net/quic/congestion_control/cubic.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/cubic.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_flags.h"
  14 #include "net/quic/quic_protocol.h"
  15 #include "net/quic/quic_time.h"
  16
  17 using std::max;
  18
  19 namespace net {
  20
  21 namespace {
  22
  23 // Constants based on TCP defaults.
  24 // The following constants are in 2^10 fractions of a second instead of ms to
  25 // allow a 10 shift right to divide.
  26 const int kCubeScale = 40;  // 1024*1024^3 (first 1024 is from 0.100^3)
  27                             // where 0.100 is 100 ms which is the scaling
  28                             // round trip time.
  29 const int kCubeCongestionWindowScale = 410;
  30 const uint64 kCubeFactor = (UINT64_C(1) << kCubeScale) /
  31     kCubeCongestionWindowScale;
  32
  33 const uint32 kDefaultNumConnections = 2;
  34 const float kBeta = 0.7f;  // Default Cubic backoff factor.
  35 // Additional backoff factor when loss occurs in the concave part of the Cubic
  36 // curve. This additional backoff factor is expected to give up bandwidth to
  37 // new concurrent flows and speed up convergence.
  38 const float kBetaLastMax = 0.85f;
  39
  40 }  // namespace
  41
  42 Cubic::Cubic(const QuicClock* clock)
  43     : clock_(clock),
  44       num_connections_(kDefaultNumConnections),
  45       epoch_(QuicTime::Zero()),
  46       last_update_time_(QuicTime::Zero()) {
  47   Reset();
  48 }
  49
  50 void Cubic::SetNumConnections(int num_connections) {
  51   num_connections_ = num_connections;
  52 }
  53
  54 float Cubic::Alpha() const {
  55   // TCPFriendly alpha is described in Section 3.3 of the CUBIC paper. Note that
  56   // beta here is a cwnd multiplier, and is equal to 1-beta from the paper.
  57   // We derive the equivalent alpha for an N-connection emulation as:
  58   const float beta = Beta();
  59   return 3 * num_connections_ * num_connections_ * (1 - beta) / (1 + beta);
  60 }
  61
  62 float Cubic::Beta() const {
  63   // kNConnectionBeta is the backoff factor after loss for our N-connection
  64   // emulation, which emulates the effective backoff of an ensemble of N
  65   // TCP-Reno connections on a single loss event. The effective multiplier is
  66   // computed as:
  67   return (num_connections_ - 1 + kBeta) / num_connections_;
  68 }
  69
  70 void Cubic::Reset() {
  71   epoch_ = QuicTime::Zero();  // Reset time.
  72   last_update_time_ = QuicTime::Zero();  // Reset time.
  73   last_congestion_window_ = 0;
  74   last_max_congestion_window_ = 0;
  75   acked_packets_count_ = 0;
  76   estimated_tcp_congestion_window_ = 0;
  77   origin_point_congestion_window_ = 0;
  78   time_to_origin_point_ = 0;
  79   last_target_congestion_window_ = 0;
  80 }
  81
  82 void Cubic::OnApplicationLimited() {
  83   // When sender is not using the available congestion window, the window does
  84   // not grow. But to be RTT-independent, Cubic assumes that the sender has been
  85   // using the entire window during the time since the beginning of the current
  86   // "epoch" (the end of the last loss recovery period). Since
  87   // application-limited periods break this assumption, we reset the epoch when
  88   // in such a period. This reset effectively freezes congestion window growth
  89   // through application-limited periods and allows Cubic growth to continue
  90   // when the entire window is being used.
  91   epoch_ = QuicTime::Zero();
  92 }
  93
  94 QuicPacketCount Cubic::CongestionWindowAfterPacketLoss(
  95     QuicPacketCount current_congestion_window) {
  96   if (current_congestion_window < last_max_congestion_window_) {
  97     // We never reached the old max, so assume we are competing with another
  98     // flow. Use our extra back off factor to allow the other flow to go up.
  99     last_max_congestion_window_ =
 100         static_cast<int>(kBetaLastMax * current_congestion_window);
 101   } else {
 102     last_max_congestion_window_ = current_congestion_window;
 103   }
 104   epoch_ = QuicTime::Zero();  // Reset time.
 105   return static_cast<int>(current_congestion_window * Beta());
 106 }
 107
 108 QuicPacketCount Cubic::CongestionWindowAfterAck(
 109     QuicPacketCount current_congestion_window,
 110     QuicTime::Delta delay_min) {
 111   acked_packets_count_ += 1;  // Packets acked.
 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_packets_count_ = 1;  // 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   QuicPacketCount delta_congestion_window = (kCubeCongestionWindowScale
 149       * offset * offset * offset) >> kCubeScale;
 150
 151   QuicPacketCount target_congestion_window =
 152       origin_point_congestion_window_ - delta_congestion_window;
 153
 154   DCHECK_LT(0u, estimated_tcp_congestion_window_);
 155   // With dynamic beta/alpha based on number of active streams, it is possible
 156   // for the required_ack_count to become much lower than acked_packets_count_
 157   // suddenly, leading to more than one iteration through the following loop.
 158   while (true) {
 159     // Update estimated TCP congestion_window.
 160     QuicPacketCount required_ack_count = static_cast<QuicPacketCount>(
 161         estimated_tcp_congestion_window_ / Alpha());
 162     if (acked_packets_count_ < required_ack_count) {
 163       break;
 164     }
 165     acked_packets_count_ -= required_ack_count;
 166     estimated_tcp_congestion_window_++;
 167   }
 168
 169   // We have a new cubic congestion window.
 170   last_target_congestion_window_ = target_congestion_window;
 171
 172   // Compute target congestion_window based on cubic target and estimated TCP
 173   // congestion_window, use highest (fastest).
 174   if (target_congestion_window < estimated_tcp_congestion_window_) {
 175     target_congestion_window = estimated_tcp_congestion_window_;
 176   }
 177
 178   DVLOG(1) << "Final target congestion_window: " << target_congestion_window;
 179   return target_congestion_window;
 180 }
 181
 182 }  // namespace net