Fixes reduction for n-connection emulation in TCP Reno.

net/quic/congestion_control/tcp_cubic_sender.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/tcp_cubic_sender.h"
 
 #include <algorithm>
 
 #include "base/metrics/histogram.h"
 #include "net/quic/congestion_control/prr_sender.h"
 #include "net/quic/congestion_control/rtt_stats.h"
 #include "net/quic/crypto/crypto_protocol.h"
 
 using std::max;
 using std::min;
 
 namespace net {
 
 namespace {
 // Constants based on TCP defaults.
 // The minimum cwnd based on RFC 3782 (TCP NewReno) for cwnd reductions on a
 // fast retransmission.  The cwnd after a timeout is still 1.
 const QuicPacketCount kMinimumCongestionWindow = 2;
 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(
     const QuicClock* clock,
     const RttStats* rtt_stats,
     bool reno,
     QuicPacketCount max_tcp_congestion_window,
     QuicConnectionStats* stats)
     : hybrid_slow_start_(clock),
       cubic_(clock, stats),
       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),
       largest_sent_at_last_cutback_(0),
       congestion_window_(kInitialCongestionWindow),
       previous_congestion_window_(0),
       slowstart_threshold_(max_tcp_congestion_window),
       previous_slowstart_threshold_(0),
       last_cutback_exited_slowstart_(false),
       max_tcp_congestion_window_(max_tcp_congestion_window) {
@@ skipping 17 matching lines @@
                                config.ReceivedInitialCongestionWindow());
     }
   }
 }
 
 void TcpCubicSender::SetNumEmulatedConnections(int num_connections) {
   num_connections_ = max(1, 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,
     const CongestionVector& acked_packets,
     const CongestionVector& lost_packets) {
   if (rtt_updated && InSlowStart() &&
       hybrid_slow_start_.ShouldExitSlowStart(rtt_stats_->latest_rtt(),
                                              rtt_stats_->MinRtt(),
                                              congestion_window_)) {
     slowstart_threshold_ = congestion_window_;
@@ skipping 38 matching lines @@
   }
   ++stats_->tcp_loss_events;
   last_cutback_exited_slowstart_ = InSlowStart();
   if (InSlowStart()) {
     ++stats_->slowstart_packets_lost;
   }
 
   prr_.OnPacketLost(bytes_in_flight);
 
   if (reno_) {
-    congestion_window_ = congestion_window_ * kRenoBeta;
+    congestion_window_ = congestion_window_ * RenoBeta();
   } else {
     congestion_window_ =
         cubic_.CongestionWindowAfterPacketLoss(congestion_window_);
   }
   slowstart_threshold_ = congestion_window_;
   // Enforce TCP's minimum congestion window of 2*MSS.
   if (congestion_window_ < kMinimumCongestionWindow) {
     congestion_window_ = kMinimumCongestionWindow;
   }
   largest_sent_at_last_cutback_ = largest_sent_sequence_number_;
@@ skipping 169 matching lines @@
   congestion_window_ = previous_congestion_window_;
   slowstart_threshold_ = previous_slowstart_threshold_;
   previous_congestion_window_ = 0;
 }
 
 CongestionControlType TcpCubicSender::GetCongestionControlType() const {
   return reno_ ? kReno : kCubic;
 }
 
 }  // namespace net