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_macros.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 QuicByteCount kMaxBurstBytes = 3 * kMaxSegmentSize;
  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     : cubic_(clock),
  38       rtt_stats_(rtt_stats),
  39       stats_(stats),
  40       reno_(reno),
  41       num_connections_(kDefaultNumConnections),
  42       congestion_window_count_(0),
  43       largest_sent_sequence_number_(0),
  44       largest_acked_sequence_number_(0),
  45       largest_sent_at_last_cutback_(0),
  46       congestion_window_(initial_tcp_congestion_window),
  47       min_congestion_window_(kDefaultMinimumCongestionWindow),
  48       min4_mode_(false),
  49       slowstart_threshold_(max_tcp_congestion_window),
  50       last_cutback_exited_slowstart_(false),
  51       max_tcp_congestion_window_(max_tcp_congestion_window),
  52       clock_(clock) {
  53 }
  54
  55 TcpCubicSender::~TcpCubicSender() {
  56   UMA_HISTOGRAM_COUNTS("Net.QuicSession.FinalTcpCwnd", congestion_window_);
  57 }
  58
  59 void TcpCubicSender::SetFromConfig(const QuicConfig& config,
  60                                    Perspective perspective) {
  61   if (perspective == Perspective::IS_SERVER) {
  62     if (config.HasReceivedConnectionOptions() &&
  63         ContainsQuicTag(config.ReceivedConnectionOptions(), kIW03)) {
  64       // Initial window experiment.
  65       congestion_window_ = 3;
  66     }
  67     if (config.HasReceivedConnectionOptions() &&
  68         ContainsQuicTag(config.ReceivedConnectionOptions(), kIW10)) {
  69       // Initial window experiment.
  70       congestion_window_ = 10;
  71     }
  72     if (config.HasReceivedConnectionOptions() &&
  73         ContainsQuicTag(config.ReceivedConnectionOptions(), kIW20)) {
  74       // Initial window experiment.
  75       congestion_window_ = 20;
  76     }
  77     if (config.HasReceivedConnectionOptions() &&
  78         ContainsQuicTag(config.ReceivedConnectionOptions(), kIW50)) {
  79       // Initial window experiment.
  80       congestion_window_ = 50;
  81     }
  82     if (config.HasReceivedConnectionOptions() &&
  83         ContainsQuicTag(config.ReceivedConnectionOptions(), kMIN1)) {
  84       // Min CWND experiment.
  85       min_congestion_window_ = 1;
  86     }
  87     if (config.HasReceivedConnectionOptions() &&
  88         ContainsQuicTag(config.ReceivedConnectionOptions(), kMIN4)) {
  89       // Min CWND of 4 experiment.
  90       min4_mode_ = true;
  91       min_congestion_window_ = 1;
  92     }
  93   }
  94 }
  95
  96 void TcpCubicSender::ResumeConnectionState(
  97     const CachedNetworkParameters& cached_network_params,
  98     bool max_bandwidth_resumption) {
  99   QuicBandwidth bandwidth = QuicBandwidth::FromBytesPerSecond(
 100       max_bandwidth_resumption
 101           ? cached_network_params.max_bandwidth_estimate_bytes_per_second()
 102           : cached_network_params.bandwidth_estimate_bytes_per_second());
 103   QuicTime::Delta rtt_ms =
 104       QuicTime::Delta::FromMilliseconds(cached_network_params.min_rtt_ms());
 105
 106   // Make sure CWND is in appropriate range (in case of bad data).
