src/net/quic/core/congestion_control/bbr_sender.cc

// Copyright 2016 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.

#include "net/quic/core/congestion_control/bbr_sender.h"

#include <algorithm>
#include <sstream>

#include "net/quic/core/congestion_control/rtt_stats.h"
#include "net/quic/core/crypto/crypto_protocol.h"
#include "net/quic/core/quic_flags.h"
#include "net/quic/platform/api/quic_bug_tracker.h"
#include "net/quic/platform/api/quic_flag_utils.h"
#include "net/quic/platform/api/quic_logging.h"

namespace net {

namespace {
// Constants based on TCP defaults.
const QuicByteCount kMaxSegmentSize = kDefaultTCPMSS;
// The minimum CWND to ensure delayed acks don't reduce bandwidth measurements.
// Does not inflate the pacing rate.
const QuicByteCount kMinimumCongestionWindow = 4 * kMaxSegmentSize;

// The gain used for the slow start, equal to 2/ln(2).
const float kHighGain = 2.885f;
// The gain used to drain the queue after the slow start.
const float kDrainGain = 1.f / kHighGain;
// The cycle of gains used during the PROBE_BW stage.
const float kPacingGain[] = {1.25, 0.75, 1, 1, 1, 1, 1, 1};

// The length of the gain cycle.
const size_t kGainCycleLength = sizeof(kPacingGain) / sizeof(kPacingGain[0]);
// The size of the bandwidth filter window, in round-trips.
const QuicRoundTripCount kBandwidthWindowSize = kGainCycleLength + 2;

// The time after which the current min_rtt value expires.
const QuicTime::Delta kMinRttExpiry = QuicTime::Delta::FromSeconds(10);
// The minimum time the connection can spend in PROBE_RTT mode.
const QuicTime::Delta kProbeRttTime = QuicTime::Delta::FromMilliseconds(200);

// If the bandwidth does not increase by the factor of |kStartupGrowthTarget|
// within |kRoundTripsWithoutGrowthBeforeExitingStartup| rounds, the connection
// will exit the STARTUP mode.
const float kStartupGrowthTarget = 1.25;
const QuicRoundTripCount kRoundTripsWithoutGrowthBeforeExitingStartup = 3;
}  // namespace

BbrSender::DebugState::DebugState(const BbrSender& sender)
    : mode(sender.mode_),
      max_bandwidth(sender.max_bandwidth_.GetBest()),
      round_trip_count(sender.round_trip_count_),
      gain_cycle_index(sender.cycle_current_offset_),
      congestion_window(sender.congestion_window_),
      is_at_full_bandwidth(sender.is_at_full_bandwidth_),
      bandwidth_at_last_round(sender.bandwidth_at_last_round_),
      rounds_without_bandwidth_gain(sender.rounds_without_bandwidth_gain_),
      min_rtt(sender.min_rtt_),
      min_rtt_timestamp(sender.min_rtt_timestamp_),
      recovery_state(sender.recovery_state_),
      recovery_window(sender.recovery_window_),
      last_sample_is_app_limited(sender.last_sample_is_app_limited_),
      end_of_app_limited_phase(sender.sampler_.end_of_app_limited_phase()) {}

BbrSender::DebugState::DebugState(const DebugState& state) = default;

