> though going the in-house approach would warrant keeping an eye on the relevant kernel commits like a hawk to avoid missing bug fixes like these. These in-house implementations tend to have less eyeballs than the kernel.

This is somewhat funny to read because this specific issue in CUBIC (sudden CWND jump upon existing quiescence) was originally discovered in Google's QUIC library and then later reported to the team working on the TCP stack. I know this because I was the one who found that bug back in 2015.

That said, congestion control algorithms are really prone to logic bugs, and very subtle changes in the algorithm can often lead to dramatically different outcomes. Because of that, there's a lot of value in running congestion control code that has been tested on a wide variety of real Internet traffic.

> I can see why they rewrote QUIC in Rust and for use in userspace

As far as I know, while they might have either way, they did not ("rewrite QUICK [...] for use in userspace"): the linux kernel implementation only landed late 2025. Quiche was started ca 2018 (that's when Cloudflare started beta-deploying QUIC, the first public alpha of quiche was january 2019).

I don't know that there even was an in-kernel implementation of quic before msquic.sys which I believe first shipped in Server 2022 circa mid 2021 (and is used as the implementation backend by MsQuic on Server 2022 and W11).