 107   QuicPacketCount new_congestion_window =
 108       bandwidth.ToBytesPerPeriod(rtt_ms) / kMaxPacketSize;
 109   congestion_window_ = max(min(new_congestion_window, kMaxCongestionWindow),
 110                            kMinCongestionWindowForBandwidthResumption);
 111 }
 112
 113 void TcpCubicSender::SetNumEmulatedConnections(int num_connections) {
 114   num_connections_ = max(1, num_connections);
 115   cubic_.SetNumConnections(num_connections_);
 116 }
 117
 118 void TcpCubicSender::SetMaxCongestionWindow(
 119     QuicByteCount max_congestion_window) {
 120   max_tcp_congestion_window_ = max_congestion_window / kMaxPacketSize;
 121 }
 122
 123 float TcpCubicSender::RenoBeta() const {
 124   // kNConnectionBeta is the backoff factor after loss for our N-connection
 125   // emulation, which emulates the effective backoff of an ensemble of N
 126   // TCP-Reno connections on a single loss event. The effective multiplier is
 127   // computed as:
 128   return (num_connections_ - 1 + kRenoBeta) / num_connections_;
 129 }
 130
 131 void TcpCubicSender::OnCongestionEvent(
 132     bool rtt_updated,
 133     QuicByteCount bytes_in_flight,
 134     const CongestionVector& acked_packets,
 135     const CongestionVector& lost_packets) {
 136   if (rtt_updated && InSlowStart() &&
 137       hybrid_slow_start_.ShouldExitSlowStart(rtt_stats_->latest_rtt(),
 138                                              rtt_stats_->min_rtt(),
 139                                              congestion_window_)) {
 140     slowstart_threshold_ = congestion_window_;
 141   }
 142   for (CongestionVector::const_iterator it = lost_packets.begin();
 143        it != lost_packets.end(); ++it) {
 144     OnPacketLost(it->first, bytes_in_flight);
 145   }
 146   for (CongestionVector::const_iterator it = acked_packets.begin();
 147        it != acked_packets.end(); ++it) {
 148     OnPacketAcked(it->first, it->second.bytes_sent, bytes_in_flight);
 149   }
 150 }
 151
 152 void TcpCubicSender::OnPacketAcked(
 153     QuicPacketSequenceNumber acked_sequence_number,
 154     QuicByteCount acked_bytes,
 155     QuicByteCount bytes_in_flight) {
 156   largest_acked_sequence_number_ = max(acked_sequence_number,
 157                                        largest_acked_sequence_number_);
 158   if (InRecovery()) {
 159     // PRR is used when in recovery.
 160     prr_.OnPacketAcked(acked_bytes);
 161     return;
 162   }
 163   MaybeIncreaseCwnd(acked_sequence_number, bytes_in_flight);
 164   // TODO(ianswett): Should this even be called when not in slow start?
 165   hybrid_slow_start_.OnPacketAcked(acked_sequence_number, InSlowStart());
 166 }
 167
 168 void TcpCubicSender::OnPacketLost(QuicPacketSequenceNumber sequence_number,
 169                                   QuicByteCount bytes_in_flight) {
 170   // TCP NewReno (RFC6582) says that once a loss occurs, any losses in packets
 171   // already sent should be treated as a single loss event, since it's expected.
 172   if (sequence_number <= largest_sent_at_last_cutback_) {
 173     if (last_cutback_exited_slowstart_) {
 174       ++stats_->slowstart_packets_lost;
 175     }
 176     DVLOG(1) << "Ignoring loss for largest_missing:" << sequence_number
 177              << " because it was sent prior to the last CWND cutback.";
 178     return;
 179   }
 180   ++stats_->tcp_loss_events;
 181   last_cutback_exited_slowstart_ = InSlowStart();
 182   if (InSlowStart()) {
 183     ++stats_->slowstart_packets_lost;
 184   }
 185
 186   prr_.OnPacketLost(bytes_in_flight);
 187
 188   if (reno_) {
 189     congestion_window_ = congestion_window_ * RenoBeta();
 190   } else {
 191     congestion_window_ =
 192         cubic_.CongestionWindowAfterPacketLoss(congestion_window_);
 193   }
 194   slowstart_threshold_ = congestion_window_;
 195   // Enforce a minimum congestion window.
 196   if (congestion_window_ < min_congestion_window_) {
 197     congestion_window_ = min_congestion_window_;
 198   }
 199   largest_sent_at_last_cutback_ = largest_sent_sequence_number_;
 200   // reset packet count from congestion avoidance mode. We start
 201   // counting again when we're out of recovery.
