Create a TcpCubicSender in QUIC that tracks the congestion window in

net/quic/congestion_control/cubic_bytes.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/cubic.h"
+#include "net/quic/congestion_control/cubic_bytes.h"
 
 #include <algorithm>
 #include <cmath>
 
 #include "base/basictypes.h"
 #include "base/logging.h"
-#include "base/time/time.h"
-#include "net/quic/quic_flags.h"
 #include "net/quic/quic_protocol.h"
 
 using std::max;
 
 namespace net {
 
 namespace {
 
 // Constants based on TCP defaults.
 // The following constants are in 2^10 fractions of a second instead of ms to
 // allow a 10 shift right to divide.
 const int kCubeScale = 40;  // 1024*1024^3 (first 1024 is from 0.100^3)
                             // where 0.100 is 100 ms which is the scaling
                             // round trip time.
 const int kCubeCongestionWindowScale = 410;
+// The cube factor for packets in bytes.
 const uint64 kCubeFactor = (GG_UINT64_C(1) << kCubeScale) /
-    kCubeCongestionWindowScale;
+                           kCubeCongestionWindowScale / kDefaultTCPMSS;
 
 const uint32 kDefaultNumConnections = 2;
 const float kBeta = 0.7f;  // Default Cubic backoff factor.
 // Additional backoff factor when loss occurs in the concave part of the Cubic
 // curve. This additional backoff factor is expected to give up bandwidth to
 // new concurrent flows and speed up convergence.
 const float kBetaLastMax = 0.85f;
 
 }  // namespace
 
-Cubic::Cubic(const QuicClock* clock)
+CubicBytes::CubicBytes(const QuicClock* clock)
     : clock_(clock),
       num_connections_(kDefaultNumConnections),
       epoch_(QuicTime::Zero()),
       last_update_time_(QuicTime::Zero()) {
   Reset();
 }
 
-void Cubic::SetNumConnections(int num_connections) {
+void CubicBytes::SetNumConnections(int num_connections) {
   num_connections_ = num_connections;
 }
 
-float Cubic::Alpha() const {
+float CubicBytes::Alpha() const {
   // TCPFriendly alpha is described in Section 3.3 of the CUBIC paper. Note that
   // beta here is a cwnd multiplier, and is equal to 1-beta from the paper.
   // We derive the equivalent alpha for an N-connection emulation as:
   const float beta = Beta();
   return 3 * num_connections_ * num_connections_ * (1 - beta) / (1 + beta);
 }
 
-float Cubic::Beta() const {
+float CubicBytes::Beta() 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 + kBeta) / num_connections_;
 }
 
-void Cubic::Reset() {
-  epoch_ = QuicTime::Zero();  // Reset time.
+void CubicBytes::Reset() {
+  epoch_ = QuicTime::Zero();             // Reset time.
   last_update_time_ = QuicTime::Zero();  // Reset time.
   last_congestion_window_ = 0;
   last_max_congestion_window_ = 0;
-  acked_packets_count_ = 0;
+  acked_bytes_count_ = 0;
   estimated_tcp_congestion_window_ = 0;
   origin_point_congestion_window_ = 0;
   time_to_origin_point_ = 0;
   last_target_congestion_window_ = 0;
 }
 
-QuicPacketCount Cubic::CongestionWindowAfterPacketLoss(
-    QuicPacketCount current_congestion_window) {
+QuicByteCount CubicBytes::CongestionWindowAfterPacketLoss(
+    QuicByteCount current_congestion_window) {
   if (current_congestion_window < last_max_congestion_window_) {
     // We never reached the old max, so assume we are competing with another
     // flow. Use our extra back off factor to allow the other flow to go up.
     last_max_congestion_window_ =
         static_cast<int>(kBetaLastMax * current_congestion_window);
   } else {
     last_max_congestion_window_ = current_congestion_window;
   }
   epoch_ = QuicTime::Zero();  // Reset time.
   return static_cast<int>(current_congestion_window * Beta());
 }
 
