quiche/recovery/gcongestion/

bbr2.rs

// Copyright (c) 2015 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

// Copyright (C) 2023, Cloudflare, Inc.
// All rights reserved.
//
// Redistribution and use in source and binary forms, with or without
// modification, are permitted provided that the following conditions are
// met:
//
//     * Redistributions of source code must retain the above copyright notice,
//       this list of conditions and the following disclaimer.
//
//     * Redistributions in binary form must reproduce the above copyright
//       notice, this list of conditions and the following disclaimer in the
//       documentation and/or other materials provided with the distribution.
//
// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS
// IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
// THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
// PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR
// CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
// EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
// PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
// PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
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// NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
// SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

mod drain;
mod mode;
mod network_model;
mod probe_bw;
mod probe_rtt;
mod startup;

use std::time::Duration;
use std::time::Instant;

use network_model::BBRv2NetworkModel;

use crate::recovery::gcongestion::Bandwidth;
use crate::recovery::RecoveryStats;

use self::mode::Mode;
use self::mode::ModeImpl;

use super::bbr::SendTimeState;
use super::Acked;
use super::BbrBwLoReductionStrategy;
use super::BbrParams;
use super::CongestionControl;
use super::Lost;
use super::RttStats;

const MAX_MODE_CHANGES_PER_CONGESTION_EVENT: usize = 4;

#[derive(Debug)]
struct Params {
    // STARTUP parameters.
    /// The gain for CWND in startup.
    startup_cwnd_gain: f32,

    startup_pacing_gain: f32,

    /// STARTUP or PROBE_UP are exited if the total bandwidth growth is less
    /// than `full_bw_threshold` in the last `startup_full_bw_rounds`` round
    /// trips.
    full_bw_threshold: f32,

    /// The number of rounds to stay in  STARTUP before exiting due to
    /// bandwidth plateau.
    startup_full_bw_rounds: usize,

    /// Number of rounds to stay in STARTUP when there's a sufficient queue that
    /// bytes_in_flight never drops below the target (1.75 * BDP).  0 indicates
    /// the feature is disabled and we never exit due to queueing.
    max_startup_queue_rounds: usize,

    /// The minimum number of loss marking events to exit STARTUP.
    startup_full_loss_count: usize,

    /// DRAIN parameters.
    drain_cwnd_gain: f32,

    drain_pacing_gain: f32,

    // PROBE_BW parameters.
    /// Max number of rounds before probing for Reno-coexistence.
    probe_bw_probe_max_rounds: usize,

    enable_reno_coexistence: bool,

    /// Multiplier to get Reno-style probe epoch duration as: k * BDP round
    /// trips. If zero, disables Reno-style BDP-scaled coexistence mechanism.
    probe_bw_probe_reno_gain: f32,

    /// Minimum duration for BBR-native probes.
    probe_bw_probe_base_duration: Duration,

    /// The minimum number of loss marking events to exit the PROBE_UP phase.
    probe_bw_full_loss_count: usize,

    /// Pacing gains.
    probe_bw_probe_up_pacing_gain: f32,
    probe_bw_probe_down_pacing_gain: f32,
    probe_bw_default_pacing_gain: f32,

    /// cwnd_gain for probe bw phases other than ProbeBW_UP
    probe_bw_cwnd_gain: f32,

    /// cwnd_gain for ProbeBW_UP
    probe_bw_up_cwnd_gain: f32,

    // PROBE_UP parameters.
    probe_up_ignore_inflight_hi: bool,

    /// Number of rounds to stay in PROBE_UP when there's a sufficient queue
    /// that bytes_in_flight never drops below the target.  0 indicates the
    /// feature is disabled and we never exit due to queueing.
    // TODO(vlad):
    max_probe_up_queue_rounds: usize,

    // PROBE_RTT parameters.
    probe_rtt_inflight_target_bdp_fraction: f32,

    /// The default period for entering PROBE_RTT
    probe_rtt_period: Duration,

    probe_rtt_duration: Duration,

    probe_rtt_pacing_gain: f32,
    probe_rtt_cwnd_gain: f32,

    // Parameters used by multiple modes.
    /// The initial value of the max ack height filter's window length.
    initial_max_ack_height_filter_window: usize,

    /// The default fraction of unutilized headroom to try to leave in path
    /// upon high loss.
    inflight_hi_headroom: f32,

    /// Estimate startup/bw probing has gone too far if loss rate exceeds this.
    loss_threshold: f32,

