audio/transport_feedback_packet_loss_tracker.cc - src - Git at Google

 /*
  *  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.
  */

 #include "audio/transport_feedback_packet_loss_tracker.h"

 #include <limits>
 #include <utility>

 #include "modules/rtp_rtcp/include/rtp_rtcp_defines.h"
 #include "modules/rtp_rtcp/source/rtcp_packet/transport_feedback.h"
 #include "rtc_base/checks.h"
 #include "rtc_base/numerics/mod_ops.h"

 namespace {
 constexpr uint16_t kSeqNumHalf = 0x8000u;
 void UpdateCounter(size_t* counter, bool increment) {
   if (increment) {
     RTC_DCHECK_LT(*counter, std::numeric_limits<std::size_t>::max());
     ++(*counter);
   } else {
     RTC_DCHECK_GT(*counter, 0);
     --(*counter);
   }
 }
 }  // namespace

 namespace webrtc {

 TransportFeedbackPacketLossTracker::TransportFeedbackPacketLossTracker(
     int64_t max_window_size_ms,
     size_t plr_min_num_acked_packets,
     size_t rplr_min_num_acked_pairs)
     : max_window_size_ms_(max_window_size_ms),
       ref_packet_status_(packet_status_window_.begin()),
       plr_state_(plr_min_num_acked_packets),
       rplr_state_(rplr_min_num_acked_pairs) {
   RTC_DCHECK_GT(max_window_size_ms, 0);
   RTC_DCHECK_GT(plr_min_num_acked_packets, 0);
   RTC_DCHECK_GT(rplr_min_num_acked_pairs, 0);
   Reset();
 }

 void TransportFeedbackPacketLossTracker::Reset() {
   acked_packets_ = 0;
   plr_state_.Reset();
   rplr_state_.Reset();
   packet_status_window_.clear();
   ref_packet_status_ = packet_status_window_.begin();
 }

 uint16_t TransportFeedbackPacketLossTracker::ReferenceSequenceNumber() const {
   RTC_DCHECK(!packet_status_window_.empty());
   return ref_packet_status_->first;
 }

 uint16_t TransportFeedbackPacketLossTracker::NewestSequenceNumber() const {
   RTC_DCHECK(!packet_status_window_.empty());
   return PreviousPacketStatus(packet_status_window_.end())->first;
 }

 void TransportFeedbackPacketLossTracker::OnPacketAdded(uint16_t seq_num,
                                                        int64_t send_time_ms) {
   // Sanity - time can't flow backwards.
   RTC_DCHECK(
       packet_status_window_.empty() ||
       PreviousPacketStatus(packet_status_window_.end())->second.send_time_ms <=
           send_time_ms);

   if (packet_status_window_.find(seq_num) != packet_status_window_.end() ||
       (!packet_status_window_.empty() &&
        ForwardDiff(seq_num, NewestSequenceNumber()) <= kSeqNumHalf)) {
     // The only way for these two to happen is when the stream lies dormant for
     // long enough for the sequence numbers to wrap. Everything in the window in
     // such a case would be too old to use.
     Reset();
   }

   // Maintain a window where the newest sequence number is at most 0x7fff away
   // from the oldest, so that would could still distinguish old/new.
   while (!packet_status_window_.empty() &&
          ForwardDiff(ref_packet_status_->first, seq_num) >= kSeqNumHalf) {
     RemoveOldestPacketStatus();
   }

   SentPacket sent_packet(send_time_ms, PacketStatus::Unacked);
   packet_status_window_.insert(packet_status_window_.end(),
                                std::make_pair(seq_num, sent_packet));

   if (packet_status_window_.size() == 1) {
     ref_packet_status_ = packet_status_window_.cbegin();
   }
 }

 void TransportFeedbackPacketLossTracker::OnPacketFeedbackVector(
     const std::vector<PacketFeedback>& packet_feedback_vector) {
   for (const PacketFeedback& packet : packet_feedback_vector) {
     const auto& it = packet_status_window_.find(packet.sequence_number);