BbrSender::BbrSender(const RttStats* rtt_stats,
                     const QuicUnackedPacketMap* unacked_packets,
                     QuicPacketCount initial_tcp_congestion_window,
                     QuicPacketCount max_tcp_congestion_window,
                     QuicRandom* random)
    : rtt_stats_(rtt_stats),
      unacked_packets_(unacked_packets),
      random_(random),
      mode_(STARTUP),
      sampler_(),
      round_trip_count_(0),
      last_sent_packet_(0),
      current_round_trip_end_(0),
      max_bandwidth_(kBandwidthWindowSize, QuicBandwidth::Zero(), 0),
      max_ack_spacing_(kBandwidthWindowSize, QuicTime::Delta::Zero(), 0),
      largest_acked_time_(QuicTime::Zero()),
      largest_acked_sent_time_(QuicTime::Zero()),
      max_ack_height_(kBandwidthWindowSize, 0, 0),
      aggregation_epoch_start_time_(QuicTime::Zero()),
      aggregation_epoch_bytes_(0),
      min_rtt_(QuicTime::Delta::Zero()),
      min_rtt_timestamp_(QuicTime::Zero()),
      congestion_window_(initial_tcp_congestion_window * kDefaultTCPMSS),
      initial_congestion_window_(initial_tcp_congestion_window *
                                 kDefaultTCPMSS),
      max_congestion_window_(max_tcp_congestion_window * kDefaultTCPMSS),
      pacing_rate_(QuicBandwidth::Zero()),
      pacing_gain_(1),
      congestion_window_gain_(1),
      congestion_window_gain_constant_(
          static_cast<float>(base::GetFlag(FLAGS_quic_bbr_cwnd_gain))),
      rtt_variance_weight_(static_cast<float>(
          base::GetFlag(FLAGS_quic_bbr_rtt_variation_weight))),
      num_startup_rtts_(kRoundTripsWithoutGrowthBeforeExitingStartup),
      cycle_current_offset_(0),
      last_cycle_start_(QuicTime::Zero()),
      is_at_full_bandwidth_(false),
      rounds_without_bandwidth_gain_(0),
      bandwidth_at_last_round_(QuicBandwidth::Zero()),
      exiting_quiescence_(false),
      exit_probe_rtt_at_(QuicTime::Zero()),
      probe_rtt_round_passed_(false),
      last_sample_is_app_limited_(false),
      recovery_state_(NOT_IN_RECOVERY),
      end_recovery_at_(0),
      recovery_window_(max_congestion_window_) {
  EnterStartupMode();
}

BbrSender::~BbrSender() {}

bool BbrSender::InSlowStart() const {
  return mode_ == STARTUP;
}

bool BbrSender::OnPacketSent(QuicTime sent_time,
                             QuicByteCount bytes_in_flight,
                             QuicPacketNumber packet_number,
                             QuicByteCount bytes,
                             HasRetransmittableData is_retransmittable) {
  last_sent_packet_ = packet_number;

  if (bytes_in_flight == 0 && sampler_.is_app_limited()) {
    exiting_quiescence_ = true;
  }

  sampler_.OnPacketSent(sent_time, packet_number, bytes, bytes_in_flight,
                        is_retransmittable);
  return is_retransmittable == HAS_RETRANSMITTABLE_DATA;
}

QuicTime::Delta BbrSender::TimeUntilSend(QuicTime /* now */,
                                         QuicByteCount bytes_in_flight) const {
  if (bytes_in_flight < GetCongestionWindow()) {
    return QuicTime::Delta::Zero();
  }
  return QuicTime::Delta::Infinite();
}

QuicBandwidth BbrSender::PacingRate(QuicByteCount /*bytes_in_flight*/) const {
  if (pacing_rate_.IsZero()) {
    return kHighGain * QuicBandwidth::FromBytesAndTimeDelta(
                           initial_congestion_window_, GetMinRtt());
  }
  return pacing_rate_;
}

QuicBandwidth BbrSender::BandwidthEstimate() const {
  return max_bandwidth_.GetBest();
}

QuicByteCount BbrSender::GetCongestionWindow() const {
  if (mode_ == PROBE_RTT) {
    return kMinimumCongestionWindow;
  }

  if (InRecovery()) {
    return std::min(congestion_window_, recovery_window_);
  }

  return congestion_window_;
}

QuicByteCount BbrSender::GetSlowStartThreshold() const {
  return 0;
}

bool BbrSender::InRecovery() const {
  return recovery_state_ != NOT_IN_RECOVERY;
}

