modules/remote_bitrate_estimator/remote_estimator_proxy.cc - src - Git at Google

 /*
  *  Copyright (c) 2015 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/remote_bitrate_estimator/remote_estimator_proxy.h"

 #include <algorithm>
 #include <limits>
 #include <memory>
 #include <utility>

 #include "api/units/data_size.h"
 #include "modules/rtp_rtcp/source/rtcp_packet/remote_estimate.h"
 #include "modules/rtp_rtcp/source/rtcp_packet/transport_feedback.h"
 #include "rtc_base/checks.h"
 #include "rtc_base/logging.h"
 #include "rtc_base/numerics/safe_minmax.h"
 #include "system_wrappers/include/clock.h"

 namespace webrtc {
 namespace {
 // The maximum allowed value for a timestamp in milliseconds. This is lower
 // than the numerical limit since we often convert to microseconds.
 constexpr int64_t kMaxTimeMs = std::numeric_limits<int64_t>::max() / 1000;
 constexpr TimeDelta kBackWindow = TimeDelta::Millis(500);
 constexpr TimeDelta kMinInterval = TimeDelta::Millis(50);
 constexpr TimeDelta kMaxInterval = TimeDelta::Millis(250);
 constexpr TimeDelta kDefaultInterval = TimeDelta::Millis(100);

 TimeDelta GetAbsoluteSendTimeDelta(uint32_t new_sendtime,
                                    uint32_t previous_sendtime) {
   static constexpr uint32_t kWrapAroundPeriod = 0x0100'0000;
   RTC_DCHECK_LT(new_sendtime, kWrapAroundPeriod);
   RTC_DCHECK_LT(previous_sendtime, kWrapAroundPeriod);
   uint32_t delta = (new_sendtime - previous_sendtime) % kWrapAroundPeriod;
   if (delta >= kWrapAroundPeriod / 2) {
     // absolute send time wraps around, thus treat deltas larger than half of
     // the wrap around period as negative. Ignore reordering of packets and
     // treat them as they have approximately the same send time.
     return TimeDelta::Zero();
   }
   return TimeDelta::Micros(int64_t{delta} * 1'000'000 / (1 << 18));
 }
 }  // namespace

 RemoteEstimatorProxy::RemoteEstimatorProxy(
     TransportFeedbackSender feedback_sender,
     NetworkStateEstimator* network_state_estimator)
     : feedback_sender_(std::move(feedback_sender)),
       last_process_time_(Timestamp::MinusInfinity()),
       network_state_estimator_(network_state_estimator),
       media_ssrc_(0),
       feedback_packet_count_(0),
       packet_overhead_(DataSize::Zero()),
       send_interval_(kDefaultInterval),
       send_periodic_feedback_(true),
       previous_abs_send_time_(0),
       abs_send_timestamp_(Timestamp::Zero()) {
   RTC_LOG(LS_INFO)
       << "Maximum interval between transport feedback RTCP messages: "
       << kMaxInterval;
 }

 RemoteEstimatorProxy::~RemoteEstimatorProxy() {}

 void RemoteEstimatorProxy::MaybeCullOldPackets(int64_t sequence_number,
                                                Timestamp arrival_time) {
   if (periodic_window_start_seq_ >=
           packet_arrival_times_.end_sequence_number() &&
       arrival_time - Timestamp::Zero() >= kBackWindow) {
     // Start new feedback packet, cull old packets.
     packet_arrival_times_.RemoveOldPackets(sequence_number,
                                            arrival_time - kBackWindow);
   }
 }

 void RemoteEstimatorProxy::IncomingPacket(int64_t arrival_time_ms,
                                           size_t payload_size,
                                           const RTPHeader& header) {
   if (arrival_time_ms < 0 || arrival_time_ms >= kMaxTimeMs) {
     RTC_LOG(LS_WARNING) << "Arrival time out of bounds: " << arrival_time_ms;
     return;
   }
   Packet packet = {.arrival_time = Timestamp::Millis(arrival_time_ms),
                    .size = DataSize::Bytes(header.headerLength + payload_size),
                    .ssrc = header.ssrc};
   if (header.extension.hasTransportSequenceNumber) {
     packet.transport_sequence_number = header.extension.transportSequenceNumber;
   }
   if (header.extension.hasAbsoluteSendTime) {
     packet.absolute_send_time_24bits = header.extension.absoluteSendTime;
   }
   packet.feedback_request = header.extension.feedback_request;

