modules/remote_bitrate_estimator/congestion_control_feedback_generator.h

/*
 *  Copyright (c) 2024 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.
 */
#ifndef MODULES_REMOTE_BITRATE_ESTIMATOR_CONGESTION_CONTROL_FEEDBACK_GENERATOR_H_
#define MODULES_REMOTE_BITRATE_ESTIMATOR_CONGESTION_CONTROL_FEEDBACK_GENERATOR_H_

#include <cstdint>
#include <map>
#include <optional>

#include "api/environment/environment.h"
#include "api/sequence_checker.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 "modules/remote_bitrate_estimator/congestion_control_feedback_tracker.h"
#include "modules/remote_bitrate_estimator/rtp_transport_feedback_generator.h"
#include "modules/rtp_rtcp/source/rtp_packet_received.h"
#include "rtc_base/experiments/field_trial_parser.h"

namespace webrtc {

// The class is responsible for generating RTCP feedback packets based on
// incoming media packets. Feedback format will comply with RFC 8888.
// https://datatracker.ietf.org/doc/rfc8888/

// Min and max duration between feedback is configurable using field
// trials, but per default, min is 25ms and max is 500ms.
//
// RTCP should not use more than 5% of the uplink link capacity.
// However, there is no good way for a feedback sender to know the
// link capacity unless media is sent in both directions. So we just assume that
// the link capacity is 10 Mbit/s or more and allow sending 500 kbit/s of
// feedback packets. This allows an approximate receive rate of 200
// Mbit/s with feedback every 25ms. (200 Mbit/s with average size of 800 bytes =
// 31250 packets/s => 40 feedback packets/s with feedback of 780 packets each)

// If possible, given the other constraints, feedback will be sent when a packet
// with marker bit is received in order to provide feedback as soon as possible
// after receiving a complete video frame. If no packet with marker bit is
// received, feedback can be delayed up to 25ms after the first packet since the
// last sent feedback. On good networks, this means that a sender may receive
// feedback for every sent frame.
class CongestionControlFeedbackGenerator
    : public RtpTransportFeedbackGenerator {
 public:
  CongestionControlFeedbackGenerator(
      const Environment& env,
      RtpTransportFeedbackGenerator::RtcpSender feedback_sender);
  ~CongestionControlFeedbackGenerator() = default;

  void OnReceivedPacket(const RtpPacketReceived& packet) override;

  void OnSendBandwidthEstimateChanged(DataRate estimate) override {}

  TimeDelta Process(Timestamp now) override;

 private:
  Timestamp NextFeedbackTime() const RTC_RUN_ON(sequence_checker_);

  void SendFeedback(Timestamp now) RTC_RUN_ON(sequence_checker_);

  void CalculateNextPossibleSendTime(DataSize feedback_size, Timestamp now)
      RTC_RUN_ON(sequence_checker_);

  const Environment env_;
  SequenceChecker sequence_checker_;
  const RtcpSender rtcp_sender_;

  FieldTrialParameter<TimeDelta> min_time_between_feedback_;
  FieldTrialParameter<TimeDelta> max_time_to_wait_for_packet_with_marker_;
  FieldTrialParameter<TimeDelta> max_time_between_feedback_;

  DataSize packet_overhead_ = DataSize::Zero();
  DataSize send_rate_debt_ = DataSize::Zero();

  std::map</*ssrc=*/uint32_t, CongestionControlFeedbackTracker>
      feedback_trackers_;

  // std::vector<PacketInfo> packets_;
  Timestamp last_feedback_sent_time_ = Timestamp::Zero();
  std::optional<Timestamp> first_arrival_time_since_feedback_;
  bool marker_bit_seen_ = false;
  Timestamp next_possible_feedback_send_time_ = Timestamp::Zero();
};

}  // namespace webrtc

#endif  // MODULES_REMOTE_BITRATE_ESTIMATOR_CONGESTION_CONTROL_FEEDBACK_GENERATOR_H_