crossdesk/src/qos/congestion_control.cpp

#include "congestion_control.h"

#include <algorithm>
#include <numeric>
#include <vector>

#include "log.h"

constexpr int64_t kLossUpdateInterval = 1000;

// Pacing-rate relative to our target send rate.
// Multiplicative factor that is applied to the target bitrate to calculate
// the number of bytes that can be transmitted per interval.
// Increasing this factor will result in lower delays in cases of bitrate
// overshoots from the encoder.
constexpr float kDefaultPaceMultiplier = 2.5f;

// If the probe result is far below the current throughput estimate
// it's unlikely that the probe is accurate, so we don't want to drop too far.
// However, if we actually are overusing, we want to drop to something slightly
// below the current throughput estimate to drain the network queues.
constexpr double kProbeDropThroughputFraction = 0.85;

CongestionControl::CongestionControl() {}

CongestionControl::~CongestionControl() {}

NetworkControlUpdate CongestionControl::OnTransportPacketsFeedback(
    TransportPacketsFeedback report) {
  if (report.packet_feedbacks.empty()) {
    // TODO(bugs.webrtc.org/10125): Design a better mechanism to safe-guard
    // against building very large network queues.
    return NetworkControlUpdate();
  }

  if (congestion_window_pushback_controller_) {
    congestion_window_pushback_controller_->UpdateOutstandingData(
        report.data_in_flight);
  }
  int64_t max_feedback_rtt = std::numeric_limits<int64_t>::min();
  int64_t min_propagation_rtt = std::numeric_limits<int64_t>::max();
  int64_t max_recv_time = std::numeric_limits<int64_t>::min();

  std::vector<PacketResult> feedbacks = report.ReceivedWithSendInfo();
  for (const auto& feedback : feedbacks)
    max_recv_time = std::max(max_recv_time, feedback.receive_time);

  for (const auto& feedback : feedbacks) {
    int64_t feedback_rtt =
        report.feedback_time - feedback.sent_packet.send_time;
    int64_t min_pending_time = max_recv_time - feedback.receive_time;
    int64_t propagation_rtt = feedback_rtt - min_pending_time;
    max_feedback_rtt = std::max(max_feedback_rtt, feedback_rtt);
    min_propagation_rtt = std::min(min_propagation_rtt, propagation_rtt);
  }

  if (max_feedback_rtt != std::numeric_limits<int64_t>::min() &&
      min_propagation_rtt != std::numeric_limits<int64_t>::max()) {
    feedback_max_rtts_.push_back(max_feedback_rtt);
    const size_t kMaxFeedbackRttWindow = 32;
    if (feedback_max_rtts_.size() > kMaxFeedbackRttWindow)
      feedback_max_rtts_.pop_front();
    // TODO(srte): Use time since last unacknowledged packet.
    bandwidth_estimation_->UpdatePropagationRtt(report.feedback_time,
                                                min_propagation_rtt);
  }
  if (packet_feedback_only_) {
    if (!feedback_max_rtts_.empty()) {
      int64_t sum_rtt_ms =
          std::accumulate(feedback_max_rtts_.begin(), feedback_max_rtts_.end(),
                          static_cast<int64_t>(0));
      int64_t mean_rtt_ms = sum_rtt_ms / feedback_max_rtts_.size();
      if (delay_based_bwe_) delay_based_bwe_->OnRttUpdate(mean_rtt_ms);
    }

    int64_t feedback_min_rtt = std::numeric_limits<int64_t>::max();
    for (const auto& packet_feedback : feedbacks) {
      int64_t pending_time = max_recv_time - packet_feedback.receive_time;
      int64_t rtt = report.feedback_time -
                    packet_feedback.sent_packet.send_time - pending_time;
      // Value used for predicting NACK round trip time in FEC controller.
      feedback_min_rtt = std::min(rtt, feedback_min_rtt);
    }
    if (feedback_min_rtt != std::numeric_limits<int64_t>::max() &&
        feedback_min_rtt != std::numeric_limits<int64_t>::min()) {
      bandwidth_estimation_->UpdateRtt(feedback_min_rtt, report.feedback_time);
    }

