🌐 TCP Congestion Control

Bottleneck bandwidth 10 pkt/RTT Round-trip time (RTT) 100 ms Buffer size 12 pkt Random loss rate 0.0 % Algorithm variant

Reno

Tahoe

CUBIC-ish

NewReno

Presets

Ideal link

Lossy wireless

Small buffer

High BDP

Phaseslow start

cwnd1 pkt

ssthresh∞

Throughput0 pkt/RTT

Buffer fill0 / 12

Drops0

The congestion window (cwnd) limits how many packets may be in flight. In slow start cwnd doubles every RTT until it reaches ssthresh; then congestion avoidance adds ~1 packet per RTT (additive increase). When cwnd exceeds bandwidth×RTT the bottleneck buffer fills; an overflow (or a random loss) signals congestion and cwnd is cut — multiplicative decrease. That AIMD cycle draws the familiar sawtooth.

cwnd vs time — slow start (steep), AIMD (linear ramp), loss (drop)

Sender → bottleneck buffer → receiver. Green = data packets, cyan = ACKs.