{"schema_version":"onlylabs.public_signal.v1","title":"Cloudflare (Workers AI) Writing: When \"idle\" isn't idle: how a Linux kernel optimization became a QUIC bug","description":"Cloudflare (Workers AI) writing signal with public source context, captured evidence pages, related signals, and category-scoped analysis context.","url":"https://onlylabs.fyi/signals/78bfe7c8-7194-45b3-a9e8-9db6918eb075","json_url":"https://onlylabs.fyi/signals/78bfe7c8-7194-45b3-a9e8-9db6918eb075/signal.json","generated_at":"2026-06-07T21:14:33.951247+00:00","org":{"slug":"cloudflare","name":"Cloudflare (Workers AI)","category":"neocloud","category_label":"Neocloud","dossier_url":"https://onlylabs.fyi/labs/cloudflare","dossier_json_url":"https://onlylabs.fyi/labs/cloudflare/dossier.json"},"related_urls":{"signal":"https://onlylabs.fyi/signals/78bfe7c8-7194-45b3-a9e8-9db6918eb075","signal_json":"https://onlylabs.fyi/signals/78bfe7c8-7194-45b3-a9e8-9db6918eb075/signal.json","source":"https://blog.cloudflare.com/quic-death-spiral-fix/","lab_dossier":"https://onlylabs.fyi/labs/cloudflare","lab_dossier_json":"https://onlylabs.fyi/labs/cloudflare/dossier.json","analysis":"https://onlylabs.fyi/analysis/cloudflare","analysis_json":"https://onlylabs.fyi/analysis/cloudflare/analysis.json","analysis_evidence_json":"https://onlylabs.fyi/analysis/cloudflare/evidence.json","category":"https://onlylabs.fyi/neoclouds","category_json":"https://onlylabs.fyi/neoclouds.json","category_feed":"https://onlylabs.fyi/neoclouds/feed.xml","category_signals_json":"https://onlylabs.fyi/signals.json?category=neocloud","topic":"https://onlylabs.fyi/topics/talking","topic_signals_json":"https://onlylabs.fyi/topics/talking/signals.json?category=neocloud","topic_feed":"https://onlylabs.fyi/topics/talking/feed.xml?category=neocloud","data_business":null},"answer_pack":{"answer":"Cloudflare (Workers AI) published 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. The story starts with a Linux kernel change aimed at bringing CUBIC into line with the app-limited exclusion described in RFC 9438 §4.2-12 — a fix to a real problem in TCP that, when ported to our QUIC implementation, surfaced unexpected behaviors in quiche. 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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. The story starts with a Linux kernel change aimed at bringing CUBIC into line with the app-limited exclusion described in RFC 9438 §4.2-12 — a fix to a real problem in TCP that, when ported to our QUIC implementation, surfaced unexpected behaviors in quiche. 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