     // Packets which aren't at least marked as unacked either do not belong to
     // this media stream, or have been shifted out of window.
     if (it == packet_status_window_.end())
       continue;

     const bool lost = packet.arrival_time_ms == PacketFeedback::kNotReceived;
     const PacketStatus packet_status =
         lost ? PacketStatus::Lost : PacketStatus::Received;

     UpdatePacketStatus(it, packet_status);
   }
 }

 absl::optional<float> TransportFeedbackPacketLossTracker::GetPacketLossRate()
     const {
   return plr_state_.GetMetric();
 }

 absl::optional<float>
 TransportFeedbackPacketLossTracker::GetRecoverablePacketLossRate() const {
   return rplr_state_.GetMetric();
 }

 void TransportFeedbackPacketLossTracker::UpdatePacketStatus(
     SentPacketStatusMap::iterator it,
     PacketStatus new_status) {
   if (it->second.status != PacketStatus::Unacked) {
     // Normally, packets are sent (inserted into window as "unacked"), then we
     // receive one feedback for them.
     // But it is possible that a packet would receive two feedbacks. Then:
     if (it->second.status == PacketStatus::Lost &&
         new_status == PacketStatus::Received) {
       // If older status said that the packet was lost but newer one says it
       // is received, we take the newer one.
       UpdateMetrics(it, false);
       it->second.status =
           PacketStatus::Unacked;  // For clarity; overwritten shortly.
     } else {
       // If the value is unchanged or if older status said that the packet was
       // received but the newer one says it is lost, we ignore it.
       // The standard allows for previously-reported packets to carry
       // no report when the reports overlap, which also looks like the
       // packet is being reported as lost.
       return;
     }
   }

   // Change from UNACKED to RECEIVED/LOST.
   it->second.status = new_status;
   UpdateMetrics(it, true);

   // Remove packets from the beginning of the window until we only hold packets,
   // be they acked or unacked, which are not more than |max_window_size_ms|
   // older from the newest packet. (If the packet we're now inserting into the
   // window isn't the newest, it would not trigger any removals; the newest
   // already removed all relevant.)
   while (ref_packet_status_ != packet_status_window_.end() &&
          (it->second.send_time_ms - ref_packet_status_->second.send_time_ms) >
              max_window_size_ms_) {
     RemoveOldestPacketStatus();
   }
 }

 void TransportFeedbackPacketLossTracker::RemoveOldestPacketStatus() {
   UpdateMetrics(ref_packet_status_, false);
   const auto it = ref_packet_status_;
   ref_packet_status_ = NextPacketStatus(it);
   packet_status_window_.erase(it);
 }

 void TransportFeedbackPacketLossTracker::UpdateMetrics(
     ConstPacketStatusIterator it,
     bool apply /* false = undo */) {
   RTC_DCHECK(it != packet_status_window_.end());
   // Metrics are dependent on feedbacks from the other side. We don't want
   // to update the metrics each time a packet is sent, except for the case
   // when it shifts old sent-but-unacked-packets out of window.
   RTC_DCHECK(!apply || it->second.status != PacketStatus::Unacked);

   if (it->second.status != PacketStatus::Unacked) {
     UpdateCounter(&acked_packets_, apply);
   }

   UpdatePlr(it, apply);
   UpdateRplr(it, apply);
 }

 void TransportFeedbackPacketLossTracker::UpdatePlr(
     ConstPacketStatusIterator it,
     bool apply /* false = undo */) {
   switch (it->second.status) {
     case PacketStatus::Unacked:
       return;
     case PacketStatus::Received:
       UpdateCounter(&plr_state_.num_received_packets_, apply);
       break;
     case PacketStatus::Lost:
       UpdateCounter(&plr_state_.num_lost_packets_, apply);
       break;
     default:
       RTC_NOTREACHED();
   }
 }

 void TransportFeedbackPacketLossTracker::UpdateRplr(
     ConstPacketStatusIterator it,
     bool apply /* false = undo */) {
   if (it->second.status == PacketStatus::Unacked) {
     // Unacked packets cannot compose a pair.
     return;
   }

