GATE CS 2008 Q56 — Transport Layer: Flow Control, Congestion Control, UDP, TCP, Sockets

In the slow start phase of the TCP congestion control algorithm, the congestion window ($cwnd$) grows as follows:

• The sender starts with a small congestion window, typically $cwnd = 1 \text{ MSS}$ (Maximum Segment Size).
• For every successful acknowledgment (ACK) received, the sender increases the congestion window by $1 \text{ MSS}$.

Let's trace the growth per Round Trip Time (RTT):

• RTT 1: Send $1$ segment. Receive $1$ ACK $\rightarrow$ $cwnd$ increases by $1$, becoming $2 \text{ MSS}$.
• RTT 2: Send $2$ segments. Receive $2$ ACKs $\rightarrow$ $cwnd$ increases by $2$, becoming $4 \text{ MSS}$.
• RTT 3: Send $4$ segments. Receive $4$ ACKs $\rightarrow$ $cwnd$ increases by $4$, becoming $8 \text{ MSS}$.

In general, after $n$ RTTs, the congestion window size is $2^n \text{ MSS}$. This doubling of the congestion window size every RTT represents exponential growth.

Therefore, the correct option is D.