Tweaked version of BBR for WebRTC.

webrtc/modules/remote_bitrate_estimator/test/estimators/bbr.cc

 /*
  *  Copyright (c) 2017 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  *
  */
@@ skipping 16 matching lines @@
 // suggests using this value.
 const float kHighGain = 2.885f;
 // BBR uses this value to drain queues created during STARTUP in one round trip
 // time.
 const float kDrainGain = 1 / kHighGain;
 // kStartupGrowthTarget and kMaxRoundsWithoutGrowth are chosen from
 // experiments, according to the design document.
 const float kStartupGrowthTarget = 1.25f;
 const int kMaxRoundsWithoutGrowth = 3;
 // Pacing gain values for Probe Bandwidth mode.
-const float kPacingGain[] = {1.25, 0.75, 1, 1, 1, 1, 1, 1};
+const float kPacingGain[] = {1.1, 0.9, 1, 1, 1, 1, 1, 1};
 const size_t kGainCycleLength = sizeof(kPacingGain) / sizeof(kPacingGain[0]);
 // Least number of rounds PROBE_RTT should last.
 const int kProbeRttDurationRounds = 1;
 // The least amount of milliseconds PROBE_RTT mode should last.
 const int kProbeRttDurationMs = 200;
 // Gain value for congestion window for assuming that network has no queues.
 const float kTargetCongestionWindowGain = 1;
 // Gain value for congestion window in PROBE_BW mode. In theory it should be
 // equal to 1, but in practice because of delayed acks and the way networks
 // work, it is nice to have some extra room in congestion window for full link
 // utilization. Value chosen by observations on different tests.
-const float kCruisingCongestionWindowGain = 1.5f;
+const float kCruisingCongestionWindowGain = 2;
 // Pacing gain specific for Recovery mode. Chosen by experiments in simulation
 // tool.
 const float kRecoveryPacingGain = 0.5f;
 // Congestion window gain specific for Recovery mode. Chosen by experiments in
 // simulation tool.
 const float kRecoveryCongestionWindowGain = 1.5f;
 // Number of rounds over which average RTT is stored for Recovery mode.
 const size_t kPastRttsFilterSize = 1;
 // Threshold to assume average RTT has increased for a round. Chosen by
 // experiments in simulation tool.
 const float kRttIncreaseThreshold = 3;
 // Threshold to assume average RTT has decreased for a round. Chosen by
 // experiments in simulation tool.
 const float kRttDecreaseThreshold = 1.5f;
+// If |kCongestionWindowThreshold| of the congestion window is filled up, tell
+// encoder to stop, to avoid building sender side queues.
+const float kCongestionWindowThreshold = 0.69f;
+// Duration we send at |kDefaultRatebps| in order to ensure BBR has data to work
+// with.
+const int64_t kDefaultDurationMs = 200;
+const int64_t kDefaultRatebps = 300000;
+// Congestion window gain for PROBE_RTT mode.
+const float kProbeRttCongestionWindowGain = 0.65f;
+// We need to be sure that data inflight has increased by at least
+// |kTargetCongestionWindowGainForHighGain| compared to the congestion window in
+// PROBE_BW's high gain phase, to make ramp-up quicker. As high gain value has
+// been decreased from 1.25 to 1.1 we need to make
+// |kTargetCongestionWindowGainForHighGain| slightly higher than the actual high
+// gain value.
+const float kTargetCongestionWindowGainForHighGain = 1.15f;
+// Encoder rate gain value for PROBE_RTT mode.
+const float kEncoderRateGainForProbeRtt = 0.1f;
 }  // namespace
 
-BbrBweSender::BbrBweSender(Clock* clock)
+BbrBweSender::BbrBweSender(BitrateObserver* observer, Clock* clock)
     : BweSender(0),
+      observer_(observer),
       clock_(clock),
       mode_(STARTUP),
       max_bandwidth_filter_(new MaxBandwidthFilter()),
       min_rtt_filter_(new MinRttFilter()),
       congestion_window_(new CongestionWindow()),
       rand_(new Random(time(NULL))),
       round_count_(0),
       round_trip_end_(0),
       full_bandwidth_reached_(false),
       cycle_start_time_ms_(0),
@@ skipping 28 matching lines @@
 
 int BbrBweSender::GetFeedbackIntervalMs() const {
   return kFeedbackIntervalsMs;
 }
 
 void BbrBweSender::CalculatePacingRate() {
   pacing_rate_bps_ =
       max_bandwidth_filter_->max_bandwidth_estimate_bps() * pacing_gain_;
 }
 
 void BbrBweSender::HandleLoss(uint64_t last_acked_packet,

[terelius, 2017/08/18 14:34:04]

Pls add comment about what this does.