   // Previous packet and current packet might compose a pair.
   if (it != ref_packet_status_) {
     const auto& prev = PreviousPacketStatus(it);
     if (prev->second.status != PacketStatus::Unacked) {
       UpdateCounter(&rplr_state_.num_acked_pairs_, apply);
       if (prev->second.status == PacketStatus::Lost &&
           it->second.status == PacketStatus::Received) {
         UpdateCounter(
             &rplr_state_.num_recoverable_losses_, apply);
       }
     }
   }

   // Current packet and next packet might compose a pair.
   const auto& next = NextPacketStatus(it);
   if (next != packet_status_window_.end() &&
       next->second.status != PacketStatus::Unacked) {
     UpdateCounter(&rplr_state_.num_acked_pairs_, apply);
     if (it->second.status == PacketStatus::Lost &&
         next->second.status == PacketStatus::Received) {
       UpdateCounter(&rplr_state_.num_recoverable_losses_, apply);
     }
   }
 }

 TransportFeedbackPacketLossTracker::ConstPacketStatusIterator
 TransportFeedbackPacketLossTracker::PreviousPacketStatus(
     ConstPacketStatusIterator it) const {
   RTC_DCHECK(it != ref_packet_status_);
   if (it == packet_status_window_.end()) {
     // This is to make PreviousPacketStatus(packet_status_window_.end()) point
     // to the last element.
     it = ref_packet_status_;
   }

   if (it == packet_status_window_.begin()) {
     // Due to the circular nature of sequence numbers, we let the iterator
     // go to the end.
     it = packet_status_window_.end();
   }
   return --it;
 }

 TransportFeedbackPacketLossTracker::ConstPacketStatusIterator
 TransportFeedbackPacketLossTracker::NextPacketStatus(
     ConstPacketStatusIterator it) const {
   RTC_DCHECK(it != packet_status_window_.end());
   ++it;
   if (it == packet_status_window_.end()) {
     // Due to the circular nature of sequence numbers, we let the iterator
     // goes back to the beginning.
     it = packet_status_window_.begin();
   }
   if (it == ref_packet_status_) {
     // This is to make the NextPacketStatus of the last element to return the
     // beyond-the-end iterator.
     it = packet_status_window_.end();
   }
   return it;
 }

 // TODO(minyue): This method checks the states of this class do not misbehave.
 // The method is used both in unit tests and a fuzzer test. The fuzzer test
 // is present to help finding potential errors. Once the fuzzer test shows no
 // error after long period, we can remove the fuzzer test, and move this method
 // to unit test.
 void TransportFeedbackPacketLossTracker::Validate() const {  // Testing only!
   RTC_CHECK_EQ(plr_state_.num_received_packets_ + plr_state_.num_lost_packets_,
                acked_packets_);
   RTC_CHECK_LE(acked_packets_, packet_status_window_.size());
   RTC_CHECK_LE(rplr_state_.num_recoverable_losses_,
                rplr_state_.num_acked_pairs_);
   RTC_CHECK_LE(rplr_state_.num_acked_pairs_, acked_packets_ - 1);

   size_t unacked_packets = 0;
   size_t received_packets = 0;
   size_t lost_packets = 0;
   size_t acked_pairs = 0;
   size_t recoverable_losses = 0;

   if (!packet_status_window_.empty()) {
     ConstPacketStatusIterator it = ref_packet_status_;
     do {
       switch (it->second.status) {
         case PacketStatus::Unacked:
           ++unacked_packets;
           break;
         case PacketStatus::Received:
           ++received_packets;
           break;
         case PacketStatus::Lost:
           ++lost_packets;
           break;
         default:
           RTC_NOTREACHED();
       }