void BbrSender::SetFromConfig(const QuicConfig& config,
                              Perspective perspective) {
  if (FLAGS_quic_reloadable_flag_quic_allow_2_rtt_bbr_startup) {
    QUIC_FLAG_COUNT(quic_reloadable_flag_quic_allow_2_rtt_bbr_startup);
    if (config.HasClientRequestedIndependentOption(k1RTT, perspective)) {
      num_startup_rtts_ = 1;
    }
    if (config.HasClientRequestedIndependentOption(k2RTT, perspective)) {
      num_startup_rtts_ = 2;
    }
  }
}

void BbrSender::OnCongestionEvent(bool /*rtt_updated*/,
                                  QuicByteCount prior_in_flight,
                                  QuicTime event_time,
                                  const CongestionVector& acked_packets,
                                  const CongestionVector& lost_packets) {
  const QuicByteCount total_bytes_acked_before = sampler_.total_bytes_acked();

  bool is_round_start = false;
  bool min_rtt_expired = false;

  DiscardLostPackets(lost_packets);

  // Input the new data into the BBR model of the connection.
  if (!acked_packets.empty()) {
    QuicPacketNumber last_acked_packet = acked_packets.rbegin()->first;
    is_round_start = UpdateRoundTripCounter(last_acked_packet);
    min_rtt_expired = UpdateBandwidthAndMinRtt(event_time, acked_packets);
    UpdateRecoveryState(last_acked_packet, !lost_packets.empty(),
                        is_round_start);
    if (FLAGS_quic_reloadable_flag_quic_bbr_ack_spacing2) {
      QUIC_FLAG_COUNT_N(quic_reloadable_flag_quic_bbr_ack_spacing2, 1, 2);
      UpdateAckSpacing(event_time, last_acked_packet, acked_packets);
    }
    if (FLAGS_quic_reloadable_flag_quic_bbr_ack_aggregation_bytes) {
      QUIC_FLAG_COUNT_N(quic_reloadable_flag_quic_bbr_ack_aggregation_bytes, 1,
                        2);
      UpdateAckAggregationBytes(
          event_time, sampler_.total_bytes_acked() - total_bytes_acked_before);
    }
  }

  // Handle logic specific to PROBE_BW mode.
  if (mode_ == PROBE_BW) {
    UpdateGainCyclePhase(event_time, prior_in_flight, !lost_packets.empty());
  }

  // Handle logic specific to STARTUP and DRAIN modes.
  if (is_round_start && !is_at_full_bandwidth_) {
    CheckIfFullBandwidthReached();
  }
  MaybeExitStartupOrDrain(event_time);

  // Handle logic specific to PROBE_RTT.
  MaybeEnterOrExitProbeRtt(event_time, is_round_start, min_rtt_expired);

  // After the model is updated, recalculate the pacing rate and congestion
  // window.
  QuicByteCount bytes_acked =
      sampler_.total_bytes_acked() - total_bytes_acked_before;
  CalculatePacingRate();
  CalculateCongestionWindow(bytes_acked);
  CalculateRecoveryWindow(bytes_acked);

  // Cleanup internal state.
  sampler_.RemoveObsoletePackets(unacked_packets_->GetLeastUnacked());
}

CongestionControlType BbrSender::GetCongestionControlType() const {
  return kBBR;
}

QuicTime::Delta BbrSender::GetMinRtt() const {
  return !min_rtt_.IsZero()
             ? min_rtt_
             : QuicTime::Delta::FromMicroseconds(rtt_stats_->initial_rtt_us());
}

QuicByteCount BbrSender::GetTargetCongestionWindow(float gain) const {
  QuicByteCount bdp = GetMinRtt() * BandwidthEstimate();
  QuicByteCount congestion_window = gain * bdp;

  // BDP estimate will be zero if no bandwidth samples are available yet.
  if (congestion_window == 0) {
    congestion_window = gain * initial_congestion_window_;
  }

  return std::max(congestion_window, kMinimumCongestionWindow);
}

void BbrSender::EnterStartupMode() {
  mode_ = STARTUP;
  pacing_gain_ = kHighGain;
  congestion_window_gain_ = kHighGain;
}

void BbrSender::EnterProbeBandwidthMode(QuicTime now) {
  mode_ = PROBE_BW;
  congestion_window_gain_ = congestion_window_gain_constant_;

