Limit QUIC's Cubic CWND increases to 1/2 the bytes acked when in

net/quic/core/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/core/congestion_control/cubic.h"
 
 #include <stdint.h>
 #include <algorithm>
 #include <cmath>
 
 #include "base/logging.h"
 #include "net/quic/core/quic_flags.h"
 #include "net/quic/core/quic_protocol.h"
 #include "net/quic/core/quic_time.h"
 
 using std::max;
+using std::min;
 
 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
@@ skipping 40 matching lines @@
   return (num_connections_ - 1 + kBeta) / num_connections_;
 }
 
 void Cubic::Reset() {
   epoch_ = QuicTime::Zero();  // Reset time.
   app_limited_start_time_ = QuicTime::Zero();
   last_update_time_ = QuicTime::Zero();  // Reset time.
   last_congestion_window_ = 0;
   last_max_congestion_window_ = 0;
   acked_packets_count_ = 0;
+  epoch_packets_count_ = 0;
   estimated_tcp_congestion_window_ = 0;
   origin_point_congestion_window_ = 0;
   time_to_origin_point_ = 0;
   last_target_congestion_window_ = 0;
 }
 
 void Cubic::OnApplicationLimited() {
   // When sender is not using the available congestion window, Cubic's epoch
   // should not continue growing. Reset the epoch when in such a period.
   epoch_ = QuicTime::Zero();
@@ skipping 10 matching lines @@
     last_max_congestion_window_ = current_congestion_window;
   }
   epoch_ = QuicTime::Zero();  // Reset time.
   return static_cast<int>(current_congestion_window * Beta());
 }
 
 QuicPacketCount Cubic::CongestionWindowAfterAck(
     QuicPacketCount current_congestion_window,
     QuicTime::Delta delay_min) {
   acked_packets_count_ += 1;  // Packets acked.
+  epoch_packets_count_ += 1;
   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 &&
       (current_time - last_update_time_ <= MaxCubicTimeInterval())) {
     return max(last_target_congestion_window_,
                estimated_tcp_congestion_window_);
   }
   last_congestion_window_ = current_congestion_window;
   last_update_time_ = current_time;
 
   if (!epoch_.IsInitialized()) {
     // First ACK after a loss event.
     epoch_ = current_time;     // Start of epoch.
     acked_packets_count_ = 1;  // Reset count.
+    epoch_packets_count_ = 1;
     // Reset estimated_tcp_congestion_window_ to be in sync with cubic.
     estimated_tcp_congestion_window_ = current_congestion_window;
     if (last_max_congestion_window_ <= current_congestion_window) {
       time_to_origin_point_ = 0;
       origin_point_congestion_window_ = current_congestion_window;
     } else {
       time_to_origin_point_ = static_cast<uint32_t>(
           cbrt(kCubeFactor *
                (last_max_congestion_window_ - current_congestion_window)));
       origin_point_congestion_window_ = last_max_congestion_window_;
     }
   }
 
   // Change the time unit from microseconds to 2^10 fractions per second. Take
   // the round trip time in account. This is done to allow us to use shift as a
   // divide operator.
   int64_t elapsed_time =
       ((current_time + delay_min - epoch_).ToMicroseconds() << 10) /
       kNumMicrosPerSecond;
 
   int64_t offset = time_to_origin_point_ - elapsed_time;
   QuicPacketCount delta_congestion_window =
       (kCubeCongestionWindowScale * offset * offset * offset) >> kCubeScale;
 
   QuicPacketCount target_congestion_window =
       origin_point_congestion_window_ - delta_congestion_window;
 
+  if (FLAGS_quic_limit_cubic_cwnd_increase) {
+    // Limit the CWND increase to half the acked packets rounded up to the
+    // nearest packet.
+    target_congestion_window =
+        min(target_congestion_window,
+            current_congestion_window + (epoch_packets_count_ + 1) / 2);
+  }
+
   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.
     QuicPacketCount required_ack_count = static_cast<QuicPacketCount>(
         estimated_tcp_congestion_window_ / Alpha());
     if (acked_packets_count_ < required_ack_count) {
       break;
     }
     acked_packets_count_ -= required_ack_count;
     estimated_tcp_congestion_window_++;
   }
+  epoch_packets_count_ = 0;
 
   // 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) << "Final target congestion_window: " << target_congestion_window;
   return target_congestion_window;
 }
 
 }  // namespace net