net/quic/congestion_control/cubic_bytes.cc

   1 // Copyright (c) 2015 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/cubic_bytes.h"
   6
   7 #include <stdint.h>
   8 #include <algorithm>
   9 #include <cmath>
  10
  11 #include "base/basictypes.h"
  12 #include "base/logging.h"
  13 #include "net/quic/quic_protocol.h"
  14
  15 using std::max;
  16
  17 namespace net {
  18
  19 namespace {
  20
  21 // Constants based on TCP defaults.
  22 // The following constants are in 2^10 fractions of a second instead of ms to
  23 // allow a 10 shift right to divide.
  24 const int kCubeScale = 40;  // 1024*1024^3 (first 1024 is from 0.100^3)
  25                             // where 0.100 is 100 ms which is the scaling
  26                             // round trip time.
  27 const int kCubeCongestionWindowScale = 410;
  28 // The cube factor for packets in bytes.
  29 const uint64 kCubeFactor = (UINT64_C(1) << kCubeScale) /
  30                            kCubeCongestionWindowScale / kDefaultTCPMSS;
  31
  32 const uint32 kDefaultNumConnections = 2;
  33 const float kBeta = 0.7f;  // Default Cubic backoff factor.
  34 // Additional backoff factor when loss occurs in the concave part of the Cubic
  35 // curve. This additional backoff factor is expected to give up bandwidth to
  36 // new concurrent flows and speed up convergence.
  37 const float kBetaLastMax = 0.85f;
  38
  39 }  // namespace
  40
  41 CubicBytes::CubicBytes(const QuicClock* clock)
  42     : clock_(clock),
  43       num_connections_(kDefaultNumConnections),
  44       epoch_(QuicTime::Zero()),
  45       last_update_time_(QuicTime::Zero()) {
  46   Reset();
  47 }
  48
  49 void CubicBytes::SetNumConnections(int num_connections) {
  50   num_connections_ = num_connections;
  51 }
  52
  53 float CubicBytes::Alpha() const {
  54   // TCPFriendly alpha is described in Section 3.3 of the CUBIC paper. Note that
  55   // beta here is a cwnd multiplier, and is equal to 1-beta from the paper.
  56   // We derive the equivalent alpha for an N-connection emulation as:
  57   const float beta = Beta();
  58   return 3 * num_connections_ * num_connections_ * (1 - beta) / (1 + beta);
  59 }
  60
  61 float CubicBytes::Beta() const {
  62   // kNConnectionBeta is the backoff factor after loss for our N-connection
  63   // emulation, which emulates the effective backoff of an ensemble of N
  64   // TCP-Reno connections on a single loss event. The effective multiplier is
  65   // computed as:
  66   return (num_connections_ - 1 + kBeta) / num_connections_;
  67 }
  68
  69 void CubicBytes::Reset() {
  70   epoch_ = QuicTime::Zero();             // Reset time.
  71   last_update_time_ = QuicTime::Zero();  // Reset time.
  72   last_congestion_window_ = 0;
  73   last_max_congestion_window_ = 0;
  74   acked_bytes_count_ = 0;
  75   estimated_tcp_congestion_window_ = 0;
  76   origin_point_congestion_window_ = 0;
  77   time_to_origin_point_ = 0;
  78   last_target_congestion_window_ = 0;
  79 }
  80
  81 void CubicBytes::OnApplicationLimited() {
  82   // When sender is not using the available congestion window, the window does
  83   // not grow. But to be RTT-independent, Cubic assumes that the sender has been
  84   // using the entire window during the time since the beginning of the current
  85   // "epoch" (the end of the last loss recovery period). Since
  86   // application-limited periods break this assumption, we reset the epoch when
  87   // in such a period. This reset effectively freezes congestion window growth
  88   // through application-limited periods and allows Cubic growth to continue
  89   // when the entire window is being used.
