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

This post details a bug discovered in Cloudflare's quiche implementation of the CUBIC congestion control algorithm for QUIC. The bug caused the congestion window (cwnd) to become permanently pinned at its minimum after a congestion collapse event, preventing recovery. The post traces the origin of the bug to a Linux kernel optimization for handling idle periods in TCP, which was ported to quiche. It explains how the user-space nature of QUIC and a subtle difference in how idle periods were handled led to the bug, resulting in rapid oscillations between congestion avoidance and recovery states. A fix, derived from a subsequent kernel patch, is presented to resolve the issue.