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Netflix has contributed an implementation of BBR to FreeBSD. See the main/sys/netinet/tcp_stacks/bbr.c source file and the commented commit by Randall Stewart from September 2019.
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BBR in QUIC
BBR has also been implemented for QUIC (including BBRv2, see above), and is in active use on Google's servers. Source code can be found in the congestion_control part of Google's QUICHE project. This is also used by Chromium, the open-source upstream project for Google Chrome. The main logic can be found in the tcp_sender.{cc,h} files.
Cloudflare has an independent implementation of QUIC and HTTP/3, written in Rust. Somewhat confusingly, this is also called QUICHE and can be found on GitHub under cloudflare/quiche. Support BBR has been announced as possible future work in a blog post about CUBIC and HyStart++ Support in quiche, but, as of November 2021, hasn't appeared in the official source tree yet.
Other
Mark Claypool has written an implementation of BBR for ns-3, a popular network simulator.
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