modules/congestion_controller/scream/scream_network_controller.cc

/*
 *  Copyright (c) 2025 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.
 */

#include "modules/congestion_controller/scream/scream_network_controller.h"

#include <algorithm>
#include <memory>
#include <optional>
#include <utility>

#include "api/transport/network_control.h"
#include "api/transport/network_types.h"
#include "api/units/data_rate.h"
#include "api/units/data_size.h"
#include "api/units/time_delta.h"
#include "api/units/timestamp.h"
#include "logging/rtc_event_log/events/rtc_event_remote_estimate.h"
#include "modules/congestion_controller/scream/scream_v2.h"
#include "rtc_base/checks.h"
#include "rtc_base/logging.h"
namespace webrtc {

static constexpr DataRate kDefaultStartRate = DataRate::KilobitsPerSec(300);

ScreamNetworkController::ScreamNetworkController(NetworkControllerConfig config)
    : env_(config.env),
      params_(env_.field_trials()),
      default_pacing_window_(config.default_pacing_time_window),
      allow_initial_bwe_before_media_(
          config.stream_based_config.enable_repeated_initial_probing),
      current_pacing_window_(config.default_pacing_time_window),
      scream_(std::in_place, env_),
      min_target_rate_(
          config.constraints.min_data_rate.value_or(DataRate::Zero())),
      max_target_rate_(
          config.constraints.max_data_rate.value_or(DataRate::PlusInfinity())),
      starting_rate_(
          config.constraints.starting_rate.value_or(kDefaultStartRate)),
      streams_config_(config.stream_based_config),
      max_seen_total_allocated_bitrate_(
          config.stream_based_config.max_total_allocated_bitrate.value_or(
              DataRate::Zero())) {
  UpdateScreamTargetBitrateConstraints();
}

void ScreamNetworkController::UpdateScreamTargetBitrateConstraints() {
  // TODO: bugs.webrtc.org/447037083 - We should also consider remote network
  // state estimates.
  scream_->SetTargetBitrateConstraints(
      min_target_rate_,
      std::min(max_target_rate_,
               remote_bitrate_report_.value_or(DataRate::PlusInfinity())),
      starting_rate_);
}

NetworkControlUpdate ScreamNetworkController::CreateFirstUpdate(Timestamp now) {
  RTC_DCHECK(network_available_);
  RTC_DCHECK(!first_update_created_);
  first_update_created_ = true;
  padding_interval_end_time_ = Timestamp::MinusInfinity();
  if (allow_initial_bwe_before_media_) {
    initial_bwe_probe_end_time_ = now + params_.initial_probing_duration.Get();
  }
  NetworkControlUpdate update = CreateUpdate(now);

  if (allow_initial_bwe_before_media_) {
    // Creating a probe packet allows padding packets to be sent. So this is
    // only used for triggering padding.
    update.probe_cluster_configs.emplace_back(ProbeClusterConfig{
        .at_time = now,
        .target_data_rate = DataRate::KilobitsPerSec(50),
        .target_duration = TimeDelta::Millis(1),
        .min_probe_delta = TimeDelta::Millis(10),
        // Use two probe packets even though one should be enough. This is a
        // workaround needed because the pacer will not generate or send padding
        // packets until after two probing packets.
        .target_probe_count = 2,
    });
  }
  return update;
}

NetworkControlUpdate ScreamNetworkController::OnNetworkAvailability(
    NetworkAvailability msg) {
  network_available_ = msg.network_available;
  if (!first_update_created_ && network_available_) {
    return CreateFirstUpdate(msg.at_time);
  }
  return NetworkControlUpdate();
}

NetworkControlUpdate ScreamNetworkController::OnNetworkRouteChange(
    NetworkRouteChange msg) {
  RTC_LOG(LS_INFO) << " OnNetworkRouteChange, resetting ScreamV2.";
  min_target_rate_ = msg.constraints.min_data_rate.value_or(min_target_rate_);
  max_target_rate_ = msg.constraints.max_data_rate.value_or(max_target_rate_);
  starting_rate_ = msg.constraints.starting_rate.value_or(starting_rate_);
  scream_.emplace(env_);
  first_update_created_ = false;
  UpdateScreamTargetBitrateConstraints();
  if (network_available_ &&
      streams_config_.max_total_allocated_bitrate > DataRate::Zero()) {
    return CreateFirstUpdate(msg.at_time);
  }
  return NetworkControlUpdate();
}

NetworkControlUpdate ScreamNetworkController::OnProcessInterval(
    ProcessInterval msg) {
  NetworkControlUpdate update;
  update.pacer_config = MaybeCreatePacerConfig(msg.at_time);
  return update;
}

NetworkControlUpdate ScreamNetworkController::OnRemoteBitrateReport(
    RemoteBitrateReport msg) {
  remote_bitrate_report_ = msg.bandwidth;
  UpdateScreamTargetBitrateConstraints();
  return CreateUpdate(msg.receive_time);
}

NetworkControlUpdate ScreamNetworkController::OnRoundTripTimeUpdate(
    RoundTripTimeUpdate msg) {
  // Scream uses Smoothed RTT from TransportFeedback.
  return NetworkControlUpdate();
}

NetworkControlUpdate ScreamNetworkController::OnSentPacket(SentPacket msg) {
  scream_->OnPacketSent(msg.data_in_flight);
  if (msg.data_in_flight > scream_->max_data_in_flight() ||
      scream_->delay_based_congestion_control().IsQueueDelayDetected()) {
    return CreateUpdate(msg.send_time);
  }
  return NetworkControlUpdate();
}

NetworkControlUpdate ScreamNetworkController::OnReceivedPacket(
    ReceivedPacket msg) {
  // Scream does not have to know about received packets.
  return NetworkControlUpdate();
}