       auto next = std::next(it);
       if (next == packet_status_window_.end())
         next = packet_status_window_.begin();

       if (next != ref_packet_status_) {  // If we have a next packet...
         RTC_CHECK_GE(next->second.send_time_ms, it->second.send_time_ms);

         if (it->second.status != PacketStatus::Unacked &&
             next->second.status != PacketStatus::Unacked) {
           ++acked_pairs;
           if (it->second.status == PacketStatus::Lost &&
               next->second.status == PacketStatus::Received) {
             ++recoverable_losses;
           }
         }
       }

       RTC_CHECK_LT(ForwardDiff(ReferenceSequenceNumber(), it->first),
                    kSeqNumHalf);

       it = next;
     } while (it != ref_packet_status_);
   }

   RTC_CHECK_EQ(plr_state_.num_received_packets_, received_packets);
   RTC_CHECK_EQ(plr_state_.num_lost_packets_, lost_packets);
   RTC_CHECK_EQ(packet_status_window_.size(),
                unacked_packets + received_packets + lost_packets);
   RTC_CHECK_EQ(rplr_state_.num_acked_pairs_, acked_pairs);
   RTC_CHECK_EQ(rplr_state_.num_recoverable_losses_, recoverable_losses);
 }

 absl::optional<float> TransportFeedbackPacketLossTracker::PlrState::GetMetric()
     const {
   const size_t total = num_lost_packets_ + num_received_packets_;
   if (total < min_num_acked_packets_) {
     return absl::nullopt;
   } else {
     return static_cast<float>(num_lost_packets_) / total;
   }
 }

 absl::optional<float> TransportFeedbackPacketLossTracker::RplrState::GetMetric()
     const {
   if (num_acked_pairs_ < min_num_acked_pairs_) {
     return absl::nullopt;
   } else {
     return static_cast<float>(num_recoverable_losses_) / num_acked_pairs_;
   }
 }

 }  // namespace webrtc
	/*
	* 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.
	*/

	#include "audio/transport_feedback_packet_loss_tracker.h"

	#include <limits>
	#include <utility>

	#include "modules/rtp_rtcp/include/rtp_rtcp_defines.h"
	#include "modules/rtp_rtcp/source/rtcp_packet/transport_feedback.h"
	#include "rtc_base/checks.h"
	#include "rtc_base/numerics/mod_ops.h"

	namespace {
	constexpr uint16_t kSeqNumHalf = 0x8000u;
	void UpdateCounter(size_t* counter, bool increment) {
	if (increment) {
	RTC_DCHECK_LT(*counter, std::numeric_limits<std::size_t>::max());
	++(*counter);
	} else {
	RTC_DCHECK_GT(*counter, 0);
	--(*counter);
	}
	}
	} // namespace

	namespace webrtc {

	TransportFeedbackPacketLossTracker::TransportFeedbackPacketLossTracker(
	int64_t max_window_size_ms,
	size_t plr_min_num_acked_packets,
	size_t rplr_min_num_acked_pairs)
	: max_window_size_ms_(max_window_size_ms),
	ref_packet_status_(packet_status_window_.begin()),
	plr_state_(plr_min_num_acked_packets),
	rplr_state_(rplr_min_num_acked_pairs) {
	RTC_DCHECK_GT(max_window_size_ms, 0);
	RTC_DCHECK_GT(plr_min_num_acked_packets, 0);
	RTC_DCHECK_GT(rplr_min_num_acked_pairs, 0);
	Reset();
	}

	void TransportFeedbackPacketLossTracker::Reset() {
	acked_packets_ = 0;
	plr_state_.Reset();
	rplr_state_.Reset();
	packet_status_window_.clear();
	ref_packet_status_ = packet_status_window_.begin();
	}