    /// A common factor for multiplicative decreases. Used for adjusting
    /// `bandwidth_lo``, `inflight_lo`` and `inflight_hi`` upon losses.
    beta: f32,

    // Experimental flags.
    add_ack_height_to_queueing_threshold: bool,

    /// Don't run PROBE_RTT on the regular schedule
    avoid_unnecessary_probe_rtt: bool,

    /// When exiting STARTUP due to loss, set `inflight_hi`` to the max of bdp
    /// and max bytes delivered in round.
    limit_inflight_hi_by_max_delivered: bool,

    startup_loss_exit_use_max_delivered_for_inflight_hi: bool,

    /// Increase `inflight_hi`` based on delievered, not inflight.
    use_bytes_delivered_for_inflight_hi: bool,

    /// Set the pacing gain to 25% larger than the recent BW increase in
    /// STARTUP.
    decrease_startup_pacing_at_end_of_round: bool,

    /// Avoid Overestimation in Bandwidth Sampler with ack aggregation
    enable_overestimate_avoidance: bool,

    /// If true, apply the fix to A0 point selection logic so the
    /// implementation is consistent with the behavior of the
    /// google/quiche implementation.
    choose_a0_point_fix: bool,

    bw_lo_mode: BwLoMode,

    /// Determines whether app limited rounds with no bandwidth growth count
    /// towards the rounds threshold to exit startup.
    ignore_app_limited_for_no_bandwidth_growth: bool,

    /// Initial pacing rate for a new connection before an RTT
    /// estimate is available.  This rate serves as an upper bound on
    /// the initial pacing rate, which is calculated by dividing the
    /// initial cwnd by the first RTT estimate.
    initial_pacing_rate_bytes_per_second: Option<u64>,

    /// If true, scale the pacing rate when updating mss when doing pmtud.
    scale_pacing_rate_by_mss: bool,
}

impl Params {
    fn with_overrides(mut self, custom_bbr_settings: &BbrParams) -> Self {
        macro_rules! apply_override {
            ($field:ident) => {
                if let Some(custom_value) = custom_bbr_settings.$field {
                    self.$field = custom_value;
                }
            };
        }

        macro_rules! apply_optional_override {
            ($field:ident) => {
                if let Some(custom_value) = custom_bbr_settings.$field {
                    self.$field = Some(custom_value);
                }
            };
        }

        apply_override!(startup_cwnd_gain);
        apply_override!(startup_pacing_gain);
        apply_override!(full_bw_threshold);
        apply_override!(startup_full_bw_rounds);
        apply_override!(startup_full_loss_count);
        apply_override!(drain_cwnd_gain);
        apply_override!(drain_pacing_gain);
        apply_override!(enable_reno_coexistence);
        apply_override!(enable_overestimate_avoidance);
        apply_override!(choose_a0_point_fix);
        apply_override!(probe_bw_probe_up_pacing_gain);
        apply_override!(probe_bw_probe_down_pacing_gain);
        apply_override!(probe_bw_cwnd_gain);
        apply_override!(probe_bw_up_cwnd_gain);
        apply_override!(probe_rtt_pacing_gain);
        apply_override!(probe_rtt_cwnd_gain);
        apply_override!(max_probe_up_queue_rounds);
        apply_override!(loss_threshold);
        apply_override!(use_bytes_delivered_for_inflight_hi);
        apply_override!(decrease_startup_pacing_at_end_of_round);
        apply_override!(ignore_app_limited_for_no_bandwidth_growth);
        apply_override!(scale_pacing_rate_by_mss);
        apply_optional_override!(initial_pacing_rate_bytes_per_second);

        if let Some(custom_value) = custom_bbr_settings.bw_lo_reduction_strategy {
            self.bw_lo_mode = custom_value.into();
        }

