net/quic/congestion_control/tcp_cubic_sender.cc

   1 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style license that can be
   3 // found in the LICENSE file.
   4
   5 #include "net/quic/congestion_control/tcp_cubic_sender.h"
   6
   7 #include <algorithm>
   8
   9 #include "base/metrics/histogram.h"
  10 #include "net/quic/congestion_control/prr_sender.h"
  11 #include "net/quic/congestion_control/rtt_stats.h"
  12 #include "net/quic/crypto/crypto_protocol.h"
  13 #include "net/quic/proto/cached_network_parameters.pb.h"
  14
  15 using std::max;
  16 using std::min;
  17
  18 namespace net {
  19
  20 namespace {
  21 // Constants based on TCP defaults.
  22 // The minimum cwnd based on RFC 3782 (TCP NewReno) for cwnd reductions on a
  23 // fast retransmission.  The cwnd after a timeout is still 1.
  24 const QuicPacketCount kDefaultMinimumCongestionWindow = 2;
  25 const QuicByteCount kMaxSegmentSize = kDefaultTCPMSS;
  26 const int kMaxBurstLength = 3;
  27 const float kRenoBeta = 0.7f;  // Reno backoff factor.
  28 const uint32 kDefaultNumConnections = 2;  // N-connection emulation.
  29 }  // namespace
  30
  31 TcpCubicSender::TcpCubicSender(const QuicClock* clock,
  32                                const RttStats* rtt_stats,
  33                                bool reno,
  34                                QuicPacketCount initial_tcp_congestion_window,
  35                                QuicPacketCount max_tcp_congestion_window,
  36                                QuicConnectionStats* stats)
  37     : hybrid_slow_start_(clock),
  38       cubic_(clock),
  39       rtt_stats_(rtt_stats),
  40       stats_(stats),
  41       reno_(reno),
  42       num_connections_(kDefaultNumConnections),
  43       congestion_window_count_(0),
  44       largest_sent_sequence_number_(0),
  45       largest_acked_sequence_number_(0),
  46       largest_sent_at_last_cutback_(0),
  47       congestion_window_(initial_tcp_congestion_window),
  48       min_congestion_window_(kDefaultMinimumCongestionWindow),
  49       min4_mode_(false),
  50       slowstart_threshold_(max_tcp_congestion_window),
  51       last_cutback_exited_slowstart_(false),
  52       max_tcp_congestion_window_(max_tcp_congestion_window),
  53       clock_(clock) {
  54 }
  55
  56 TcpCubicSender::~TcpCubicSender() {
  57   UMA_HISTOGRAM_COUNTS("Net.QuicSession.FinalTcpCwnd", congestion_window_);
  58 }
  59
  60 void TcpCubicSender::SetFromConfig(const QuicConfig& config,
  61                                    Perspective perspective) {
  62   if (perspective == Perspective::IS_SERVER) {
  63     if (config.HasReceivedConnectionOptions() &&
  64         ContainsQuicTag(config.ReceivedConnectionOptions(), kIW10)) {
  65       // Initial window experiment.
  66       congestion_window_ = 10;
  67     }
  68     if (config.HasReceivedConnectionOptions() &&
  69         ContainsQuicTag(config.ReceivedConnectionOptions(), kMIN1)) {
  70       // Min CWND experiment.
  71       min_congestion_window_ = 1;
  72     }
  73     if (config.HasReceivedConnectionOptions() &&
  74         ContainsQuicTag(config.ReceivedConnectionOptions(), kMIN4)) {
  75       // Min CWND of 4 experiment.
  76       min4_mode_ = true;
  77       min_congestion_window_ = 1;
  78     }
  79   }
  80 }
  81
  82 bool TcpCubicSender::ResumeConnectionState(
  83     const CachedNetworkParameters& cached_network_params,
  84     bool max_bandwidth_resumption) {
  85   // If the previous bandwidth estimate is less than an hour old, store in
  86   // preparation for doing bandwidth resumption.
  87   int64 seconds_since_estimate =
  88       clock_->WallNow().ToUNIXSeconds() - cached_network_params.timestamp();
  89   if (seconds_since_estimate > kNumSecondsPerHour) {
  90     return false;
  91   }
  92
  93   QuicBandwidth bandwidth = QuicBandwidth::FromBytesPerSecond(
  94       max_bandwidth_resumption
  95           ? cached_network_params.max_bandwidth_estimate_bytes_per_second()
  96           : cached_network_params.bandwidth_estimate_bytes_per_second());
  97   QuicTime::Delta rtt_ms =
  98       QuicTime::Delta::FromMilliseconds(cached_network_params.min_rtt_ms());
  99
 100   // Make sure CWND is in appropriate range (in case of bad data).