   IncomingPacket(packet);
 }

 void RemoteEstimatorProxy::IncomingPacket(Packet packet) {
   MutexLock lock(&lock_);
   media_ssrc_ = packet.ssrc;
   int64_t seq = 0;

   if (packet.transport_sequence_number.has_value()) {
     seq = unwrapper_.Unwrap(*packet.transport_sequence_number);

     if (send_periodic_feedback_) {
       MaybeCullOldPackets(seq, packet.arrival_time);

       if (!periodic_window_start_seq_ || seq < *periodic_window_start_seq_) {
         periodic_window_start_seq_ = seq;
       }
     }

     // We are only interested in the first time a packet is received.
     if (packet_arrival_times_.has_received(seq)) {
       return;
     }

     packet_arrival_times_.AddPacket(seq, packet.arrival_time);

     // Limit the range of sequence numbers to send feedback for.
     if (!periodic_window_start_seq_.has_value() ||
         periodic_window_start_seq_.value() <
             packet_arrival_times_.begin_sequence_number()) {
       periodic_window_start_seq_ =
           packet_arrival_times_.begin_sequence_number();
     }

     if (packet.feedback_request) {
       // Send feedback packet immediately.
       SendFeedbackOnRequest(seq, *packet.feedback_request);
     }
   }

   if (network_state_estimator_ && packet.absolute_send_time_24bits) {
     PacketResult packet_result;
     packet_result.receive_time = packet.arrival_time;
     abs_send_timestamp_ += GetAbsoluteSendTimeDelta(
         *packet.absolute_send_time_24bits, previous_abs_send_time_);
     previous_abs_send_time_ = *packet.absolute_send_time_24bits;
     packet_result.sent_packet.send_time = abs_send_timestamp_;
     packet_result.sent_packet.size = packet.size + packet_overhead_;
     packet_result.sent_packet.sequence_number = seq;
     network_state_estimator_->OnReceivedPacket(packet_result);
   }
 }

 TimeDelta RemoteEstimatorProxy::Process(Timestamp now) {
   MutexLock lock(&lock_);
   if (!send_periodic_feedback_) {
     return TimeDelta::PlusInfinity();
   }
   Timestamp next_process_time = last_process_time_ + send_interval_;
   if (now >= next_process_time) {
     last_process_time_ = now;
     SendPeriodicFeedbacks();
     return send_interval_;
   }

   return next_process_time - now;
 }

 void RemoteEstimatorProxy::OnBitrateChanged(int bitrate_bps) {
   // TwccReportSize = Ipv4(20B) + UDP(8B) + SRTP(10B) +
   // AverageTwccReport(30B)
   // TwccReport size at 50ms interval is 24 byte.
   // TwccReport size at 250ms interval is 36 byte.
   // AverageTwccReport = (TwccReport(50ms) + TwccReport(250ms)) / 2
   constexpr DataSize kTwccReportSize = DataSize::Bytes(20 + 8 + 10 + 30);
   constexpr DataRate kMinTwccRate = kTwccReportSize / kMaxInterval;

   // Let TWCC reports occupy 5% of total bandwidth.
   DataRate twcc_bitrate = DataRate::BitsPerSec(0.05 * bitrate_bps);

   // Check upper send_interval bound by checking bitrate to avoid overflow when
   // dividing by small bitrate, in particular avoid dividing by zero bitrate.
   TimeDelta send_interval =
       twcc_bitrate <= kMinTwccRate
           ? kMaxInterval
           : std::max(kTwccReportSize / twcc_bitrate, kMinInterval);

   MutexLock lock(&lock_);
   send_interval_ = send_interval;
 }

 void RemoteEstimatorProxy::SetSendPeriodicFeedback(
     bool send_periodic_feedback) {
   MutexLock lock(&lock_);
   send_periodic_feedback_ = send_periodic_feedback;
 }

 void RemoteEstimatorProxy::SetTransportOverhead(DataSize overhead_per_packet) {
   MutexLock lock(&lock_);
   packet_overhead_ = overhead_per_packet;
 }

 void RemoteEstimatorProxy::SendPeriodicFeedbacks() {
   // `periodic_window_start_seq_` is the first sequence number to include in
   // the current feedback packet. Some older may still be in the map, in case
   // a reordering happens and we need to retransmit them.
   if (!periodic_window_start_seq_)
     return;