        self
    }
}

const DEFAULT_PARAMS: Params = Params {
    startup_cwnd_gain: 2.0,

    startup_pacing_gain: 2.773,

    full_bw_threshold: 1.25,

    startup_full_bw_rounds: 3,

    max_startup_queue_rounds: 0,

    startup_full_loss_count: 8,

    drain_cwnd_gain: 2.0,

    drain_pacing_gain: 1.0 / 2.885,

    probe_bw_probe_max_rounds: 63,

    enable_reno_coexistence: true,

    probe_bw_probe_reno_gain: 1.0,

    probe_bw_probe_base_duration: Duration::from_millis(2000),

    probe_bw_full_loss_count: 2,

    probe_bw_probe_up_pacing_gain: 1.25,

    probe_bw_probe_down_pacing_gain: 0.9, // BBRv3

    probe_bw_default_pacing_gain: 1.0,

    probe_bw_cwnd_gain: 2.25, // BBRv3

    probe_bw_up_cwnd_gain: 2.25, // BBRv3

    probe_up_ignore_inflight_hi: false,

    max_probe_up_queue_rounds: 2,

    probe_rtt_inflight_target_bdp_fraction: 0.5,

    probe_rtt_period: Duration::from_millis(10000),

    probe_rtt_duration: Duration::from_millis(200),

    probe_rtt_pacing_gain: 1.0,

    probe_rtt_cwnd_gain: 1.0,

    initial_max_ack_height_filter_window: 10,

    inflight_hi_headroom: 0.15,

    loss_threshold: 0.015,

    beta: 0.3,

    add_ack_height_to_queueing_threshold: false,

    avoid_unnecessary_probe_rtt: true,

    limit_inflight_hi_by_max_delivered: true,

    startup_loss_exit_use_max_delivered_for_inflight_hi: true,

    use_bytes_delivered_for_inflight_hi: true,

    decrease_startup_pacing_at_end_of_round: true,

    enable_overestimate_avoidance: true,

    choose_a0_point_fix: false,

    bw_lo_mode: BwLoMode::InflightReduction,

    ignore_app_limited_for_no_bandwidth_growth: false,

    initial_pacing_rate_bytes_per_second: None,

    scale_pacing_rate_by_mss: false,
};

#[derive(Debug, PartialEq)]
enum BwLoMode {
    Default,
    MinRttReduction,
    InflightReduction,
    CwndReduction,
}

impl From<BbrBwLoReductionStrategy> for BwLoMode {
    fn from(value: BbrBwLoReductionStrategy) -> Self {
        match value {
            BbrBwLoReductionStrategy::Default => BwLoMode::Default,
            BbrBwLoReductionStrategy::MinRttReduction =>
                BwLoMode::MinRttReduction,
            BbrBwLoReductionStrategy::InflightReduction =>
                BwLoMode::InflightReduction,
            BbrBwLoReductionStrategy::CwndReduction => BwLoMode::CwndReduction,
        }
    }
}

#[derive(Debug)]
struct Limits<T: Ord> {
    lo: T,
    hi: T,
}

impl<T: Ord + Clone + Copy> Limits<T> {
    fn min(&self) -> T {
        self.lo
    }

    fn apply_limits(&self, val: T) -> T {
        val.max(self.lo).min(self.hi)
    }
}

impl<T: Ord + Clone + Copy + From<u8>> Limits<T> {
    pub(crate) fn no_greater_than(val: T) -> Self {
        Self {
            lo: T::from(0),
            hi: val,
        }
    }
}

fn initial_pacing_rate(
    cwnd_in_bytes: usize, smoothed_rtt: Duration, params: &Params,
) -> Bandwidth {
    if let Some(pacing_rate) = params.initial_pacing_rate_bytes_per_second {
        return Bandwidth::from_bytes_per_second(pacing_rate);
    }

    Bandwidth::from_bytes_and_time_delta(cwnd_in_bytes, smoothed_rtt) * 2.885
}

#[derive(Debug)]
pub(crate) struct BBRv2 {
    mode: Mode,
    cwnd: usize,
    mss: usize,

    pacing_rate: Bandwidth,

    cwnd_limits: Limits<usize>,

    initial_cwnd: usize,

    last_sample_is_app_limited: bool,
    has_non_app_limited_sample: bool,
    last_quiescence_start: Option<Instant>,
    params: Params,
}

struct BBRv2CongestionEvent {
    event_time: Instant,

    /// The congestion window prior to the processing of the ack/loss events.
    prior_cwnd: usize,
    /// Total bytes inflight before the processing of the ack/loss events.
    prior_bytes_in_flight: usize,

    /// Total bytes inflight after the processing of the ack/loss events.
    bytes_in_flight: usize,
    /// Total bytes acked from acks in this event.
    bytes_acked: usize,
    /// Total bytes lost from losses in this event.
    bytes_lost: usize,

    /// Whether acked_packets indicates the end of a round trip.
    end_of_round_trip: bool,
    // When the event happened, whether the sender is probing for bandwidth.
    is_probing_for_bandwidth: bool,

