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What this shows

An interactive visualization of TCP's congestion window (cwnd) growing and shrinking over time under TCP Reno or TCP Cubic congestion control, through Slow Start, Congestion Avoidance, and Fast Recovery.

How to use it

  • Click any point on the chart to inject a loss event — a soft loss (3 duplicate ACKs) or a hard loss (RTO timeout) — and see how cwnd reacts.
  • Switch between TCP Reno and TCP Cubic to compare their growth curves.
  • Hover a point to inspect its exact cwnd, ssthresh, and phase.
  • Use the gear icon to toggle phase shading, RTT points, and limit lines, adjust the simulated network limit and RTT count, and zoom/pan the chart.
  • Drag the chart to pan, or check the legend below for a full key to every line, marker, and shaded region.

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TCP Congestion Control Visualizer

Show:
145 MSS
200
Zoom All Pan RTT 0–200
Click any RTT point to inject a loss event · Hover to inspect · Drag to pan
Loss event
Choose loss type to inject.
Legend ▼
Graph Points
cwnd value
Soft loss — 3 dup ACKs
Hard loss — RTO timeout
Lines & Limits
Network limit (unknown to sender)
ssthresh — slow start threshold
wmax — Cubic window max
Phases
Slow Start
Fast Recovery
Congestion Avoidance
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