   std::unique_ptr<rtcp::RemoteEstimate> remote_estimate;
   if (network_state_estimator_) {
     absl::optional<NetworkStateEstimate> state_estimate =
         network_state_estimator_->GetCurrentEstimate();
     if (state_estimate) {
       remote_estimate = std::make_unique<rtcp::RemoteEstimate>();
       remote_estimate->SetEstimate(state_estimate.value());
     }
   }

   int64_t packet_arrival_times_end_seq =
       packet_arrival_times_.end_sequence_number();
   while (periodic_window_start_seq_ < packet_arrival_times_end_seq) {
     auto feedback_packet = MaybeBuildFeedbackPacket(
         /*include_timestamps=*/true, periodic_window_start_seq_.value(),
         packet_arrival_times_end_seq,
         /*is_periodic_update=*/true);

     if (feedback_packet == nullptr) {
       break;
     }

     RTC_DCHECK(feedback_sender_ != nullptr);

     std::vector<std::unique_ptr<rtcp::RtcpPacket>> packets;
     if (remote_estimate) {
       packets.push_back(std::move(remote_estimate));
     }
     packets.push_back(std::move(feedback_packet));

     feedback_sender_(std::move(packets));
     // Note: Don't erase items from packet_arrival_times_ after sending, in
     // case they need to be re-sent after a reordering. Removal will be
     // handled by OnPacketArrival once packets are too old.
   }
 }

 void RemoteEstimatorProxy::SendFeedbackOnRequest(
     int64_t sequence_number,
     const FeedbackRequest& feedback_request) {
   if (feedback_request.sequence_count == 0) {
     return;
   }

   int64_t first_sequence_number =
       sequence_number - feedback_request.sequence_count + 1;

   auto feedback_packet = MaybeBuildFeedbackPacket(
       feedback_request.include_timestamps, first_sequence_number,
       sequence_number + 1, /*is_periodic_update=*/false);

   // This is called when a packet has just been added.
   RTC_DCHECK(feedback_packet != nullptr);

   // Clear up to the first packet that is included in this feedback packet.
   packet_arrival_times_.EraseTo(first_sequence_number);

   RTC_DCHECK(feedback_sender_ != nullptr);
   std::vector<std::unique_ptr<rtcp::RtcpPacket>> packets;
   packets.push_back(std::move(feedback_packet));
   feedback_sender_(std::move(packets));
 }

 std::unique_ptr<rtcp::TransportFeedback>
 RemoteEstimatorProxy::MaybeBuildFeedbackPacket(
     bool include_timestamps,
     int64_t begin_sequence_number_inclusive,
     int64_t end_sequence_number_exclusive,
     bool is_periodic_update) {
   RTC_DCHECK_LT(begin_sequence_number_inclusive, end_sequence_number_exclusive);

   int64_t start_seq =
       packet_arrival_times_.clamp(begin_sequence_number_inclusive);

   int64_t end_seq = packet_arrival_times_.clamp(end_sequence_number_exclusive);

   // Create the packet on demand, as it's not certain that there are packets
   // in the range that have been received.
   std::unique_ptr<rtcp::TransportFeedback> feedback_packet = nullptr;

   int64_t next_sequence_number = begin_sequence_number_inclusive;

   for (int64_t seq = start_seq; seq < end_seq; ++seq) {
     PacketArrivalTimeMap::PacketArrivalTime packet =
         packet_arrival_times_.FindNextAtOrAfter(seq);
     seq = packet.sequence_number;
     if (seq >= end_seq) {
       break;
     }

     if (feedback_packet == nullptr) {
       feedback_packet =
           std::make_unique<rtcp::TransportFeedback>(include_timestamps);
       feedback_packet->SetMediaSsrc(media_ssrc_);
       // Base sequence number is the expected first sequence number. This is
       // known, but we might not have actually received it, so the base time
       // shall be the time of the first received packet in the feedback.
       feedback_packet->SetBase(
           static_cast<uint16_t>(begin_sequence_number_inclusive & 0xFFFF),
           packet.arrival_time);
       feedback_packet->SetFeedbackSequenceNumber(feedback_packet_count_++);
     }

     if (!feedback_packet->AddReceivedPacket(static_cast<uint16_t>(seq & 0xFFFF),
                                             packet.arrival_time)) {
       // Could not add timestamp, feedback packet might be full. Return and
       // try again with a fresh packet.
       break;
     }

     next_sequence_number = seq + 1;
   }
   if (is_periodic_update) {
     periodic_window_start_seq_ = next_sequence_number;
   }
   return feedback_packet;
 }

