Fixes QUIC's Cubic sender to use correct alpha when in Reno mode.

net/quic/congestion_control/cubic.cc

 // Copyright (c) 2012 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 <algorithm>
 
 #include "base/basictypes.h"
 #include "base/logging.h"
 #include "base/time/time.h"
 #include "net/quic/congestion_control/cube_root.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;
 const uint64 kCubeFactor = (GG_UINT64_C(1) << kCubeScale) /
     kCubeCongestionWindowScale;
-const uint32 kBetaSPDY = 939;  // Back off factor after loss for SPDY, reduces
-                               // the CWND by 1/12th.
-const uint32 kBetaLastMax = 871;  // Additional back off factor after loss for
-                                  // the stored max value.
+
+const uint32 kNumConnections = 2;
+const float kBeta = 0.7;  // 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 = 0.85;
+
+// 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:
+const float kNConnectionBeta = (kNumConnections - 1 + kBeta) / kNumConnections;
+
+// TCPFriendly alpha is described in Section 3.3 of the CUBIC paper. Note that
+// kBeta here is a cwnd multiplier, and is equal to 1-beta from the CUBIC paper.
+// We derive the equivalent kNConnectionAlpha for an N-connection emulation as:
+const float kNConnectionAlpha = 3 * kNumConnections * kNumConnections *
+      (1 - kNConnectionBeta) / (1 + kNConnectionBeta);
+// TODO(jri): Compute kNConnectionBeta and kNConnectionAlpha from
+// number of active streams.
 }  // namespace
 
 Cubic::Cubic(const QuicClock* clock)
     : clock_(clock),
       epoch_(QuicTime::Zero()),
       last_update_time_(QuicTime::Zero()) {
   Reset();
 }
 
 void Cubic::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;
   estimated_tcp_congestion_window_ = 0;
   origin_point_congestion_window_ = 0;
   time_to_origin_point_ = 0;
   last_target_congestion_window_ = 0;
 }
 
 QuicTcpCongestionWindow Cubic::CongestionWindowAfterPacketLoss(
     QuicTcpCongestionWindow 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_ =
-        (kBetaLastMax * current_congestion_window) >> 10;
+        static_cast<int>(kBetaLastMax * current_congestion_window);
   } else {
     last_max_congestion_window_ = current_congestion_window;
   }
   epoch_ = QuicTime::Zero();  // Reset time.
-  return (current_congestion_window * kBetaSPDY) >> 10;
+  return static_cast<int>(current_congestion_window * kNConnectionBeta);
 }
 
 QuicTcpCongestionWindow Cubic::CongestionWindowAfterAck(
     QuicTcpCongestionWindow current_congestion_window,
     QuicTime::Delta delay_min) {
   acked_packets_count_ += 1;  // Packets acked.
   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 &&
@@ skipping 31 matching lines @@
   int64 offset = time_to_origin_point_ - elapsed_time;
   QuicTcpCongestionWindow delta_congestion_window = (kCubeCongestionWindowScale
       * offset * offset * offset) >> kCubeScale;
 
   QuicTcpCongestionWindow target_congestion_window =
       origin_point_congestion_window_ - delta_congestion_window;
 
   // We have a new cubic congestion window.
   last_target_congestion_window_ = target_congestion_window;
 
-  // Update estimated TCP congestion_window.
-  // Note: we do a normal Reno congestion avoidance calculation not the
-  // calculation described in section 3.3 TCP-friendly region of the document.
-  while (acked_packets_count_ >= estimated_tcp_congestion_window_) {
-    acked_packets_count_ -= estimated_tcp_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.
+    uint32 required_ack_count =
+        estimated_tcp_congestion_window_ / kNConnectionAlpha;
+    if (acked_packets_count_ < required_ack_count) {
+      break;
+    }
+    acked_packets_count_ -= required_ack_count;
     estimated_tcp_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