feat(core,host): adaptive bitrate — mid-stream encoder re-targeting when set to Automatic
New SetBitrate (0x05) / BitrateChanged (0x06) control messages: the host clamps like the Hello request, acks the resolved rate, and rebuilds the ENCODER ONLY in place (same mode, first new-rate frame is an IDR — the proven mode-switch resync, minus the pipeline churn). The client side is an AIMD controller (core abr.rs) in the data-plane pump, armed only when the user's bitrate is Automatic (Hello bitrate_kbps == 0): ×0.7 after two bad 750 ms windows (FEC-unrecoverable frames, ≥2% loss, one-way-delay rise above its rolling baseline — the pre-loss bufferbloat signal off the clock-skew handshake — or a jump-to-live flush), ~+6% after ~10 s clean, ceiling = the session's starting rate, 3 s cooldown, self-disables against a host that never acks (older build). Division of labour: adaptive FEC keeps answering fast random loss; bitrate now answers persistent congestion, closing the FEC death-spiral gap. The web-console sample reports the live rate. Also: join_host_port() brackets bare IPv6 literals before SocketAddr parsing (parse-side IPv6 groundwork, pairs with the next commit). Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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//! Adaptive bitrate: the client-side AIMD controller behind the "Automatic" bitrate setting.
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//!
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//! Runs inside [`crate::client`]'s data-plane pump on the same 750 ms cadence as the adaptive-FEC
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//! [`crate::quic::LossReport`], deciding when to ask the host for a different encoder bitrate via
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//! [`crate::quic::SetBitrate`]. Division of labour with adaptive FEC: **FEC answers fast, random
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//! loss** (Wi-Fi bursts, RF noise — recoverable redundancy is the right tool); **bitrate answers
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//! persistent congestion** (the link simply can't carry the rate — more FEC only adds load). The
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//! controller therefore reacts to *sustained* signals only:
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//!
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//! - **unrecoverable frames** — loss exceeded the FEC budget (the stream visibly froze/recovered);
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//! - **heavy loss** — a window whose shard loss is beyond what FEC should be left to absorb alone;
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//! - **one-way-delay rise** — capture→received latency (host-clock skew corrected) climbing above
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//! its rolling baseline: standing queue growth, the *pre-loss* signature of a saturated link
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//! (bufferbloat) — this is the early-warning signal loss-based control lacks;
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//! - **a jump-to-live flush** — the pump discarded its backlog, the strongest "we were behind"
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//! evidence there is.
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//!
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//! AIMD shape: two consecutive bad windows ⇒ multiplicative decrease (×0.7, floored); ~10 s of
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//! clean windows ⇒ additive-ish increase (+~6 %, ceilinged at the session's starting rate — the
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//! controller recovers *back to* what was negotiated, never beyond it). Changes are rate-limited
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//! (each one costs the IDR the host's rebuilt encoder opens with) and the whole controller
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//! disables itself against a host that never answers [`crate::quic::BitrateChanged`] (an older
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//! build that ignores unknown control messages).
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use std::collections::VecDeque;
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use std::time::{Duration, Instant};
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/// Never ask for less than this — below it the stream is unusable anyway and the floor keeps a
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/// mis-measured window from cratering the session.
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const FLOOR_KBPS: u32 = 5_000;
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/// Consecutive bad windows before a decrease — one window can be a scheduler blip or a single
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/// Wi-Fi scan; two in a row (1.5 s) is a condition.
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const BAD_WINDOWS_TO_DECREASE: u32 = 2;
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/// Consecutive clean windows before probing back up (~10 s at the 750 ms cadence): recovery is
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/// deliberately much slower than backoff, classic AIMD.
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const CLEAN_WINDOWS_TO_INCREASE: u32 = 13;
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/// Minimum gap between requested changes — every accepted change costs an encoder rebuild + IDR
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/// on the host, and back-to-back steps would outrun the ack/effect round trip.
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const CHANGE_COOLDOWN: Duration = Duration::from_secs(3);
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/// Window shard loss beyond which the window counts bad even without an unrecoverable frame:
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/// 2 % sustained is congestion territory, not the random tail FEC exists for.
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const HEAVY_LOSS_PPM: u32 = 20_000;
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/// How far the window's mean one-way delay may sit above the rolling baseline before it counts
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/// as queue growth. 25 ms is far beyond jitter at any streamable frame rate.
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const OWD_RISE_US: i64 = 25_000;
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/// Rolling window (in 750 ms report windows, ~30 s) whose minimum mean is the OWD baseline.
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/// Long enough to remember the uncongested floor, short enough to follow genuine path changes.
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const BASELINE_WINDOWS: usize = 40;
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/// Requests sent without a single [`crate::quic::BitrateChanged`] ack before concluding the host
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/// predates bitrate renegotiation and going quiet for the rest of the session.
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const MAX_UNACKED: u32 = 3;
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/// One decision per report window; `Some(kbps)` = send a [`crate::quic::SetBitrate`].
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pub(crate) struct BitrateController {
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/// `false` = permanently off (explicit user bitrate, an old host, or ack silence).