 101   QuicPacketCount new_congestion_window =
 102       bandwidth.ToBytesPerPeriod(rtt_ms) / kMaxPacketSize;
 103   congestion_window_ = max(min(new_congestion_window, kMaxTcpCongestionWindow),
 104                            kMinCongestionWindowForBandwidthResumption);
 105
 106   // TODO(rjshade): Set appropriate CWND when previous connection was in slow
 107   // start at time of estimate.
 108   return true;
 109 }
 110
 111 void TcpCubicSender::SetNumEmulatedConnections(int num_connections) {
 112   num_connections_ = max(1, num_connections);
 113   cubic_.SetNumConnections(num_connections_);
 114 }
 115
 116 void TcpCubicSender::SetMaxCongestionWindow(
 117     QuicByteCount max_congestion_window) {
 118   max_tcp_congestion_window_ = max_congestion_window / kMaxPacketSize;
 119 }
 120
 121 float TcpCubicSender::RenoBeta() const {
 122   // kNConnectionBeta is the backoff factor after loss for our N-connection
 123   // emulation, which emulates the effective backoff of an ensemble of N
 124   // TCP-Reno connections on a single loss event. The effective multiplier is
 125   // computed as:
 126   return (num_connections_ - 1 + kRenoBeta) / num_connections_;
 127 }
 128
 129 void TcpCubicSender::OnCongestionEvent(
 130     bool rtt_updated,
 131     QuicByteCount bytes_in_flight,
 132     const CongestionVector& acked_packets,
 133     const CongestionVector& lost_packets) {
 134   if (rtt_updated && InSlowStart() &&
 135       hybrid_slow_start_.ShouldExitSlowStart(rtt_stats_->latest_rtt(),
 136                                              rtt_stats_->min_rtt(),
 137                                              congestion_window_)) {
 138     slowstart_threshold_ = congestion_window_;
 139   }
 140   for (CongestionVector::const_iterator it = lost_packets.begin();
 141        it != lost_packets.end(); ++it) {
 142     OnPacketLost(it->first, bytes_in_flight);
 143   }
 144   for (CongestionVector::const_iterator it = acked_packets.begin();
 145        it != acked_packets.end(); ++it) {
 146     OnPacketAcked(it->first, it->second.bytes_sent, bytes_in_flight);
 147   }
 148 }
 149
 150 void TcpCubicSender::OnPacketAcked(
 151     QuicPacketSequenceNumber acked_sequence_number,
 152     QuicByteCount acked_bytes,
 153     QuicByteCount bytes_in_flight) {
 154   largest_acked_sequence_number_ = max(acked_sequence_number,
 155                                        largest_acked_sequence_number_);
 156   if (InRecovery()) {
 157     // PRR is used when in recovery.
 158     prr_.OnPacketAcked(acked_bytes);
 159     return;
 160   }
 161   MaybeIncreaseCwnd(acked_sequence_number, bytes_in_flight);
 162   // TODO(ianswett): Should this even be called when not in slow start?
 163   hybrid_slow_start_.OnPacketAcked(acked_sequence_number, InSlowStart());
 164 }
 165
 166 void TcpCubicSender::OnPacketLost(QuicPacketSequenceNumber sequence_number,
 167                                   QuicByteCount bytes_in_flight) {
 168   // TCP NewReno (RFC6582) says that once a loss occurs, any losses in packets
 169   // already sent should be treated as a single loss event, since it's expected.
 170   if (sequence_number <= largest_sent_at_last_cutback_) {
 171     if (last_cutback_exited_slowstart_) {
 172       ++stats_->slowstart_packets_lost;
 173     }
 174     DVLOG(1) << "Ignoring loss for largest_missing:" << sequence_number
 175              << " because it was sent prior to the last CWND cutback.";
 176     return;
 177   }
 178   ++stats_->tcp_loss_events;
 179   last_cutback_exited_slowstart_ = InSlowStart();
 180   if (InSlowStart()) {
 181     ++stats_->slowstart_packets_lost;
 182   }
 183
 184   prr_.OnPacketLost(bytes_in_flight);
 185
 186   if (reno_) {
 187     congestion_window_ = congestion_window_ * RenoBeta();
 188   } else {
 189     congestion_window_ =
 190         cubic_.CongestionWindowAfterPacketLoss(congestion_window_);
 191   }
 192   slowstart_threshold_ = congestion_window_;
 193   // Enforce a minimum congestion window.
