GATE CS 2012 Q45 — Transport Layer: Flow Control, Congestion Control, UDP, TCP, Sockets

We can trace the congestion window size ($ext{cwnd}$) and slow-start threshold ($ext{ssthresh}$) step-by-step through each transmission:

• Initial State: $ext{cwnd} = 2\text{ MSS}$, $ext{ssthresh} = 8\text{ MSS}$.
• 1st Transmission: $ext{cwnd} = 2\text{ MSS}$. Since $ext{cwnd} < \text{ssthresh}$, it is in the Slow Start phase. After successful transmission, $ext{cwnd}$ doubles to $4\text{ MSS}$.
• 2nd Transmission: $ext{cwnd} = 4\text{ MSS}$. Since $ext{cwnd} < \text{ssthresh}$, it is in Slow Start. After successful transmission, $ext{cwnd}$ doubles to $8\text{ MSS}$.
• 3rd Transmission: $ext{cwnd} = 8\text{ MSS}$. Since $ext{cwnd} \ge \text{ssthresh}$, it enters the Congestion Avoidance phase. After successful transmission, $ext{cwnd}$ increases additively by $1$ to $9\text{ MSS}$.
• 4th Transmission: $ext{cwnd} = 9\text{ MSS}$. After successful transmission, $ext{cwnd}$ increases to $10\text{ MSS}$.
• 5th Transmission: $ext{cwnd} = 10\text{ MSS}$. A timeout occurs during this transmission:
  • New $ext{ssthresh} = \max(\text{cwnd}/2, 2) = 10/2 = 5\text{ MSS}$.
  • $ext{cwnd}$ is reset to the initial slow start value of $2\text{ MSS}$.
• 6th Transmission: $ext{cwnd} = 2\text{ MSS}$. Since $ext{cwnd} < \text{ssthresh}$ ($2 < 5$), it is in Slow Start. After successful transmission, $ext{cwnd}$ doubles to $4\text{ MSS}$.
• 7th Transmission: $ext{cwnd} = 4\text{ MSS}$. Since $ext{cwnd} < \text{ssthresh}$, it is in Slow Start. Upon success, doubling would exceed $ext{ssthresh}$ ($5$), so $ext{cwnd}$ is capped at $ext{ssthresh} = 5\text{ MSS}$.
• 8th Transmission: $ext{cwnd} = 5\text{ MSS}$. Since $ext{cwnd} \ge \text{ssthresh}$, it is in Congestion Avoidance. After successful transmission, $ext{cwnd}$ increases to $6\text{ MSS}$.
• 9th Transmission: $ext{cwnd} = 6\text{ MSS}$. After successful transmission, $ext{cwnd}$ increases to $7\text{ MSS}$.
• 10th Transmission: $ext{cwnd} = 7\text{ MSS}$.

Thus, the congestion window size at the start of the tenth transmission (or at the end of the ninth transmission) is $7 ext{ MSS}$.