GATE CS 2020 Q55 — Transport Layer: Flow Control, Congestion Control, UDP, TCP, Sockets

Initially, at $t = 0$, the TCP connection is established and the client starts with a congestion window ($\text{cwnd}$) of $1 \text{ MSS} = 2 \text{ KB}$.

Since the slow-start threshold ($\text{ssthresh}$) is $32 \text{ KB}$, the TCP connection operates in the Slow-Start phase as long as $\text{cwnd} < \text{ssthresh}$. In this phase, $\text{cwnd}$ doubles every round-trip time (RTT) of $6$ ms:

• At $t = 0$ ms: $\text{cwnd} = 2 \text{ KB}$
• At $t = 6$ ms: $\text{cwnd} = 4 \text{ KB}$
• At $t = 12$ ms: $\text{cwnd} = 8 \text{ KB}$
• At $t = 18$ ms: $\text{cwnd} = 16 \text{ KB}$
• At $t = 24$ ms: $\text{cwnd} = 32 \text{ KB}$

At $t = 24$ ms, $\text{cwnd}$ reaches $\text{ssthresh} = 32 \text{ KB}$. The TCP connection now enters the Congestion Avoidance phase, where $\text{cwnd}$ increases linearly by $1 \text{ MSS} = 2 \text{ KB}$ per RTT:

• At $t = 30$ ms: $\text{cwnd} = 32 + 2 = 34 \text{ KB}$
• At $t = 36$ ms: $\text{cwnd} = 34 + 2 = 36 \text{ KB}$
• At $t = 42$ ms: $\text{cwnd} = 36 + 2 = 38 \text{ KB}$
• At $t = 48$ ms: $\text{cwnd} = 38 + 2 = 40 \text{ KB}$
• At $t = 54$ ms: $\text{cwnd} = 40 + 2 = 42 \text{ KB}$
• At $t = 60$ ms: $\text{cwnd} = 42 + 2 = 44 \text{ KB}$

Since the receiver advertised window ($\text{rwnd} = 50 \text{ KB}$) is greater than $44 \text{ KB}$, the congestion window size is not limited by $\text{rwnd}$. Thus, the size of the congestion window at $t + 60$ ms is $44 \text{ KB}$.