    expected_packets_since_last_loss_update_ +=
        report.PacketsWithFeedback().size();
    for (const auto& packet_feedback : report.PacketsWithFeedback()) {
      if (!packet_feedback.IsReceived())
        lost_packets_since_last_loss_update_ += 1;
    }
    if (report.feedback_time > next_loss_update_) {
      next_loss_update_ = report.feedback_time + kLossUpdateInterval;
      bandwidth_estimation_->UpdatePacketsLost(
          lost_packets_since_last_loss_update_,
          expected_packets_since_last_loss_update_, report.feedback_time);
      expected_packets_since_last_loss_update_ = 0;
      lost_packets_since_last_loss_update_ = 0;
    }
  }
  std::optional<int64_t> alr_start_time =
      alr_detector_->GetApplicationLimitedRegionStartTime();

  if (previously_in_alr_ && !alr_start_time.has_value()) {
    int64_t now_ms = report.feedback_time;
    acknowledged_bitrate_estimator_->SetAlrEndedTime(report.feedback_time);
    probe_controller_->SetAlrEndedTimeMs(now_ms);
  }
  previously_in_alr_ = alr_start_time.has_value();
  acknowledged_bitrate_estimator_->IncomingPacketFeedbackVector(
      report.SortedByReceiveTime());
  auto acknowledged_bitrate = acknowledged_bitrate_estimator_->bitrate();
  bandwidth_estimation_->SetAcknowledgedRate(acknowledged_bitrate,
                                             report.feedback_time);
  for (const auto& feedback : report.SortedByReceiveTime()) {
    if (feedback.sent_packet.pacing_info.probe_cluster_id !=
        PacedPacketInfo::kNotAProbe) {
      probe_bitrate_estimator_->HandleProbeAndEstimateBitrate(feedback);
    }
  }

  std::optional<int64_t> probe_bitrate =
      probe_bitrate_estimator_->FetchAndResetLastEstimatedBitrate();
  if (ignore_probes_lower_than_network_estimate_ && probe_bitrate &&
      estimate_ && *probe_bitrate < delay_based_bwe_->last_estimate() &&
      *probe_bitrate < estimate_->link_capacity_lower) {
    probe_bitrate.reset();
  }
  if (limit_probes_lower_than_throughput_estimate_ && probe_bitrate &&
      acknowledged_bitrate) {
    // Limit the backoff to something slightly below the acknowledged
    // bitrate. ("Slightly below" because we want to drain the queues
    // if we are actually overusing.)
    // The acknowledged bitrate shouldn't normally be higher than the delay
    // based estimate, but it could happen e.g. due to packet bursts or
    // encoder overshoot. We use std::min to ensure that a probe result
    // below the current BWE never causes an increase.
    int64_t limit =
        std::min(delay_based_bwe_->last_estimate(),
                 *acknowledged_bitrate * kProbeDropThroughputFraction);
    probe_bitrate = std::max(*probe_bitrate, limit);
  }

  NetworkControlUpdate update;
  bool recovered_from_overuse = false;

  DelayBasedBwe::Result result;
  result = delay_based_bwe_->IncomingPacketFeedbackVector(
      report, acknowledged_bitrate, probe_bitrate, estimate_,
      alr_start_time.has_value());

  if (result.updated) {
    if (result.probe) {
      bandwidth_estimation_->SetSendBitrate(result.target_bitrate,
                                            report.feedback_time);
    }
    // Since SetSendBitrate now resets the delay-based estimate, we have to
    // call UpdateDelayBasedEstimate after SetSendBitrate.
    bandwidth_estimation_->UpdateDelayBasedEstimate(report.feedback_time,
                                                    result.target_bitrate);
  }
  bandwidth_estimation_->UpdateLossBasedEstimator(
      report, result.delay_detector_state, probe_bitrate,
      alr_start_time.has_value());
  if (result.updated) {
    // Update the estimate in the ProbeController, in case we want to probe.
    MaybeTriggerOnNetworkChanged(&update, report.feedback_time);
  }

  recovered_from_overuse = result.recovered_from_overuse;

  if (recovered_from_overuse) {
    probe_controller_->SetAlrStartTimeMs(alr_start_time);
    auto probes = probe_controller_->RequestProbe(report.feedback_time);
    update.probe_cluster_configs.insert(update.probe_cluster_configs.end(),
                                        probes.begin(), probes.end());
  }

  // No valid RTT could be because send-side BWE isn't used, in which case
  // we don't try to limit the outstanding packets.
  if (rate_control_settings_.UseCongestionWindow() &&
      max_feedback_rtt.IsFinite()) {
    UpdateCongestionWindowSize();
  }
  if (congestion_window_pushback_controller_ && current_data_window_) {
    congestion_window_pushback_controller_->SetDataWindow(
        *current_data_window_);
  } else {
    update.congestion_window = current_data_window_;
  }

  return update;
}