net/quic/congestion_control/tcp_cubic_sender.cc - mojo - Git at Google

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.

 #include "net/quic/congestion_control/tcp_cubic_sender.h"

 #include <algorithm>

 #include "base/metrics/histogram.h"
 #include "net/quic/congestion_control/rtt_stats.h"
 #include "net/quic/crypto/crypto_protocol.h"

 using std::max;
 using std::min;

 namespace net {

 namespace {
 // Constants based on TCP defaults.
 // The minimum cwnd based on RFC 3782 (TCP NewReno) for cwnd reductions on a
 // fast retransmission.  The cwnd after a timeout is still 1.
 const QuicTcpCongestionWindow kMinimumCongestionWindow = 2;
 const QuicByteCount kMaxSegmentSize = kDefaultTCPMSS;
 const int64 kInitialCongestionWindow = 10;
 const int kMaxBurstLength = 3;
 };  // namespace

 TcpCubicSender::TcpCubicSender(
     const QuicClock* clock,
     const RttStats* rtt_stats,
     bool reno,
     QuicTcpCongestionWindow max_tcp_congestion_window,
     QuicConnectionStats* stats)
     : hybrid_slow_start_(clock),
       cubic_(clock, stats),
       rtt_stats_(rtt_stats),
       stats_(stats),
       reno_(reno),
       num_connections_(2),
       congestion_window_count_(0),
       receive_window_(kDefaultSocketReceiveBuffer),
       prr_out_(0),
       prr_delivered_(0),
       ack_count_since_loss_(0),
       bytes_in_flight_before_loss_(0),
       largest_sent_sequence_number_(0),
       largest_acked_sequence_number_(0),
       largest_sent_at_last_cutback_(0),
       congestion_window_(kInitialCongestionWindow),
       previous_congestion_window_(0),
       slowstart_threshold_(max_tcp_congestion_window),
       previous_slowstart_threshold_(0),
       last_cutback_exited_slowstart_(false),
       max_tcp_congestion_window_(max_tcp_congestion_window) {
 }

 TcpCubicSender::~TcpCubicSender() {
   UMA_HISTOGRAM_COUNTS("Net.QuicSession.FinalTcpCwnd", congestion_window_);
 }

 void TcpCubicSender::SetFromConfig(const QuicConfig& config, bool is_server) {
   if (is_server) {
     if (config.HasReceivedConnectionOptions() &&
         ContainsQuicTag(config.ReceivedConnectionOptions(), kIW10)) {
       // Initial window experiment.  Ignore the initial congestion
       // window suggested by the client and use the default ICWND of
       // 10 instead.
       congestion_window_ = kInitialCongestionWindow;
     } else if (config.HasReceivedInitialCongestionWindow()) {
       // Set the initial window size.
       congestion_window_ = min(kMaxInitialWindow,
                                config.ReceivedInitialCongestionWindow());
     }
   }
   if (config.HasReceivedSocketReceiveBuffer()) {
     // Set the initial socket receive buffer size in bytes.
     receive_window_ = config.ReceivedSocketReceiveBuffer();
   }
 }

 void TcpCubicSender::SetNumEmulatedConnections(int num_connections) {
   num_connections_ = max(1, num_connections);
   cubic_.SetNumConnections(num_connections_);
 }

 void TcpCubicSender::OnIncomingQuicCongestionFeedbackFrame(
     const QuicCongestionFeedbackFrame& feedback,
     QuicTime feedback_receive_time) {
   if (feedback.type == kTCP) {
     receive_window_ = feedback.tcp.receive_window;
   }
 }

 void TcpCubicSender::OnCongestionEvent(
     bool rtt_updated,
     QuicByteCount bytes_in_flight,
     const CongestionVector& acked_packets,
     const CongestionVector& lost_packets) {
   if (rtt_updated && InSlowStart() &&
       hybrid_slow_start_.ShouldExitSlowStart(rtt_stats_->latest_rtt(),
                                              rtt_stats_->min_rtt(),
                                              congestion_window_)) {
     slowstart_threshold_ = congestion_window_;
   }
   for (CongestionVector::const_iterator it = lost_packets.begin();
        it != lost_packets.end(); ++it) {
     OnPacketLost(it->first, bytes_in_flight);
   }
   for (CongestionVector::const_iterator it = acked_packets.begin();
        it != acked_packets.end(); ++it) {
     OnPacketAcked(it->first, it->second.bytes_sent, bytes_in_flight);
   }
 }