[gnish1, 2017/08/18 14:53:50]

Done.

                               uint64_t recently_acked_packet) {
   // Logic specific to wrapping sequence numbers.
   if (!last_acked_packet)

[terelius, 2017/08/18 14:34:04]

Needed?

[gnish1, 2017/08/18 14:53:50]

Done.

     return;
   for (uint16_t i = last_acked_packet + 1;
        AheadOrAt<uint16_t>(recently_acked_packet - 1, i); i++) {
     congestion_window_->AckReceived(packet_stats_[i].payload_size_bytes);
+    observer_->OnBytesAcked(packet_stats_[i].payload_size_bytes);
   }
 }
 
 void BbrBweSender::AddToPastRtts(int64_t rtt_sample_ms) {
   uint64_t last_round = 0;
   if (!past_rtts_.empty())
     last_round = past_rtts_.back().round;
 
   // Try to add the sample to the last round.
   if (last_round == round_count_ && !past_rtts_.empty()) {
     past_rtts_.back().sum_of_rtts_ms += rtt_sample_ms;
     past_rtts_.back().num_samples++;
   } else {
     // If the sample belongs to a new round, keep number of rounds in the window
     // equal to |kPastRttsFilterSize|.
     if (past_rtts_.size() == kPastRttsFilterSize)
       past_rtts_.pop_front();
     past_rtts_.push_back(
         BbrBweSender::AverageRtt(rtt_sample_ms, 1, round_count_));
   }
 }
 
