Describe the Slow Start phase of TCP congestion control and its purpose.

• A. cwnd starts small (≈1 MSS) and grows exponentially each RTT until a threshold is reached.
• B. cwnd starts large and immediately saturates.
• C. cwnd remains constant throughout.
• D. Slow Start phase reduces cwnd.

Answer: (A)

Slow Start is the phase that rapidly increases the amount of data that can be in flight to quickly discover how much capacity the path can handle. It starts with a very small cwnd (about one MSS) and, as each acknowledgment arrives, cwnd grows so that the in-flight data roughly doubles each round-trip time. This exponential growth continues until cwnd reaches a threshold (ssthresh). The purpose is to probe available network capacity fast while limiting the risk of overwhelming the network; once the threshold is reached, the algorithm switches to a slower, linear increase in cwnd during congestion avoidance. If congestion is detected, cwnd is reduced and Slow Start may restart, but the basic idea is to ramp up quickly from a small start value to find the path’s capacity.