  // Pick a random offset for the gain cycle out of {0, 2..7} range. 1 is
  // excluded because in that case increased gain and decreased gain would not
  // follow each other.
  cycle_current_offset_ =
      random_->RandUint64() % (sizeof(kGainCycleLength) - 1);
  if (cycle_current_offset_ >= 1) {
    cycle_current_offset_ += 1;
  }

  last_cycle_start_ = now;
  pacing_gain_ = kPacingGain[cycle_current_offset_];
}

void BbrSender::DiscardLostPackets(const CongestionVector& lost_packets) {
  for (const auto& packet : lost_packets) {
    sampler_.OnPacketLost(packet.first);
  }
}

bool BbrSender::UpdateRoundTripCounter(QuicPacketNumber last_acked_packet) {
  if (last_acked_packet > current_round_trip_end_) {
    round_trip_count_++;
    current_round_trip_end_ = last_sent_packet_;
    return true;
  }

  return false;
}

bool BbrSender::UpdateBandwidthAndMinRtt(
    QuicTime now,
    const CongestionVector& acked_packets) {
  QuicTime::Delta sample_min_rtt = QuicTime::Delta::Infinite();
  for (const auto& packet : acked_packets) {
    BandwidthSample bandwidth_sample =
        sampler_.OnPacketAcknowledged(now, packet.first);
    last_sample_is_app_limited_ = bandwidth_sample.is_app_limited;
    if (!bandwidth_sample.rtt.IsZero()) {
      sample_min_rtt = std::min(sample_min_rtt, bandwidth_sample.rtt);
    }

    if (!bandwidth_sample.is_app_limited ||
        bandwidth_sample.bandwidth > BandwidthEstimate()) {
      max_bandwidth_.Update(bandwidth_sample.bandwidth, round_trip_count_);
    }
  }

  // If none of the RTT samples are valid, return immediately.
  if (sample_min_rtt.IsInfinite()) {
    return false;
  }

  // Do not expire min_rtt if none was ever available.
  bool min_rtt_expired =
      !min_rtt_.IsZero() && (now > (min_rtt_timestamp_ + kMinRttExpiry));

  if (min_rtt_expired || sample_min_rtt < min_rtt_ || min_rtt_.IsZero()) {
    QUIC_DVLOG(2) << "Min RTT updated, old value: " << min_rtt_
                  << ", new value: " << sample_min_rtt
                  << ", current time: " << now.ToDebuggingValue();

    min_rtt_ = sample_min_rtt;
    min_rtt_timestamp_ = now;
  }

  return min_rtt_expired;
}

void BbrSender::UpdateGainCyclePhase(QuicTime now,
                                     QuicByteCount prior_in_flight,
                                     bool has_losses) {
  // In most cases, the cycle is advanced after an RTT passes.
  bool should_advance_gain_cycling = now - last_cycle_start_ > GetMinRtt();

  // If the pacing gain is above 1.0, the connection is trying to probe the
  // bandwidth by increasing the number of bytes in flight to at least
  // pacing_gain * BDP.  Make sure that it actually reaches the target, as long
  // as there are no losses suggesting that the buffers are not able to hold
  // that much.
  if (pacing_gain_ > 1.0 && !has_losses &&
      prior_in_flight < GetTargetCongestionWindow(pacing_gain_)) {
    should_advance_gain_cycling = false;
  }

  // If pacing gain is below 1.0, the connection is trying to drain the extra
  // queue which could have been incurred by probing prior to it.  If the number
  // of bytes in flight falls down to the estimated BDP value earlier, conclude
  // that the queue has been successfully drained and exit this cycle early.
  if (pacing_gain_ < 1.0 && prior_in_flight <= GetTargetCongestionWindow(1)) {
    should_advance_gain_cycling = true;
  }

  if (should_advance_gain_cycling) {
    cycle_current_offset_ = (cycle_current_offset_ + 1) % kGainCycleLength;
    last_cycle_start_ = now;
    pacing_gain_ = kPacingGain[cycle_current_offset_];
  }
}

void BbrSender::CheckIfFullBandwidthReached() {
  if (last_sample_is_app_limited_) {
    return;
  }

