Create a TcpCubicSender in QUIC that tracks the congestion window in

net/quic/congestion_control/tcp_cubic_bytes_sender.cc

-// Copyright (c) 2012 The Chromium Authors. All rights reserved.
+// Copyright (c) 2015 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 "net/quic/congestion_control/tcp_cubic_bytes_sender.h"
 
 #include <algorithm>
 
-#include "base/metrics/histogram.h"
 #include "net/quic/congestion_control/prr_sender.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 QuicPacketCount kDefaultMinimumCongestionWindow = 2;
+// fast retransmission.
+const QuicByteCount kMinimumCongestionWindow = 2 * kDefaultTCPMSS;
 const QuicByteCount kMaxSegmentSize = kDefaultTCPMSS;
 const int kMaxBurstLength = 3;
-const float kRenoBeta = 0.7f;  // Reno backoff factor.
+const float kRenoBeta = 0.7f;             // Reno backoff factor.
 const uint32 kDefaultNumConnections = 2;  // N-connection emulation.
 }  // namespace
 
-TcpCubicSender::TcpCubicSender(const QuicClock* clock,
-                               const RttStats* rtt_stats,
-                               bool reno,
-                               QuicPacketCount initial_tcp_congestion_window,
-                               QuicConnectionStats* stats)
+TcpCubicBytesSender::TcpCubicBytesSender(
+    const QuicClock* clock,
+    const RttStats* rtt_stats,
+    bool reno,
+    QuicPacketCount initial_tcp_congestion_window,
+    QuicConnectionStats* stats)
     : hybrid_slow_start_(clock),
       cubic_(clock),
       rtt_stats_(rtt_stats),
       stats_(stats),
       reno_(reno),
       num_connections_(kDefaultNumConnections),
       num_acked_packets_(0),
       largest_sent_sequence_number_(0),
       largest_acked_sequence_number_(0),
       largest_sent_at_last_cutback_(0),
-      congestion_window_(initial_tcp_congestion_window),
-      min_congestion_window_(kDefaultMinimumCongestionWindow),
+      congestion_window_(initial_tcp_congestion_window * kMaxSegmentSize),
       slowstart_threshold_(std::numeric_limits<uint64>::max()),
       last_cutback_exited_slowstart_(false),
       clock_(clock) {
 }
 
-TcpCubicSender::~TcpCubicSender() {
-  UMA_HISTOGRAM_COUNTS("Net.QuicSession.FinalTcpCwnd", congestion_window_);
+TcpCubicBytesSender::~TcpCubicBytesSender() {
 }
 
-void TcpCubicSender::SetFromConfig(const QuicConfig& config,
-                                   Perspective perspective,
-                                   bool using_pacing) {
+void TcpCubicBytesSender::SetFromConfig(const QuicConfig& config,
+                                        Perspective perspective,
+                                        bool using_pacing) {
   if (perspective == Perspective::IS_SERVER) {
     if (config.HasReceivedConnectionOptions() &&
         ContainsQuicTag(config.ReceivedConnectionOptions(), kIW10)) {
       // Initial window experiment.
-      congestion_window_ = 10;
-    }
-    if (config.HasReceivedConnectionOptions() &&
-        ContainsQuicTag(config.ReceivedConnectionOptions(), kMIN1)) {
-      // Min CWND experiment.
-      min_congestion_window_ = 1;
+      congestion_window_ = 10 * kMaxSegmentSize;
     }
     if (using_pacing) {
       // Disable the ack train mode in hystart when pacing is enabled, since it
       // may be falsely triggered.
       hybrid_slow_start_.set_ack_train_detection(false);
     }
   }
 }
 
-bool TcpCubicSender::ResumeConnectionState(
+bool TcpCubicBytesSender::ResumeConnectionState(
     const CachedNetworkParameters& cached_network_params) {
   // If the previous bandwidth estimate is less than an hour old, store in
   // preparation for doing bandwidth resumption.
   int64 seconds_since_estimate =
       clock_->WallNow().ToUNIXSeconds() - cached_network_params.timestamp();
   if (seconds_since_estimate > kNumSecondsPerHour) {
     return false;
   }
 
   QuicBandwidth bandwidth = QuicBandwidth::FromBytesPerSecond(
       cached_network_params.bandwidth_estimate_bytes_per_second());
   QuicTime::Delta rtt_ms =
       QuicTime::Delta::FromMilliseconds(cached_network_params.min_rtt_ms());
 
   // Make sure CWND is in appropriate range (in case of bad data).
-  QuicPacketCount new_congestion_window =
-      bandwidth.ToBytesPerPeriod(rtt_ms) / kMaxPacketSize;
-  congestion_window_ = max(
-      min(new_congestion_window, kMaxCongestionWindowForBandwidthResumption),
-      kMinCongestionWindowForBandwidthResumption);
+  QuicByteCount new_congestion_window = bandwidth.ToBytesPerPeriod(rtt_ms);
+  congestion_window_ =
+      max(min(new_congestion_window,
+              kMaxCongestionWindowForBandwidthResumption * kMaxSegmentSize),
+          kMinCongestionWindowForBandwidthResumption * kMaxSegmentSize);
 