	uint16_t TransportFeedbackPacketLossTracker::ReferenceSequenceNumber() const {
	RTC_DCHECK(!packet_status_window_.empty());
	return ref_packet_status_->first;
	}

	uint16_t TransportFeedbackPacketLossTracker::NewestSequenceNumber() const {
	RTC_DCHECK(!packet_status_window_.empty());
	return PreviousPacketStatus(packet_status_window_.end())->first;
	}

	void TransportFeedbackPacketLossTracker::OnPacketAdded(uint16_t seq_num,
	int64_t send_time_ms) {
	// Sanity - time can't flow backwards.
	RTC_DCHECK(
	packet_status_window_.empty() \|\|
	PreviousPacketStatus(packet_status_window_.end())->second.send_time_ms <=
	send_time_ms);

	if (packet_status_window_.find(seq_num) != packet_status_window_.end() \|\|
	(!packet_status_window_.empty() &&
	ForwardDiff(seq_num, NewestSequenceNumber()) <= kSeqNumHalf)) {
	// The only way for these two to happen is when the stream lies dormant for
	// long enough for the sequence numbers to wrap. Everything in the window in
	// such a case would be too old to use.
	Reset();
	}

	// Maintain a window where the newest sequence number is at most 0x7fff away
	// from the oldest, so that would could still distinguish old/new.
	while (!packet_status_window_.empty() &&
	ForwardDiff(ref_packet_status_->first, seq_num) >= kSeqNumHalf) {
	RemoveOldestPacketStatus();
	}

	SentPacket sent_packet(send_time_ms, PacketStatus::Unacked);
	packet_status_window_.insert(packet_status_window_.end(),
	std::make_pair(seq_num, sent_packet));

	if (packet_status_window_.size() == 1) {
	ref_packet_status_ = packet_status_window_.cbegin();
	}
	}

	void TransportFeedbackPacketLossTracker::OnPacketFeedbackVector(
	const std::vector<PacketFeedback>& packet_feedback_vector) {
	for (const PacketFeedback& packet : packet_feedback_vector) {
	const auto& it = packet_status_window_.find(packet.sequence_number);

	// Packets which aren't at least marked as unacked either do not belong to
	// this media stream, or have been shifted out of window.
	if (it == packet_status_window_.end())
	continue;

	const bool lost = packet.arrival_time_ms == PacketFeedback::kNotReceived;
	const PacketStatus packet_status =
	lost ? PacketStatus::Lost : PacketStatus::Received;

	UpdatePacketStatus(it, packet_status);
	}
	}

	absl::optional<float> TransportFeedbackPacketLossTracker::GetPacketLossRate()
	const {
	return plr_state_.GetMetric();
	}

	absl::optional<float>
	TransportFeedbackPacketLossTracker::GetRecoverablePacketLossRate() const {
	return rplr_state_.GetMetric();
	}

	void TransportFeedbackPacketLossTracker::UpdatePacketStatus(
	SentPacketStatusMap::iterator it,
	PacketStatus new_status) {
	if (it->second.status != PacketStatus::Unacked) {
	// Normally, packets are sent (inserted into window as "unacked"), then we
	// receive one feedback for them.
	// But it is possible that a packet would receive two feedbacks. Then:
	if (it->second.status == PacketStatus::Lost &&
	new_status == PacketStatus::Received) {
	// If older status said that the packet was lost but newer one says it
	// is received, we take the newer one.
	UpdateMetrics(it, false);
	it->second.status =
	PacketStatus::Unacked; // For clarity; overwritten shortly.
	} else {
	// If the value is unchanged or if older status said that the packet was
	// received but the newer one says it is lost, we ignore it.
	// The standard allows for previously-reported packets to carry
	// no report when the reports overlap, which also looks like the
	// packet is being reported as lost.
	return;
	}
	}

	// Change from UNACKED to RECEIVED/LOST.
	it->second.status = new_status;
	UpdateMetrics(it, true);

