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This post analyzes a critical bug in CUBIC, Linux's default TCP and QUIC congestion control algorithm, where the congestion window becomes permanently pinned at its minimum after a congestion collapse event.
•CUBIC governs bandwidth probing, loss detection, and recovery for most TCP and QUIC connections on the public Internet
•A test failure occurring 61% of the time revealed that CUBIC failed to recover after the loss phase ended
•The root cause traced back to a 2017 Linux kernel optimization that adjusts the epoch timestamp during app idle periods
•When ported to quiche (QUIC implementation in user space), the optimization incorrectly interpreted zero bytes_in_flight as an idle condition
•The bug manifests as rapid oscillation (999 state transitions in 6.7 seconds) between recovery and congestion avoidance states, with cwnd locked at minimum
This summary was automatically generated by AI based on the original article and may not be fully accurate.
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