Adding upstream version 1:115.7.0.upstream/1%115.7.0upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

7 files changed, 1885 insertions, 0 deletions

diff --git a/third_party/rust/neqo-transport/src/cc/classic_cc.rs b/third_party/rust/neqo-transport/src/cc/classic_cc.rs
new file mode 100644
index 0000000000..fbaa5e1227
--- /dev/null
+++ b/third_party/rust/neqo-transport/src/cc/classic_cc.rs
@@ -0,0 +1,1079 @@
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Congestion control
+#![deny(clippy::pedantic)]
+
+use std::cmp::{max, min};
+use std::fmt::{self, Debug, Display};
+use std::time::{Duration, Instant};
+
+use super::CongestionControl;
+
+use crate::cc::MAX_DATAGRAM_SIZE;
+use crate::qlog::{self, QlogMetric};
+use crate::sender::PACING_BURST_SIZE;
+use crate::tracking::SentPacket;
+use neqo_common::{const_max, const_min, qdebug, qinfo, qlog::NeqoQlog, qtrace};
+
+pub const CWND_INITIAL_PKTS: usize = 10;
+pub const CWND_INITIAL: usize = const_min(
+    CWND_INITIAL_PKTS * MAX_DATAGRAM_SIZE,
+    const_max(2 * MAX_DATAGRAM_SIZE, 14720),
+);
+pub const CWND_MIN: usize = MAX_DATAGRAM_SIZE * 2;
+const PERSISTENT_CONG_THRESH: u32 = 3;
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+enum State {
+    /// In either slow start or congestion avoidance, not recovery.
+    SlowStart,
+    /// In congestion avoidance.
+    CongestionAvoidance,
+    /// In a recovery period, but no packets have been sent yet.  This is a
+    /// transient state because we want to exempt the first packet sent after
+    /// entering recovery from the congestion window.
+    RecoveryStart,
+    /// In a recovery period, with the first packet sent at this time.
+    Recovery,
+    /// Start of persistent congestion, which is transient, like `RecoveryStart`.
+    PersistentCongestion,
+}
+
+impl State {
+    pub fn in_recovery(self) -> bool {
+        matches!(self, Self::RecoveryStart | Self::Recovery)
+    }
+
+    pub fn in_slow_start(self) -> bool {
+        self == Self::SlowStart
+    }
+
+    /// These states are transient, we tell qlog on entry, but not on exit.
+    pub fn transient(self) -> bool {
+        matches!(self, Self::RecoveryStart | Self::PersistentCongestion)
+    }
+
+    /// Update a transient state to the true state.
+    pub fn update(&mut self) {
+        *self = match self {
+            Self::PersistentCongestion => Self::SlowStart,
+            Self::RecoveryStart => Self::Recovery,
+            _ => unreachable!(),
+        };
+    }
+
+    pub fn to_qlog(self) -> &'static str {
+        match self {
+            Self::SlowStart | Self::PersistentCongestion => "slow_start",
+            Self::CongestionAvoidance => "congestion_avoidance",
+            Self::Recovery | Self::RecoveryStart => "recovery",
+        }
+    }
+}
+
+pub trait WindowAdjustment: Display + Debug {
+    /// This is called when an ack is received.
+    /// The function calculates the amount of acked bytes congestion controller needs
+    /// to collect before increasing its cwnd by `MAX_DATAGRAM_SIZE`.
+    fn bytes_for_cwnd_increase(
+        &mut self,
+        curr_cwnd: usize,
+        new_acked_bytes: usize,
+        min_rtt: Duration,
+        now: Instant,
+    ) -> usize;
+    /// This function is called when a congestion event has beed detected and it
+    /// returns new (decreased) values of `curr_cwnd` and `acked_bytes`.
+    /// This value can be very small; the calling code is responsible for ensuring that the
+    /// congestion window doesn't drop below the minimum of `CWND_MIN`.
+    fn reduce_cwnd(&mut self, curr_cwnd: usize, acked_bytes: usize) -> (usize, usize);
+    /// Cubic needs this signal to reset its epoch.
+    fn on_app_limited(&mut self);
+    #[cfg(test)]
+    fn last_max_cwnd(&self) -> f64;
+    #[cfg(test)]
+    fn set_last_max_cwnd(&mut self, last_max_cwnd: f64);
+}
+
+#[derive(Debug)]
+pub struct ClassicCongestionControl<T> {
+    cc_algorithm: T,
+    state: State,
+    congestion_window: usize, // = kInitialWindow
+    bytes_in_flight: usize,
+    acked_bytes: usize,
+    ssthresh: usize,
+    recovery_start: Option<Instant>,
+
+    qlog: NeqoQlog,
+}
+
+impl<T: WindowAdjustment> Display for ClassicCongestionControl<T> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(
+            f,
+            "{} CongCtrl {}/{} ssthresh {}",
+            self.cc_algorithm, self.bytes_in_flight, self.congestion_window, self.ssthresh,
+        )?;
+        Ok(())
+    }
+}
+
+impl<T: WindowAdjustment> CongestionControl for ClassicCongestionControl<T> {
+    fn set_qlog(&mut self, qlog: NeqoQlog) {
+        self.qlog = qlog;
+    }
+
+    #[must_use]
+    fn cwnd(&self) -> usize {
+        self.congestion_window
+    }
+
+    #[must_use]
+    fn bytes_in_flight(&self) -> usize {
+        self.bytes_in_flight
+    }
+
+    #[must_use]
+    fn cwnd_avail(&self) -> usize {
+        // BIF can be higher than cwnd due to PTO packets, which are sent even
+        // if avail is 0, but still count towards BIF.
+        self.congestion_window.saturating_sub(self.bytes_in_flight)
+    }
+
+    // Multi-packet version of OnPacketAckedCC
+    fn on_packets_acked(&mut self, acked_pkts: &[SentPacket], min_rtt: Duration, now: Instant) {
+        // Check whether we are app limited before acked packets are removed
+        // from bytes_in_flight.
+        let is_app_limited = self.app_limited();
+        qtrace!(
+            [self],
+            "limited={}, bytes_in_flight={}, cwnd={}, state={:?} pacing_burst_size={}",
+            is_app_limited,
+            self.bytes_in_flight,
+            self.congestion_window,
+            self.state,
+            MAX_DATAGRAM_SIZE * PACING_BURST_SIZE,
+        );
+
+        let mut new_acked = 0;
+        for pkt in acked_pkts.iter().filter(|pkt| pkt.cc_outstanding()) {
+            assert!(self.bytes_in_flight >= pkt.size);
+            self.bytes_in_flight -= pkt.size;
+
+            if !self.after_recovery_start(pkt) {
+                // Do not increase congestion window for packets sent before
+                // recovery last started.
+                continue;
+            }
+
+            if self.state.in_recovery() {
+                self.set_state(State::CongestionAvoidance);
+                qlog::metrics_updated(&mut self.qlog, &[QlogMetric::InRecovery(false)]);
+            }
+
+            new_acked += pkt.size;
+        }
+
+        if is_app_limited {
+            self.cc_algorithm.on_app_limited();
+            return;
+        }
+
+        qtrace!([self], "ACK received, acked_bytes = {}", self.acked_bytes);
+
+        // Slow start, up to the slow start threshold.
+        if self.congestion_window < self.ssthresh {
+            self.acked_bytes += new_acked;
+            let increase = min(self.ssthresh - self.congestion_window, self.acked_bytes);
+            self.congestion_window += increase;
+            self.acked_bytes -= increase;
+            qinfo!([self], "slow start += {}", increase);
+            if self.congestion_window == self.ssthresh {
+                // This doesn't look like it is necessary, but it can happen
+                // after persistent congestion.
+                self.set_state(State::CongestionAvoidance);
+            }
+        }
+        // Congestion avoidance, above the slow start threshold.
+        if self.congestion_window >= self.ssthresh {
+            // The following function return the amount acked bytes a controller needs
+            // to collect to be allowed to increase its cwnd by MAX_DATAGRAM_SIZE.
+            let bytes_for_increase = self.cc_algorithm.bytes_for_cwnd_increase(
+                self.congestion_window,
+                new_acked,
+                min_rtt,
+                now,
+            );
+            debug_assert!(bytes_for_increase > 0);
+            // If enough credit has been accumulated already, apply them gradually.
+            // If we have sudden increase in allowed rate we actually increase cwnd gently.
+            if self.acked_bytes >= bytes_for_increase {
+                self.acked_bytes = 0;
+                self.congestion_window += MAX_DATAGRAM_SIZE;
+            }
+            self.acked_bytes += new_acked;
+            if self.acked_bytes >= bytes_for_increase {
+                self.acked_bytes -= bytes_for_increase;
+                self.congestion_window += MAX_DATAGRAM_SIZE; // or is this the current MTU?
+            }
+            // The number of bytes we require can go down over time with Cubic.
+            // That might result in an excessive rate of increase, so limit the number of unused
+            // acknowledged bytes after increasing the congestion window twice.
+            self.acked_bytes = min(bytes_for_increase, self.acked_bytes);
+        }
+        qlog::metrics_updated(
+            &mut self.qlog,
+            &[
+                QlogMetric::CongestionWindow(self.congestion_window),
+                QlogMetric::BytesInFlight(self.bytes_in_flight),
+            ],
+        );
+    }
+
+    /// Update congestion controller state based on lost packets.
