Cloudflare fixes CUBIC congestion control bug in QUIC implementation that permanently pinned performance at minimum
By
sbulaev
Master baker tier. Every paragraph earns its place on the tray.
Summary
Cloudflare engineers discovered and fixed a bug in the CUBIC congestion control algorithm (RFC 9438) used in their QUIC implementation (quiche). The bug caused the congestion window (cwnd) to become permanently pinned at its minimum floor value, severely degrading performance. The root cause was that the kernel's idle detection mechanism incorrectly treated normal RTT wait times as application idleness, triggering an unnecessary cwnd reset. The fix involved correctly measuring idle periods to distinguish between actual application idleness and normal network round-trip time waits.
Key quotes
· 3 pulledCUBIC, standardized in RFC 9438, is the default congestion controller in Linux, and as a result governs how most TCP and QUIC connections on the public Internet probe for available bandwidth.
In this post, we'll tell the story of a bug in which CUBIC's congestion window (cwnd) gets permanently pinned at its minimum and never recovers.
The fix involved correctly measuring idle periods to distinguish RTT wait times from actual application idleness.
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