	// Remove packets from the beginning of the window until we only hold packets,
	// be they acked or unacked, which are not more than \|max_window_size_ms\|
	// older from the newest packet. (If the packet we're now inserting into the
	// window isn't the newest, it would not trigger any removals; the newest
	// already removed all relevant.)
	while (ref_packet_status_ != packet_status_window_.end() &&
	(it->second.send_time_ms - ref_packet_status_->second.send_time_ms) >
	max_window_size_ms_) {
	RemoveOldestPacketStatus();
	}
	}

	void TransportFeedbackPacketLossTracker::RemoveOldestPacketStatus() {
	UpdateMetrics(ref_packet_status_, false);
	const auto it = ref_packet_status_;
	ref_packet_status_ = NextPacketStatus(it);
	packet_status_window_.erase(it);
	}

	void TransportFeedbackPacketLossTracker::UpdateMetrics(
	ConstPacketStatusIterator it,
	bool apply /* false = undo */) {
	RTC_DCHECK(it != packet_status_window_.end());
	// Metrics are dependent on feedbacks from the other side. We don't want
	// to update the metrics each time a packet is sent, except for the case
	// when it shifts old sent-but-unacked-packets out of window.
	RTC_DCHECK(!apply \|\| it->second.status != PacketStatus::Unacked);

	if (it->second.status != PacketStatus::Unacked) {
	UpdateCounter(&acked_packets_, apply);
	}

	UpdatePlr(it, apply);
	UpdateRplr(it, apply);
	}

	void TransportFeedbackPacketLossTracker::UpdatePlr(
	ConstPacketStatusIterator it,
	bool apply /* false = undo */) {
	switch (it->second.status) {
	case PacketStatus::Unacked:
	return;
	case PacketStatus::Received:
	UpdateCounter(&plr_state_.num_received_packets_, apply);
	break;
	case PacketStatus::Lost:
	UpdateCounter(&plr_state_.num_lost_packets_, apply);
	break;
	default:
	RTC_NOTREACHED();
	}
	}

	void TransportFeedbackPacketLossTracker::UpdateRplr(
	ConstPacketStatusIterator it,
	bool apply /* false = undo */) {
	if (it->second.status == PacketStatus::Unacked) {
	// Unacked packets cannot compose a pair.
	return;
	}

	// Previous packet and current packet might compose a pair.
	if (it != ref_packet_status_) {
	const auto& prev = PreviousPacketStatus(it);
	if (prev->second.status != PacketStatus::Unacked) {
	UpdateCounter(&rplr_state_.num_acked_pairs_, apply);
	if (prev->second.status == PacketStatus::Lost &&
	it->second.status == PacketStatus::Received) {
	UpdateCounter(
	&rplr_state_.num_recoverable_losses_, apply);
	}
	}
	}

	// Current packet and next packet might compose a pair.
	const auto& next = NextPacketStatus(it);
	if (next != packet_status_window_.end() &&
	next->second.status != PacketStatus::Unacked) {
	UpdateCounter(&rplr_state_.num_acked_pairs_, apply);
	if (it->second.status == PacketStatus::Lost &&
	next->second.status == PacketStatus::Received) {
	UpdateCounter(&rplr_state_.num_recoverable_losses_, apply);
	}
	}
	}

	TransportFeedbackPacketLossTracker::ConstPacketStatusIterator
	TransportFeedbackPacketLossTracker::PreviousPacketStatus(
	ConstPacketStatusIterator it) const {
	RTC_DCHECK(it != ref_packet_status_);
	if (it == packet_status_window_.end()) {
	// This is to make PreviousPacketStatus(packet_status_window_.end()) point
	// to the last element.
	it = ref_packet_status_;
	}

	if (it == packet_status_window_.begin()) {
	// Due to the circular nature of sequence numbers, we let the iterator
	// go to the end.
	it = packet_status_window_.end();
	}
	return --it;
	}