    // Maximum bandwidth of all bandwidth samples from acked_packets.
    // This sample may be app-limited, and will be None if there are no newly
    // acknowledged inflight packets.
    sample_max_bandwidth: Option<Bandwidth>,

    /// Minimum rtt of all bandwidth samples from acked_packets.
    /// None if acked_packets is empty.
    sample_min_rtt: Option<Duration>,

    /// The send state of the largest packet in acked_packets, unless it is
    /// empty. If acked_packets is empty, it's the send state of the largest
    /// packet in lost_packets.
    last_packet_send_state: SendTimeState,
}

impl BBRv2CongestionEvent {
    fn new(
        event_time: Instant, prior_cwnd: usize, prior_bytes_in_flight: usize,
        is_probing_for_bandwidth: bool,
    ) -> Self {
        BBRv2CongestionEvent {
            event_time,
            prior_cwnd,
            prior_bytes_in_flight,
            is_probing_for_bandwidth,
            bytes_in_flight: 0,
            bytes_acked: 0,
            bytes_lost: 0,
            end_of_round_trip: false,
            last_packet_send_state: Default::default(),
            sample_max_bandwidth: None,
            sample_min_rtt: None,
        }
    }
}

impl BBRv2 {
    pub fn new(
        initial_congestion_window: usize, max_congestion_window: usize,
        max_segment_size: usize, smoothed_rtt: Duration,
        custom_bbr_params: Option<&BbrParams>,
    ) -> Self {
        let cwnd = initial_congestion_window * max_segment_size;
        let params = if let Some(custom_bbr_settings) = custom_bbr_params {
            DEFAULT_PARAMS.with_overrides(custom_bbr_settings)
        } else {
            DEFAULT_PARAMS
        };

        BBRv2 {
            mode: Mode::startup(BBRv2NetworkModel::new(&params, smoothed_rtt)),
            cwnd,
            pacing_rate: initial_pacing_rate(cwnd, smoothed_rtt, &params),
            cwnd_limits: Limits {
                lo: initial_congestion_window * max_segment_size,
                hi: max_congestion_window * max_segment_size,
            },
            initial_cwnd: initial_congestion_window * max_segment_size,
            last_sample_is_app_limited: false,
            has_non_app_limited_sample: false,
            last_quiescence_start: None,
            mss: max_segment_size,
            params,
        }
    }

    fn on_exit_quiescence(&mut self, now: Instant) {
        if let Some(last_quiescence_start) = self.last_quiescence_start.take() {
            self.mode.do_on_exit_quiescence(
                now,
                last_quiescence_start,
                &self.params,
            )
        }
    }

    fn get_target_congestion_window(&self, gain: f32) -> usize {
        let network_model = self.mode.network_model();
        network_model
            .bdp(network_model.bandwidth_estimate(), gain)
            .max(self.cwnd_limits.min())
    }

    fn update_pacing_rate(&mut self, bytes_acked: usize) {
        let network_model = self.mode.network_model();
        let bandwidth_estimate = match network_model.bandwidth_estimate() {
            e if e == Bandwidth::zero() => return,
            e => e,
        };

        if network_model.total_bytes_acked() == bytes_acked {
            // After the first ACK, cwnd is still the initial congestion window.
            self.pacing_rate = Bandwidth::from_bytes_and_time_delta(
                self.cwnd,
                network_model.min_rtt(),
            );

            if let Some(pacing_rate) =
                self.params.initial_pacing_rate_bytes_per_second
            {
                // Do not allow the pacing rate calculated from the first RTT
                // measurement to be higher than the configured initial pacing
                // rate.
                let initial_pacing_rate =
                    Bandwidth::from_bytes_per_second(pacing_rate);
                self.pacing_rate = self.pacing_rate.min(initial_pacing_rate);
            }

            return;
        }

        let target_rate = bandwidth_estimate * network_model.pacing_gain();
        if network_model.full_bandwidth_reached() {
            self.pacing_rate = target_rate;
            return;
        }

        if self.params.decrease_startup_pacing_at_end_of_round &&
            network_model.pacing_gain() < self.params.startup_pacing_gain
        {
            self.pacing_rate = target_rate;
            return;
        }

        if self.params.bw_lo_mode != BwLoMode::Default &&
            network_model.loss_events_in_round() > 0
        {
            self.pacing_rate = target_rate;
            return;
        }