 void TcpCubicSender::OnPacketAcked(
     QuicPacketSequenceNumber acked_sequence_number,
     QuicByteCount acked_bytes,
     QuicByteCount bytes_in_flight) {
   largest_acked_sequence_number_ = max(acked_sequence_number,
                                        largest_acked_sequence_number_);
   if (InRecovery()) {
     PrrOnPacketAcked(acked_bytes);
     return;
   }
   MaybeIncreaseCwnd(acked_sequence_number, bytes_in_flight);
   // TODO(ianswett): Should this even be called when not in slow start?
   hybrid_slow_start_.OnPacketAcked(acked_sequence_number, InSlowStart());
 }

 void TcpCubicSender::OnPacketLost(QuicPacketSequenceNumber sequence_number,
                                   QuicByteCount bytes_in_flight) {
   // TCP NewReno (RFC6582) says that once a loss occurs, any losses in packets
   // already sent should be treated as a single loss event, since it's expected.
   if (sequence_number <= largest_sent_at_last_cutback_) {
     if (last_cutback_exited_slowstart_) {
       ++stats_->slowstart_packets_lost;
     }
     DVLOG(1) << "Ignoring loss for largest_missing:" << sequence_number
              << " because it was sent prior to the last CWND cutback.";
     return;
   }
   ++stats_->tcp_loss_events;
   last_cutback_exited_slowstart_ = InSlowStart();
   if (InSlowStart()) {
     ++stats_->slowstart_packets_lost;
   }
   PrrOnPacketLost(bytes_in_flight);

   if (reno_) {
     congestion_window_ = congestion_window_ >> 1;
   } else {
     congestion_window_ =
         cubic_.CongestionWindowAfterPacketLoss(congestion_window_);
   }
   slowstart_threshold_ = congestion_window_;
   // Enforce TCP's minimum congestion window of 2*MSS.
   if (congestion_window_ < kMinimumCongestionWindow) {
     congestion_window_ = kMinimumCongestionWindow;
   }
   largest_sent_at_last_cutback_ = largest_sent_sequence_number_;
   // reset packet count from congestion avoidance mode. We start
   // counting again when we're out of recovery.
   congestion_window_count_ = 0;
   DVLOG(1) << "Incoming loss; congestion window: " << congestion_window_
            << " slowstart threshold: " << slowstart_threshold_;
 }

 bool TcpCubicSender::OnPacketSent(QuicTime /*sent_time*/,
                                   QuicByteCount /*bytes_in_flight*/,
                                   QuicPacketSequenceNumber sequence_number,
                                   QuicByteCount bytes,
                                   HasRetransmittableData is_retransmittable) {
   // Only update bytes_in_flight_ for data packets.
   if (is_retransmittable != HAS_RETRANSMITTABLE_DATA) {
     return false;
   }

   prr_out_ += bytes;
   DCHECK_LT(largest_sent_sequence_number_, sequence_number);
   largest_sent_sequence_number_ = sequence_number;
   hybrid_slow_start_.OnPacketSent(sequence_number);
   return true;
 }

 QuicTime::Delta TcpCubicSender::TimeUntilSend(
     QuicTime /* now */,
     QuicByteCount bytes_in_flight,
     HasRetransmittableData has_retransmittable_data) const {
   if (has_retransmittable_data == NO_RETRANSMITTABLE_DATA) {
     // For TCP we can always send an ACK immediately.
     return QuicTime::Delta::Zero();
   }
   if (InRecovery()) {
     return PrrTimeUntilSend(bytes_in_flight);
   }
   if (SendWindow() > bytes_in_flight) {
     return QuicTime::Delta::Zero();
   }
   return QuicTime::Delta::Infinite();
 }

 QuicByteCount TcpCubicSender::SendWindow() const {
   // What's the current send window in bytes.
   return min(receive_window_, GetCongestionWindow());
 }

 QuicBandwidth TcpCubicSender::BandwidthEstimate() const {
   return QuicBandwidth::FromBytesAndTimeDelta(GetCongestionWindow(),
                                               rtt_stats_->SmoothedRtt());
 }

 bool TcpCubicSender::HasReliableBandwidthEstimate() const {
   return !InSlowStart() && !InRecovery();
 }

