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 QuicPacketCount Cubic::CongestionWindowAfterPacketLoss(
  83     QuicPacketCount current_congestion_window) {
  84   if (current_congestion_window < last_max_congestion_window_) {
  85     // We never reached the old max, so assume we are competing with another
  86     // flow. Use our extra back off factor to allow the other flow to go up.
  87     last_max_congestion_window_ =
  88         static_cast<int>(kBetaLastMax * current_congestion_window);
  89   } else {
  90     last_max_congestion_window_ = current_congestion_window;
  91   }
  92   epoch_ = QuicTime::Zero();  // Reset time.
  93   return static_cast<int>(current_congestion_window * Beta());
  94 }
  95
  96 QuicPacketCount Cubic::CongestionWindowAfterAck(
  97     QuicPacketCount current_congestion_window,
  98     QuicTime::Delta delay_min) {
  99   acked_packets_count_ += 1;  // Packets acked.
 100   QuicTime current_time = clock_->ApproximateNow();
 101
 102   // Cubic is "independent" of RTT, the update is limited by the time elapsed.
 103   if (last_congestion_window_ == current_congestion_window &&
 104       (current_time.Subtract(last_update_time_) <= MaxCubicTimeInterval())) {
 105     return max(last_target_congestion_window_,
 106                estimated_tcp_congestion_window_);
 107   }
 108   last_congestion_window_ = current_congestion_window;
 109   last_update_time_ = current_time;
 110
 111   if (!epoch_.IsInitialized()) {
 112     // First ACK after a loss event.
 113     DVLOG(1) << "Start of epoch";
 114     epoch_ = current_time;  // Start of epoch.
 115     acked_packets_count_ = 1;  // Reset count.
 116     // Reset estimated_tcp_congestion_window_ to be in sync with cubic.
 117     estimated_tcp_congestion_window_ = current_congestion_window;
 118     if (last_max_congestion_window_ <= current_congestion_window) {
 119       time_to_origin_point_ = 0;
 120       origin_point_congestion_window_ = current_congestion_window;
 121     } else {
 122       time_to_origin_point_ =
 123           static_cast<uint32>(cbrt(kCubeFactor * (last_max_congestion_window_ -
 124                                                   current_congestion_window)));
 125       origin_point_congestion_window_ = last_max_congestion_window_;
 126     }
 127   }
 128   // Change the time unit from microseconds to 2^10 fractions per second. Take
 129   // the round trip time in account. This is done to allow us to use shift as a
 130   // divide operator.
 131   int64 elapsed_time =
 132       (current_time.Add(delay_min).Subtract(epoch_).ToMicroseconds() << 10) /
 133       kNumMicrosPerSecond;
 134
 135   int64 offset = time_to_origin_point_ - elapsed_time;
 136   QuicPacketCount delta_congestion_window = (kCubeCongestionWindowScale
 137       * offset * offset * offset) >> kCubeScale;
 138
 139   QuicPacketCount target_congestion_window =
 140       origin_point_congestion_window_ - delta_congestion_window;
 141
 142   DCHECK_LT(0u, estimated_tcp_congestion_window_);
 143   // With dynamic beta/alpha based on number of active streams, it is possible
 144   // for the required_ack_count to become much lower than acked_packets_count_
 145   // suddenly, leading to more than one iteration through the following loop.
 146   while (true) {
 147     // Update estimated TCP congestion_window.
 148     QuicPacketCount required_ack_count = static_cast<QuicPacketCount>(
 149         estimated_tcp_congestion_window_ / Alpha());
 150     if (acked_packets_count_ < required_ack_count) {
 151       break;
 152     }
 153     acked_packets_count_ -= required_ack_count;
 154     estimated_tcp_congestion_window_++;
 155   }
 156
 157   // We have a new cubic congestion window.
 158   last_target_congestion_window_ = target_congestion_window;
 159
 160   // Compute target congestion_window based on cubic target and estimated TCP
 161   // congestion_window, use highest (fastest).
 162   if (target_congestion_window < estimated_tcp_congestion_window_) {
 163     target_congestion_window = estimated_tcp_congestion_window_;
 164   }
 165
 166   DVLOG(1) << "Target congestion_window: " << target_congestion_window;
 167   return target_congestion_window;
 168 }
 169
 170 }  // namespace net