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CUBIC reacts to loss; BBR estimates bandwidth and pacing. Watch how each responds.
What you're seeing
CUBIC (red) uses packet loss as the congestion signal — additive increase until loss, then multiplicative decrease. On a slightly lossy link it spends a lot of time recovering, losing throughput.
BBR (green) estimates the bottleneck bandwidth and round-trip time, paces at that rate. Doesn't mis-attribute random loss to congestion. Throughput stays much higher on lossy networks.
Slide the loss rate to see how each algorithm reacts.
★ KEY TAKEAWAY
BBR (model-based) keeps throughput high under loss; CUBIC (loss-based) backs off aggressively. Big difference on lossy networks.
▶ WHAT TO TRY
- Slide Loss rate from 0% to 50%.
- Watch CUBIC (red) crash while BBR (green) stays high.
- YouTube switched to BBR for exactly this reason.