+    fn on_packets_lost(
+        &mut self,
+        first_rtt_sample_time: Option<Instant>,
+        prev_largest_acked_sent: Option<Instant>,
+        pto: Duration,
+        lost_packets: &[SentPacket],
+    ) -> bool {
+        if lost_packets.is_empty() {
+            return false;
+        }
+
+        for pkt in lost_packets.iter().filter(|pkt| pkt.cc_in_flight()) {
+            assert!(self.bytes_in_flight >= pkt.size);
+            self.bytes_in_flight -= pkt.size;
+        }
+        qlog::metrics_updated(
+            &mut self.qlog,
+            &[QlogMetric::BytesInFlight(self.bytes_in_flight)],
+        );
+
+        qdebug!([self], "Pkts lost {}", lost_packets.len());
+
+        let congestion = self.on_congestion_event(lost_packets.last().unwrap());
+        let persistent_congestion = self.detect_persistent_congestion(
+            first_rtt_sample_time,
+            prev_largest_acked_sent,
+            pto,
+            lost_packets,
+        );
+        congestion || persistent_congestion
+    }
+
+    fn discard(&mut self, pkt: &SentPacket) {
+        if pkt.cc_outstanding() {
+            assert!(self.bytes_in_flight >= pkt.size);
+            self.bytes_in_flight -= pkt.size;
+            qlog::metrics_updated(
+                &mut self.qlog,
+                &[QlogMetric::BytesInFlight(self.bytes_in_flight)],
+            );
+            qtrace!([self], "Ignore pkt with size {}", pkt.size);
+        }
+    }
+
+    fn discard_in_flight(&mut self) {
+        self.bytes_in_flight = 0;
+        qlog::metrics_updated(
+            &mut self.qlog,
+            &[QlogMetric::BytesInFlight(self.bytes_in_flight)],
+        );
+    }
+
+    fn on_packet_sent(&mut self, pkt: &SentPacket) {
+        // Record the recovery time and exit any transient state.
+        if self.state.transient() {
+            self.recovery_start = Some(pkt.time_sent);
+            self.state.update();
+        }
+
+        if !pkt.cc_in_flight() {
+            return;
+        }
+
+        self.bytes_in_flight += pkt.size;
+        qdebug!(
+            [self],
+            "Pkt Sent len {}, bif {}, cwnd {}",
+            pkt.size,
+            self.bytes_in_flight,
+            self.congestion_window
+        );
+        qlog::metrics_updated(
+            &mut self.qlog,
+            &[QlogMetric::BytesInFlight(self.bytes_in_flight)],
+        );
+    }
+
+    /// Whether a packet can be sent immediately as a result of entering recovery.
+    fn recovery_packet(&self) -> bool {
+        self.state == State::RecoveryStart
+    }
+}
+
+impl<T: WindowAdjustment> ClassicCongestionControl<T> {
+    pub fn new(cc_algorithm: T) -> Self {
+        Self {
+            cc_algorithm,
+            state: State::SlowStart,
+            congestion_window: CWND_INITIAL,
+            bytes_in_flight: 0,
+            acked_bytes: 0,
+            ssthresh: usize::MAX,
+            recovery_start: None,
+            qlog: NeqoQlog::disabled(),
+        }
+    }
+
+    #[cfg(test)]
+    #[must_use]
+    pub fn ssthresh(&self) -> usize {
+        self.ssthresh
+    }
+
+    #[cfg(test)]
+    pub fn set_ssthresh(&mut self, v: usize) {
+        self.ssthresh = v;
+    }
+
+    #[cfg(test)]
+    pub fn last_max_cwnd(&self) -> f64 {
+        self.cc_algorithm.last_max_cwnd()
+    }
+
+    #[cfg(test)]
+    pub fn set_last_max_cwnd(&mut self, last_max_cwnd: f64) {
+        self.cc_algorithm.set_last_max_cwnd(last_max_cwnd);
+    }
+
+    #[cfg(test)]
+    pub fn acked_bytes(&self) -> usize {
+        self.acked_bytes
+    }
+
+    fn set_state(&mut self, state: State) {
+        if self.state != state {
+            qdebug!([self], "state -> {:?}", state);
+            let old_state = self.state;
+            self.qlog.add_event(|| {
+                // No need to tell qlog about exit from transient states.
+                if old_state.transient() {
+                    None
+                } else {
+                    Some(::qlog::event::Event::congestion_state_updated(
+                        Some(old_state.to_qlog().to_owned()),
+                        state.to_qlog().to_owned(),
+                    ))
+                }
+            });
+            self.state = state;
+        }
+    }
+
+    fn detect_persistent_congestion(
+        &mut self,
+        first_rtt_sample_time: Option<Instant>,
+        prev_largest_acked_sent: Option<Instant>,
+        pto: Duration,
+        lost_packets: &[SentPacket],
+    ) -> bool {
+        if first_rtt_sample_time.is_none() {
+            return false;
+        }
+
+        let pc_period = pto * PERSISTENT_CONG_THRESH;
+
+        let mut last_pn = 1 << 62; // Impossibly large, but not enough to overflow.
+        let mut start = None;
+
+        // Look for the first lost packet after the previous largest acknowledged.
+        // Ignore packets that weren't ack-eliciting for the start of this range.
+        // Also, make sure to ignore any packets sent before we got an RTT estimate
+        // as we might not have sent PTO packets soon enough after those.
+        let cutoff = max(first_rtt_sample_time, prev_largest_acked_sent);
+        for p in lost_packets
+            .iter()
+            .skip_while(|p| Some(p.time_sent) < cutoff)
+        {
+            if p.pn != last_pn + 1 {
+                // Not a contiguous range of lost packets, start over.
+                start = None;
+            }
+            last_pn = p.pn;
+            if !p.cc_in_flight() {
+                // Not interesting, keep looking.
+                continue;
+            }
+            if let Some(t) = start {
+                if p.time_sent.checked_duration_since(t).unwrap() > pc_period {
+                    qinfo!([self], "persistent congestion");
+                    self.congestion_window = CWND_MIN;
+                    self.acked_bytes = 0;
+                    self.set_state(State::PersistentCongestion);
+                    qlog::metrics_updated(
+                        &mut self.qlog,
+                        &[QlogMetric::CongestionWindow(self.congestion_window)],
+                    );
+                    return true;
+                }
+            } else {
+                start = Some(p.time_sent);
+            }
+        }
+        false
+    }
+
+    #[must_use]
+    fn after_recovery_start(&mut self, packet: &SentPacket) -> bool {
+        // At the start of the first recovery period, if the state is
+        // transient, all packets will have been sent before recovery.
+        self.recovery_start
+            .map_or(!self.state.transient(), |t| packet.time_sent >= t)
+    }
+
+    /// Handle a congestion event.
+    /// Returns true if this was a true congestion event.
+    fn on_congestion_event(&mut self, last_packet: &SentPacket) -> bool {
+        // Start a new congestion event if lost packet was sent after the start
+        // of the previous congestion recovery period.
+        if !self.after_recovery_start(last_packet) {
+            return false;
+        }
+
+        let (cwnd, acked_bytes) = self
+            .cc_algorithm
+            .reduce_cwnd(self.congestion_window, self.acked_bytes);
+        self.congestion_window = max(cwnd, CWND_MIN);
+        self.acked_bytes = acked_bytes;
+        self.ssthresh = self.congestion_window;
+        qinfo!(
+            [self],
+            "Cong event -> recovery; cwnd {}, ssthresh {}",
+            self.congestion_window,
+            self.ssthresh
+        );
+        qlog::metrics_updated(
+            &mut self.qlog,
+            &[
+                QlogMetric::CongestionWindow(self.congestion_window),
+                QlogMetric::SsThresh(self.ssthresh),
+                QlogMetric::InRecovery(true),
+            ],
+        );
+        self.set_state(State::RecoveryStart);
+        true
+    }
+
+    #[allow(clippy::unused_self)]
+    fn app_limited(&self) -> bool {
+        if self.bytes_in_flight >= self.congestion_window {
+            false
+        } else if self.state.in_slow_start() {
+            // Allow for potential doubling of the congestion window during slow start.
+            // That is, the application might not have been able to send enough to respond
+            // to increases to the congestion window.
+            self.bytes_in_flight < self.congestion_window / 2
+        } else {
+            // We're not limited if the in-flight data is within a single burst of the
+            // congestion window.
+            (self.bytes_in_flight + MAX_DATAGRAM_SIZE * PACING_BURST_SIZE) < self.congestion_window
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::{
+        ClassicCongestionControl, WindowAdjustment, CWND_INITIAL, CWND_MIN, PERSISTENT_CONG_THRESH,
+    };
+    use crate::cc::cubic::{Cubic, CUBIC_BETA_USIZE_DIVISOR, CUBIC_BETA_USIZE_QUOTIENT};
+    use crate::cc::new_reno::NewReno;
+    use crate::cc::{
+        CongestionControl, CongestionControlAlgorithm, CWND_INITIAL_PKTS, MAX_DATAGRAM_SIZE,
+    };
+    use crate::packet::{PacketNumber, PacketType};
+    use crate::tracking::SentPacket;
+    use std::convert::TryFrom;
+    use std::time::{Duration, Instant};
+    use test_fixture::now;
+
+    const PTO: Duration = Duration::from_millis(100);
+    const RTT: Duration = Duration::from_millis(98);
+    const ZERO: Duration = Duration::from_secs(0);
+    const EPSILON: Duration = Duration::from_nanos(1);
+    const GAP: Duration = Duration::from_secs(1);
+    /// The largest time between packets without causing persistent congestion.