 void BbrBweSender::GiveFeedback(const FeedbackPacket& feedback) {
   int64_t now_ms = clock_->TimeInMilliseconds();
   last_packet_ack_time_ = now_ms;
   const BbrBweFeedback& fb = static_cast<const BbrBweFeedback&>(feedback);
   // feedback_vector holds values of acknowledged packets' sequence numbers.
-  const std::vector<uint64_t>& feedback_vector = fb.packet_feedback_vector();
+  const std::vector<uint16_t>& feedback_vector = fb.packet_feedback_vector();
   // Go through all the packets acked, update variables/containers accordingly.
   for (uint16_t sequence_number : feedback_vector) {
     // Completing packet information with a recently received ack.
     PacketStats* packet = &packet_stats_[sequence_number];
     bytes_acked_ += packet->payload_size_bytes;
     packet->data_sent_bytes = bytes_sent_;
     packet->last_sent_packet_send_time_ms = last_packet_send_time_;
     packet->data_acked_bytes = bytes_acked_;
     packet->ack_time_ms = now_ms;
     // Logic specific to applying "bucket" to high gain, in order to have
     // quicker ramp-up. We check if we started receiving acks for the packets
     // sent during high gain phase.
     if (packet->sequence_number == first_packet_seq_num_during_high_gain_) {
       first_packet_ack_time_during_high_gain_ms_ = now_ms;
       // Substracting half of the packet's size to avoid overestimation.
       data_acked_before_high_gain_started_bytes_ =
           bytes_acked_ - packet->payload_size_bytes / 2;
     }
     // If the last packet of high gain phase has been acked, high gain phase is
     // over.
     if (packet->sequence_number == last_packet_seq_num_during_high_gain_) {
       last_packet_ack_time_during_high_gain_ms_ = now_ms;
       data_acked_before_high_gain_ended_bytes_ =
           bytes_acked_ - packet->payload_size_bytes / 2;
       high_gain_over_ = true;
     }
-    // Notify pacer that an ack was received, to adjust data inflight.
-    // TODO(gnish): Add implementation for BitrateObserver class, to notify
-    // pacer about incoming acks.
+    observer_->OnBytesAcked(packet->payload_size_bytes);
     congestion_window_->AckReceived(packet->payload_size_bytes);
     HandleLoss(last_packet_acked_sequence_number_, packet->sequence_number);
     last_packet_acked_sequence_number_ = packet->sequence_number;
     // Logic for wrapping sequence numbers. If round started with packet number
     // x, it can never end on y, if x > y. That could happen when sequence
     // numbers are wrapped after some point.
     if (packet->sequence_number == 0)
       round_trip_end_ = 0;
   }
   // Check if new round started for the connection.
   bool new_round_started = false;
   if (!feedback_vector.empty()) {
     if (last_packet_acked_sequence_number_ > round_trip_end_) {
       new_round_started = true;
       round_count_++;
       round_trip_end_ = last_packet_sent_sequence_number_;
     }
   }
-  bool min_rtt_expired = false;
-  min_rtt_expired =
-      UpdateBandwidthAndMinRtt(now_ms, feedback_vector, bytes_acked_);
+  TryEnteringProbeRtt(now_ms);
+  UpdateBandwidthAndMinRtt(now_ms, feedback_vector, bytes_acked_);
   if (new_round_started && !full_bandwidth_reached_) {
     full_bandwidth_reached_ = max_bandwidth_filter_->FullBandwidthReached(
         kStartupGrowthTarget, kMaxRoundsWithoutGrowth);
   }
   switch (mode_) {
     break;
     case STARTUP:
       TryExitingStartup();
       break;
     case DRAIN:
       TryExitingDrain(now_ms);
       break;
     case PROBE_BW:
       TryUpdatingCyclePhase(now_ms);
       break;
     case PROBE_RTT:
       TryExitingProbeRtt(now_ms, round_count_);
       break;
     case RECOVERY:
       TryExitingRecovery(new_round_started);
       break;
   }
-  TryEnteringProbeRtt(now_ms);
   TryEnteringRecovery(new_round_started);  // Comment this line to disable
                                            // entering Recovery mode.
-  for (uint64_t f : feedback_vector)
+  for (uint16_t f : feedback_vector)
     AddToPastRtts(packet_stats_[f].ack_time_ms - packet_stats_[f].send_time_ms);
   CalculatePacingRate();
+  size_t cwnd = congestion_window_->GetCongestionWindow(
+      mode_, max_bandwidth_filter_->max_bandwidth_estimate_bps(),
+      min_rtt_filter_->min_rtt_ms(), congestion_window_gain_);
   // Make sure we don't get stuck when pacing_rate is 0, because of simulation
   // tool specifics.
   if (!pacing_rate_bps_)
     pacing_rate_bps_ = 100;
   BWE_TEST_LOGGING_PLOT(1, "SendRate", now_ms, pacing_rate_bps_ / 1000);
-  // TODO(gnish): Add implementation for BitrateObserver class to update pacing
-  // rate for the pacer and the encoder.
+  int64_t rate_for_pacer_bps = pacing_rate_bps_;
+  // Encoder producing slightly more due to rounding errors, so we ask for 2%

[terelius, 2017/08/18 14:34:03]

No longer true?

[gnish1, 2017/08/18 14:53:50]

Done.

+  // less.
+  int64_t rate_for_encoder_bps = pacing_rate_bps_;
+  if (congestion_window_->data_inflight() >= cwnd * kCongestionWindowThreshold)
+    rate_for_encoder_bps = 0;
+  // We dont completely stop sending during PROBE_RTT, so we need encoder to
+  // produce something, another way of doing this would be telling encoder to
+  // stop and send padding instead of actual data.
+  if (mode_ == PROBE_RTT)
+    rate_for_encoder_bps = rate_for_pacer_bps * kEncoderRateGainForProbeRtt;
+  // Send for 300 kbps for first 200 ms, so that BBR has data to work with.
+  if (now_ms <= kDefaultDurationMs)
+    observer_->OnNetworkChanged(
+        kDefaultRatebps, kDefaultRatebps, false,
+        clock_->TimeInMicroseconds() + kFeedbackIntervalsMs * 1000, cwnd);
+  else
+    observer_->OnNetworkChanged(
+        rate_for_encoder_bps, rate_for_pacer_bps, mode_ == PROBE_RTT,
+        clock_->TimeInMicroseconds() + kFeedbackIntervalsMs * 1000, cwnd);
 }
 
 size_t BbrBweSender::TargetCongestionWindow(float gain) {
   size_t target_congestion_window =
       congestion_window_->GetTargetCongestionWindow(
           max_bandwidth_filter_->max_bandwidth_estimate_bps(),
           min_rtt_filter_->min_rtt_ms(), gain);
   return target_congestion_window;
 }
 