 202   congestion_window_count_ = 0;
 203   DVLOG(1) << "Incoming loss; congestion window: " << congestion_window_
 204            << " slowstart threshold: " << slowstart_threshold_;
 205 }
 206
 207 bool TcpCubicSender::OnPacketSent(QuicTime /*sent_time*/,
 208                                   QuicByteCount /*bytes_in_flight*/,
 209                                   QuicPacketSequenceNumber sequence_number,
 210                                   QuicByteCount bytes,
 211                                   HasRetransmittableData is_retransmittable) {
 212   if (InSlowStart()) {
 213     ++(stats_->slowstart_packets_sent);
 214   }
 215
 216   // Only update bytes_in_flight_ for data packets.
 217   if (is_retransmittable != HAS_RETRANSMITTABLE_DATA) {
 218     return false;
 219   }
 220   if (InRecovery()) {
 221     // PRR is used when in recovery.
 222     prr_.OnPacketSent(bytes);
 223   }
 224   DCHECK_LT(largest_sent_sequence_number_, sequence_number);
 225   largest_sent_sequence_number_ = sequence_number;
 226   hybrid_slow_start_.OnPacketSent(sequence_number);
 227   return true;
 228 }
 229
 230 QuicTime::Delta TcpCubicSender::TimeUntilSend(
 231     QuicTime /* now */,
 232     QuicByteCount bytes_in_flight,
 233     HasRetransmittableData has_retransmittable_data) const {
 234   if (has_retransmittable_data == NO_RETRANSMITTABLE_DATA) {
 235     // For TCP we can always send an ACK immediately.
 236     return QuicTime::Delta::Zero();
 237   }
 238   if (InRecovery()) {
 239     // PRR is used when in recovery.
 240     return prr_.TimeUntilSend(GetCongestionWindow(), bytes_in_flight,
 241                               slowstart_threshold_ * kMaxSegmentSize);
 242   }
 243   if (GetCongestionWindow() > bytes_in_flight) {
 244     return QuicTime::Delta::Zero();
 245   }
 246   if (min4_mode_ && bytes_in_flight < 4 * kMaxSegmentSize) {
 247     return QuicTime::Delta::Zero();
 248   }
 249   return QuicTime::Delta::Infinite();
 250 }
 251
 252 QuicBandwidth TcpCubicSender::PacingRate() const {
 253   // We pace at twice the rate of the underlying sender's bandwidth estimate
 254   // during slow start and 1.25x during congestion avoidance to ensure pacing
 255   // doesn't prevent us from filling the window.
 256   QuicTime::Delta srtt = rtt_stats_->smoothed_rtt();
 257   if (srtt.IsZero()) {
 258     srtt = QuicTime::Delta::FromMicroseconds(rtt_stats_->initial_rtt_us());
 259   }
 260   const QuicBandwidth bandwidth =
 261       QuicBandwidth::FromBytesAndTimeDelta(GetCongestionWindow(), srtt);
 262   return bandwidth.Scale(InSlowStart() ? 2 : 1.25);
 263 }
 264
 265 QuicBandwidth TcpCubicSender::BandwidthEstimate() const {
 266   QuicTime::Delta srtt = rtt_stats_->smoothed_rtt();
 267   if (srtt.IsZero()) {
 268     // If we haven't measured an rtt, the bandwidth estimate is unknown.