+    const SUB_PC: Duration = Duration::from_millis(100 * PERSISTENT_CONG_THRESH as u64);
+    /// The minimum time between packets to cause persistent congestion.
+    /// Uses an odd expression because `Duration` arithmetic isn't `const`.
+    const PC: Duration = Duration::from_nanos(100_000_000 * (PERSISTENT_CONG_THRESH as u64) + 1);
+
+    fn cwnd_is_default(cc: &ClassicCongestionControl<NewReno>) {
+        assert_eq!(cc.cwnd(), CWND_INITIAL);
+        assert_eq!(cc.ssthresh(), usize::MAX);
+    }
+
+    fn cwnd_is_halved(cc: &ClassicCongestionControl<NewReno>) {
+        assert_eq!(cc.cwnd(), CWND_INITIAL / 2);
+        assert_eq!(cc.ssthresh(), CWND_INITIAL / 2);
+    }
+
+    fn lost(pn: PacketNumber, ack_eliciting: bool, t: Duration) -> SentPacket {
+        SentPacket::new(
+            PacketType::Short,
+            pn,
+            now() + t,
+            ack_eliciting,
+            Vec::new(),
+            100,
+        )
+    }
+
+    fn congestion_control(cc: CongestionControlAlgorithm) -> Box<dyn CongestionControl> {
+        match cc {
+            CongestionControlAlgorithm::NewReno => {
+                Box::new(ClassicCongestionControl::new(NewReno::default()))
+            }
+            CongestionControlAlgorithm::Cubic => {
+                Box::new(ClassicCongestionControl::new(Cubic::default()))
+            }
+        }
+    }
+
+    fn persistent_congestion_by_algorithm(
+        cc_alg: CongestionControlAlgorithm,
+        reduced_cwnd: usize,
+        lost_packets: &[SentPacket],
+        persistent_expected: bool,
+    ) {
+        let mut cc = congestion_control(cc_alg);
+        for p in lost_packets {
+            cc.on_packet_sent(p);
+        }
+
+        cc.on_packets_lost(Some(now()), None, PTO, lost_packets);
+
+        let persistent = if cc.cwnd() == reduced_cwnd {
+            false
+        } else if cc.cwnd() == CWND_MIN {
+            true
+        } else {
+            panic!("unexpected cwnd");
+        };
+        assert_eq!(persistent, persistent_expected);
+    }
+
+    fn persistent_congestion(lost_packets: &[SentPacket], persistent_expected: bool) {
+        persistent_congestion_by_algorithm(
+            CongestionControlAlgorithm::NewReno,
+            CWND_INITIAL / 2,
+            lost_packets,
+            persistent_expected,
+        );
+        persistent_congestion_by_algorithm(
+            CongestionControlAlgorithm::Cubic,
+            CWND_INITIAL * CUBIC_BETA_USIZE_QUOTIENT / CUBIC_BETA_USIZE_DIVISOR,
+            lost_packets,
+            persistent_expected,
+        );
+    }
+
+    /// A span of exactly the PC threshold only reduces the window on loss.
+    #[test]
+    fn persistent_congestion_none() {
+        persistent_congestion(&[lost(1, true, ZERO), lost(2, true, SUB_PC)], false);
+    }
+
+    /// A span of just more than the PC threshold causes persistent congestion.
+    #[test]
+    fn persistent_congestion_simple() {
+        persistent_congestion(&[lost(1, true, ZERO), lost(2, true, PC)], true);
+    }
+
+    /// Both packets need to be ack-eliciting.
+    #[test]
+    fn persistent_congestion_non_ack_eliciting() {
+        persistent_congestion(&[lost(1, false, ZERO), lost(2, true, PC)], false);
+        persistent_congestion(&[lost(1, true, ZERO), lost(2, false, PC)], false);
+    }
+
+    /// Packets in the middle, of any type, are OK.
+    #[test]
+    fn persistent_congestion_middle() {
+        persistent_congestion(
+            &[lost(1, true, ZERO), lost(2, false, RTT), lost(3, true, PC)],
+            true,
+        );
+        persistent_congestion(
+            &[lost(1, true, ZERO), lost(2, true, RTT), lost(3, true, PC)],
+            true,
+        );
+    }
+
+    /// Leading non-ack-eliciting packets are skipped.
+    #[test]
+    fn persistent_congestion_leading_non_ack_eliciting() {
+        persistent_congestion(
+            &[lost(1, false, ZERO), lost(2, true, RTT), lost(3, true, PC)],
+            false,
+        );
+        persistent_congestion(
+            &[
+                lost(1, false, ZERO),
+                lost(2, true, RTT),
+                lost(3, true, RTT + PC),
+            ],
+            true,
+        );
+    }
+
+    /// Trailing non-ack-eliciting packets aren't relevant.
+    #[test]
+    fn persistent_congestion_trailing_non_ack_eliciting() {
+        persistent_congestion(
+            &[
+                lost(1, true, ZERO),
+                lost(2, true, PC),
+                lost(3, false, PC + EPSILON),
+            ],
+            true,
+        );
+        persistent_congestion(
+            &[
+                lost(1, true, ZERO),
+                lost(2, true, SUB_PC),
+                lost(3, false, PC),
+            ],
+            false,
+        );
+    }
+
+    /// Gaps in the middle, of any type, restart the count.
+    #[test]
+    fn persistent_congestion_gap_reset() {
+        persistent_congestion(&[lost(1, true, ZERO), lost(3, true, PC)], false);
+        persistent_congestion(
+            &[
+                lost(1, true, ZERO),
+                lost(2, true, RTT),
+                lost(4, true, GAP),
+                lost(5, true, GAP + PTO * PERSISTENT_CONG_THRESH),
+            ],
+            false,
+        );
+    }
+
+    /// A span either side of a gap will cause persistent congestion.
+    #[test]
+    fn persistent_congestion_gap_or() {
+        persistent_congestion(
+            &[
+                lost(1, true, ZERO),
+                lost(2, true, PC),
+                lost(4, true, GAP),
+                lost(5, true, GAP + PTO),
+            ],
+            true,
+        );
+        persistent_congestion(
+            &[
+                lost(1, true, ZERO),
+                lost(2, true, PTO),
+                lost(4, true, GAP),
+                lost(5, true, GAP + PC),
+            ],
+            true,
+        );
+    }
+
+    /// A gap only restarts after an ack-eliciting packet.
+    #[test]
+    fn persistent_congestion_gap_non_ack_eliciting() {
+        persistent_congestion(
+            &[
+                lost(1, true, ZERO),
+                lost(2, true, PTO),
+                lost(4, false, GAP),
+                lost(5, true, GAP + PC),
+            ],
+            false,
+        );
+        persistent_congestion(
+            &[
+                lost(1, true, ZERO),
+                lost(2, true, PTO),
+                lost(4, false, GAP),
+                lost(5, true, GAP + RTT),
+                lost(6, true, GAP + RTT + SUB_PC),
+            ],
+            false,
+        );
+        persistent_congestion(
+            &[
+                lost(1, true, ZERO),
+                lost(2, true, PTO),
+                lost(4, false, GAP),
+                lost(5, true, GAP + RTT),
+                lost(6, true, GAP + RTT + PC),
+            ],
+            true,
+        );
+    }
+
+    /// Get a time, in multiples of `PTO`, relative to `now()`.
+    fn by_pto(t: u32) -> Instant {
+        now() + (PTO * t)
+    }
+
+    /// Make packets that will be made lost.
+    /// `times` is the time of sending, in multiples of `PTO`, relative to `now()`.
+    fn make_lost(times: &[u32]) -> Vec<SentPacket> {
+        times
+            .iter()
+            .enumerate()
+            .map(|(i, &t)| {
+                SentPacket::new(
+                    PacketType::Short,
+                    u64::try_from(i).unwrap(),
+                    by_pto(t),
+                    true,
+                    Vec::new(),
+                    1000,
+                )
+            })
+            .collect::<Vec<_>>()
+    }
+
+    /// Call `detect_persistent_congestion` using times relative to now and the fixed PTO time.
+    /// `last_ack` and `rtt_time` are times in multiples of `PTO`, relative to `now()`,
+    /// for the time of the largest acknowledged and the first RTT sample, respectively.
+    fn persistent_congestion_by_pto<T: WindowAdjustment>(
+        mut cc: ClassicCongestionControl<T>,
+        last_ack: u32,
+        rtt_time: u32,
+        lost: &[SentPacket],
+    ) -> bool {
+        assert_eq!(cc.cwnd(), CWND_INITIAL);
+
+        let last_ack = Some(by_pto(last_ack));
+        let rtt_time = Some(by_pto(rtt_time));
+
+        // Persistent congestion is never declared if the RTT time is `None`.
+        cc.detect_persistent_congestion(None, None, PTO, lost);
+        assert_eq!(cc.cwnd(), CWND_INITIAL);
+        cc.detect_persistent_congestion(None, last_ack, PTO, lost);
+        assert_eq!(cc.cwnd(), CWND_INITIAL);
+
+        cc.detect_persistent_congestion(rtt_time, last_ack, PTO, lost);
+        cc.cwnd() == CWND_MIN
+    }
+
+    /// No persistent congestion can be had if there are no lost packets.
