When "idle" isn't idle: how a Linux kernel optimization became a QUIC bug

**TL;DR:** When "idle" isn't idle: how a Linux kernel optimization became a QUIC bug

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CUBIC, 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, back off when they detect loss, and recover afterward. At Cloudflare, our open-source implementation of QUIC, quiche , uses CUBIC as its default congestion controller, meaning this code is in the critical path for a significant share of the traffic we serve.

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 from a congestion collapse event. 2-12 — a fix to a real problem in TCP that, when ported to our QUIC implementation, surfaced unexpected behaviors in quiche. It has a happy ending: an elegant (near-)one-line fix that broke the cycle.

CUBIC's logic in a nutshell Before we dive into the core problem, a quick refresher on Congestion Control Algorithms (CCAs) may help to set the stage. The central knob a CCA turns is the congestion window ( cwnd ): the sender-side cap

Source: Cloudflare Blog

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Last updated: June 16, 2026.

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