        // By default, the pacing rate never decreases in STARTUP.
        self.pacing_rate = self.pacing_rate.max(target_rate);
    }

    fn update_congestion_window(&mut self, bytes_acked: usize) {
        let network_model = self.mode.network_model();
        let mut target_cwnd =
            self.get_target_congestion_window(network_model.cwnd_gain());

        let prior_cwnd = self.cwnd;
        if network_model.full_bandwidth_reached() {
            target_cwnd += network_model.max_ack_height();
            self.cwnd = target_cwnd.min(prior_cwnd + bytes_acked);
        } else if prior_cwnd < target_cwnd || prior_cwnd < 2 * self.initial_cwnd {
            self.cwnd = prior_cwnd + bytes_acked;
        }

        self.cwnd = self
            .mode
            .get_cwnd_limits(&self.params)
            .apply_limits(self.cwnd);
        self.cwnd = self.cwnd_limits.apply_limits(self.cwnd);
    }

    fn on_enter_quiescence(&mut self, time: Instant) {
        self.last_quiescence_start = Some(time);
    }

    fn target_bytes_inflight(&self) -> usize {
        let network_model = &self.mode.network_model();
        let bdp = network_model.bdp1(network_model.bandwidth_estimate());
        bdp.min(self.get_congestion_window())
    }
}

impl CongestionControl for BBRv2 {
    #[cfg(feature = "qlog")]
    fn state_str(&self) -> &'static str {
        self.mode.state_str()
    }

    fn get_congestion_window(&self) -> usize {
        self.cwnd
    }

    fn get_congestion_window_in_packets(&self) -> usize {
        self.cwnd / self.mss
    }

    fn can_send(&self, bytes_in_flight: usize) -> bool {
        bytes_in_flight < self.get_congestion_window()
    }

    fn on_packet_sent(
        &mut self, sent_time: Instant, bytes_in_flight: usize,
        packet_number: u64, bytes: usize, is_retransmissible: bool,
        rtt_stats: &RttStats,
    ) {
        if bytes_in_flight == 0 && self.params.avoid_unnecessary_probe_rtt {
            self.on_exit_quiescence(sent_time);
        }

        let network_model = self.mode.network_model_mut();
        network_model.on_packet_sent(
            sent_time,
            bytes_in_flight,
            packet_number,
            bytes,
            is_retransmissible,
            rtt_stats,
        );
    }

    fn on_congestion_event(
        &mut self, _rtt_updated: bool, prior_in_flight: usize,
        _bytes_in_flight: usize, event_time: Instant, acked_packets: &[Acked],
        lost_packets: &[Lost], least_unacked: u64, _rtt_stats: &RttStats,
        recovery_stats: &mut RecoveryStats,
    ) {
        let mut congestion_event = BBRv2CongestionEvent::new(
            event_time,
            self.cwnd,
            prior_in_flight,
            self.mode.is_probing_for_bandwidth(),
        );

        let network_model = self.mode.network_model_mut();
        network_model.on_congestion_event_start(
            acked_packets,
            lost_packets,
            &mut congestion_event,
            &self.params,
        );

        // Number of mode changes allowed for this congestion event.
        let mut mode_changes_allowed = MAX_MODE_CHANGES_PER_CONGESTION_EVENT;
        while mode_changes_allowed > 0 &&
            self.mode.do_on_congestion_event(
                prior_in_flight,
                event_time,
                acked_packets,
                lost_packets,
                &mut congestion_event,
                self.target_bytes_inflight(),
                &self.params,
                recovery_stats,
                self.get_congestion_window(),
            )
        {
            mode_changes_allowed -= 1;
        }

        self.update_pacing_rate(congestion_event.bytes_acked);

        self.update_congestion_window(congestion_event.bytes_acked);

        let network_model = self.mode.network_model_mut();
        network_model
            .on_congestion_event_finish(least_unacked, &congestion_event);
        self.last_sample_is_app_limited =
            congestion_event.last_packet_send_state.is_app_limited;
        if !self.last_sample_is_app_limited {
            self.has_non_app_limited_sample = true;
        }
        if congestion_event.bytes_in_flight == 0 &&
            self.params.avoid_unnecessary_probe_rtt
        {
            self.on_enter_quiescence(event_time);
        }
    }

    fn on_packet_neutered(&mut self, packet_number: u64) {
        let network_model = self.mode.network_model_mut();
        network_model.on_packet_neutered(packet_number);
    }

