Reno · CUBIC · BBR

Three answers to one question.

"How fast can I send before the network breaks?" Each algorithm answers differently:

• Reno probes by halving on every loss — the classic AIMD sawtooth.
• CUBIC follows a cubic curve back to its previous peak, recovering faster than Reno on high-BDP paths.
• BBR ignores loss entirely — it models bandwidth and propagation delay, paces to the model, and re-measures both periodically.

Try this:

1. 1Drag LOSS to 0.5%. Watch Reno's cwnd carve a sawtooth — that's AIMD on display.
2. 2Push LOSS to 2%. Reno collapses; CUBIC holds higher; BBR barely flinches because it doesn't react to loss.
3. 3Crank RTT to 200ms. BBR's PROBE_BW pacing remains visible; the loss-based algorithms slow their growth.
4. 4Watch the PHASE chip on each card. CUBIC stays in cubic; Reno toggles between slow-start / congestion-avoidance; BBR cycles through startup → drain → probe-bw.

Why this matters:

ISPs run fiber. The TCP stack on every customer's laptop chooses one of these (or a relative). On a clean fiber link with low loss, all three look similar; the differences appear when wifi sneezes, when buffer-bloat shows up at a peering point, or when an upstream provider starts dropping at 0.5%.

Streaming services adopted BBR because it doesn't need loss to "find the rate" — it holds high throughput on paths that would have collapsed Reno.