  QuicBandwidth target = bandwidth_at_last_round_ * kStartupGrowthTarget;
  if (BandwidthEstimate() >= target) {
    bandwidth_at_last_round_ = BandwidthEstimate();
    rounds_without_bandwidth_gain_ = 0;
    return;
  }

  rounds_without_bandwidth_gain_++;
  if (rounds_without_bandwidth_gain_ >= num_startup_rtts_) {
    is_at_full_bandwidth_ = true;
  }
}

void BbrSender::MaybeExitStartupOrDrain(QuicTime now) {
  if (mode_ == STARTUP && is_at_full_bandwidth_) {
    mode_ = DRAIN;
    pacing_gain_ = kDrainGain;
    congestion_window_gain_ = kHighGain;
  }
  if (mode_ == DRAIN &&
      unacked_packets_->bytes_in_flight() <= GetTargetCongestionWindow(1)) {
    EnterProbeBandwidthMode(now);
  }
}

void BbrSender::MaybeEnterOrExitProbeRtt(QuicTime now,
                                         bool is_round_start,
                                         bool min_rtt_expired) {
  if (min_rtt_expired && !exiting_quiescence_ && mode_ != PROBE_RTT) {
    mode_ = PROBE_RTT;
    pacing_gain_ = 1;
    // Do not decide on the time to exit PROBE_RTT until the |bytes_in_flight|
    // is at the target small value.
    exit_probe_rtt_at_ = QuicTime::Zero();
  }

  if (mode_ == PROBE_RTT) {
    sampler_.OnAppLimited();

    if (exit_probe_rtt_at_ == QuicTime::Zero()) {
      // If the window has reached the appropriate size, schedule exiting
      // PROBE_RTT.  The CWND during PROBE_RTT is kMinimumCongestionWindow, but
      // we allow an extra packet since QUIC checks CWND before sending a
      // packet.
      if (unacked_packets_->bytes_in_flight() <
          kMinimumCongestionWindow + kMaxPacketSize) {
        exit_probe_rtt_at_ = now + kProbeRttTime;
        probe_rtt_round_passed_ = false;
      }
    } else {
      if (is_round_start) {
        probe_rtt_round_passed_ = true;
      }
      if (now >= exit_probe_rtt_at_ && probe_rtt_round_passed_) {
        min_rtt_timestamp_ = now;
        if (!is_at_full_bandwidth_) {
          EnterStartupMode();
        } else {
          EnterProbeBandwidthMode(now);
        }
      }
    }
  }

  exiting_quiescence_ = false;
}

void BbrSender::UpdateRecoveryState(QuicPacketNumber last_acked_packet,
                                    bool has_losses,
                                    bool is_round_start) {
  // Exit recovery when there are no losses for a round.
  if (has_losses) {
    end_recovery_at_ = last_sent_packet_;
  }

  switch (recovery_state_) {
    case NOT_IN_RECOVERY:
      // Enter conservation on the first loss.
      if (has_losses) {
        recovery_state_ = CONSERVATION;
        // Since the conservation phase is meant to be lasting for a whole
        // round, extend the current round as if it were started right now.
        current_round_trip_end_ = last_sent_packet_;
      }
      break;

    case CONSERVATION:
      if (is_round_start) {
        recovery_state_ = GROWTH;
      }

    case GROWTH:
      // Exit recovery if appropriate.
      if (!has_losses && last_acked_packet > end_recovery_at_) {
        recovery_state_ = NOT_IN_RECOVERY;
      }
      break;
  }
}