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enabled: bool,
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/// The rate we believe the host encodes at (updated by acks; requests are not assumed).
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current_kbps: u32,
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/// The session's starting (negotiated) rate — the recovery ceiling.
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ceiling_kbps: u32,
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floor_kbps: u32,
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/// Recent window mean OWDs (µs); the rolling min is the uncongested baseline.
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owd_means: VecDeque<i64>,
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bad_windows: u32,
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clean_windows: u32,
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last_change: Option<Instant>,
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/// Requests since the last ack — reaching [`MAX_UNACKED`] disables the controller.
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unacked: u32,
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}
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impl BitrateController {
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/// `start_kbps` is the Welcome-resolved session bitrate when the user chose Automatic, or `0`
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/// to build a permanently-disabled controller (explicit bitrate / an old host that didn't
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/// echo one — no known ceiling to work against).
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pub(crate) fn new(start_kbps: u32) -> Self {
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BitrateController {
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enabled: start_kbps > 0,
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current_kbps: start_kbps,
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ceiling_kbps: start_kbps,
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floor_kbps: FLOOR_KBPS.min(start_kbps.max(1)),
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owd_means: VecDeque::with_capacity(BASELINE_WINDOWS),
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bad_windows: 0,
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clean_windows: 0,
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last_change: None,
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unacked: 0,
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}
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}
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/// The host's [`crate::quic::BitrateChanged`] ack: its clamp is authoritative for what the
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/// encoder now targets, and any ack proves the host renegotiates (resets the silence counter).
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pub(crate) fn on_ack(&mut self, kbps: u32) {
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if kbps > 0 {
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self.current_kbps = kbps;
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}
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self.unacked = 0;
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}
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/// Feed one report window; returns the rate to request now, if any. `dropped` = frames that
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/// went FEC-unrecoverable in the window, `loss_ppm` the window's [`crate::quic::LossReport`]
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/// figure, `owd_mean_us` the window's mean skew-corrected capture→received latency (`None`
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/// without a clock handshake), `flushed` = the pump's jump-to-live fired in the window.
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pub(crate) fn on_window(
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&mut self,
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now: Instant,
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dropped: u64,
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loss_ppm: u32,
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owd_mean_us: Option<i64>,
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flushed: bool,
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) -> Option<u32> {
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if !self.enabled {
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return None;
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}
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if self.unacked >= MAX_UNACKED {
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// The host never answered: an older build. Go quiet instead of spamming a message it
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// logs as unknown every few seconds.
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self.enabled = false;
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tracing::info!("adaptive bitrate off — host never acked a SetBitrate (older host)");
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return None;
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}
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// OWD: compare against the rolling-min baseline of PRIOR windows (so a rising window
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// doesn't drag its own baseline up), then record it.
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let owd_bad = match owd_mean_us {
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Some(mean) => {
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let bad = self
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.owd_means
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.iter()
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.min()
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.is_some_and(|&base| mean > base + OWD_RISE_US);
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if self.owd_means.len() == BASELINE_WINDOWS {
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self.owd_means.pop_front();
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}
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self.owd_means.push_back(mean);
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bad
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}
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None => false,
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};
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let bad = dropped > 0 || loss_ppm >= HEAVY_LOSS_PPM || owd_bad || flushed;
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if bad {
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self.bad_windows += 1;
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self.clean_windows = 0;
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} else {
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self.clean_windows += 1;
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self.bad_windows = 0;
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}
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let cooled = self
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.last_change
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.is_none_or(|t| now.duration_since(t) >= CHANGE_COOLDOWN);
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if !cooled {
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return None;
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}
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if self.bad_windows >= BAD_WINDOWS_TO_DECREASE && self.current_kbps > self.floor_kbps {
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let next = ((self.current_kbps as u64 * 7 / 10) as u32).max(self.floor_kbps);
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self.bad_windows = 0;
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return self.request(next, now);
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}
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if self.clean_windows >= CLEAN_WINDOWS_TO_INCREASE && self.current_kbps < self.ceiling_kbps
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{
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let next = (self.current_kbps + self.current_kbps / 16 + 1).min(self.ceiling_kbps);
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self.clean_windows = 0;
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return self.request(next, now);
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}
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None
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}
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fn request(&mut self, kbps: u32, now: Instant) -> Option<u32> {
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self.last_change = Some(now);
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self.unacked += 1;
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// `current_kbps` is NOT updated here — the host's ack is authoritative. A lost/ignored
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// request just recomputes from the same base next time (and counts toward MAX_UNACKED).
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Some(kbps)
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}
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}
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#[cfg(test)]
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mod tests {
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use super::*;
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/// A window cadence matching the pump's 750 ms tick, safely past the change cooldown when
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/// stepped 5× between decisions.
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const TICK: Duration = Duration::from_millis(750);
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fn ticks(start: Instant, n: u32) -> Instant {
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start + TICK * n
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}
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/// Drive `n` clean windows, asserting no decision fires before the clean threshold.
