uses $\text{cwnd} = 4\text{ MSS}$.

Thus, the congestion window size during the 3rd RTT is 4 MSS.