+    #[test]
+    fn persistent_congestion_no_lost() {
+        let lost = make_lost(&[]);
+        assert!(!persistent_congestion_by_pto(
+            ClassicCongestionControl::new(NewReno::default()),
+            0,
+            0,
+            &lost
+        ));
+        assert!(!persistent_congestion_by_pto(
+            ClassicCongestionControl::new(Cubic::default()),
+            0,
+            0,
+            &lost
+        ));
+    }
+
+    /// No persistent congestion can be had if there is only one lost packet.
+    #[test]
+    fn persistent_congestion_one_lost() {
+        let lost = make_lost(&[1]);
+        assert!(!persistent_congestion_by_pto(
+            ClassicCongestionControl::new(NewReno::default()),
+            0,
+            0,
+            &lost
+        ));
+        assert!(!persistent_congestion_by_pto(
+            ClassicCongestionControl::new(Cubic::default()),
+            0,
+            0,
+            &lost
+        ));
+    }
+
+    /// Persistent congestion can't happen based on old packets.
+    #[test]
+    fn persistent_congestion_past() {
+        // Packets sent prior to either the last acknowledged or the first RTT
+        // sample are not considered.  So 0 is ignored.
+        let lost = make_lost(&[0, PERSISTENT_CONG_THRESH + 1, PERSISTENT_CONG_THRESH + 2]);
+        assert!(!persistent_congestion_by_pto(
+            ClassicCongestionControl::new(NewReno::default()),
+            1,
+            1,
+            &lost
+        ));
+        assert!(!persistent_congestion_by_pto(
+            ClassicCongestionControl::new(NewReno::default()),
+            0,
+            1,
+            &lost
+        ));
+        assert!(!persistent_congestion_by_pto(
+            ClassicCongestionControl::new(NewReno::default()),
+            1,
+            0,
+            &lost
+        ));
+        assert!(!persistent_congestion_by_pto(
+            ClassicCongestionControl::new(Cubic::default()),
+            1,
+            1,
+            &lost
+        ));
+        assert!(!persistent_congestion_by_pto(
+            ClassicCongestionControl::new(Cubic::default()),
+            0,
+            1,
+            &lost
+        ));
+        assert!(!persistent_congestion_by_pto(
+            ClassicCongestionControl::new(Cubic::default()),
+            1,
+            0,
+            &lost
+        ));
+    }
+
+    /// Persistent congestion doesn't start unless the packet is ack-eliciting.
+    #[test]
+    fn persistent_congestion_ack_eliciting() {
+        let mut lost = make_lost(&[1, PERSISTENT_CONG_THRESH + 2]);
+        lost[0] = SentPacket::new(
+            lost[0].pt,
+            lost[0].pn,
+            lost[0].time_sent,
+            false,
+            Vec::new(),
+            lost[0].size,
+        );
+        assert!(!persistent_congestion_by_pto(
+            ClassicCongestionControl::new(NewReno::default()),
+            0,
+            0,
+            &lost
+        ));
+        assert!(!persistent_congestion_by_pto(
+            ClassicCongestionControl::new(Cubic::default()),
+            0,
+            0,
+            &lost
+        ));
+    }
+
+    /// Detect persistent congestion.  Note that the first lost packet needs to have a time
+    /// greater than the previously acknowledged packet AND the first RTT sample.  And the
+    /// difference in times needs to be greater than the persistent congestion threshold.
+    #[test]
+    fn persistent_congestion_min() {
+        let lost = make_lost(&[1, PERSISTENT_CONG_THRESH + 2]);
+        assert!(persistent_congestion_by_pto(
+            ClassicCongestionControl::new(NewReno::default()),
+            0,
+            0,
+            &lost
+        ));
+        assert!(persistent_congestion_by_pto(
+            ClassicCongestionControl::new(Cubic::default()),
+            0,
+            0,
+            &lost
+        ));
+    }
+
+    /// Make sure that not having a previous largest acknowledged also results
+    /// in detecting persistent congestion.  (This is not expected to happen, but
+    /// the code permits it).
+    #[test]
+    fn persistent_congestion_no_prev_ack_newreno() {
+        let lost = make_lost(&[1, PERSISTENT_CONG_THRESH + 2]);
+        let mut cc = ClassicCongestionControl::new(NewReno::default());
+        cc.detect_persistent_congestion(Some(by_pto(0)), None, PTO, &lost);
+        assert_eq!(cc.cwnd(), CWND_MIN);
+    }
+
+    #[test]
+    fn persistent_congestion_no_prev_ack_cubic() {
+        let lost = make_lost(&[1, PERSISTENT_CONG_THRESH + 2]);
+        let mut cc = ClassicCongestionControl::new(Cubic::default());
+        cc.detect_persistent_congestion(Some(by_pto(0)), None, PTO, &lost);
+        assert_eq!(cc.cwnd(), CWND_MIN);
+    }
+
+    /// The code asserts on ordering errors.
+    #[test]
+    #[should_panic]
+    fn persistent_congestion_unsorted_newreno() {
+        let lost = make_lost(&[PERSISTENT_CONG_THRESH + 2, 1]);
+        assert!(!persistent_congestion_by_pto(
+            ClassicCongestionControl::new(NewReno::default()),
+            0,
+            0,
+            &lost
+        ));
+    }
+
+    /// The code asserts on ordering errors.
+    #[test]
+    #[should_panic]
+    fn persistent_congestion_unsorted_cubic() {
+        let lost = make_lost(&[PERSISTENT_CONG_THRESH + 2, 1]);
+        assert!(!persistent_congestion_by_pto(
+            ClassicCongestionControl::new(Cubic::default()),
+            0,
+            0,
+            &lost
+        ));
+    }
+
+    #[test]
+    fn app_limited_slow_start() {
+        const LESS_THAN_CWND_PKTS: usize = 4;
+        let mut cc = ClassicCongestionControl::new(NewReno::default());
+
+        for i in 0..CWND_INITIAL_PKTS {
+            let sent = SentPacket::new(
+                PacketType::Short,
+                u64::try_from(i).unwrap(), // pn
+                now(),                     // time sent
+                true,                      // ack eliciting
+                Vec::new(),                // tokens
+                MAX_DATAGRAM_SIZE,         // size
+            );
+            cc.on_packet_sent(&sent);
+        }
+        assert_eq!(cc.bytes_in_flight(), CWND_INITIAL);
+
+        for i in 0..LESS_THAN_CWND_PKTS {
+            let acked = SentPacket::new(
+                PacketType::Short,
+                u64::try_from(i).unwrap(), // pn
+                now(),                     // time sent
+                true,                      // ack eliciting
+                Vec::new(),                // tokens
+                MAX_DATAGRAM_SIZE,         // size
+            );
+            cc.on_packets_acked(&[acked], RTT, now());
+
+            assert_eq!(
+                cc.bytes_in_flight(),
+                (CWND_INITIAL_PKTS - i - 1) * MAX_DATAGRAM_SIZE
+            );
+            assert_eq!(cc.cwnd(), (CWND_INITIAL_PKTS + i + 1) * MAX_DATAGRAM_SIZE);
+        }
+
+        // Now we are app limited
+        for i in 4..CWND_INITIAL_PKTS {
+            let p = [SentPacket::new(
+                PacketType::Short,
+                u64::try_from(i).unwrap(), // pn
+                now(),                     // time sent
+                true,                      // ack eliciting
+                Vec::new(),                // tokens
+                MAX_DATAGRAM_SIZE,         // size
+            )];
+            cc.on_packets_acked(&p, RTT, now());
+
+            assert_eq!(
+                cc.bytes_in_flight(),
+                (CWND_INITIAL_PKTS - i - 1) * MAX_DATAGRAM_SIZE
+            );
+            assert_eq!(cc.cwnd(), (CWND_INITIAL_PKTS + 4) * MAX_DATAGRAM_SIZE);
+        }
+    }
+
+    #[test]
+    fn app_limited_congestion_avoidance() {
+        const CWND_PKTS_CA: usize = CWND_INITIAL_PKTS / 2;
+
+        let mut cc = ClassicCongestionControl::new(NewReno::default());
+
+        // Change state to congestion avoidance by introducing loss.
+
+        let p_lost = SentPacket::new(
+            PacketType::Short,
+            1,                 // pn
+            now(),             // time sent
+            true,              // ack eliciting
+            Vec::new(),        // tokens
+            MAX_DATAGRAM_SIZE, // size
+        );
+        cc.on_packet_sent(&p_lost);
+        cwnd_is_default(&cc);
+        cc.on_packets_lost(Some(now()), None, PTO, &[p_lost]);
+        cwnd_is_halved(&cc);
+        let p_not_lost = SentPacket::new(
+            PacketType::Short,
+            1,                 // pn
+            now(),             // time sent
+            true,              // ack eliciting
+            Vec::new(),        // tokens
+            MAX_DATAGRAM_SIZE, // size
+        );
+        cc.on_packet_sent(&p_not_lost);
+        cc.on_packets_acked(&[p_not_lost], RTT, now());
+        cwnd_is_halved(&cc);
+        // cc is app limited therefore cwnd in not increased.
+        assert_eq!(cc.acked_bytes, 0);
+
+        // Now we are in the congestion avoidance state.