	TransportFeedbackPacketLossTracker::ConstPacketStatusIterator
	TransportFeedbackPacketLossTracker::NextPacketStatus(
	ConstPacketStatusIterator it) const {
	RTC_DCHECK(it != packet_status_window_.end());
	++it;
	if (it == packet_status_window_.end()) {
	// Due to the circular nature of sequence numbers, we let the iterator
	// goes back to the beginning.
	it = packet_status_window_.begin();
	}
	if (it == ref_packet_status_) {
	// This is to make the NextPacketStatus of the last element to return the
	// beyond-the-end iterator.
	it = packet_status_window_.end();
	}
	return it;
	}

	// TODO(minyue): This method checks the states of this class do not misbehave.
	// The method is used both in unit tests and a fuzzer test. The fuzzer test
	// is present to help finding potential errors. Once the fuzzer test shows no
	// error after long period, we can remove the fuzzer test, and move this method
	// to unit test.
	void TransportFeedbackPacketLossTracker::Validate() const { // Testing only!
	RTC_CHECK_EQ(plr_state_.num_received_packets_ + plr_state_.num_lost_packets_,
	acked_packets_);
	RTC_CHECK_LE(acked_packets_, packet_status_window_.size());
	RTC_CHECK_LE(rplr_state_.num_recoverable_losses_,
	rplr_state_.num_acked_pairs_);
	RTC_CHECK_LE(rplr_state_.num_acked_pairs_, acked_packets_ - 1);

	size_t unacked_packets = 0;
	size_t received_packets = 0;
	size_t lost_packets = 0;
	size_t acked_pairs = 0;
	size_t recoverable_losses = 0;

	if (!packet_status_window_.empty()) {
	ConstPacketStatusIterator it = ref_packet_status_;
	do {
	switch (it->second.status) {
	case PacketStatus::Unacked:
	++unacked_packets;
	break;
	case PacketStatus::Received:
	++received_packets;
	break;
	case PacketStatus::Lost:
	++lost_packets;
	break;
	default:
	RTC_NOTREACHED();
	}

	auto next = std::next(it);
	if (next == packet_status_window_.end())
	next = packet_status_window_.begin();

	if (next != ref_packet_status_) { // If we have a next packet...
	RTC_CHECK_GE(next->second.send_time_ms, it->second.send_time_ms);

	if (it->second.status != PacketStatus::Unacked &&
	next->second.status != PacketStatus::Unacked) {
	++acked_pairs;
	if (it->second.status == PacketStatus::Lost &&
	next->second.status == PacketStatus::Received) {
	++recoverable_losses;
	}
	}
	}

	RTC_CHECK_LT(ForwardDiff(ReferenceSequenceNumber(), it->first),
	kSeqNumHalf);

	it = next;
	} while (it != ref_packet_status_);
	}

	RTC_CHECK_EQ(plr_state_.num_received_packets_, received_packets);
	RTC_CHECK_EQ(plr_state_.num_lost_packets_, lost_packets);
	RTC_CHECK_EQ(packet_status_window_.size(),
	unacked_packets + received_packets + lost_packets);
	RTC_CHECK_EQ(rplr_state_.num_acked_pairs_, acked_pairs);
	RTC_CHECK_EQ(rplr_state_.num_recoverable_losses_, recoverable_losses);
	}

	absl::optional<float> TransportFeedbackPacketLossTracker::PlrState::GetMetric()
	const {
	const size_t total = num_lost_packets_ + num_received_packets_;
	if (total < min_num_acked_packets_) {
	return absl::nullopt;
	} else {
	return static_cast<float>(num_lost_packets_) / total;
	}
	}

	absl::optional<float> TransportFeedbackPacketLossTracker::RplrState::GetMetric()
	const {
	if (num_acked_pairs_ < min_num_acked_pairs_) {
	return absl::nullopt;
	} else {
	return static_cast<float>(num_recoverable_losses_) / num_acked_pairs_;
	}
	}

	} // namespace webrtc