 QuicTime::Delta TcpCubicSender::RetransmissionDelay() const {
   if (!rtt_stats_->HasUpdates()) {
     return QuicTime::Delta::Zero();
   }
   return QuicTime::Delta::FromMicroseconds(
       rtt_stats_->SmoothedRtt().ToMicroseconds() +
       4 * rtt_stats_->mean_deviation().ToMicroseconds());
 }

 QuicByteCount TcpCubicSender::GetCongestionWindow() const {
   return congestion_window_ * kMaxSegmentSize;
 }

 bool TcpCubicSender::InSlowStart() const {
   return congestion_window_ < slowstart_threshold_;
 }

 QuicByteCount TcpCubicSender::GetSlowStartThreshold() const {
   return slowstart_threshold_ * kMaxSegmentSize;
 }

 bool TcpCubicSender::IsCwndLimited(QuicByteCount bytes_in_flight) const {
   const QuicByteCount congestion_window_bytes = congestion_window_ *
       kMaxSegmentSize;
   if (bytes_in_flight >= congestion_window_bytes) {
     return true;
   }
   const QuicByteCount max_burst = kMaxBurstLength * kMaxSegmentSize;
   const QuicByteCount available_bytes =
       congestion_window_bytes - bytes_in_flight;
   const bool slow_start_limited = InSlowStart() &&
       bytes_in_flight >  congestion_window_bytes / 2;
   return slow_start_limited || available_bytes <= max_burst;
 }

 bool TcpCubicSender::InRecovery() const {
   return largest_acked_sequence_number_ <= largest_sent_at_last_cutback_ &&
       largest_acked_sequence_number_ != 0;
 }

 // Called when we receive an ack. Normal TCP tracks how many packets one ack
 // represents, but quic has a separate ack for each packet.
 void TcpCubicSender::MaybeIncreaseCwnd(
     QuicPacketSequenceNumber acked_sequence_number,
     QuicByteCount bytes_in_flight) {
   LOG_IF(DFATAL, InRecovery()) << "Never increase the CWND during recovery.";
   if (!IsCwndLimited(bytes_in_flight)) {
     // We don't update the congestion window unless we are close to using the
     // window we have available.
     return;
   }
   if (InSlowStart()) {
     // congestion_window_cnt is the number of acks since last change of snd_cwnd
     if (congestion_window_ < max_tcp_congestion_window_) {
       // TCP slow start, exponential growth, increase by one for each ACK.
       ++congestion_window_;
     }
     DVLOG(1) << "Slow start; congestion window: " << congestion_window_
              << " slowstart threshold: " << slowstart_threshold_;
     return;
   }
   if (congestion_window_ >= max_tcp_congestion_window_) {
     return;
   }
   // Congestion avoidance
   if (reno_) {
     // Classic Reno congestion avoidance.
     ++congestion_window_count_;
     // Divide by num_connections to smoothly increase the CWND at a faster
     // rate than conventional Reno.
     if (congestion_window_count_ * num_connections_ >= congestion_window_) {
       ++congestion_window_;
       congestion_window_count_ = 0;
     }

     DVLOG(1) << "Reno; congestion window: " << congestion_window_
              << " slowstart threshold: " << slowstart_threshold_
              << " congestion window count: " << congestion_window_count_;
   } else {
     congestion_window_ = min(max_tcp_congestion_window_,
                              cubic_.CongestionWindowAfterAck(
                                  congestion_window_, rtt_stats_->min_rtt()));
     DVLOG(1) << "Cubic; congestion window: " << congestion_window_
              << " slowstart threshold: " << slowstart_threshold_;
   }
 }

 void TcpCubicSender::OnRetransmissionTimeout(bool packets_retransmitted) {
   largest_sent_at_last_cutback_ = 0;
   if (!packets_retransmitted) {
     return;
   }
   cubic_.Reset();
   hybrid_slow_start_.Restart();
   previous_slowstart_threshold_ = slowstart_threshold_;
   slowstart_threshold_ = congestion_window_ / 2;
   previous_congestion_window_ = congestion_window_;
   congestion_window_ = kMinimumCongestionWindow;
 }

 void TcpCubicSender::RevertRetransmissionTimeout() {
   if (previous_congestion_window_ == 0) {
     LOG(DFATAL) << "No previous congestion window to revert to.";
     return;
   }
   congestion_window_ = previous_congestion_window_;
   slowstart_threshold_ = previous_slowstart_threshold_;
   previous_congestion_window_ = 0;
 }

 void TcpCubicSender::PrrOnPacketLost(QuicByteCount bytes_in_flight) {
   prr_out_ = 0;
   bytes_in_flight_before_loss_ = bytes_in_flight;
   prr_delivered_ = 0;
   ack_count_since_loss_ = 0;
 }

 void TcpCubicSender::PrrOnPacketAcked(QuicByteCount acked_bytes) {
   prr_delivered_ += acked_bytes;
   ++ack_count_since_loss_;
 }

