GATE CS 2018 Q14 — Transport Layer: Flow Control, Congestion Control, UDP, TCP, Sockets

To determine the correct statement, let us analyze the behavior of the TCP congestion window ($cwnd$) during the Slow Start phase:

1. Per-ACK Behavior: For every successful individual acknowledgment (ACK) received, the sender increases the $cwnd$ by $1\text{ MSS}$.
   Therefore, Statement (i) is false (it increases by $1\text{ MSS}$, not $2$) and Statement (ii) is false (it does not double on every individual ACK).
2. Per-RTT Behavior: Suppose the current congestion window size is $W$ segments. The sender transmits $W$ segments in one Round Trip Time (RTT).
   • Upon receiving $W$ successful ACKs during this RTT, the window size increases by $W \times 1\text{ MSS}$.
   • The new congestion window size becomes $W + W = 2W$ segments.
   Thus, the congestion window approximately doubles every RTT (exponential growth).
   Therefore, Statement (iv) is true.
3. Congestion Avoidance Phase: Increasing the $cwnd$ by $1\text{ MSS}$ every RTT (additive increase) is the characteristic of the Congestion Avoidance phase, not the Slow Start phase.
   Therefore, Statement (iii) is false.

Since only statement (iv) is true, the correct option is C.