 }  // namespace webrtc
	/*
	* Copyright (c) 2015 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/remote_bitrate_estimator/remote_estimator_proxy.h"

	#include <algorithm>
	#include <limits>
	#include <memory>
	#include <utility>

	#include "api/units/data_size.h"
	#include "modules/rtp_rtcp/source/rtcp_packet/remote_estimate.h"
	#include "modules/rtp_rtcp/source/rtcp_packet/transport_feedback.h"
	#include "rtc_base/checks.h"
	#include "rtc_base/logging.h"
	#include "rtc_base/numerics/safe_minmax.h"
	#include "system_wrappers/include/clock.h"

	namespace webrtc {
	namespace {
	// The maximum allowed value for a timestamp in milliseconds. This is lower
	// than the numerical limit since we often convert to microseconds.
	constexpr int64_t kMaxTimeMs = std::numeric_limits<int64_t>::max() / 1000;
	constexpr TimeDelta kBackWindow = TimeDelta::Millis(500);
	constexpr TimeDelta kMinInterval = TimeDelta::Millis(50);
	constexpr TimeDelta kMaxInterval = TimeDelta::Millis(250);
	constexpr TimeDelta kDefaultInterval = TimeDelta::Millis(100);

	TimeDelta GetAbsoluteSendTimeDelta(uint32_t new_sendtime,
	uint32_t previous_sendtime) {
	static constexpr uint32_t kWrapAroundPeriod = 0x0100'0000;
	RTC_DCHECK_LT(new_sendtime, kWrapAroundPeriod);
	RTC_DCHECK_LT(previous_sendtime, kWrapAroundPeriod);
	uint32_t delta = (new_sendtime - previous_sendtime) % kWrapAroundPeriod;
	if (delta >= kWrapAroundPeriod / 2) {
	// absolute send time wraps around, thus treat deltas larger than half of
	// the wrap around period as negative. Ignore reordering of packets and
	// treat them as they have approximately the same send time.
	return TimeDelta::Zero();
	}
	return TimeDelta::Micros(int64_t{delta} * 1'000'000 / (1 << 18));
	}
	} // namespace

	RemoteEstimatorProxy::RemoteEstimatorProxy(
	TransportFeedbackSender feedback_sender,
	NetworkStateEstimator* network_state_estimator)
	: feedback_sender_(std::move(feedback_sender)),
	last_process_time_(Timestamp::MinusInfinity()),
	network_state_estimator_(network_state_estimator),
	media_ssrc_(0),
	feedback_packet_count_(0),
	packet_overhead_(DataSize::Zero()),
	send_interval_(kDefaultInterval),
	send_periodic_feedback_(true),
	previous_abs_send_time_(0),
	abs_send_timestamp_(Timestamp::Zero()) {
	RTC_LOG(LS_INFO)
	<< "Maximum interval between transport feedback RTCP messages: "
	<< kMaxInterval;
	}

	RemoteEstimatorProxy::~RemoteEstimatorProxy() {}

	void RemoteEstimatorProxy::MaybeCullOldPackets(int64_t sequence_number,
	Timestamp arrival_time) {
	if (periodic_window_start_seq_ >=
	packet_arrival_times_.end_sequence_number() &&
	arrival_time - Timestamp::Zero() >= kBackWindow) {
	// Start new feedback packet, cull old packets.
	packet_arrival_times_.RemoveOldPackets(sequence_number,
	arrival_time - kBackWindow);
	}
	}

	void RemoteEstimatorProxy::IncomingPacket(int64_t arrival_time_ms,
	size_t payload_size,
	const RTPHeader& header) {
	if (arrival_time_ms < 0 \|\| arrival_time_ms >= kMaxTimeMs) {
	RTC_LOG(LS_WARNING) << "Arrival time out of bounds: " << arrival_time_ms;
	return;
	}
	Packet packet = {.arrival_time = Timestamp::Millis(arrival_time_ms),
	.size = DataSize::Bytes(header.headerLength + payload_size),
	.ssrc = header.ssrc};
	if (header.extension.hasTransportSequenceNumber) {
	packet.transport_sequence_number = header.extension.transportSequenceNumber;
	}
	if (header.extension.hasAbsoluteSendTime) {
	packet.absolute_send_time_24bits = header.extension.absoluteSendTime;
	}
	packet.feedback_request = header.extension.feedback_request;

