Consider the following statements regarding the slow start phase of the TCP congestion control algorithm. Note that $cwnd$ stands for the TCP congestion window and MSS denotes the Maximum Segment Size. (i) The $cwnd$ increases by 2 MSS on every successful acknowledgment. (ii) The $cwnd$ approximately doubles on every successful acknowledgement. (iii) The $cwnd$ increases by 1 MSS every round trip time. (iv) The $cwnd$ approximately doubles every round trip time. Which one of the following is correct?

During the Slow Start phase of TCP congestion control:

1. For every successful acknowledgment (ACK) received, the congestion window ($cwnd$) increases by $1\text{ MSS}$ ($cwnd = cwnd + 1\text{ MSS}$).

2. Since the sender transmits an entire window of packets per round trip time (RTT) and each acknowledged packet increments $cwnd$ by $1\text{ MSS}$, the value of $cwnd$ approximately doubles every round trip time (RTT) (exponential growth).

Evaluating the given statements:

• (i) is false because $cwnd$ increases by $1\text{ MSS}$ per ACK, not $2\text{ MSS}$.

• (ii) is false because $cwnd$ increases additively by $1\text{ MSS}$ per ACK, not doubling per ACK.

• (iii) is false because $cwnd$ increasing by $1\text{ MSS}$ per RTT is the behavior of the Congestion Avoidance phase (Additive Increase), not Slow Start.

• (iv) is true because exponential growth results in $cwnd$ doubling every RTT.

Hence, Only (iv) is true, which corresponds to option (C).