   // TODO(rjshade): Set appropriate CWND when previous connection was in slow
   // start at time of estimate.
   return true;
 }
 
-void TcpCubicSender::SetNumEmulatedConnections(int num_connections) {
+void TcpCubicBytesSender::SetNumEmulatedConnections(int num_connections) {
   num_connections_ = max(1, num_connections);
   cubic_.SetNumConnections(num_connections_);
 }
 
-float TcpCubicSender::RenoBeta() const {
+float TcpCubicBytesSender::RenoBeta() const {
   // kNConnectionBeta is the backoff factor after loss for our N-connection
   // emulation, which emulates the effective backoff of an ensemble of N
   // TCP-Reno connections on a single loss event. The effective multiplier is
   // computed as:
   return (num_connections_ - 1 + kRenoBeta) / num_connections_;
 }
 
-void TcpCubicSender::OnCongestionEvent(
+void TcpCubicBytesSender::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_)) {
+      hybrid_slow_start_.ShouldExitSlowStart(
+          rtt_stats_->latest_rtt(), rtt_stats_->min_rtt(),
+          congestion_window_ / kMaxSegmentSize)) {
     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(
+void TcpCubicBytesSender::OnPacketAcked(
     QuicPacketSequenceNumber acked_sequence_number,
     QuicByteCount acked_bytes,
     QuicByteCount bytes_in_flight) {
-  largest_acked_sequence_number_ = max(acked_sequence_number,
-                                       largest_acked_sequence_number_);
+  largest_acked_sequence_number_ =
+      max(acked_sequence_number, largest_acked_sequence_number_);
   if (InRecovery()) {
     // PRR is used when in recovery.
     prr_.OnPacketAcked(acked_bytes);
     return;
   }
-  MaybeIncreaseCwnd(acked_sequence_number, bytes_in_flight);
+  MaybeIncreaseCwnd(acked_sequence_number, acked_bytes, 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) {
+void TcpCubicBytesSender::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;
   }
 
   prr_.OnPacketLost(bytes_in_flight);
 
   if (reno_) {
     congestion_window_ = congestion_window_ * RenoBeta();
   } else {
     congestion_window_ =
         cubic_.CongestionWindowAfterPacketLoss(congestion_window_);
   }
   slowstart_threshold_ = congestion_window_;
-  // Enforce a minimum congestion window.
-  if (congestion_window_ < min_congestion_window_) {
-    congestion_window_ = min_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.
+  // Reset packet count from congestion avoidance mode. We start counting again
+  // when we're out of recovery.
   num_acked_packets_ = 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) {
+bool TcpCubicBytesSender::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;
   }
   if (InRecovery()) {
     // PRR is used when in recovery.
     prr_.OnPacketSent(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::Delta TcpCubicBytesSender::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()) {
     // PRR is used when in recovery.
     return prr_.TimeUntilSend(GetCongestionWindow(), bytes_in_flight,
-                              slowstart_threshold_ * kMaxSegmentSize);
+                              slowstart_threshold_);
   }
   if (GetCongestionWindow() > bytes_in_flight) {
     return QuicTime::Delta::Zero();
   }
   return QuicTime::Delta::Infinite();
 }
 
-QuicBandwidth TcpCubicSender::PacingRate() const {
+QuicBandwidth TcpCubicBytesSender::PacingRate() const {
   // We pace at twice the rate of the underlying sender's bandwidth estimate
   // during slow start and 1.25x during congestion avoidance to ensure pacing
   // doesn't prevent us from filling the window.
   QuicTime::Delta srtt = rtt_stats_->smoothed_rtt();
   if (srtt.IsZero()) {
     srtt = QuicTime::Delta::FromMicroseconds(rtt_stats_->initial_rtt_us());
   }
   const QuicBandwidth bandwidth =
       QuicBandwidth::FromBytesAndTimeDelta(GetCongestionWindow(), srtt);
   return bandwidth.Scale(InSlowStart() ? 2 : 1.25);
 }
 
-QuicBandwidth TcpCubicSender::BandwidthEstimate() const {
+QuicBandwidth TcpCubicBytesSender::BandwidthEstimate() const {
   QuicTime::Delta srtt = rtt_stats_->smoothed_rtt();
   if (srtt.IsZero()) {
     // If we haven't measured an rtt, the bandwidth estimate is unknown.
     return QuicBandwidth::Zero();
   }
   return QuicBandwidth::FromBytesAndTimeDelta(GetCongestionWindow(), srtt);
 }
 
-bool TcpCubicSender::HasReliableBandwidthEstimate() const {
+bool TcpCubicBytesSender::HasReliableBandwidthEstimate() const {
   return !InSlowStart() && !InRecovery() &&
-      !rtt_stats_->smoothed_rtt().IsZero();;
+         !rtt_stats_->smoothed_rtt().IsZero();
 }
 