 QuicTime::Delta TcpCubicSender::PrrTimeUntilSend(
     QuicByteCount bytes_in_flight) const {
   DCHECK(InRecovery());
   // Return QuicTime::Zero In order to ensure limited transmit always works.
   if (prr_out_ == 0 || bytes_in_flight < kMaxSegmentSize) {
     return QuicTime::Delta::Zero();
   }
   if (SendWindow() > bytes_in_flight) {
     // During PRR-SSRB, limit outgoing packets to 1 extra MSS per ack, instead
     // of sending the entire available window. This prevents burst retransmits
     // when more packets are lost than the CWND reduction.
     //   limit = MAX(prr_delivered - prr_out, DeliveredData) + MSS
     if (prr_delivered_ + ack_count_since_loss_ * kMaxSegmentSize <= prr_out_) {
       return QuicTime::Delta::Infinite();
     }
     return QuicTime::Delta::Zero();
   }
   // Implement Proportional Rate Reduction (RFC6937)
   // Checks a simplified version of the PRR formula that doesn't use division:
   // AvailableSendWindow =
   //   CEIL(prr_delivered * ssthresh / BytesInFlightAtLoss) - prr_sent
   if (prr_delivered_ * slowstart_threshold_ * kMaxSegmentSize >
           prr_out_ * bytes_in_flight_before_loss_) {
     return QuicTime::Delta::Zero();
   }
   return QuicTime::Delta::Infinite();
 }

 CongestionControlType TcpCubicSender::GetCongestionControlType() const {
   return reno_ ? kReno : kCubic;
 }

 }  // namespace net
	// Copyright (c) 2012 The Chromium Authors. All rights reserved.
	// Use of this source code is governed by a BSD-style license that can be
	// found in the LICENSE file.

	#include "net/quic/congestion_control/tcp_cubic_sender.h"

	#include <algorithm>

	#include "base/metrics/histogram.h"
	#include "net/quic/congestion_control/rtt_stats.h"
	#include "net/quic/crypto/crypto_protocol.h"

	using std::max;
	using std::min;

	namespace net {

	namespace {
	// Constants based on TCP defaults.
	// The minimum cwnd based on RFC 3782 (TCP NewReno) for cwnd reductions on a
	// fast retransmission. The cwnd after a timeout is still 1.
	const QuicTcpCongestionWindow kMinimumCongestionWindow = 2;
	const QuicByteCount kMaxSegmentSize = kDefaultTCPMSS;
	const int64 kInitialCongestionWindow = 10;
	const int kMaxBurstLength = 3;
	}; // namespace

	TcpCubicSender::TcpCubicSender(
	const QuicClock* clock,
	const RttStats* rtt_stats,
	bool reno,
	QuicTcpCongestionWindow max_tcp_congestion_window,
	QuicConnectionStats* stats)
	: hybrid_slow_start_(clock),
	cubic_(clock, stats),
	rtt_stats_(rtt_stats),
	stats_(stats),
	reno_(reno),
	num_connections_(2),
	congestion_window_count_(0),
	receive_window_(kDefaultSocketReceiveBuffer),
	prr_out_(0),
	prr_delivered_(0),
	ack_count_since_loss_(0),
	bytes_in_flight_before_loss_(0),
	largest_sent_sequence_number_(0),
	largest_acked_sequence_number_(0),
	largest_sent_at_last_cutback_(0),
	congestion_window_(kInitialCongestionWindow),
	previous_congestion_window_(0),
	slowstart_threshold_(max_tcp_congestion_window),
	previous_slowstart_threshold_(0),
	last_cutback_exited_slowstart_(false),
	max_tcp_congestion_window_(max_tcp_congestion_window) {
	}