 rtc::Optional<int64_t> BbrBweSender::CalculateBandwidthSample(
     size_t data_sent_bytes,
     int64_t send_time_delta_ms,
     size_t data_acked_bytes,
     int64_t ack_time_delta_ms) {
   rtc::Optional<int64_t> bandwidth_sample;
   if (send_time_delta_ms > 0)
-    *bandwidth_sample = data_sent_bytes * 8000 / send_time_delta_ms;
+    bandwidth_sample.emplace(data_sent_bytes * 8000 / send_time_delta_ms);
   rtc::Optional<int64_t> ack_rate;
   if (ack_time_delta_ms > 0)
-    *ack_rate = data_acked_bytes * 8000 / ack_time_delta_ms;
+    ack_rate.emplace(data_acked_bytes * 8000 / ack_time_delta_ms);
   // If send rate couldn't be calculated automaticaly set |bandwidth_sample| to
   // ack_rate.
   if (!bandwidth_sample)
     bandwidth_sample = ack_rate;
   if (bandwidth_sample && ack_rate)
-    *bandwidth_sample = std::min(*bandwidth_sample, *ack_rate);
+    bandwidth_sample.emplace(std::min(*bandwidth_sample, *ack_rate));
   return bandwidth_sample;
 }
 
 void BbrBweSender::AddSampleForHighGain() {
   if (!high_gain_over_)
     return;
   high_gain_over_ = false;
   // Calculate data sent/acked and time elapsed only for packets sent during
   // high gain phase.
   size_t data_sent_bytes = data_sent_before_high_gain_ended_bytes_ -
                            data_sent_before_high_gain_started_bytes_;
   int64_t send_time_delta_ms = last_packet_send_time_during_high_gain_ms_ -
                                *first_packet_send_time_during_high_gain_ms_;
   size_t data_acked_bytes = data_acked_before_high_gain_ended_bytes_ -
                             data_acked_before_high_gain_started_bytes_;
   int64_t ack_time_delta_ms = last_packet_ack_time_during_high_gain_ms_ -
                               first_packet_ack_time_during_high_gain_ms_;
   rtc::Optional<int64_t> bandwidth_sample = CalculateBandwidthSample(
       data_sent_bytes, send_time_delta_ms, data_acked_bytes, ack_time_delta_ms);
   if (bandwidth_sample)
     max_bandwidth_filter_->AddBandwidthSample(*bandwidth_sample, round_count_);
   first_packet_send_time_during_high_gain_ms_.reset();
 }
 
-bool BbrBweSender::UpdateBandwidthAndMinRtt(
+void BbrBweSender::UpdateBandwidthAndMinRtt(
     int64_t now_ms,
-    const std::vector<uint64_t>& feedback_vector,
+    const std::vector<uint16_t>& feedback_vector,
     int64_t bytes_acked) {
   rtc::Optional<int64_t> min_rtt_sample_ms;
-  for (uint64_t f : feedback_vector) {
+  for (uint16_t f : feedback_vector) {
     PacketStats packet = packet_stats_[f];
     size_t data_sent_bytes =
         packet.data_sent_bytes - packet.data_sent_before_last_sent_packet_bytes;
     int64_t send_time_delta_ms =
         packet.last_sent_packet_send_time_ms - packet.send_time_ms;
     size_t data_acked_bytes = packet.data_acked_bytes -
                               packet.data_acked_before_last_acked_packet_bytes;
     int64_t ack_time_delta_ms =
         packet.ack_time_ms - packet.last_acked_packet_ack_time_ms;
     rtc::Optional<int64_t> bandwidth_sample =
         CalculateBandwidthSample(data_sent_bytes, send_time_delta_ms,
                                  data_acked_bytes, ack_time_delta_ms);
     if (bandwidth_sample)
       max_bandwidth_filter_->AddBandwidthSample(*bandwidth_sample,
                                                 round_count_);
-    AddSampleForHighGain();  // Comment to disable bucket for high gain.
+    // AddSampleForHighGain();  // Comment to disable bucket for high gain.
     if (!min_rtt_sample_ms)
-      *min_rtt_sample_ms = packet.ack_time_ms - packet.send_time_ms;
+      min_rtt_sample_ms.emplace(packet.ack_time_ms - packet.send_time_ms);
     else
       *min_rtt_sample_ms = std::min(*min_rtt_sample_ms,
                                     packet.ack_time_ms - packet.send_time_ms);
     BWE_TEST_LOGGING_PLOT(1, "MinRtt", now_ms,
                           packet.ack_time_ms - packet.send_time_ms);
   }
-  if (!min_rtt_sample_ms)
-    return false;
-  min_rtt_filter_->AddRttSample(*min_rtt_sample_ms, now_ms);
-  bool min_rtt_expired = min_rtt_filter_->MinRttExpired(now_ms);
-  return min_rtt_expired;
+  // We only feed RTT samples into the min_rtt filter which were not produced
+  // during 1.1 gain phase, to ensure they contain no queueing delay. But if the
+  // rtt sample from 1.1 gain phase improves the current estimate then we should
+  // make it as a new best estimate.
+  if (pacing_gain_ <= 1.0f || !min_rtt_filter_->min_rtt_ms() ||
+      *min_rtt_filter_->min_rtt_ms() >= *min_rtt_sample_ms)
+    min_rtt_filter_->AddRttSample(*min_rtt_sample_ms, now_ms);
 }
 