-QuicPacketCount Cubic::CongestionWindowAfterAck(
-    QuicPacketCount current_congestion_window,
+QuicByteCount CubicBytes::CongestionWindowAfterAck(
+    QuicByteCount acked_bytes,
+    QuicByteCount current_congestion_window,
     QuicTime::Delta delay_min) {
-  acked_packets_count_ += 1;  // Packets acked.
+  acked_bytes_count_ += acked_bytes;
   QuicTime current_time = clock_->ApproximateNow();
 
   // Cubic is "independent" of RTT, the update is limited by the time elapsed.
   if (last_congestion_window_ == current_congestion_window &&
       (current_time.Subtract(last_update_time_) <= MaxCubicTimeInterval())) {
     return max(last_target_congestion_window_,
                estimated_tcp_congestion_window_);
   }
   last_congestion_window_ = current_congestion_window;
   last_update_time_ = current_time;
 
   if (!epoch_.IsInitialized()) {
     // First ACK after a loss event.
     DVLOG(1) << "Start of epoch";
-    epoch_ = current_time;  // Start of epoch.
-    acked_packets_count_ = 1;  // Reset count.
+    epoch_ = current_time;             // Start of epoch.
+    acked_bytes_count_ = acked_bytes;  // Reset count.
     // Reset estimated_tcp_congestion_window_ to be in sync with cubic.
     estimated_tcp_congestion_window_ = current_congestion_window;
     if (last_max_congestion_window_ <= current_congestion_window) {
       time_to_origin_point_ = 0;
       origin_point_congestion_window_ = current_congestion_window;
     } else {
       time_to_origin_point_ =
           static_cast<uint32>(cbrt(kCubeFactor * (last_max_congestion_window_ -
                                                   current_congestion_window)));
-      origin_point_congestion_window_ =
-          last_max_congestion_window_;
+      origin_point_congestion_window_ = last_max_congestion_window_;
     }
   }
   // Change the time unit from microseconds to 2^10 fractions per second. Take
   // the round trip time in account. This is done to allow us to use shift as a
   // divide operator.
   int64 elapsed_time =
       (current_time.Add(delay_min).Subtract(epoch_).ToMicroseconds() << 10) /
-      base::Time::kMicrosecondsPerSecond;
+      kNumMicrosPerSecond;
 
   int64 offset = time_to_origin_point_ - elapsed_time;
-  QuicPacketCount delta_congestion_window = (kCubeCongestionWindowScale
-      * offset * offset * offset) >> kCubeScale;
+  QuicByteCount delta_congestion_window =
+      ((kCubeCongestionWindowScale * offset * offset * offset) >> kCubeScale) *
+      kDefaultTCPMSS;
 
-  QuicPacketCount target_congestion_window =
+  QuicByteCount target_congestion_window =
       origin_point_congestion_window_ - delta_congestion_window;
 
   DCHECK_LT(0u, estimated_tcp_congestion_window_);
-  // With dynamic beta/alpha based on number of active streams, it is possible
-  // for the required_ack_count to become much lower than acked_packets_count_
-  // suddenly, leading to more than one iteration through the following loop.
-  while (true) {
-    // Update estimated TCP congestion_window.
-    QuicPacketCount required_ack_count = static_cast<QuicPacketCount>(
-        estimated_tcp_congestion_window_ / Alpha());
-    if (acked_packets_count_ < required_ack_count) {
-      break;
-    }
-    acked_packets_count_ -= required_ack_count;
-    estimated_tcp_congestion_window_++;
-  }
+  // Increase the window by Alpha * 1 MSS of bytes every time we ack an
+  // estimated tcp window of bytes.
+  estimated_tcp_congestion_window_ += acked_bytes_count_ *
+                                      (Alpha() * kDefaultTCPMSS) /
+                                      estimated_tcp_congestion_window_;
+  acked_bytes_count_ = 0;
 
   // We have a new cubic congestion window.
   last_target_congestion_window_ = target_congestion_window;
 
   // Compute target congestion_window based on cubic target and estimated TCP
   // congestion_window, use highest (fastest).
   if (target_congestion_window < estimated_tcp_congestion_window_) {
     target_congestion_window = estimated_tcp_congestion_window_;
   }
 
   DVLOG(1) << "Target congestion_window: " << target_congestion_window;
   return target_congestion_window;
 }
 
 }  // namespace net