	TcpCubicSender::~TcpCubicSender() {
	UMA_HISTOGRAM_COUNTS("Net.QuicSession.FinalTcpCwnd", congestion_window_);
	}

	void TcpCubicSender::SetFromConfig(const QuicConfig& config, bool is_server) {
	if (is_server) {
	if (config.HasReceivedConnectionOptions() &&
	ContainsQuicTag(config.ReceivedConnectionOptions(), kIW10)) {
	// Initial window experiment. Ignore the initial congestion
	// window suggested by the client and use the default ICWND of
	// 10 instead.
	congestion_window_ = kInitialCongestionWindow;
	} else if (config.HasReceivedInitialCongestionWindow()) {
	// Set the initial window size.
	congestion_window_ = min(kMaxInitialWindow,
	config.ReceivedInitialCongestionWindow());
	}
	}
	if (config.HasReceivedSocketReceiveBuffer()) {
	// Set the initial socket receive buffer size in bytes.
	receive_window_ = config.ReceivedSocketReceiveBuffer();
	}
	}

	void TcpCubicSender::SetNumEmulatedConnections(int num_connections) {
	num_connections_ = max(1, num_connections);
	cubic_.SetNumConnections(num_connections_);
	}

	void TcpCubicSender::OnIncomingQuicCongestionFeedbackFrame(
	const QuicCongestionFeedbackFrame& feedback,
	QuicTime feedback_receive_time) {
	if (feedback.type == kTCP) {
	receive_window_ = feedback.tcp.receive_window;
	}
	}

	void TcpCubicSender::OnCongestionEvent(
	bool rtt_updated,
	QuicByteCount bytes_in_flight,
	const CongestionVector& acked_packets,
	const CongestionVector& lost_packets) {
	if (rtt_updated && InSlowStart() &&
	hybrid_slow_start_.ShouldExitSlowStart(rtt_stats_->latest_rtt(),
	rtt_stats_->min_rtt(),
	congestion_window_)) {
	slowstart_threshold_ = congestion_window_;
	}
	for (CongestionVector::const_iterator it = lost_packets.begin();
	it != lost_packets.end(); ++it) {
	OnPacketLost(it->first, bytes_in_flight);
	}
	for (CongestionVector::const_iterator it = acked_packets.begin();
	it != acked_packets.end(); ++it) {
	OnPacketAcked(it->first, it->second.bytes_sent, bytes_in_flight);
	}
	}

	void TcpCubicSender::OnPacketAcked(
	QuicPacketSequenceNumber acked_sequence_number,
	QuicByteCount acked_bytes,
	QuicByteCount bytes_in_flight) {
	largest_acked_sequence_number_ = max(acked_sequence_number,
	largest_acked_sequence_number_);
	if (InRecovery()) {
	PrrOnPacketAcked(acked_bytes);
	return;
	}
	MaybeIncreaseCwnd(acked_sequence_number, bytes_in_flight);
	// TODO(ianswett): Should this even be called when not in slow start?
	hybrid_slow_start_.OnPacketAcked(acked_sequence_number, InSlowStart());
	}

	void TcpCubicSender::OnPacketLost(QuicPacketSequenceNumber sequence_number,
	QuicByteCount bytes_in_flight) {
	// TCP NewReno (RFC6582) says that once a loss occurs, any losses in packets
	// already sent should be treated as a single loss event, since it's expected.
	if (sequence_number <= largest_sent_at_last_cutback_) {
	if (last_cutback_exited_slowstart_) {
	++stats_->slowstart_packets_lost;
	}
	DVLOG(1) << "Ignoring loss for largest_missing:" << sequence_number
	<< " because it was sent prior to the last CWND cutback.";
	return;
	}
	++stats_->tcp_loss_events;
	last_cutback_exited_slowstart_ = InSlowStart();
	if (InSlowStart()) {
	++stats_->slowstart_packets_lost;
	}
	PrrOnPacketLost(bytes_in_flight);

	if (reno_) {
	congestion_window_ = congestion_window_ >> 1;
	} else {
	congestion_window_ =
	cubic_.CongestionWindowAfterPacketLoss(congestion_window_);
	}
	slowstart_threshold_ = congestion_window_;
	// Enforce TCP's minimum congestion window of 2*MSS.
	if (congestion_window_ < kMinimumCongestionWindow) {
	congestion_window_ = kMinimumCongestionWindow;
	}
	largest_sent_at_last_cutback_ = largest_sent_sequence_number_;
	// reset packet count from congestion avoidance mode. We start
	// counting again when we're out of recovery.
	congestion_window_count_ = 0;
	DVLOG(1) << "Incoming loss; congestion window: " << congestion_window_
	<< " slowstart threshold: " << slowstart_threshold_;
	}