 269     return QuicBandwidth::Zero();
 270   }
 271   return QuicBandwidth::FromBytesAndTimeDelta(GetCongestionWindow(), srtt);
 272 }
 273
 274 QuicTime::Delta TcpCubicSender::RetransmissionDelay() const {
 275   if (rtt_stats_->smoothed_rtt().IsZero()) {
 276     return QuicTime::Delta::Zero();
 277   }
 278   return rtt_stats_->smoothed_rtt().Add(
 279       rtt_stats_->mean_deviation().Multiply(4));
 280 }
 281
 282 QuicByteCount TcpCubicSender::GetCongestionWindow() const {
 283   return congestion_window_ * kMaxSegmentSize;
 284 }
 285
 286 bool TcpCubicSender::InSlowStart() const {
 287   return congestion_window_ < slowstart_threshold_;
 288 }
 289
 290 QuicByteCount TcpCubicSender::GetSlowStartThreshold() const {
 291   return slowstart_threshold_ * kMaxSegmentSize;
 292 }
 293
 294 bool TcpCubicSender::IsCwndLimited(QuicByteCount bytes_in_flight) const {
 295   const QuicByteCount congestion_window_bytes = GetCongestionWindow();
 296   if (bytes_in_flight >= congestion_window_bytes) {
 297     return true;
 298   }
 299   const QuicByteCount available_bytes =
 300       congestion_window_bytes - bytes_in_flight;
 301   const bool slow_start_limited = InSlowStart() &&
 302       bytes_in_flight >  congestion_window_bytes / 2;
 303   return slow_start_limited || available_bytes <= kMaxBurstBytes;
 304 }
 305
 306 bool TcpCubicSender::InRecovery() const {
 307   return largest_acked_sequence_number_ <= largest_sent_at_last_cutback_ &&
 308       largest_acked_sequence_number_ != 0;
 309 }
 310
 311 // Called when we receive an ack. Normal TCP tracks how many packets one ack
 312 // represents, but quic has a separate ack for each packet.
 313 void TcpCubicSender::MaybeIncreaseCwnd(
 314     QuicPacketSequenceNumber acked_sequence_number,
 315     QuicByteCount bytes_in_flight) {
 316   LOG_IF(DFATAL, InRecovery()) << "Never increase the CWND during recovery.";
 317   if (!IsCwndLimited(bytes_in_flight)) {
 318     // We don't update the congestion window unless we are close to using the
 319     // window we have available.
 320     return;
 321   }
 322   if (congestion_window_ >= max_tcp_congestion_window_) {
 323     return;
 324   }
 325   if (InSlowStart()) {
 326     // TCP slow start, exponential growth, increase by one for each ACK.
 327     ++congestion_window_;
 328     DVLOG(1) << "Slow start; congestion window: " << congestion_window_
 329              << " slowstart threshold: " << slowstart_threshold_;
 330     return;
 331   }
 332   // Congestion avoidance
 333   if (reno_) {
 334     // Classic Reno congestion avoidance.
 335     ++congestion_window_count_;
 336     // Divide by num_connections to smoothly increase the CWND at a faster
 337     // rate than conventional Reno.
 338     if (congestion_window_count_ * num_connections_ >= congestion_window_) {
 339       ++congestion_window_;
 340       congestion_window_count_ = 0;
 341     }
 342
 343     DVLOG(1) << "Reno; congestion window: " << congestion_window_
 344              << " slowstart threshold: " << slowstart_threshold_
 345              << " congestion window count: " << congestion_window_count_;
 346   } else {
 347     congestion_window_ = min(max_tcp_congestion_window_,
 348                              cubic_.CongestionWindowAfterAck(
 349                                  congestion_window_, rtt_stats_->min_rtt()));
 350     DVLOG(1) << "Cubic; congestion window: " << congestion_window_
 351              << " slowstart threshold: " << slowstart_threshold_;
 352   }
 353 }
 354
 355 void TcpCubicSender::OnRetransmissionTimeout(bool packets_retransmitted) {
 356   largest_sent_at_last_cutback_ = 0;
 357   if (!packets_retransmitted) {
 358     return;
 359   }
 360   cubic_.Reset();
 361   hybrid_slow_start_.Restart();
 362   slowstart_threshold_ = congestion_window_ / 2;
 363   congestion_window_ = min_congestion_window_;
 364 }
 365
 366 CongestionControlType TcpCubicSender::GetCongestionControlType() const {
 367   return reno_ ? kReno : kCubic;
 368 }
 369
 370 }  // namespace net