    fn on_retransmission_timeout(&mut self, _packets_retransmitted: bool) {}

    fn on_connection_migration(&mut self) {}

    fn is_in_recovery(&self) -> bool {
        // TODO(vlad): is this true?
        self.last_quiescence_start.is_none()
    }

    fn is_cwnd_limited(&self, bytes_in_flight: usize) -> bool {
        bytes_in_flight >= self.get_congestion_window()
    }

    fn pacing_rate(
        &self, _bytes_in_flight: usize, _rtt_stats: &RttStats,
    ) -> Bandwidth {
        self.pacing_rate
    }

    fn bandwidth_estimate(&self, _rtt_stats: &RttStats) -> Bandwidth {
        let network_model = self.mode.network_model();
        network_model.bandwidth_estimate()
    }

    fn max_bandwidth(&self) -> Bandwidth {
        self.mode.network_model().max_bandwidth()
    }

    fn update_mss(&mut self, new_mss: usize) {
        self.cwnd_limits.hi = (self.cwnd_limits.hi as u64 * new_mss as u64 /
            self.mss as u64) as usize;
        self.cwnd_limits.lo = (self.cwnd_limits.lo as u64 * new_mss as u64 /
            self.mss as u64) as usize;
        self.cwnd =
            (self.cwnd as u64 * new_mss as u64 / self.mss as u64) as usize;
        self.initial_cwnd = (self.initial_cwnd as u64 * new_mss as u64 /
            self.mss as u64) as usize;
        if self.params.scale_pacing_rate_by_mss {
            self.pacing_rate =
                self.pacing_rate * (new_mss as f64 / self.mss as f64);
        }
        self.mss = new_mss;
    }

    fn on_app_limited(&mut self, bytes_in_flight: usize) {
        if bytes_in_flight >= self.get_congestion_window() {
            return;
        }

        let network_model = self.mode.network_model_mut();
        network_model.on_app_limited()
    }

    fn limit_cwnd(&mut self, max_cwnd: usize) {
        self.cwnd_limits.hi = max_cwnd
    }
}

#[cfg(test)]
mod tests {
    use rstest::rstest;

    use super::*;

    #[rstest]
    fn update_mss(#[values(false, true)] scale_pacing_rate_by_mss: bool) {
        const INIT_PACKET_SIZE: usize = 1200;
        const INIT_WINDOW_PACKETS: usize = 10;
        const MAX_WINDOW_PACKETS: usize = 10000;
        const INIT_CWND: usize = INIT_WINDOW_PACKETS * INIT_PACKET_SIZE;
        const MAX_CWND: usize = MAX_WINDOW_PACKETS * INIT_PACKET_SIZE;
        let initial_rtt = Duration::from_millis(333);
        let bbr_params = &BbrParams {
            scale_pacing_rate_by_mss: Some(scale_pacing_rate_by_mss),
            ..Default::default()
        };

        const NEW_PACKET_SIZE: usize = 1450;
        const NEW_CWND: usize = INIT_WINDOW_PACKETS * NEW_PACKET_SIZE;
        const NEW_MAX_CWND: usize = MAX_WINDOW_PACKETS * NEW_PACKET_SIZE;

        let mut bbr2 = BBRv2::new(
            INIT_WINDOW_PACKETS,
            MAX_WINDOW_PACKETS,
            INIT_PACKET_SIZE,
            initial_rtt,
            Some(bbr_params),
        );

        assert_eq!(bbr2.cwnd_limits.lo, INIT_CWND);
        assert_eq!(bbr2.cwnd_limits.hi, MAX_CWND);
        assert_eq!(bbr2.cwnd, INIT_CWND);
        assert_eq!(
            bbr2.pacing_rate.to_bytes_per_period(initial_rtt),
            (2.88499 * INIT_CWND as f64) as u64
        );

        bbr2.update_mss(NEW_PACKET_SIZE);

        assert_eq!(bbr2.cwnd_limits.lo, NEW_CWND);
        assert_eq!(bbr2.cwnd_limits.hi, NEW_MAX_CWND);
        assert_eq!(bbr2.cwnd, NEW_CWND);
        let pacing_cwnd = if scale_pacing_rate_by_mss {
            NEW_CWND
        } else {
            INIT_CWND
        };
        assert_eq!(
            bbr2.pacing_rate.to_bytes_per_period(initial_rtt),
            (2.88499 * pacing_cwnd as f64) as u64
        );
    }
}