relnote: Fixes bug in QUIC's cubic code that caused inflation in

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_macros.h"
 #include "net/quic/congestion_control/prr_sender.h"
 #include "net/quic/congestion_control/rtt_stats.h"
 #include "net/quic/crypto/crypto_protocol.h"
 #include "net/quic/proto/cached_network_parameters.pb.h"
+#include "net/quic/quic_flags.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.
@@ skipping 281 matching lines @@
   return largest_acked_packet_number_ <= largest_sent_at_last_cutback_ &&
          largest_acked_packet_number_ != 0;
 }
 
 // Called when we receive an ack. Normal TCP tracks how many packets one ack
 // represents, but quic has a separate ack for each packet.
 void TcpCubicSender::MaybeIncreaseCwnd(QuicPacketNumber acked_packet_number,
                                        QuicByteCount bytes_in_flight) {
   LOG_IF(DFATAL, InRecovery()) << "Never increase the CWND during recovery.";
   if (!IsCwndLimited(bytes_in_flight)) {
-    // We don't update the congestion window unless we are close to using the
-    // window we have available.
+    // Do not increase the congestion window unless the sender is close to using
+    // the current window.
+    if (FLAGS_reset_cubic_epoch_when_app_limited) {
+      cubic_.OnApplicationLimited();
+    }
     return;
   }
   if (congestion_window_ >= max_tcp_congestion_window_) {
     return;
   }
   if (InSlowStart()) {
     // TCP slow start, exponential growth, increase by one for each ACK.
     ++congestion_window_;
     DVLOG(1) << "Slow start; congestion window: " << congestion_window_
              << " slowstart threshold: " << slowstart_threshold_;
@@ skipping 31 matching lines @@
   hybrid_slow_start_.Restart();
   slowstart_threshold_ = congestion_window_ / 2;
   congestion_window_ = min_congestion_window_;
 }
 
 CongestionControlType TcpCubicSender::GetCongestionControlType() const {
   return reno_ ? kReno : kCubic;
 }
 
 }  // namespace net