	IncomingPacket(packet);
	}

	void RemoteEstimatorProxy::IncomingPacket(Packet packet) {
	MutexLock lock(&lock_);
	media_ssrc_ = packet.ssrc;
	int64_t seq = 0;

	if (packet.transport_sequence_number.has_value()) {
	seq = unwrapper_.Unwrap(*packet.transport_sequence_number);

	if (send_periodic_feedback_) {
	MaybeCullOldPackets(seq, packet.arrival_time);

	if (!periodic_window_start_seq_ \|\| seq < *periodic_window_start_seq_) {
	periodic_window_start_seq_ = seq;
	}
	}

	// We are only interested in the first time a packet is received.
	if (packet_arrival_times_.has_received(seq)) {
	return;
	}

	packet_arrival_times_.AddPacket(seq, packet.arrival_time);

	// Limit the range of sequence numbers to send feedback for.
	if (!periodic_window_start_seq_.has_value() \|\|
	periodic_window_start_seq_.value() <
	packet_arrival_times_.begin_sequence_number()) {
	periodic_window_start_seq_ =
	packet_arrival_times_.begin_sequence_number();
	}

	if (packet.feedback_request) {
	// Send feedback packet immediately.
	SendFeedbackOnRequest(seq, *packet.feedback_request);
	}
	}

	if (network_state_estimator_ && packet.absolute_send_time_24bits) {
	PacketResult packet_result;
	packet_result.receive_time = packet.arrival_time;
	abs_send_timestamp_ += GetAbsoluteSendTimeDelta(
	*packet.absolute_send_time_24bits, previous_abs_send_time_);
	previous_abs_send_time_ = *packet.absolute_send_time_24bits;
	packet_result.sent_packet.send_time = abs_send_timestamp_;
	packet_result.sent_packet.size = packet.size + packet_overhead_;
	packet_result.sent_packet.sequence_number = seq;
	network_state_estimator_->OnReceivedPacket(packet_result);
	}
	}

	TimeDelta RemoteEstimatorProxy::Process(Timestamp now) {
	MutexLock lock(&lock_);
	if (!send_periodic_feedback_) {
	return TimeDelta::PlusInfinity();
	}
	Timestamp next_process_time = last_process_time_ + send_interval_;
	if (now >= next_process_time) {
	last_process_time_ = now;
	SendPeriodicFeedbacks();
	return send_interval_;
	}

	return next_process_time - now;
	}

	void RemoteEstimatorProxy::OnBitrateChanged(int bitrate_bps) {
	// TwccReportSize = Ipv4(20B) + UDP(8B) + SRTP(10B) +
	// AverageTwccReport(30B)
	// TwccReport size at 50ms interval is 24 byte.
	// TwccReport size at 250ms interval is 36 byte.
	// AverageTwccReport = (TwccReport(50ms) + TwccReport(250ms)) / 2
	constexpr DataSize kTwccReportSize = DataSize::Bytes(20 + 8 + 10 + 30);
	constexpr DataRate kMinTwccRate = kTwccReportSize / kMaxInterval;

	// Let TWCC reports occupy 5% of total bandwidth.
	DataRate twcc_bitrate = DataRate::BitsPerSec(0.05 * bitrate_bps);

	// Check upper send_interval bound by checking bitrate to avoid overflow when
	// dividing by small bitrate, in particular avoid dividing by zero bitrate.
	TimeDelta send_interval =
	twcc_bitrate <= kMinTwccRate
	? kMaxInterval
	: std::max(kTwccReportSize / twcc_bitrate, kMinInterval);

	MutexLock lock(&lock_);
	send_interval_ = send_interval;
	}

	void RemoteEstimatorProxy::SetSendPeriodicFeedback(
	bool send_periodic_feedback) {
	MutexLock lock(&lock_);
	send_periodic_feedback_ = send_periodic_feedback;
	}

	void RemoteEstimatorProxy::SetTransportOverhead(DataSize overhead_per_packet) {
	MutexLock lock(&lock_);
	packet_overhead_ = overhead_per_packet;
	}

	void RemoteEstimatorProxy::SendPeriodicFeedbacks() {
	// `periodic_window_start_seq_` is the first sequence number to include in
	// the current feedback packet. Some older may still be in the map, in case
	// a reordering happens and we need to retransmit them.
	if (!periodic_window_start_seq_)
	return;

