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TCP Congestion Control: CUBIC vs BBR

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The congestion control arms race

TCP's job is to send as fast as the network allows without causing congestion. For 30 years, the answer was simple: grow the congestion window until you see packet loss, then reduce it. CUBIC refines this with a mathematical growth function.

BBR fundamentally changes the approach: instead of waiting for loss as a signal, it measures the network's bandwidth and RTT to directly target the optimal sending rate — filling the pipe without building queues.

Packets in motion — click to inspect:

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