	bool TcpCubicSender::OnPacketSent(QuicTime /sent_time/,
	QuicByteCount /bytes_in_flight/,
	QuicPacketSequenceNumber sequence_number,
	QuicByteCount bytes,
	HasRetransmittableData is_retransmittable) {
	// Only update bytes_in_flight_ for data packets.
	if (is_retransmittable != HAS_RETRANSMITTABLE_DATA) {
	return false;
	}

	prr_out_ += bytes;
	DCHECK_LT(largest_sent_sequence_number_, sequence_number);
	largest_sent_sequence_number_ = sequence_number;
	hybrid_slow_start_.OnPacketSent(sequence_number);
	return true;
	}

	QuicTime::Delta TcpCubicSender::TimeUntilSend(
	QuicTime /* now */,
	QuicByteCount bytes_in_flight,
	HasRetransmittableData has_retransmittable_data) const {
	if (has_retransmittable_data == NO_RETRANSMITTABLE_DATA) {
	// For TCP we can always send an ACK immediately.
	return QuicTime::Delta::Zero();
	}
	if (InRecovery()) {
	return PrrTimeUntilSend(bytes_in_flight);
	}
	if (SendWindow() > bytes_in_flight) {
	return QuicTime::Delta::Zero();
	}
	return QuicTime::Delta::Infinite();
	}

	QuicByteCount TcpCubicSender::SendWindow() const {
	// What's the current send window in bytes.
	return min(receive_window_, GetCongestionWindow());
	}

	QuicBandwidth TcpCubicSender::BandwidthEstimate() const {
	return QuicBandwidth::FromBytesAndTimeDelta(GetCongestionWindow(),
	rtt_stats_->SmoothedRtt());
	}

	bool TcpCubicSender::HasReliableBandwidthEstimate() const {
	return !InSlowStart() && !InRecovery();
	}

	QuicTime::Delta TcpCubicSender::RetransmissionDelay() const {
	if (!rtt_stats_->HasUpdates()) {
	return QuicTime::Delta::Zero();
	}
	return QuicTime::Delta::FromMicroseconds(
	rtt_stats_->SmoothedRtt().ToMicroseconds() +
	4 * rtt_stats_->mean_deviation().ToMicroseconds());
	}

	QuicByteCount TcpCubicSender::GetCongestionWindow() const {
	return congestion_window_ * kMaxSegmentSize;
	}

	bool TcpCubicSender::InSlowStart() const {
	return congestion_window_ < slowstart_threshold_;
	}

	QuicByteCount TcpCubicSender::GetSlowStartThreshold() const {
	return slowstart_threshold_ * kMaxSegmentSize;
	}

	bool TcpCubicSender::IsCwndLimited(QuicByteCount bytes_in_flight) const {
	const QuicByteCount congestion_window_bytes = congestion_window_ *
	kMaxSegmentSize;
	if (bytes_in_flight >= congestion_window_bytes) {
	return true;
	}
	const QuicByteCount max_burst = kMaxBurstLength * kMaxSegmentSize;
	const QuicByteCount available_bytes =
	congestion_window_bytes - bytes_in_flight;
	const bool slow_start_limited = InSlowStart() &&
	bytes_in_flight > congestion_window_bytes / 2;
	return slow_start_limited \|\| available_bytes <= max_burst;
	}

	bool TcpCubicSender::InRecovery() const {
	return largest_acked_sequence_number_ <= largest_sent_at_last_cutback_ &&
	largest_acked_sequence_number_ != 0;
	}

