quinn-congestions

Differences from the upstream quinn BBR

The bbr module is a port of the BBR implementation from quinn upstream (quinn-proto 0.12), adapted to the Controller trait of the bbrv3 branch (Tipuch/quinn). Compared with the upstream BBR, it has three differences:

1. min RTT tracking uses rtt.min() instead of rtt.get()

   In on_ack, min_rtt is updated whenever the estimator's historical minimum (rtt.min()) is lower, instead of the smoothed RTT (rtt.get()). This tracks the true minimum RTT more aggressively, matching the min_rtt semantics of the BBR spec more closely.

2. RNG switched from StdRng to Pcg32

   The random number generator used to pick the random gain-cycle offset in ProbeBw is now Pcg32 (from rand_pcg, seeded via Pcg32::from_rng(&mut rand::rng())) — the same RNG the bbrv3 branch's bbr3 uses — instead of rand::rngs::StdRng::from_os_rng(). This also moved the crate from rand 0.9 to the rand 0.10 ecosystem (RngExt, rand_pcg).

3. Controller trait signature adapted for the bbrv3 branch

   on_ack, on_end_acks and on_congestion_event accept an extra _space: SpaceId argument, which is required by the Controller trait of the bbrv3 branch but not by upstream quinn.

Everything else — the state machine (Startup / Drain / ProbeBw / ProbeRtt), recovery logic, ACK aggregation and all tuning constants (K_PACING_GAIN, K_DEFAULT_HIGH_GAIN, ...) — is identical to the upstream implementation.