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fn run_clean(c: &mut BitrateController, start: Instant, from: u32, n: u32) -> Option<u32> {
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let mut out = None;
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for i in from..from + n {
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out = c.on_window(ticks(start, i), 0, 0, Some(10_000), false);
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if out.is_some() {
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return out;
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}
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}
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out
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}
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#[test]
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fn disabled_when_not_automatic_or_old_host() {
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// start 0 = explicit user bitrate or a host that didn't echo one → permanently off.
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let mut c = BitrateController::new(0);
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let now = Instant::now();
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assert_eq!(c.on_window(now, 5, 900_000, Some(500_000), true), None);
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}
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#[test]
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fn two_bad_windows_step_down_multiplicatively() {
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let mut c = BitrateController::new(20_000);
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let start = Instant::now();
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// One bad window is a blip — no reaction.
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assert_eq!(c.on_window(ticks(start, 0), 1, 0, None, false), None);
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// The second consecutive bad window backs off ×0.7.
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assert_eq!(c.on_window(ticks(start, 1), 1, 0, None, false), Some(14_000));
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c.on_ack(14_000);
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// Still bad after the cooldown → another ×0.7 step from the ACKED rate.
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assert_eq!(c.on_window(ticks(start, 6), 1, 0, None, false), None); // bad #1 again
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assert_eq!(c.on_window(ticks(start, 7), 1, 0, None, false), Some(9_800));
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}
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#[test]
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fn cooldown_blocks_back_to_back_steps() {
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let mut c = BitrateController::new(20_000);
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let start = Instant::now();
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assert_eq!(c.on_window(ticks(start, 0), 1, 0, None, false), None);
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assert_eq!(c.on_window(ticks(start, 1), 1, 0, None, false), Some(14_000));
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c.on_ack(14_000);
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// Two more bad windows land INSIDE the 3 s cooldown (ticks 2,3 = 1.5/2.25 s) → held.
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assert_eq!(c.on_window(ticks(start, 2), 1, 0, None, false), None);
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assert_eq!(c.on_window(ticks(start, 3), 1, 0, None, false), None);
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}
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#[test]
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fn floor_is_never_crossed() {
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let mut c = BitrateController::new(6_000);
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let start = Instant::now();
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assert_eq!(c.on_window(ticks(start, 0), 1, 0, None, false), None);
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// ×0.7 of 6000 = 4200 < floor → clamped to 5000.
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assert_eq!(c.on_window(ticks(start, 1), 1, 0, None, false), Some(5_000));
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c.on_ack(5_000);
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// At the floor, further bad windows request nothing.
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assert_eq!(c.on_window(ticks(start, 6), 1, 0, None, false), None);
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assert_eq!(c.on_window(ticks(start, 7), 1, 0, None, false), None);
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}
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#[test]
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fn sustained_clean_recovers_toward_ceiling_only() {
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let mut c = BitrateController::new(20_000);
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let start = Instant::now();
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assert_eq!(c.on_window(ticks(start, 0), 1, 0, None, false), None);
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assert_eq!(c.on_window(ticks(start, 1), 1, 0, None, false), Some(14_000));
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c.on_ack(14_000);
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// 13 clean windows → one additive step up (14000 + 14000/16 + 1 = 14876).
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let up = run_clean(&mut c, start, 2, 13);
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assert_eq!(up, Some(14_876));
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c.on_ack(14_876);
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// Fully recovered → clean windows at the ceiling stay quiet (never probe past start).
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c.on_ack(20_000);
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assert_eq!(run_clean(&mut c, start, 40, 20), None);
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}
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#[test]
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fn owd_rise_alone_is_a_congestion_signal() {
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let mut c = BitrateController::new(20_000);
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let start = Instant::now();
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// Establish a ~10 ms baseline over a few clean windows.
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for i in 0..4 {
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assert_eq!(c.on_window(ticks(start, i), 0, 0, Some(10_000), false), None);
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}
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// Delay climbs 40 ms above baseline with ZERO loss — bufferbloat. Two windows → back off.
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assert_eq!(c.on_window(ticks(start, 4), 0, 0, Some(50_000), false), None);
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assert_eq!(
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c.on_window(ticks(start, 5), 0, 0, Some(52_000), false),
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Some(14_000)
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);
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}
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#[test]
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fn ack_silence_disables_the_controller() {
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let mut c = BitrateController::new(20_000);
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let start = Instant::now();
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let mut sent = 0;
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let mut i = 0;
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// Keep every window bad and never ack: exactly MAX_UNACKED requests, then silence.
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while i < 60 {
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if c.on_window(ticks(start, i), 1, 0, None, false).is_some() {
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sent += 1;
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}
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i += 1;
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}
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assert_eq!(sent, MAX_UNACKED);
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}
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#[test]
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fn flush_counts_as_a_bad_window() {
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let mut c = BitrateController::new(20_000);
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let start = Instant::now();
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assert_eq!(c.on_window(ticks(start, 0), 0, 0, None, true), None);
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assert_eq!(c.on_window(ticks(start, 1), 0, 0, None, true), Some(14_000));
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}
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}
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