  90   epoch_ = QuicTime::Zero();
  91 }
  92
  93 QuicByteCount CubicBytes::CongestionWindowAfterPacketLoss(
  94     QuicByteCount current_congestion_window) {
  95   if (current_congestion_window < last_max_congestion_window_) {
  96     // We never reached the old max, so assume we are competing with another
  97     // flow. Use our extra back off factor to allow the other flow to go up.
  98     last_max_congestion_window_ =
  99         static_cast<int>(kBetaLastMax * current_congestion_window);
 100   } else {
 101     last_max_congestion_window_ = current_congestion_window;
 102   }
 103   epoch_ = QuicTime::Zero();  // Reset time.
 104   return static_cast<int>(current_congestion_window * Beta());
 105 }
 106
 107 QuicByteCount CubicBytes::CongestionWindowAfterAck(
 108     QuicByteCount acked_bytes,
 109     QuicByteCount current_congestion_window,
 110     QuicTime::Delta delay_min) {
 111   acked_bytes_count_ += acked_bytes;
 112   QuicTime current_time = clock_->ApproximateNow();
 113
 114   // Cubic is "independent" of RTT, the update is limited by the time elapsed.
 115   if (last_congestion_window_ == current_congestion_window &&
 116       (current_time.Subtract(last_update_time_) <= MaxCubicTimeInterval())) {
 117     return max(last_target_congestion_window_,
 118                estimated_tcp_congestion_window_);
 119   }
 120   last_congestion_window_ = current_congestion_window;
 121   last_update_time_ = current_time;
 122
 123   if (!epoch_.IsInitialized()) {
 124     // First ACK after a loss event.
 125     DVLOG(1) << "Start of epoch";
 126     epoch_ = current_time;             // Start of epoch.
 127     acked_bytes_count_ = acked_bytes;  // Reset count.
 128     // Reset estimated_tcp_congestion_window_ to be in sync with cubic.
 129     estimated_tcp_congestion_window_ = current_congestion_window;
 130     if (last_max_congestion_window_ <= current_congestion_window) {
 131       time_to_origin_point_ = 0;
 132       origin_point_congestion_window_ = current_congestion_window;
 133     } else {
 134       time_to_origin_point_ =
 135           static_cast<uint32>(cbrt(kCubeFactor * (last_max_congestion_window_ -
 136                                                   current_congestion_window)));
 137       origin_point_congestion_window_ = last_max_congestion_window_;
 138     }
 139   }
 140   // Change the time unit from microseconds to 2^10 fractions per second. Take
 141   // the round trip time in account. This is done to allow us to use shift as a
 142   // divide operator.
 143   int64 elapsed_time =
 144       (current_time.Add(delay_min).Subtract(epoch_).ToMicroseconds() << 10) /
 145       kNumMicrosPerSecond;
 146
 147   int64 offset = time_to_origin_point_ - elapsed_time;
 148   QuicByteCount delta_congestion_window =
 149       ((kCubeCongestionWindowScale * offset * offset * offset) >> kCubeScale) *
 150       kDefaultTCPMSS;
 151
 152   QuicByteCount target_congestion_window =
 153       origin_point_congestion_window_ - delta_congestion_window;
 154
 155   DCHECK_LT(0u, estimated_tcp_congestion_window_);
 156   // Increase the window by Alpha * 1 MSS of bytes every time we ack an
 157   // estimated tcp window of bytes.
 158   estimated_tcp_congestion_window_ += acked_bytes_count_ *
 159                                       (Alpha() * kDefaultTCPMSS) /
 160                                       estimated_tcp_congestion_window_;
 161   acked_bytes_count_ = 0;
 162
 163   // We have a new cubic congestion window.
 164   last_target_congestion_window_ = target_congestion_window;
 165
 166   // Compute target congestion_window based on cubic target and estimated TCP
 167   // congestion_window, use highest (fastest).
 168   if (target_congestion_window < estimated_tcp_congestion_window_) {
 169     target_congestion_window = estimated_tcp_congestion_window_;
 170   }
 171
 172   DVLOG(1) << "Final target congestion_window: " << target_congestion_window;
 173   return target_congestion_window;
 174 }
 175
 176 }  // namespace net