 194   if (congestion_window_ < min_congestion_window_) {
 195     congestion_window_ = min_congestion_window_;
 196   }
 197   largest_sent_at_last_cutback_ = largest_sent_sequence_number_;
 198   // reset packet count from congestion avoidance mode. We start
 199   // counting again when we're out of recovery.
 200   congestion_window_count_ = 0;
 201   DVLOG(1) << "Incoming loss; congestion window: " << congestion_window_
 202            << " slowstart threshold: " << slowstart_threshold_;
 203 }
 204
 205 bool TcpCubicSender::OnPacketSent(QuicTime /*sent_time*/,
 206                                   QuicByteCount /*bytes_in_flight*/,
 207                                   QuicPacketSequenceNumber sequence_number,
 208                                   QuicByteCount bytes,
 209                                   HasRetransmittableData is_retransmittable) {
 210   if (InSlowStart()) {
 211     ++(stats_->slowstart_packets_sent);
 212   }
 213
 214   // Only update bytes_in_flight_ for data packets.
 215   if (is_retransmittable != HAS_RETRANSMITTABLE_DATA) {
 216     return false;
 217   }
 218   if (InRecovery()) {
 219     // PRR is used when in recovery.
 220     prr_.OnPacketSent(bytes);
 221   }
 222   DCHECK_LT(largest_sent_sequence_number_, sequence_number);
 223   largest_sent_sequence_number_ = sequence_number;
 224   hybrid_slow_start_.OnPacketSent(sequence_number);
 225   return true;
 226 }
 227
 228 QuicTime::Delta TcpCubicSender::TimeUntilSend(
 229     QuicTime /* now */,
 230     QuicByteCount bytes_in_flight,
 231     HasRetransmittableData has_retransmittable_data) const {
 232   if (has_retransmittable_data == NO_RETRANSMITTABLE_DATA) {
 233     // For TCP we can always send an ACK immediately.
 234     return QuicTime::Delta::Zero();
 235   }
 236   if (InRecovery()) {
 237     // PRR is used when in recovery.
 238     return prr_.TimeUntilSend(GetCongestionWindow(), bytes_in_flight,
 239                               slowstart_threshold_ * kMaxSegmentSize);
 240   }
 241   if (GetCongestionWindow() > bytes_in_flight) {
 242     return QuicTime::Delta::Zero();
 243   }
 244   if (min4_mode_ && bytes_in_flight < 4 * kMaxSegmentSize) {
 245     return QuicTime::Delta::Zero();
 246   }
 247   return QuicTime::Delta::Infinite();
 248 }
 249
 250 QuicBandwidth TcpCubicSender::PacingRate() const {
 251   // We pace at twice the rate of the underlying sender's bandwidth estimate
 252   // during slow start and 1.25x during congestion avoidance to ensure pacing
 253   // doesn't prevent us from filling the window.
 254   QuicTime::Delta srtt = rtt_stats_->smoothed_rtt();
 255   if (srtt.IsZero()) {
 256     srtt = QuicTime::Delta::FromMicroseconds(rtt_stats_->initial_rtt_us());
 257   }
 258   const QuicBandwidth bandwidth =
 259       QuicBandwidth::FromBytesAndTimeDelta(GetCongestionWindow(), srtt);
 260   return bandwidth.Scale(InSlowStart() ? 2 : 1.25);
 261 }
 262
 263 QuicBandwidth TcpCubicSender::BandwidthEstimate() const {
 264   QuicTime::Delta srtt = rtt_stats_->smoothed_rtt();
 265   if (srtt.IsZero()) {
 266     // If we haven't measured an rtt, the bandwidth estimate is unknown.