+        let mut pkts = Vec::new();
+        for i in 0..CWND_PKTS_CA {
+            let p = SentPacket::new(
+                PacketType::Short,
+                u64::try_from(i + 3).unwrap(), // pn
+                now(),                         // time sent
+                true,                          // ack eliciting
+                Vec::new(),                    // tokens
+                MAX_DATAGRAM_SIZE,             // size
+            );
+            cc.on_packet_sent(&p);
+            pkts.push(p);
+        }
+        assert_eq!(cc.bytes_in_flight(), CWND_INITIAL / 2);
+
+        for i in 0..CWND_PKTS_CA - 2 {
+            cc.on_packets_acked(&pkts[i..=i], RTT, now());
+
+            assert_eq!(
+                cc.bytes_in_flight(),
+                (CWND_PKTS_CA - i - 1) * MAX_DATAGRAM_SIZE
+            );
+            assert_eq!(cc.cwnd(), CWND_PKTS_CA * MAX_DATAGRAM_SIZE);
+            assert_eq!(cc.acked_bytes, MAX_DATAGRAM_SIZE * (i + 1));
+        }
+
+        // Now we are app limited
+        for i in CWND_PKTS_CA - 2..CWND_PKTS_CA {
+            cc.on_packets_acked(&pkts[i..=i], RTT, now());
+
+            assert_eq!(
+                cc.bytes_in_flight(),
+                (CWND_PKTS_CA - i - 1) * MAX_DATAGRAM_SIZE
+            );
+            assert_eq!(cc.cwnd(), CWND_PKTS_CA * MAX_DATAGRAM_SIZE);
+            assert_eq!(cc.acked_bytes, MAX_DATAGRAM_SIZE * 3);
+        }
+    }
+}
diff --git a/third_party/rust/neqo-transport/src/cc/cubic.rs b/third_party/rust/neqo-transport/src/cc/cubic.rs
new file mode 100644
index 0000000000..ab3fedb74f
--- /dev/null
+++ b/third_party/rust/neqo-transport/src/cc/cubic.rs
@@ -0,0 +1,211 @@
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![deny(clippy::pedantic)]
+
+use std::fmt::{self, Display};
+use std::time::{Duration, Instant};
+
+use crate::cc::{classic_cc::WindowAdjustment, MAX_DATAGRAM_SIZE_F64};
+use neqo_common::qtrace;
+use std::convert::TryFrom;
+
+// CUBIC congestion control
+
+// C is a constant fixed to determine the aggressiveness of window
+// increase  in high BDP networks.
+pub const CUBIC_C: f64 = 0.4;
+pub const CUBIC_ALPHA: f64 = 3.0 * (1.0 - 0.7) / (1.0 + 0.7);
+
+// CUBIC_BETA = 0.7;
+pub const CUBIC_BETA_USIZE_QUOTIENT: usize = 7;
+pub const CUBIC_BETA_USIZE_DIVISOR: usize = 10;
+
+/// The fast convergence ratio further reduces the congestion window when a congestion event
+/// occurs before reaching the previous `W_max`.
+pub const CUBIC_FAST_CONVERGENCE: f64 = 0.85; // (1.0 + CUBIC_BETA) / 2.0;
+
+/// The minimum number of multiples of the datagram size that need
+/// to be received to cause an increase in the congestion window.
+/// When there is no loss, Cubic can return to exponential increase, but
+/// this value reduces the magnitude of the resulting growth by a constant factor.
+/// A value of 1.0 would mean a return to the rate used in slow start.
+const EXPONENTIAL_GROWTH_REDUCTION: f64 = 2.0;
+
+/// Convert an integer congestion window value into a floating point value.
+/// This has the effect of reducing larger values to `1<<53`.
+/// If you have a congestion window that large, something is probably wrong.
+fn convert_to_f64(v: usize) -> f64 {
+    let mut f_64 = f64::try_from(u32::try_from(v >> 21).unwrap_or(u32::MAX)).unwrap();
+    f_64 *= 2_097_152.0; // f_64 <<= 21
+    f_64 += f64::try_from(u32::try_from(v & 0x1f_ffff).unwrap()).unwrap();
+    f_64
+}
+
+#[derive(Debug)]
+pub struct Cubic {
+    last_max_cwnd: f64,
+    estimated_tcp_cwnd: f64,
+    k: f64,
+    w_max: f64,
+    ca_epoch_start: Option<Instant>,
+    tcp_acked_bytes: f64,
+}
+
+impl Default for Cubic {
+    fn default() -> Self {
+        Self {
+            last_max_cwnd: 0.0,
+            estimated_tcp_cwnd: 0.0,
+            k: 0.0,
+            w_max: 0.0,
+            ca_epoch_start: None,
+            tcp_acked_bytes: 0.0,
+        }
+    }
+}
+
+impl Display for Cubic {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(
+            f,
+            "Cubic [last_max_cwnd: {}, k: {}, w_max: {}, ca_epoch_start: {:?}]",
+            self.last_max_cwnd, self.k, self.w_max, self.ca_epoch_start
+        )?;
+        Ok(())
+    }
+}
+
+#[allow(clippy::doc_markdown)]
+impl Cubic {
+    /// Original equations is:
+    /// K = cubic_root(W_max*(1-beta_cubic)/C) (Eq. 2 RFC8312)
+    /// W_max is number of segments of the maximum segment size (MSS).
+    ///
+    /// K is actually the time that W_cubic(t) = C*(t-K)^3 + W_max (Eq. 1) would
+    /// take to increase to W_max. We use bytes not MSS units, therefore this
+    /// equation will be: W_cubic(t) = C*MSS*(t-K)^3 + W_max.
+    ///
+    /// From that equation we can calculate K as:
+    /// K = cubic_root((W_max - W_cubic) / C / MSS);
+    fn calc_k(&self, curr_cwnd: f64) -> f64 {
+        ((self.w_max - curr_cwnd) / CUBIC_C / MAX_DATAGRAM_SIZE_F64).cbrt()
+    }
+
+    /// W_cubic(t) = C*(t-K)^3 + W_max (Eq. 1)
+    /// t is relative to the start of the congestion avoidance phase and it is in seconds.
+    fn w_cubic(&self, t: f64) -> f64 {
+        CUBIC_C * (t - self.k).powi(3) * MAX_DATAGRAM_SIZE_F64 + self.w_max
+    }
+
+    fn start_epoch(&mut self, curr_cwnd_f64: f64, new_acked_f64: f64, now: Instant) {
+        self.ca_epoch_start = Some(now);
+        // reset tcp_acked_bytes and estimated_tcp_cwnd;
+        self.tcp_acked_bytes = new_acked_f64;
+        self.estimated_tcp_cwnd = curr_cwnd_f64;
+        if self.last_max_cwnd <= curr_cwnd_f64 {
+            self.w_max = curr_cwnd_f64;
+            self.k = 0.0;
+        } else {
+            self.w_max = self.last_max_cwnd;
+            self.k = self.calc_k(curr_cwnd_f64);
+        }
+        qtrace!([self], "New epoch");
+    }
+}
+
+impl WindowAdjustment for Cubic {
+    // This is because of the cast in the last line from f64 to usize.
+    #[allow(clippy::cast_possible_truncation)]
+    #[allow(clippy::cast_sign_loss)]
+    fn bytes_for_cwnd_increase(
+        &mut self,
+        curr_cwnd: usize,
+        new_acked_bytes: usize,
+        min_rtt: Duration,
+        now: Instant,
+    ) -> usize {
+        let curr_cwnd_f64 = convert_to_f64(curr_cwnd);
+        let new_acked_f64 = convert_to_f64(new_acked_bytes);
+        if self.ca_epoch_start.is_none() {
+            // This is a start of a new congestion avoidance phase.
+            self.start_epoch(curr_cwnd_f64, new_acked_f64, now);
+        } else {
+            self.tcp_acked_bytes += new_acked_f64;
+        }
+
+        let time_ca = self
+            .ca_epoch_start
+            .map_or(min_rtt, |t| {
+                if now + min_rtt < t {
+                    // This only happens when processing old packets
+                    // that were saved and replayed with old timestamps.
+                    min_rtt
+                } else {
+                    now + min_rtt - t
+                }
+            })
+            .as_secs_f64();
+        let target_cubic = self.w_cubic(time_ca);
+
+        let tcp_cnt = self.estimated_tcp_cwnd / CUBIC_ALPHA;
+        while self.tcp_acked_bytes > tcp_cnt {
+            self.tcp_acked_bytes -= tcp_cnt;
+            self.estimated_tcp_cwnd += MAX_DATAGRAM_SIZE_F64;
+        }
+
+        let target_cwnd = target_cubic.max(self.estimated_tcp_cwnd);
+
+        // Calculate the number of bytes that would need to be acknowledged for an increase
+        // of `MAX_DATAGRAM_SIZE` to match the increase of `target - cwnd / cwnd` as defined
+        // in the specification (Sections 4.4 and 4.5).
+        // The amount of data required therefore reduces asymptotically as the target increases.
+        // If the target is not significantly higher than the congestion window, require a very large
+        // amount of acknowledged data (effectively block increases).
+        let mut acked_to_increase =
+            MAX_DATAGRAM_SIZE_F64 * curr_cwnd_f64 / (target_cwnd - curr_cwnd_f64).max(1.0);
+
+        // Limit increase to max 1 MSS per EXPONENTIAL_GROWTH_REDUCTION ack packets.
