Allow the number of connections QUIC's send algorithms simulate to

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"
@@ skipping 10 matching lines @@
 // 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 kNumConnections = 2;
+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;
 
-// 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, QuicConnectionStats* stats)
     : clock_(clock),
+      num_connections_(kDefaultNumConnections),
       epoch_(QuicTime::Zero()),
       last_update_time_(QuicTime::Zero()),
       stats_(stats) {
   Reset();
 }
 
+void Cubic::SetNumConnections(int num_connections) {
+  num_connections_ = num_connections;
+}
+
+float Cubic::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 {
+  // 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.
   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;
@@ skipping 21 matching lines @@
     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_ =
         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 * kNConnectionBeta);
+  return static_cast<int>(current_congestion_window * Beta());
 }
 
 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 35 matching lines @@
   QuicTcpCongestionWindow 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.
     uint32 required_ack_count =
-        estimated_tcp_congestion_window_ / kNConnectionAlpha;
+        estimated_tcp_congestion_window_ / Alpha();
     if (acked_packets_count_ < required_ack_count) {
       break;
     }
     acked_packets_count_ -= required_ack_count;
     estimated_tcp_congestion_window_++;
   }
 
   // Update cubic mode and reno mode stats in QuicConnectionStats.
   UpdateCongestionControlStats(target_congestion_window,
                                estimated_tcp_congestion_window_);
 
   // 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