 267     return QuicBandwidth::Zero();
 268   }
 269   return QuicBandwidth::FromBytesAndTimeDelta(GetCongestionWindow(), srtt);
 270 }
 271
 272 bool TcpCubicSender::HasReliableBandwidthEstimate() const {
 273   return !InSlowStart() && !InRecovery() &&
 274       !rtt_stats_->smoothed_rtt().IsZero();;
 275 }
 276
 277 QuicTime::Delta TcpCubicSender::RetransmissionDelay() const {
 278   if (rtt_stats_->smoothed_rtt().IsZero()) {
 279     return QuicTime::Delta::Zero();
 280   }
 281   return rtt_stats_->smoothed_rtt().Add(
 282       rtt_stats_->mean_deviation().Multiply(4));
 283 }
 284
 285 QuicByteCount TcpCubicSender::GetCongestionWindow() const {
 286   return congestion_window_ * kMaxSegmentSize;
 287 }
 288
 289 bool TcpCubicSender::InSlowStart() const {
 290   return congestion_window_ < slowstart_threshold_;
 291 }
 292
 293 QuicByteCount TcpCubicSender::GetSlowStartThreshold() const {
 294   return slowstart_threshold_ * kMaxSegmentSize;
 295 }
 296
 297 bool TcpCubicSender::IsCwndLimited(QuicByteCount bytes_in_flight) const {
 298   const QuicByteCount congestion_window_bytes = congestion_window_ *
 299       kMaxSegmentSize;
 300   if (bytes_in_flight >= congestion_window_bytes) {
 301     return true;
 302   }
 303   const QuicByteCount max_burst = kMaxBurstLength * kMaxSegmentSize;
 304   const QuicByteCount available_bytes =
 305       congestion_window_bytes - bytes_in_flight;
 306   const bool slow_start_limited = InSlowStart() &&
 307       bytes_in_flight >  congestion_window_bytes / 2;
 308   return slow_start_limited || available_bytes <= max_burst;
 309 }
 310
 311 bool TcpCubicSender::InRecovery() const {
 312   return largest_acked_sequence_number_ <= largest_sent_at_last_cutback_ &&
 313       largest_acked_sequence_number_ != 0;
 314 }
 315
 316 // Called when we receive an ack. Normal TCP tracks how many packets one ack
 317 // represents, but quic has a separate ack for each packet.
 318 void TcpCubicSender::MaybeIncreaseCwnd(
 319     QuicPacketSequenceNumber acked_sequence_number,
 320     QuicByteCount bytes_in_flight) {
 321   LOG_IF(DFATAL, InRecovery()) << "Never increase the CWND during recovery.";
 322   if (!IsCwndLimited(bytes_in_flight)) {
 323     // We don't update the congestion window unless we are close to using the
 324     // window we have available.
 325     return;
 326   }
 327   if (InSlowStart()) {
 328     // congestion_window_cnt is the number of acks since last change of snd_cwnd
 329     if (congestion_window_ < max_tcp_congestion_window_) {
 330       // TCP slow start, exponential growth, increase by one for each ACK.
 331       ++congestion_window_;
 332     }
 333     DVLOG(1) << "Slow start; congestion window: " << congestion_window_
 334              << " slowstart threshold: " << slowstart_threshold_;
 335     return;
 336   }
 337   if (congestion_window_ >= max_tcp_congestion_window_) {
 338     return;
 339   }
 340   // Congestion avoidance
 341   if (reno_) {
 342     // Classic Reno congestion avoidance.
 343     ++congestion_window_count_;
 344     // Divide by num_connections to smoothly increase the CWND at a faster
 345     // rate than conventional Reno.
 346     if (congestion_window_count_ * num_connections_ >= congestion_window_) {
 347       ++congestion_window_;
 348       congestion_window_count_ = 0;
 349     }
 350
 351     DVLOG(1) << "Reno; congestion window: " << congestion_window_
 352              << " slowstart threshold: " << slowstart_threshold_
 353              << " congestion window count: " << congestion_window_count_;
 354   } else {
 355     congestion_window_ = min(max_tcp_congestion_window_,
 356                              cubic_.CongestionWindowAfterAck(
 357                                  congestion_window_, rtt_stats_->min_rtt()));
 358     DVLOG(1) << "Cubic; congestion window: " << congestion_window_
 359              << " slowstart threshold: " << slowstart_threshold_;
 360   }
 361 }
 362
 363 void TcpCubicSender::OnRetransmissionTimeout(bool packets_retransmitted) {
 364   largest_sent_at_last_cutback_ = 0;
 365   if (!packets_retransmitted) {
 366     return;
 367   }
 368   cubic_.Reset();
 369   hybrid_slow_start_.Restart();
 370   slowstart_threshold_ = congestion_window_ / 2;
 371   congestion_window_ = min_congestion_window_;
 372 }
 373
 374 CongestionControlType TcpCubicSender::GetCongestionControlType() const {
 375   return reno_ ? kReno : kCubic;
 376 }
 377
 378 }  // namespace net