+        // This effectively limits target_cwnd to (1 + 1 / EXPONENTIAL_GROWTH_REDUCTION) cwnd.
+        acked_to_increase =
+            acked_to_increase.max(EXPONENTIAL_GROWTH_REDUCTION * MAX_DATAGRAM_SIZE_F64);
+        acked_to_increase as usize
+    }
+
+    fn reduce_cwnd(&mut self, curr_cwnd: usize, acked_bytes: usize) -> (usize, usize) {
+        let curr_cwnd_f64 = convert_to_f64(curr_cwnd);
+        // Fast Convergence
+        // If congestion event occurs before the maximum congestion window before the last congestion event,
+        // we reduce the the maximum congestion window and thereby W_max.
+        // check cwnd + MAX_DATAGRAM_SIZE instead of cwnd because with cwnd in bytes, cwnd may be slightly off.
+        self.last_max_cwnd = if curr_cwnd_f64 + MAX_DATAGRAM_SIZE_F64 < self.last_max_cwnd {
+            curr_cwnd_f64 * CUBIC_FAST_CONVERGENCE
+        } else {
+            curr_cwnd_f64
+        };
+        self.ca_epoch_start = None;
+        (
+            curr_cwnd * CUBIC_BETA_USIZE_QUOTIENT / CUBIC_BETA_USIZE_DIVISOR,
+            acked_bytes * CUBIC_BETA_USIZE_QUOTIENT / CUBIC_BETA_USIZE_DIVISOR,
+        )
+    }
+
+    fn on_app_limited(&mut self) {
+        // Reset ca_epoch_start. Let it start again when the congestion controller
+        // exits the app-limited period.
+        self.ca_epoch_start = None;
+    }
+
+    #[cfg(test)]
+    fn last_max_cwnd(&self) -> f64 {
+        self.last_max_cwnd
+    }
+
+    #[cfg(test)]
+    fn set_last_max_cwnd(&mut self, last_max_cwnd: f64) {
+        self.last_max_cwnd = last_max_cwnd;
+    }
+}
diff --git a/third_party/rust/neqo-transport/src/cc/mod.rs b/third_party/rust/neqo-transport/src/cc/mod.rs
new file mode 100644
index 0000000000..50a3e73c82
--- /dev/null
+++ b/third_party/rust/neqo-transport/src/cc/mod.rs
@@ -0,0 +1,84 @@
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Congestion control
+#![deny(clippy::pedantic)]
+
+use crate::path::PATH_MTU_V6;
+use crate::tracking::SentPacket;
+use crate::Error;
+use neqo_common::qlog::NeqoQlog;
+
+use std::fmt::{Debug, Display};
+use std::str::FromStr;
+use std::time::{Duration, Instant};
+
+mod classic_cc;
+mod cubic;
+mod new_reno;
+
+pub use classic_cc::ClassicCongestionControl;
+pub use classic_cc::{CWND_INITIAL, CWND_INITIAL_PKTS, CWND_MIN};
+pub use cubic::Cubic;
+pub use new_reno::NewReno;
+
+pub const MAX_DATAGRAM_SIZE: usize = PATH_MTU_V6;
+pub const MAX_DATAGRAM_SIZE_F64: f64 = 1337.0;
+
+pub trait CongestionControl: Display + Debug {
+    fn set_qlog(&mut self, qlog: NeqoQlog);
+
+    #[must_use]
+    fn cwnd(&self) -> usize;
+
+    #[must_use]
+    fn bytes_in_flight(&self) -> usize;
+
+    #[must_use]
+    fn cwnd_avail(&self) -> usize;
+
+    fn on_packets_acked(&mut self, acked_pkts: &[SentPacket], min_rtt: Duration, now: Instant);
+
+    /// Returns true if the congestion window was reduced.
+    fn on_packets_lost(
+        &mut self,
+        first_rtt_sample_time: Option<Instant>,
+        prev_largest_acked_sent: Option<Instant>,
+        pto: Duration,
+        lost_packets: &[SentPacket],
+    ) -> bool;
+
+    #[must_use]
+    fn recovery_packet(&self) -> bool;
+
+    fn discard(&mut self, pkt: &SentPacket);
+
+    fn on_packet_sent(&mut self, pkt: &SentPacket);
+
+    fn discard_in_flight(&mut self);
+}
+
+#[derive(Debug, Copy, Clone)]
+pub enum CongestionControlAlgorithm {
+    NewReno,
+    Cubic,
+}
+
+// A `FromStr` implementation so that this can be used in command-line interfaces.
+impl FromStr for CongestionControlAlgorithm {
+    type Err = Error;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        match s.trim().to_ascii_lowercase().as_str() {
+            "newreno" | "reno" => Ok(Self::NewReno),
+            "cubic" => Ok(Self::Cubic),
+            _ => Err(Error::InvalidInput),
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests;
diff --git a/third_party/rust/neqo-transport/src/cc/new_reno.rs b/third_party/rust/neqo-transport/src/cc/new_reno.rs
new file mode 100644
index 0000000000..d34cdfbab9
--- /dev/null
+++ b/third_party/rust/neqo-transport/src/cc/new_reno.rs
@@ -0,0 +1,49 @@
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Congestion control
+#![deny(clippy::pedantic)]
+
+use std::fmt::{self, Display};
+
+use crate::cc::classic_cc::WindowAdjustment;
+use std::time::{Duration, Instant};
+
+#[derive(Debug, Default)]
+pub struct NewReno {}
+
+impl Display for NewReno {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "NewReno")?;
+        Ok(())
+    }
+}
+
+impl WindowAdjustment for NewReno {
+    fn bytes_for_cwnd_increase(
+        &mut self,
+        curr_cwnd: usize,
+        _new_acked_bytes: usize,
+        _min_rtt: Duration,
+        _now: Instant,
+    ) -> usize {
+        curr_cwnd
+    }
+
+    fn reduce_cwnd(&mut self, curr_cwnd: usize, acked_bytes: usize) -> (usize, usize) {
+        (curr_cwnd / 2, acked_bytes / 2)
+    }
+
+    fn on_app_limited(&mut self) {}
+
+    #[cfg(test)]
+    fn last_max_cwnd(&self) -> f64 {
+        0.0
+    }
+
+    #[cfg(test)]
+    fn set_last_max_cwnd(&mut self, _last_max_cwnd: f64) {}
+}
diff --git a/third_party/rust/neqo-transport/src/cc/tests/cubic.rs b/third_party/rust/neqo-transport/src/cc/tests/cubic.rs
new file mode 100644
index 0000000000..c8d7fe58de
--- /dev/null
+++ b/third_party/rust/neqo-transport/src/cc/tests/cubic.rs
@@ -0,0 +1,324 @@
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![allow(clippy::cast_possible_truncation)]
+#![allow(clippy::cast_sign_loss)]
+
+use crate::cc::{
+    classic_cc::{ClassicCongestionControl, CWND_INITIAL},
+    cubic::{
+        Cubic, CUBIC_ALPHA, CUBIC_BETA_USIZE_DIVISOR, CUBIC_BETA_USIZE_QUOTIENT, CUBIC_C,
+        CUBIC_FAST_CONVERGENCE,
+    },
+    CongestionControl, MAX_DATAGRAM_SIZE, MAX_DATAGRAM_SIZE_F64,
+};
+use crate::packet::PacketType;
+use crate::tracking::SentPacket;
+use std::convert::TryFrom;
+use std::ops::Sub;
+use std::time::{Duration, Instant};
+use test_fixture::now;
+
+const RTT: Duration = Duration::from_millis(100);
+const CWND_INITIAL_F64: f64 = 10.0 * MAX_DATAGRAM_SIZE_F64;
+const CWND_INITIAL_10_F64: f64 = 10.0 * CWND_INITIAL_F64;
+const CWND_INITIAL_10: usize = 10 * CWND_INITIAL;
+const CWND_AFTER_LOSS: usize = CWND_INITIAL * CUBIC_BETA_USIZE_QUOTIENT / CUBIC_BETA_USIZE_DIVISOR;
+const CWND_AFTER_LOSS_SLOW_START: usize =
+    (CWND_INITIAL + MAX_DATAGRAM_SIZE) * CUBIC_BETA_USIZE_QUOTIENT / CUBIC_BETA_USIZE_DIVISOR;
+
+fn fill_cwnd(cc: &mut ClassicCongestionControl<Cubic>, mut next_pn: u64, now: Instant) -> u64 {
+    while cc.bytes_in_flight() < cc.cwnd() {
+        let sent = SentPacket::new(
+            PacketType::Short,
+            next_pn,           // pn
+            now,               // time sent
+            true,              // ack eliciting
+            Vec::new(),        // tokens
+            MAX_DATAGRAM_SIZE, // size
+        );
+        cc.on_packet_sent(&sent);
+        next_pn += 1;
+    }
+    next_pn
+}
+
+fn ack_packet(cc: &mut ClassicCongestionControl<Cubic>, pn: u64, now: Instant) {
+    let acked = SentPacket::new(
+        PacketType::Short,
+        pn,                // pn
+        now,               // time sent
+        true,              // ack eliciting
+        Vec::new(),        // tokens
+        MAX_DATAGRAM_SIZE, // size
+    );
+    cc.on_packets_acked(&[acked], RTT, now);
+}
+
+fn packet_lost(cc: &mut ClassicCongestionControl<Cubic>, pn: u64) {
+    const PTO: Duration = Duration::from_millis(120);
+    let p_lost = SentPacket::new(
+        PacketType::Short,
+        pn,                // pn
+        now(),             // time sent
+        true,              // ack eliciting
+        Vec::new(),        // tokens
+        MAX_DATAGRAM_SIZE, // size
+    );
+    cc.on_packets_lost(None, None, PTO, &[p_lost]);
+}
+
+fn expected_tcp_acks(cwnd_rtt_start: usize) -> u64 {
+    (f64::try_from(i32::try_from(cwnd_rtt_start).unwrap()).unwrap()
+        / MAX_DATAGRAM_SIZE_F64
+        / CUBIC_ALPHA)
+        .round() as u64
+}
+
+#[test]
+fn tcp_phase() {
+    let mut cubic = ClassicCongestionControl::new(Cubic::default());
+
+    // change to congestion avoidance state.