NetworkControlUpdate ScreamNetworkController::OnStreamsConfig(
    StreamsConfig msg) {
  RTC_LOG_IF(LS_VERBOSE, msg.max_total_allocated_bitrate.has_value())
      << "OnStreamsConfig: max_total_allocated_bitrate="
      << *msg.max_total_allocated_bitrate;
  streams_config_ = msg;
  max_seen_total_allocated_bitrate_ = std::max(
      max_seen_total_allocated_bitrate_,
      streams_config_.max_total_allocated_bitrate.value_or(DataRate::Zero()));
  if (!first_update_created_ && network_available_ &&
      streams_config_.max_total_allocated_bitrate > DataRate::Zero()) {
    return CreateFirstUpdate(msg.at_time);
  }
  return NetworkControlUpdate();
}

NetworkControlUpdate ScreamNetworkController::OnTargetRateConstraints(
    TargetRateConstraints msg) {
  min_target_rate_ = msg.min_data_rate.value_or(min_target_rate_);
  max_target_rate_ = msg.max_data_rate.value_or(max_target_rate_);
  starting_rate_ = msg.starting_rate.value_or(starting_rate_);
  UpdateScreamTargetBitrateConstraints();
  // No need to change target rate immediately. Wait until next feedback.
  return NetworkControlUpdate();
}

NetworkControlUpdate ScreamNetworkController::OnTransportLossReport(
    TransportLossReport msg) {
  return NetworkControlUpdate();
}

NetworkControlUpdate ScreamNetworkController::OnNetworkStateEstimate(
    NetworkStateEstimate msg) {
  // TODO: bugs.webrtc.org/447037083 - Implement;
  env_.event_log().Log(std::make_unique<RtcEventRemoteEstimate>(
      msg.link_capacity_lower, msg.link_capacity_upper));

  return NetworkControlUpdate();
}

NetworkControlUpdate ScreamNetworkController::OnTransportPacketsFeedback(
    TransportPacketsFeedback msg) {
  scream_->OnTransportPacketsFeedback(msg);
  return CreateUpdate(msg.feedback_time);
}

NetworkControlUpdate ScreamNetworkController::CreateUpdate(Timestamp now) {
  NetworkControlUpdate update;
  if (scream_->target_rate() != reported_target_rate_) {
    reported_target_rate_ = scream_->target_rate();
    TargetTransferRate target_rate_msg;
    target_rate_msg.at_time = now;
    target_rate_msg.target_rate = scream_->target_rate();
    target_rate_msg.network_estimate.at_time = now;
    target_rate_msg.network_estimate.round_trip_time = scream_->rtt();
    // TODO: bugs.webrtc.org/447037083 - bwe_period must currently be set but
    // it seems like it is not used for anything sensible. Try to remove it.
    target_rate_msg.network_estimate.bwe_period = TimeDelta::Millis(25);
    update.target_rate = target_rate_msg;
  }
  update.pacer_config = MaybeCreatePacerConfig(now);
  update.congestion_window = scream_->max_data_in_flight();
  return update;
}

std::optional<PacerConfig> ScreamNetworkController::MaybeCreatePacerConfig(
    Timestamp now) {
  // Allow sending packets in larger bursts if some time has passed since last
  // congestion event.
  TimeDelta pacing_window =
      (env_.clock().CurrentTime() -
           scream_->last_reaction_to_congestion_time() >
       params_.allow_large_pacing_bursts_after_congestion_time.Get())
          ? default_pacing_window_
          : TimeDelta::Millis(10);

  bool allow_padding = false;
  if (params_.time_between_periodic_padding.Get().IsFinite() &&
      now - scream_->last_reference_window_decrease_time() >
          params_.allow_padding_after_last_congestion_time) {
    if (now < padding_interval_end_time_) {
      // We are currently inside an active padding duration.
      allow_padding = true;
    } else if (now - padding_interval_end_time_ >=
               params_.time_between_periodic_padding.Get()) {
      // Enough time has passed since the end of the last padding interval;
      // start a new one.
      padding_interval_end_time_ =
          now + params_.periodic_padding_duration.Get();
      allow_padding = true;
    }
  } else if (now < padding_interval_end_time_) {
    // Stop padding immediately if a congestion event occurred recently.
    padding_interval_end_time_ = now;
  }

  DataRate padding_rate = DataRate::Zero();
  if (allow_padding) {
    // Padding is allowed.
    DataRate max_padding_rate =
        std::min({max_target_rate_, max_seen_total_allocated_bitrate_,
                  2 * streams_config_.max_total_allocated_bitrate.value_or(
                          DataRate::Zero()),
                  remote_bitrate_report_.value_or(DataRate::PlusInfinity())});
    if (max_padding_rate.IsZero() && now < initial_bwe_probe_end_time_) {
      // If initial BWE probing is allowed, probe up to the max target rate.
      max_padding_rate = max_target_rate_;
    }
    DataRate target_rate = scream_->target_rate();
    padding_rate =
        target_rate < max_padding_rate ? target_rate : DataRate::Zero();
  }

  DataRate pacing_rate = scream_->pacing_rate();
  if (padding_rate != reported_padding_rate_ ||
      pacing_rate != reported_pacing_rate_ ||
      current_pacing_window_ != pacing_window) {
    RTC_LOG_IF(LS_VERBOSE, current_pacing_window_ != pacing_window)
        << "Pacing window changed: " << pacing_window;
    reported_padding_rate_ = padding_rate;
    reported_pacing_rate_ = pacing_rate;
    current_pacing_window_ = pacing_window;

    return PacerConfig::Create(now, pacing_rate, padding_rate,
                               current_pacing_window_);
  }
  return std::nullopt;
}

}  // namespace webrtc