 void BbrBweSender::EnterStartup() {
   mode_ = STARTUP;
   pacing_gain_ = kHighGain;
   congestion_window_gain_ = kHighGain;
 }
 
 void BbrBweSender::TryExitingStartup() {
   if (full_bandwidth_reached_) {
@@ skipping 25 matching lines @@
 
 void BbrBweSender::TryUpdatingCyclePhase(int64_t now_ms) {
   // Each phase should last rougly min_rtt ms time.
   bool advance_cycle_phase = false;
   if (min_rtt_filter_->min_rtt_ms())
     advance_cycle_phase =
         now_ms - cycle_start_time_ms_ > *min_rtt_filter_->min_rtt_ms();
   // If BBR was probing and it couldn't increase data inflight sufficiently in
   // one min_rtt time, continue probing. BBR design doc isn't clear about this,
   // but condition helps in quicker ramp-up and performs better.
-  if (pacing_gain_ > 1.0 && congestion_window_->data_inflight() <
-                                TargetCongestionWindow(pacing_gain_))
+  if (pacing_gain_ > 1.0 &&
+      congestion_window_->data_inflight() <
+          TargetCongestionWindow(kTargetCongestionWindowGainForHighGain))
     advance_cycle_phase = false;
   // If BBR has already drained queues there is no point in continuing draining
   // phase.
   if (pacing_gain_ < 1.0 &&
       congestion_window_->data_inflight() <= TargetCongestionWindow(1))
     advance_cycle_phase = true;
   if (advance_cycle_phase) {
     cycle_index_++;
     cycle_index_ %= kGainCycleLength;
     pacing_gain_ = kPacingGain[cycle_index_];
     cycle_start_time_ms_ = now_ms;
   }
 }
 
 void BbrBweSender::TryEnteringProbeRtt(int64_t now_ms) {
   if (min_rtt_filter_->MinRttExpired(now_ms) && mode_ != PROBE_RTT) {
     mode_ = PROBE_RTT;
     pacing_gain_ = 1;
+    congestion_window_gain_ = kProbeRttCongestionWindowGain;
     probe_rtt_start_time_ms_ = now_ms;
     minimum_congestion_window_start_time_ms_.reset();
   }
 }
 
 // |minimum_congestion_window_start_time_|'s value is set to the first moment
 // when data inflight was less then
 // |CongestionWindow::kMinimumCongestionWindowBytes|, we should make sure that
 // BBR has been in PROBE_RTT mode for at least one round or 200ms.
 void BbrBweSender::TryExitingProbeRtt(int64_t now_ms, int64_t round) {
   if (!minimum_congestion_window_start_time_ms_) {
     if (congestion_window_->data_inflight() <=
-        CongestionWindow::kMinimumCongestionWindowBytes) {
-      *minimum_congestion_window_start_time_ms_ = now_ms;
+        TargetCongestionWindow(kProbeRttCongestionWindowGain)) {
+      minimum_congestion_window_start_time_ms_.emplace(now_ms);
       minimum_congestion_window_start_round_ = round;
     }
   } else {
     if (now_ms - *minimum_congestion_window_start_time_ms_ >=
             kProbeRttDurationMs &&
         round - minimum_congestion_window_start_round_ >=
-            kProbeRttDurationRounds)
+            kProbeRttDurationRounds) {
       EnterProbeBw(now_ms);
+    }
   }
 }
 
