Fixes reduction for n-connection emulation in TCP Reno.

net/quic/congestion_control/tcp_cubic_sender.cc

Index: net/quic/congestion_control/tcp_cubic_sender.cc
diff --git a/net/quic/congestion_control/tcp_cubic_sender.cc b/net/quic/congestion_control/tcp_cubic_sender.cc
index c55d6205ef0d71098fa26af46b3434e79f048b11..44e4b3e8aaebeb28f790130a68c05793c4d7df80 100644
--- a/net/quic/congestion_control/tcp_cubic_sender.cc
+++ b/net/quic/congestion_control/tcp_cubic_sender.cc
@@ -25,6 +25,7 @@ const QuicByteCount kMaxSegmentSize = kDefaultTCPMSS;
 const int64 kInitialCongestionWindow = 10;
 const int kMaxBurstLength = 3;
 const float kRenoBeta = 0.7f;  // Reno backoff factor.
+const uint32 kDefaultNumConnections = 2;  // N-connection emulation.
 }  // namespace
 
 TcpCubicSender::TcpCubicSender(
@@ -38,7 +39,7 @@ TcpCubicSender::TcpCubicSender(
       rtt_stats_(rtt_stats),
       stats_(stats),
       reno_(reno),
-      num_connections_(2),
+      num_connections_(kDefaultNumConnections),
       congestion_window_count_(0),
       largest_sent_sequence_number_(0),
       largest_acked_sequence_number_(0),
@@ -76,6 +77,14 @@ void TcpCubicSender::SetNumEmulatedConnections(int num_connections) {
   cubic_.SetNumConnections(num_connections_);
 }
 
+float TcpCubicSender::RenoBeta() 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 + kRenoBeta) / num_connections_;
+}
+
 void TcpCubicSender::OnCongestionEvent(
     bool rtt_updated,
     QuicByteCount bytes_in_flight,
@@ -134,7 +143,7 @@ void TcpCubicSender::OnPacketLost(QuicPacketSequenceNumber sequence_number,
   prr_.OnPacketLost(bytes_in_flight);
 
   if (reno_) {
-    congestion_window_ = congestion_window_ * kRenoBeta;
+    congestion_window_ = congestion_window_ * RenoBeta();
   } else {
     congestion_window_ =
         cubic_.CongestionWindowAfterPacketLoss(congestion_window_);