// TODO(ianswett): Move this logic into BandwidthSampler.
void BbrSender::UpdateAckSpacing(QuicTime ack_time,
                                 QuicPacketNumber largest_newly_acked,
                                 const CongestionVector& acked_packets) {
  // Ignore acks of reordered packets.
  if (largest_newly_acked < unacked_packets_->largest_observed()) {
    return;
  }
  // Ignore acks of only one packet to filter out delayed acks.
  if (acked_packets.size() == 1) {
    return;
  }
  QuicTime largest_newly_acked_sent_time =
      unacked_packets_->GetTransmissionInfo(largest_newly_acked).sent_time;
  // Initialize on the first ack.
  if (!largest_acked_time_.IsInitialized()) {
    largest_acked_time_ = ack_time;
    largest_acked_sent_time_ = largest_newly_acked_sent_time;
    return;
  }
  QuicTime::Delta ack_delta = ack_time - largest_acked_time_;
  QuicTime::Delta send_delta =
      largest_newly_acked_sent_time - largest_acked_sent_time_;
  largest_acked_time_ = ack_time;
  largest_acked_sent_time_ = largest_newly_acked_sent_time;
  if (ack_delta <= send_delta) {
    return;
  }

  // Limit the ack spacing to SRTT to filter outliers.
  QuicTime::Delta ack_spacing =
      std::min(ack_delta - send_delta, rtt_stats_->smoothed_rtt());
  max_ack_spacing_.Update(ack_spacing, round_trip_count_);
}

// TODO(ianswett): Move this logic into BandwidthSampler.
void BbrSender::UpdateAckAggregationBytes(QuicTime ack_time,
                                          QuicByteCount newly_acked_bytes) {
  // Compute how many bytes are expected to be delivered, assuming max bandwidth
  // is correct.
  QuicByteCount expected_bytes_acked =
      max_bandwidth_.GetBest() * (ack_time - aggregation_epoch_start_time_);
  // Reset the current aggregation epoch as soon as the ack arrival rate is less
  // than or equal to the max bandwidth.
  if (aggregation_epoch_bytes_ <= expected_bytes_acked) {
    // Reset to start measuring a new aggregation epoch.
    aggregation_epoch_bytes_ = newly_acked_bytes;
    aggregation_epoch_start_time_ = ack_time;
    return;
  }

  // Compute how many extra bytes were delivered vs max bandwidth.
  // Include the bytes most recently acknowledged to account for stretch acks.
  aggregation_epoch_bytes_ += newly_acked_bytes;
  max_ack_height_.Update(aggregation_epoch_bytes_ - expected_bytes_acked,
                         round_trip_count_);
}

void BbrSender::CalculatePacingRate() {
  if (BandwidthEstimate().IsZero()) {
    return;
  }

  QuicBandwidth target_rate = pacing_gain_ * BandwidthEstimate();
  if (is_at_full_bandwidth_) {
    pacing_rate_ = target_rate;
    return;
  }

  // Pace at the rate of initial_window / RTT as soon as RTT measurements are
  // available.
  if (pacing_rate_.IsZero() && !rtt_stats_->min_rtt().IsZero()) {
    pacing_rate_ = QuicBandwidth::FromBytesAndTimeDelta(
        initial_congestion_window_, rtt_stats_->min_rtt());
    return;
  }

  // Do not decrease the pacing rate during the startup.
  pacing_rate_ = std::max(pacing_rate_, target_rate);
}

void BbrSender::CalculateCongestionWindow(QuicByteCount bytes_acked) {
  if (mode_ == PROBE_RTT) {
    return;
  }

  QuicByteCount target_window =
      GetTargetCongestionWindow(congestion_window_gain_);

  if (rtt_variance_weight_ > 0.f && !BandwidthEstimate().IsZero()) {
    target_window += rtt_variance_weight_ * rtt_stats_->mean_deviation() *
                     BandwidthEstimate();
  } else if (FLAGS_quic_reloadable_flag_quic_bbr_ack_spacing2 &&
             is_at_full_bandwidth_) {
    QUIC_FLAG_COUNT_N(quic_reloadable_flag_quic_bbr_ack_spacing2, 2, 2);
    // Add CWND for inter-ack spacing once STARTUP has been exited.
    target_window += max_ack_spacing_.GetBest() * BandwidthEstimate();
  } else if (FLAGS_quic_reloadable_flag_quic_bbr_ack_aggregation_bytes &&
             is_at_full_bandwidth_) {
    QUIC_FLAG_COUNT_N(quic_reloadable_flag_quic_bbr_ack_aggregation_bytes, 2,
                      2);
    target_window += max_ack_height_.GetBest();
  }

