TCP Congestion Control

How fast should a sender go on a network it cannot see? Probe upward, back off on loss — and watch the congestion window saw.

Slow start: ×2 per RTT

Avoidance: +1 per RTT

Loss: ÷2

Animation Speed: 900ms per RTT

cwnd

ssthresh

phase

slow start

slow startcongestion avoidancessthresh▲ 3 dup ACKs× timeoutTahoe, same losses

Round 1: segments 1–1 in flight

The connection has just been established. cwnd starts at 1 segment and ssthresh at 16, a guess at where the network's capacity lies. Round 1: cwnd = 1, ssthresh = 16 (slow start). All 1 segment is acknowledged. In slow start each ACK grows cwnd by one segment, so the window doubles every round trip: 1 → 2.

Losses and Threshold

Presets

Initial ssthresh

Loss in round

Click a round to cycle: no loss → three duplicate ACKs (amber) → timeout (red).

The Rules

No loss, cwnd < ssthreshcwnd ×2 (capped at ssthresh)

No loss, cwnd ≥ ssthreshcwnd + 1

3 dup ACKs — Renossthresh = cwnd/2, cwnd = ssthresh

3 dup ACKs — Tahoessthresh = cwnd/2, cwnd = 1

Timeout — bothssthresh = cwnd/2, cwnd = 1

Real stacks start with ssthresh effectively unlimited, so the first loss ends slow start; most of Linux today runs CUBIC, which keeps this loss response but grows cwnd along a cubic curve.

Knowledge Check

Question 1 of 3

A TCP Reno sender has cwnd = 20 segments when three duplicate ACKs arrive. What are ssthresh and cwnd afterwards (ignoring the temporary +3 inflation)?

DSAverse

TCP Congestion Control

Losses and Threshold

The Rules

Knowledge Check

See also

TCP Congestion Control

Losses and Threshold

The Rules

Knowledge Check

See also