-QuicTime::Delta TcpCubicSender::RetransmissionDelay() const {
+QuicTime::Delta TcpCubicBytesSender::RetransmissionDelay() const {
   if (rtt_stats_->smoothed_rtt().IsZero()) {
     return QuicTime::Delta::Zero();
   }
   return rtt_stats_->smoothed_rtt().Add(
       rtt_stats_->mean_deviation().Multiply(4));
 }
 
-QuicByteCount TcpCubicSender::GetCongestionWindow() const {
-  return congestion_window_ * kMaxSegmentSize;
+QuicByteCount TcpCubicBytesSender::GetCongestionWindow() const {
+  return congestion_window_;
 }
 
-bool TcpCubicSender::InSlowStart() const {
+bool TcpCubicBytesSender::InSlowStart() const {
   return congestion_window_ < slowstart_threshold_;
 }
 
-QuicByteCount TcpCubicSender::GetSlowStartThreshold() const {
-  return slowstart_threshold_ * kMaxSegmentSize;
+QuicByteCount TcpCubicBytesSender::GetSlowStartThreshold() const {
+  return slowstart_threshold_;
 }
 
-bool TcpCubicSender::IsCwndLimited(QuicByteCount bytes_in_flight) const {
-  const QuicByteCount congestion_window_bytes = congestion_window_ *
-      kMaxSegmentSize;
-  if (bytes_in_flight >= congestion_window_bytes) {
+bool TcpCubicBytesSender::IsCwndLimited(QuicByteCount bytes_in_flight) const {
+  if (bytes_in_flight >= congestion_window_) {
     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;
+  const QuicByteCount available_bytes = congestion_window_ - bytes_in_flight;
+  const bool slow_start_limited =
+      InSlowStart() && bytes_in_flight > congestion_window_ / 2;
   return slow_start_limited || available_bytes <= max_burst;
 }
 
-bool TcpCubicSender::InRecovery() const {
+bool TcpCubicBytesSender::InRecovery() const {
   return largest_acked_sequence_number_ <= largest_sent_at_last_cutback_ &&
-      largest_acked_sequence_number_ != 0;
+         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(
+void TcpCubicBytesSender::MaybeIncreaseCwnd(
     QuicPacketSequenceNumber acked_sequence_number,
+    QuicByteCount acked_bytes,
     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()) {
     // TCP slow start, exponential growth, increase by one for each ACK.
-    ++congestion_window_;
+    congestion_window_ += kMaxSegmentSize;
     DVLOG(1) << "Slow start; congestion window: " << congestion_window_
              << " slowstart threshold: " << slowstart_threshold_;
     return;
   }
-  // Congestion avoidance
+  // Congestion avoidance.
   if (reno_) {
     // Classic Reno congestion avoidance.
     ++num_acked_packets_;
-    // Divide by num_connections to smoothly increase the CWND at a faster
-    // rate than conventional Reno.
-    if (num_acked_packets_ * num_connections_ >= congestion_window_) {
-      ++congestion_window_;
+    // Divide by num_connections to smoothly increase the CWND at a faster rate
+    // than conventional Reno.
+    if (num_acked_packets_ * num_connections_ >=
+        congestion_window_ / kMaxSegmentSize) {
+      congestion_window_ += kMaxSegmentSize;
       num_acked_packets_ = 0;
     }
 
     DVLOG(1) << "Reno; congestion window: " << congestion_window_
              << " slowstart threshold: " << slowstart_threshold_
              << " congestion window count: " << num_acked_packets_;
   } else {
-    congestion_window_ = cubic_.CongestionWindowAfterAck(congestion_window_,
-                                                         rtt_stats_->min_rtt());
+    congestion_window_ = cubic_.CongestionWindowAfterAck(
+        acked_bytes, congestion_window_, rtt_stats_->min_rtt());
     DVLOG(1) << "Cubic; congestion window: " << congestion_window_
              << " slowstart threshold: " << slowstart_threshold_;
   }
 }
 
-void TcpCubicSender::OnRetransmissionTimeout(bool packets_retransmitted) {
+void TcpCubicBytesSender::OnRetransmissionTimeout(bool packets_retransmitted) {
   largest_sent_at_last_cutback_ = 0;
   if (!packets_retransmitted) {
     return;
   }
   cubic_.Reset();
   hybrid_slow_start_.Restart();
   slowstart_threshold_ = congestion_window_ / 2;
-  congestion_window_ = min_congestion_window_;
+  congestion_window_ = kMinimumCongestionWindow;
 }
 
-CongestionControlType TcpCubicSender::GetCongestionControlType() const {
+CongestionControlType TcpCubicBytesSender::GetCongestionControlType() const {
   return reno_ ? kReno : kCubic;
 }
 
 }  // namespace net