 void BbrBweSender::TryEnteringRecovery(bool new_round_started) {
-  // If we are already in Recovery don't try to enter.
-  if (mode_ == RECOVERY || !new_round_started || !full_bandwidth_reached_)
+  if (mode_ == RECOVERY || !new_round_started || !full_bandwidth_reached_ ||
+      !min_rtt_filter_->min_rtt_ms())
     return;
   uint64_t increased_rtt_round_counter = 0;
   // If average RTT for past |kPastRttsFilterSize| rounds has been more than
   // some multiplier of min_rtt_ms enter Recovery.
   for (BbrBweSender::AverageRtt i : past_rtts_) {
     if (i.sum_of_rtts_ms / (int64_t)i.num_samples >=
         *min_rtt_filter_->min_rtt_ms() * kRttIncreaseThreshold)
       increased_rtt_round_counter++;
   }
   if (increased_rtt_round_counter < kPastRttsFilterSize)
@@ skipping 27 matching lines @@
       bytes_sent_ += media_packet->payload_size();
       PacketStats packet_stats = PacketStats(
           media_packet->sequence_number(), 0,
           media_packet->sender_timestamp_ms(), 0, last_packet_ack_time_,
           media_packet->payload_size(), 0, bytes_sent_, 0, bytes_acked_);
       packet_stats_[media_packet->sequence_number()] = packet_stats;
       last_packet_send_time_ = media_packet->sender_timestamp_ms();
       last_packet_sent_sequence_number_ = media_packet->sequence_number();
       // If this is the first packet sent for high gain phase, save data for it.
       if (!first_packet_send_time_during_high_gain_ms_ && pacing_gain_ > 1) {
-        *first_packet_send_time_during_high_gain_ms_ = last_packet_send_time_;
+        first_packet_send_time_during_high_gain_ms_.emplace(
+            last_packet_send_time_);
         data_sent_before_high_gain_started_bytes_ =
             bytes_sent_ - media_packet->payload_size() / 2;
         first_packet_seq_num_during_high_gain_ =
             media_packet->sequence_number();
       }
       // This condition ensures that |last_packet_seq_num_during_high_gain_|
       // will contain a sequence number of the last packet sent during high gain
       // phase.
       if (pacing_gain_ > 1) {
         last_packet_send_time_during_high_gain_ms_ = last_packet_send_time_;
         data_sent_before_high_gain_ended_bytes_ =
             bytes_sent_ - media_packet->payload_size() / 2;
         last_packet_seq_num_during_high_gain_ = media_packet->sequence_number();
       }
       congestion_window_->PacketSent(media_packet->payload_size());
     }
   }
 }
 
 void BbrBweSender::Process() {}
 
 BbrBweReceiver::BbrBweReceiver(int flow_id)
-    : BweReceiver(flow_id, kReceivingRateTimeWindowMs), clock_(0) {}
+    : BweReceiver(flow_id, kReceivingRateTimeWindowMs),
+      clock_(0),
+      packet_feedbacks_(),
+      last_feedback_ms_(0) {}
 
 BbrBweReceiver::~BbrBweReceiver() {}
 
 void BbrBweReceiver::ReceivePacket(int64_t arrival_time_ms,
-                                   const MediaPacket& media_packet) {}
+                                   const MediaPacket& media_packet) {
+  packet_feedbacks_.push_back(media_packet.sequence_number());
+  BweReceiver::ReceivePacket(arrival_time_ms, media_packet);
+}
 
 FeedbackPacket* BbrBweReceiver::GetFeedback(int64_t now_ms) {
-  return nullptr;
+  last_feedback_ms_ = now_ms;
+  int64_t corrected_send_time_ms = 0L;
+  if (!received_packets_.empty()) {
+    PacketIdentifierNode* latest = *(received_packets_.begin());
+    corrected_send_time_ms =
+        latest->send_time_ms + now_ms - latest->arrival_time_ms;
+  }
+  FeedbackPacket* fb = new BbrBweFeedback(
+      flow_id_, now_ms * 1000, corrected_send_time_ms, packet_feedbacks_);
+  packet_feedbacks_.clear();
+  return fb;
 }
 }  // namespace bwe
 }  // namespace testing
 }  // namespace webrtc