What is the purpose of Slow Start in TCP congestion control?

• A. To rapidly probe network capacity by increasing cwnd exponentially at the start.
• B. To gradually decrease the sending rate.
• C. To reset the connection.
• D. To terminate the connection.

Answer: (A)

Slow Start is the phase that lets TCP quickly discover how much data can be sent without causing congestion. When a connection begins (or after certain congestion events), the sender starts with a small amount of data in flight and then rapidly increases it. Each ACK nudges the congestion window (cwnd) up by roughly one maximum segment size (MSS), so cwnd grows roughly exponentially with each round trip. This fast ramp-up helps the sender utilize available bandwidth as soon as possible.

The idea is to probe the path’s capacity without staying at a tiny rate forever. Once cwnd reaches a threshold (ssthresh), TCP shifts to Congestion Avoidance, which grows cwnd more slowly to prevent overshoot. If a loss is detected, the sender reduces cwnd and restarts probing, which protects the network from sustained congestion. The other behaviors—gradually decreasing the rate, resetting, or terminating the connection—do not achieve the purpose of quickly and safely discovering the path’s capacity.