How does TCP congestion control work?

TCP congestion control prevents network overload. Algorithms: Slow Start (exponential growth until threshold), Congestion Avoidance (linear growth), Fast Retransmit (retransmit on 3 duplicate ACKs), Fast Recovery (don't reset to slow start). Congestion window (cwnd) limits data in flight. On timeout: cwnd = 1, slow start. On 3 dup-ACKs: cwnd halved, fast recovery. Modern variants: CUBIC (default in Linux), BBR (Google, models bandwidth and RTT).