	// Called when we receive an ack. Normal TCP tracks how many packets one ack
	// represents, but quic has a separate ack for each packet.
	void TcpCubicSender::MaybeIncreaseCwnd(
	QuicPacketSequenceNumber acked_sequence_number,
	QuicByteCount bytes_in_flight) {
	LOG_IF(DFATAL, InRecovery()) << "Never increase the CWND during recovery.";
	if (!IsCwndLimited(bytes_in_flight)) {
	// We don't update the congestion window unless we are close to using the
	// window we have available.
	return;
	}
	if (InSlowStart()) {
	// congestion_window_cnt is the number of acks since last change of snd_cwnd
	if (congestion_window_ < max_tcp_congestion_window_) {
	// TCP slow start, exponential growth, increase by one for each ACK.
	++congestion_window_;
	}
	DVLOG(1) << "Slow start; congestion window: " << congestion_window_
	<< " slowstart threshold: " << slowstart_threshold_;
	return;
	}
	if (congestion_window_ >= max_tcp_congestion_window_) {
	return;
	}
	// Congestion avoidance
	if (reno_) {
	// Classic Reno congestion avoidance.
	++congestion_window_count_;
	// Divide by num_connections to smoothly increase the CWND at a faster
	// rate than conventional Reno.
	if (congestion_window_count_ * num_connections_ >= congestion_window_) {
	++congestion_window_;
	congestion_window_count_ = 0;
	}

	DVLOG(1) << "Reno; congestion window: " << congestion_window_
	<< " slowstart threshold: " << slowstart_threshold_
	<< " congestion window count: " << congestion_window_count_;
	} else {
	congestion_window_ = min(max_tcp_congestion_window_,
	cubic_.CongestionWindowAfterAck(
	congestion_window_, rtt_stats_->min_rtt()));
	DVLOG(1) << "Cubic; congestion window: " << congestion_window_
	<< " slowstart threshold: " << slowstart_threshold_;
	}
	}

	void TcpCubicSender::OnRetransmissionTimeout(bool packets_retransmitted) {
	largest_sent_at_last_cutback_ = 0;
	if (!packets_retransmitted) {
	return;
	}
	cubic_.Reset();
	hybrid_slow_start_.Restart();
	previous_slowstart_threshold_ = slowstart_threshold_;
	slowstart_threshold_ = congestion_window_ / 2;
	previous_congestion_window_ = congestion_window_;
	congestion_window_ = kMinimumCongestionWindow;
	}

	void TcpCubicSender::RevertRetransmissionTimeout() {
	if (previous_congestion_window_ == 0) {
	LOG(DFATAL) << "No previous congestion window to revert to.";
	return;
	}
	congestion_window_ = previous_congestion_window_;
	slowstart_threshold_ = previous_slowstart_threshold_;
	previous_congestion_window_ = 0;
	}

	void TcpCubicSender::PrrOnPacketLost(QuicByteCount bytes_in_flight) {
	prr_out_ = 0;
	bytes_in_flight_before_loss_ = bytes_in_flight;
	prr_delivered_ = 0;
	ack_count_since_loss_ = 0;
	}

	void TcpCubicSender::PrrOnPacketAcked(QuicByteCount acked_bytes) {
	prr_delivered_ += acked_bytes;
	++ack_count_since_loss_;
	}

	QuicTime::Delta TcpCubicSender::PrrTimeUntilSend(
	QuicByteCount bytes_in_flight) const {
	DCHECK(InRecovery());
	// Return QuicTime::Zero In order to ensure limited transmit always works.
	if (prr_out_ == 0 \|\| bytes_in_flight < kMaxSegmentSize) {
	return QuicTime::Delta::Zero();
	}
	if (SendWindow() > bytes_in_flight) {
	// During PRR-SSRB, limit outgoing packets to 1 extra MSS per ack, instead
	// of sending the entire available window. This prevents burst retransmits
	// when more packets are lost than the CWND reduction.
	// limit = MAX(prr_delivered - prr_out, DeliveredData) + MSS
	if (prr_delivered_ + ack_count_since_loss_ * kMaxSegmentSize <= prr_out_) {
	return QuicTime::Delta::Infinite();
	}
	return QuicTime::Delta::Zero();
	}
	// Implement Proportional Rate Reduction (RFC6937)
	// Checks a simplified version of the PRR formula that doesn't use division:
	// AvailableSendWindow =
	// CEIL(prr_delivered * ssthresh / BytesInFlightAtLoss) - prr_sent
	if (prr_delivered_ * slowstart_threshold_ * kMaxSegmentSize >
	prr_out_ * bytes_in_flight_before_loss_) {
	return QuicTime::Delta::Zero();
	}
	return QuicTime::Delta::Infinite();
	}

	CongestionControlType TcpCubicSender::GetCongestionControlType() const {
	return reno_ ? kReno : kCubic;
	}

	} // namespace net