+    cubic.set_ssthresh(1);
+
+    let mut now = now();
+    let start_time = now;
+    // helper variables to remember the next packet number to be sent/acked.
+    let mut next_pn_send = 0;
+    let mut next_pn_ack = 0;
+
+    next_pn_send = fill_cwnd(&mut cubic, next_pn_send, now);
+
+    // This will start with TCP phase.
+    // in this phase cwnd is increase by CUBIC_ALPHA every RTT. We can look at it as
+    // increase of MAX_DATAGRAM_SIZE every 1 / CUBIC_ALPHA RTTs.
+    // The phase will end when cwnd calculated with cubic equation is equal to TCP estimate:
+    // CUBIC_C * (n * RTT / CUBIC_ALPHA)^3 * MAX_DATAGRAM_SIZE = n * MAX_DATAGRAM_SIZE
+    // from this n = sqrt(CUBIC_ALPHA^3/ (CUBIC_C * RTT^3)).
+    let num_tcp_increases = (CUBIC_ALPHA.powi(3) / (CUBIC_C * RTT.as_secs_f64().powi(3)))
+        .sqrt()
+        .floor() as u64;
+
+    for _ in 0..num_tcp_increases {
+        let cwnd_rtt_start = cubic.cwnd();
+        //Expected acks during a period of RTT / CUBIC_ALPHA.
+        let acks = expected_tcp_acks(cwnd_rtt_start);
+        // The time between acks if they are ideally paced over a RTT.
+        let time_increase = RTT / u32::try_from(cwnd_rtt_start / MAX_DATAGRAM_SIZE).unwrap();
+
+        for _ in 0..acks {
+            now += time_increase;
+            ack_packet(&mut cubic, next_pn_ack, now);
+            next_pn_ack += 1;
+            next_pn_send = fill_cwnd(&mut cubic, next_pn_send, now);
+        }
+
+        assert_eq!(cubic.cwnd() - cwnd_rtt_start, MAX_DATAGRAM_SIZE);
+    }
+
+    // The next increase will be according to the cubic equation.
+
+    let cwnd_rtt_start = cubic.cwnd();
+    // cwnd_rtt_start has change, therefore calculate new time_increase (the time
+    // between acks if they are ideally paced over a RTT).
+    let time_increase = RTT / u32::try_from(cwnd_rtt_start / MAX_DATAGRAM_SIZE).unwrap();
+    let mut num_acks = 0; // count the number of acks. until cwnd is increased by MAX_DATAGRAM_SIZE.
+
+    while cwnd_rtt_start == cubic.cwnd() {
+        num_acks += 1;
+        now += time_increase;
+        ack_packet(&mut cubic, next_pn_ack, now);
+        next_pn_ack += 1;
+        next_pn_send = fill_cwnd(&mut cubic, next_pn_send, now);
+    }
+
+    // Make sure that the increase is not according to TCP equation, i.e., that it took
+    // less than RTT / CUBIC_ALPHA.
+    let expected_ack_tcp_increase = expected_tcp_acks(cwnd_rtt_start);
+    assert!(num_acks < expected_ack_tcp_increase);
+
+    // This first increase after a TCP phase may be shorter than what it would take by a regular cubic phase,
+    // because of the proper byte counting and the credit it already had before entering this phase. Therefore
+    // We will perform another round and compare it to expected increase using the cubic equation.
+
+    let cwnd_rtt_start_after_tcp = cubic.cwnd();
+    let elapsed_time = now - start_time;
+
+    // calculate new time_increase.
+    let time_increase = RTT / u32::try_from(cwnd_rtt_start_after_tcp / MAX_DATAGRAM_SIZE).unwrap();
+    let mut num_acks2 = 0; // count the number of acks. until cwnd is increased by MAX_DATAGRAM_SIZE.
+
+    while cwnd_rtt_start_after_tcp == cubic.cwnd() {
+        num_acks2 += 1;
+        now += time_increase;
+        ack_packet(&mut cubic, next_pn_ack, now);
+        next_pn_ack += 1;
+        next_pn_send = fill_cwnd(&mut cubic, next_pn_send, now);
+    }
+
+    let expected_ack_tcp_increase2 = expected_tcp_acks(cwnd_rtt_start_after_tcp);
+    assert!(num_acks2 < expected_ack_tcp_increase2);
+
+    // The time needed to increase cwnd by MAX_DATAGRAM_SIZE using the cubic equation will be calculates from:
+    // W_cubic(elapsed_time + t_to_increase) - W_cubis(elapsed_time) = MAX_DATAGRAM_SIZE =>
+    // CUBIC_C * (elapsed_time + t_to_increase)^3 * MAX_DATAGRAM_SIZE + CWND_INITIAL -
+    //     CUBIC_C * elapsed_time^3 * MAX_DATAGRAM_SIZE + CWND_INITIAL = MAX_DATAGRAM_SIZE =>
+    // t_to_increase = cbrt((1 + CUBIC_C * elapsed_time^3) / CUBIC_C) - elapsed_time
+    // (t_to_increase is in seconds)
+    // number of ack needed is t_to_increase / time_increase.
+    let expected_ack_cubic_increase =
+        ((((1.0 + CUBIC_C * (elapsed_time).as_secs_f64().powi(3)) / CUBIC_C).cbrt()
+            - elapsed_time.as_secs_f64())
+            / time_increase.as_secs_f64())
+        .ceil() as u64;
+    // num_acks is very close to the calculated value. The exact value is hard to calculate
+    // because the proportional increase(i.e. curr_cwnd_f64 / (target - curr_cwnd_f64) * MAX_DATAGRAM_SIZE_F64)
+    // and the byte counting.
+    assert_eq!(num_acks2, expected_ack_cubic_increase + 2);
+}
+
+#[test]
+fn cubic_phase() {
+    let mut cubic = ClassicCongestionControl::new(Cubic::default());
+    // Set last_max_cwnd to a higher number make sure that cc is the cubic phase (cwnd is calculated by the cubic equation).
+    cubic.set_last_max_cwnd(CWND_INITIAL_10_F64);
+    // Set ssthresh to something small to make sure that cc is in the congection avoidance phase.
+    cubic.set_ssthresh(1);
+    let mut now = now();
+    let mut next_pn_send = 0;
+    let mut next_pn_ack = 0;
+
+    next_pn_send = fill_cwnd(&mut cubic, next_pn_send, now);
+
+    let k = ((CWND_INITIAL_10_F64 - CWND_INITIAL_F64) / CUBIC_C / MAX_DATAGRAM_SIZE_F64).cbrt();
+    let epoch_start = now;
+
+    // The number of RTT until W_max is reached.
+    let num_rtts_w_max = (k / RTT.as_secs_f64()).round() as u64;
+    for _ in 0..num_rtts_w_max {
+        let cwnd_rtt_start = cubic.cwnd();
+        //Expected acks
+        let acks = cwnd_rtt_start / MAX_DATAGRAM_SIZE;
+        let time_increase = RTT / u32::try_from(acks).unwrap();
+        for _ in 0..acks {
+            now += time_increase;
+            ack_packet(&mut cubic, next_pn_ack, now);
+            next_pn_ack += 1;
+            next_pn_send = fill_cwnd(&mut cubic, next_pn_send, now);
+        }
+
+        let expected =
+            (CUBIC_C * ((now - epoch_start).as_secs_f64() - k).powi(3) * MAX_DATAGRAM_SIZE_F64
+                + CWND_INITIAL_10_F64)
+                .round() as usize;
+
+        assert_within(cubic.cwnd(), expected, MAX_DATAGRAM_SIZE);
+    }
+    assert_eq!(cubic.cwnd(), CWND_INITIAL_10);
+}
+
+fn assert_within<T: Sub<Output = T> + PartialOrd + Copy>(value: T, expected: T, margin: T) {
+    if value >= expected {
+        assert!(value - expected < margin);
+    } else {
+        assert!(expected - value < margin);
+    }
+}
+
+#[test]
+fn congestion_event_slow_start() {
+    let mut cubic = ClassicCongestionControl::new(Cubic::default());
+
+    let _ = fill_cwnd(&mut cubic, 0, now());
+    ack_packet(&mut cubic, 0, now());
+
+    assert_within(cubic.last_max_cwnd(), 0.0, f64::EPSILON);
+
+    // cwnd is increased by 1 in slow start phase, after an ack.