  // Instead of immediately setting the target CWND as the new one, BBR grows
  // the CWND towards |target_window| by only increasing it |bytes_acked| at a
  // time.
  if (is_at_full_bandwidth_) {
    congestion_window_ =
        std::min(target_window, congestion_window_ + bytes_acked);
  } else if (congestion_window_ < target_window ||
             sampler_.total_bytes_acked() < initial_congestion_window_) {
    // If the connection is not yet out of startup phase, do not decrease the
    // window.
    congestion_window_ = congestion_window_ + bytes_acked;
  }

  // Enforce the limits on the congestion window.
  congestion_window_ = std::max(congestion_window_, kMinimumCongestionWindow);
  congestion_window_ = std::min(congestion_window_, max_congestion_window_);
}

void BbrSender::CalculateRecoveryWindow(QuicByteCount bytes_acked) {
  switch (recovery_state_) {
    case CONSERVATION:
      recovery_window_ = unacked_packets_->bytes_in_flight() + bytes_acked;
      break;
    case GROWTH:
      recovery_window_ = unacked_packets_->bytes_in_flight() + 2 * bytes_acked;
      break;
    default:
      break;
  }
  recovery_window_ = std::max(kMinimumCongestionWindow, recovery_window_);
}

std::string BbrSender::GetDebugState() const {
  std::ostringstream stream;
  stream << ExportDebugState();
  return stream.str();
}

void BbrSender::OnApplicationLimited(QuicByteCount bytes_in_flight) {
  if (bytes_in_flight >= GetCongestionWindow()) {
    return;
  }

  sampler_.OnAppLimited();
  QUIC_DVLOG(2) << "Becoming application limited. Last sent packet: "
                << last_sent_packet_ << ", CWND: " << GetCongestionWindow();
}

BbrSender::DebugState BbrSender::ExportDebugState() const {
  return DebugState(*this);
}

static std::string ModeToString(BbrSender::Mode mode) {
  switch (mode) {
    case BbrSender::STARTUP:
      return "STARTUP";
    case BbrSender::DRAIN:
      return "DRAIN";
    case BbrSender::PROBE_BW:
      return "PROBE_BW";
    case BbrSender::PROBE_RTT:
      return "PROBE_RTT";
  }
  return "???";
}

std::ostream& operator<<(std::ostream& os, const BbrSender::Mode& mode) {
  os << ModeToString(mode);
  return os;
}

std::ostream& operator<<(std::ostream& os, const BbrSender::DebugState& state) {
  os << "Mode: " << ModeToString(state.mode) << std::endl;
  os << "Maximum bandwidth: " << state.max_bandwidth << std::endl;
  os << "Round trip counter: " << state.round_trip_count << std::endl;
  os << "Gain cycle index: " << static_cast<int>(state.gain_cycle_index)
     << std::endl;
  os << "Congestion window: " << state.congestion_window << " bytes"
     << std::endl;

  if (state.mode == BbrSender::STARTUP) {
    os << "(startup) Bandwidth at last round: " << state.bandwidth_at_last_round
       << std::endl;
    os << "(startup) Rounds without gain: "
       << state.rounds_without_bandwidth_gain << std::endl;
  }

  os << "Minimum RTT: " << state.min_rtt << std::endl;
  os << "Minimum RTT timestamp: " << state.min_rtt_timestamp.ToDebuggingValue()
     << std::endl;

  os << "Last sample is app-limited: "
     << (state.last_sample_is_app_limited ? "yes" : "no");

  return os;
}

}  // namespace net