	std::unique_ptr<rtcp::RemoteEstimate> remote_estimate;
	if (network_state_estimator_) {
	absl::optional<NetworkStateEstimate> state_estimate =
	network_state_estimator_->GetCurrentEstimate();
	if (state_estimate) {
	remote_estimate = std::make_unique<rtcp::RemoteEstimate>();
	remote_estimate->SetEstimate(state_estimate.value());
	}
	}

	int64_t packet_arrival_times_end_seq =
	packet_arrival_times_.end_sequence_number();
	while (periodic_window_start_seq_ < packet_arrival_times_end_seq) {
	auto feedback_packet = MaybeBuildFeedbackPacket(
	/include_timestamps=/true, periodic_window_start_seq_.value(),
	packet_arrival_times_end_seq,
	/is_periodic_update=/true);

	if (feedback_packet == nullptr) {
	break;
	}

	RTC_DCHECK(feedback_sender_ != nullptr);

	std::vector<std::unique_ptr<rtcp::RtcpPacket>> packets;
	if (remote_estimate) {
	packets.push_back(std::move(remote_estimate));
	}
	packets.push_back(std::move(feedback_packet));

	feedback_sender_(std::move(packets));
	// Note: Don't erase items from packet_arrival_times_ after sending, in
	// case they need to be re-sent after a reordering. Removal will be
	// handled by OnPacketArrival once packets are too old.
	}
	}

	void RemoteEstimatorProxy::SendFeedbackOnRequest(
	int64_t sequence_number,
	const FeedbackRequest& feedback_request) {
	if (feedback_request.sequence_count == 0) {
	return;
	}

	int64_t first_sequence_number =
	sequence_number - feedback_request.sequence_count + 1;

	auto feedback_packet = MaybeBuildFeedbackPacket(
	feedback_request.include_timestamps, first_sequence_number,
	sequence_number + 1, /is_periodic_update=/false);

	// This is called when a packet has just been added.
	RTC_DCHECK(feedback_packet != nullptr);

	// Clear up to the first packet that is included in this feedback packet.
	packet_arrival_times_.EraseTo(first_sequence_number);

	RTC_DCHECK(feedback_sender_ != nullptr);
	std::vector<std::unique_ptr<rtcp::RtcpPacket>> packets;
	packets.push_back(std::move(feedback_packet));
	feedback_sender_(std::move(packets));
	}

	std::unique_ptr<rtcp::TransportFeedback>
	RemoteEstimatorProxy::MaybeBuildFeedbackPacket(
	bool include_timestamps,
	int64_t begin_sequence_number_inclusive,
	int64_t end_sequence_number_exclusive,
	bool is_periodic_update) {
	RTC_DCHECK_LT(begin_sequence_number_inclusive, end_sequence_number_exclusive);

	int64_t start_seq =
	packet_arrival_times_.clamp(begin_sequence_number_inclusive);

	int64_t end_seq = packet_arrival_times_.clamp(end_sequence_number_exclusive);

	// Create the packet on demand, as it's not certain that there are packets
	// in the range that have been received.
	std::unique_ptr<rtcp::TransportFeedback> feedback_packet = nullptr;

	int64_t next_sequence_number = begin_sequence_number_inclusive;

	for (int64_t seq = start_seq; seq < end_seq; ++seq) {
	PacketArrivalTimeMap::PacketArrivalTime packet =
	packet_arrival_times_.FindNextAtOrAfter(seq);
	seq = packet.sequence_number;
	if (seq >= end_seq) {
	break;
	}

	if (feedback_packet == nullptr) {
	feedback_packet =
	std::make_unique<rtcp::TransportFeedback>(include_timestamps);
	feedback_packet->SetMediaSsrc(media_ssrc_);
	// Base sequence number is the expected first sequence number. This is
	// known, but we might not have actually received it, so the base time
	// shall be the time of the first received packet in the feedback.
	feedback_packet->SetBase(
	static_cast<uint16_t>(begin_sequence_number_inclusive & 0xFFFF),
	packet.arrival_time);
	feedback_packet->SetFeedbackSequenceNumber(feedback_packet_count_++);
	}

	if (!feedback_packet->AddReceivedPacket(static_cast<uint16_t>(seq & 0xFFFF),
	packet.arrival_time)) {
	// Could not add timestamp, feedback packet might be full. Return and
	// try again with a fresh packet.
	break;
	}

	next_sequence_number = seq + 1;
	}
	if (is_periodic_update) {
	periodic_window_start_seq_ = next_sequence_number;
	}
	return feedback_packet;
	}

	} // namespace webrtc