When idle isn't idle: how a Linux kernel optimization became a QUIC bug
Summary
Cloudflare explains how a Linux kernel optimization for the CUBIC congestion control, ported to the QUIC-based quiche implementation, created a bug where the congestion window gets stuck at its minimum and cannot recover after loss. The issue manifested as a test where 60% of runs failed to complete within the timeout, while Reno recovered properly. A small, targeted fix across idle-time measurement restored normal cwnd growth.