+    assert_eq!(cubic.cwnd(), CWND_INITIAL + MAX_DATAGRAM_SIZE);
+
+    // Trigger a congestion_event in slow start phase
+    packet_lost(&mut cubic, 1);
+
+    // last_max_cwnd is equal to cwnd before decrease.
+    assert_within(
+        cubic.last_max_cwnd(),
+        CWND_INITIAL_F64 + MAX_DATAGRAM_SIZE_F64,
+        f64::EPSILON,
+    );
+    assert_eq!(cubic.cwnd(), CWND_AFTER_LOSS_SLOW_START);
+}
+
+#[test]
+fn congestion_event_congestion_avoidance() {
+    let mut cubic = ClassicCongestionControl::new(Cubic::default());
+
+    // Set ssthresh to something small to make sure that cc is in the congection avoidance phase.
+    cubic.set_ssthresh(1);
+
+    // Set last_max_cwnd to something smaller than cwnd so that the fast convergence is not triggered.
+    cubic.set_last_max_cwnd(3.0 * MAX_DATAGRAM_SIZE_F64);
+
+    let _ = fill_cwnd(&mut cubic, 0, now());
+    ack_packet(&mut cubic, 0, now());
+
+    assert_eq!(cubic.cwnd(), CWND_INITIAL);
+
+    // Trigger a congestion_event in slow start phase
+    packet_lost(&mut cubic, 1);
+
+    assert_within(cubic.last_max_cwnd(), CWND_INITIAL_F64, f64::EPSILON);
+    assert_eq!(cubic.cwnd(), CWND_AFTER_LOSS);
+}
+
+#[test]
+fn congestion_event_congestion_avoidance_2() {
+    let mut cubic = ClassicCongestionControl::new(Cubic::default());
+
+    // Set ssthresh to something small to make sure that cc is in the congection avoidance phase.
+    cubic.set_ssthresh(1);
+
+    // Set last_max_cwnd to something higher than cwnd so that the fast convergence is triggered.
+    cubic.set_last_max_cwnd(CWND_INITIAL_10_F64);
+
+    let _ = fill_cwnd(&mut cubic, 0, now());
+    ack_packet(&mut cubic, 0, now());
+
+    assert_within(cubic.last_max_cwnd(), CWND_INITIAL_10_F64, f64::EPSILON);
+    assert_eq!(cubic.cwnd(), CWND_INITIAL);
+
+    // Trigger a congestion_event.
+    packet_lost(&mut cubic, 1);
+
+    assert_within(
+        cubic.last_max_cwnd(),
+        CWND_INITIAL_F64 * CUBIC_FAST_CONVERGENCE,
+        f64::EPSILON,
+    );
+    assert_eq!(cubic.cwnd(), CWND_AFTER_LOSS);
+}
+
+#[test]
+fn congestion_event_congestion_avoidance_test_no_overflow() {
+    const PTO: Duration = Duration::from_millis(120);
+    let mut cubic = ClassicCongestionControl::new(Cubic::default());
+
+    // Set ssthresh to something small to make sure that cc is in the congection avoidance phase.
+    cubic.set_ssthresh(1);
+
+    // Set last_max_cwnd to something higher than cwnd so that the fast convergence is triggered.
+    cubic.set_last_max_cwnd(CWND_INITIAL_10_F64);
+
+    let _ = fill_cwnd(&mut cubic, 0, now());
+    ack_packet(&mut cubic, 1, now());
+
+    assert_within(cubic.last_max_cwnd(), CWND_INITIAL_10_F64, f64::EPSILON);
+    assert_eq!(cubic.cwnd(), CWND_INITIAL);
+
+    // Now ack packet that was send earlier.
+    ack_packet(&mut cubic, 0, now().checked_sub(PTO).unwrap());
+}
diff --git a/third_party/rust/neqo-transport/src/cc/tests/mod.rs b/third_party/rust/neqo-transport/src/cc/tests/mod.rs
new file mode 100644
index 0000000000..238a7ad012
--- /dev/null
+++ b/third_party/rust/neqo-transport/src/cc/tests/mod.rs
@@ -0,0 +1,7 @@
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+mod cubic;
+mod new_reno;
diff --git a/third_party/rust/neqo-transport/src/cc/tests/new_reno.rs b/third_party/rust/neqo-transport/src/cc/tests/new_reno.rs
new file mode 100644
index 0000000000..0b678ca55e
--- /dev/null
+++ b/third_party/rust/neqo-transport/src/cc/tests/new_reno.rs
@@ -0,0 +1,131 @@
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Congestion control
+#![deny(clippy::pedantic)]
+
+use crate::cc::new_reno::NewReno;
+use crate::cc::{ClassicCongestionControl, CongestionControl, CWND_INITIAL, MAX_DATAGRAM_SIZE};
+use crate::packet::PacketType;
+use crate::tracking::SentPacket;
+use std::time::Duration;
+use test_fixture::now;
+
+const PTO: Duration = Duration::from_millis(100);
+const RTT: Duration = Duration::from_millis(98);
+
+fn cwnd_is_default(cc: &ClassicCongestionControl<NewReno>) {
+    assert_eq!(cc.cwnd(), CWND_INITIAL);
+    assert_eq!(cc.ssthresh(), usize::MAX);
+}
+
+fn cwnd_is_halved(cc: &ClassicCongestionControl<NewReno>) {
+    assert_eq!(cc.cwnd(), CWND_INITIAL / 2);
+    assert_eq!(cc.ssthresh(), CWND_INITIAL / 2);
+}
+
+#[test]
+fn issue_876() {
+    let mut cc = ClassicCongestionControl::new(NewReno::default());
+    let time_now = now();
+    let time_before = time_now.checked_sub(Duration::from_millis(100)).unwrap();
+    let time_after = time_now + Duration::from_millis(150);
+
+    let sent_packets = &[
+        SentPacket::new(
+            PacketType::Short,
+            1,                     // pn
+            time_before,           // time sent
+            true,                  // ack eliciting
+            Vec::new(),            // tokens
+            MAX_DATAGRAM_SIZE - 1, // size
+        ),
+        SentPacket::new(
+            PacketType::Short,
+            2,                     // pn
+            time_before,           // time sent
+            true,                  // ack eliciting
+            Vec::new(),            // tokens
+            MAX_DATAGRAM_SIZE - 2, // size
+        ),
+        SentPacket::new(
+            PacketType::Short,
+            3,                 // pn
+            time_before,       // time sent
+            true,              // ack eliciting
+            Vec::new(),        // tokens
+            MAX_DATAGRAM_SIZE, // size
+        ),
+        SentPacket::new(
+            PacketType::Short,
+            4,                 // pn
+            time_before,       // time sent
+            true,              // ack eliciting
+            Vec::new(),        // tokens
+            MAX_DATAGRAM_SIZE, // size
+        ),
+        SentPacket::new(
+            PacketType::Short,
+            5,                 // pn
+            time_before,       // time sent
+            true,              // ack eliciting
+            Vec::new(),        // tokens
+            MAX_DATAGRAM_SIZE, // size
+        ),
+        SentPacket::new(
+            PacketType::Short,
+            6,                 // pn
+            time_before,       // time sent
+            true,              // ack eliciting
+            Vec::new(),        // tokens
+            MAX_DATAGRAM_SIZE, // size
+        ),
+        SentPacket::new(
+            PacketType::Short,
+            7,                     // pn
+            time_after,            // time sent
+            true,                  // ack eliciting
+            Vec::new(),            // tokens
+            MAX_DATAGRAM_SIZE - 3, // size
+        ),
+    ];
+
+    // Send some more packets so that the cc is not app-limited.
+    for p in &sent_packets[..6] {
+        cc.on_packet_sent(p);
+    }
+    assert_eq!(cc.acked_bytes(), 0);
+    cwnd_is_default(&cc);
+    assert_eq!(cc.bytes_in_flight(), 6 * MAX_DATAGRAM_SIZE - 3);
+
+    cc.on_packets_lost(Some(time_now), None, PTO, &sent_packets[0..1]);
+
+    // We are now in recovery
+    assert!(cc.recovery_packet());
+    assert_eq!(cc.acked_bytes(), 0);
+    cwnd_is_halved(&cc);
+    assert_eq!(cc.bytes_in_flight(), 5 * MAX_DATAGRAM_SIZE - 2);
+
+    // Send a packet after recovery starts
+    cc.on_packet_sent(&sent_packets[6]);
+    assert!(!cc.recovery_packet());
+    cwnd_is_halved(&cc);
+    assert_eq!(cc.acked_bytes(), 0);
+    assert_eq!(cc.bytes_in_flight(), 6 * MAX_DATAGRAM_SIZE - 5);
+
+    // and ack it. cwnd increases slightly
+    cc.on_packets_acked(&sent_packets[6..], RTT, time_now);
+    assert_eq!(cc.acked_bytes(), sent_packets[6].size);
+    cwnd_is_halved(&cc);
+    assert_eq!(cc.bytes_in_flight(), 5 * MAX_DATAGRAM_SIZE - 2);
+
+    // Packet from before is lost. Should not hurt cwnd.
+    cc.on_packets_lost(Some(time_now), None, PTO, &sent_packets[1..2]);
+    assert!(!cc.recovery_packet());
+    assert_eq!(cc.acked_bytes(), sent_packets[6].size);
+    cwnd_is_halved(&cc);
+    assert_eq!(cc.bytes_in_flight(), 4 * MAX_DATAGRAM_SIZE);
+}