relnote: Adds Slow Start with Large Reduction (SSLR) option in QUIC to do a large reduction of the …

net/quic/congestion_control/tcp_cubic_bytes_sender.cc

 // Copyright (c) 2015 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_bytes_sender.h"
 
 #include <algorithm>
 
 #include "net/quic/congestion_control/prr_sender.h"
 #include "net/quic/congestion_control/rtt_stats.h"
@@ skipping 34 matching lines @@
       largest_sent_at_last_cutback_(0),
       congestion_window_(initial_tcp_congestion_window * kDefaultTCPMSS),
       min_congestion_window_(kDefaultMinimumCongestionWindow),
       min4_mode_(false),
       max_congestion_window_(max_congestion_window * kDefaultTCPMSS),
       slowstart_threshold_(max_congestion_window * kDefaultTCPMSS),
       last_cutback_exited_slowstart_(false),
       initial_tcp_congestion_window_(initial_tcp_congestion_window *
                                      kDefaultTCPMSS),
       initial_max_tcp_congestion_window_(max_congestion_window *
-                                         kDefaultTCPMSS) {}
+                                         kDefaultTCPMSS),
+      slow_start_large_reduction_(false) {}
 
 TcpCubicBytesSender::~TcpCubicBytesSender() {}
 
 void TcpCubicBytesSender::SetFromConfig(const QuicConfig& config,
                                         Perspective perspective) {
   if (perspective == Perspective::IS_SERVER) {
     if (config.HasReceivedConnectionOptions() &&
         ContainsQuicTag(config.ReceivedConnectionOptions(), kIW10)) {
       // Initial window experiment.
       congestion_window_ = 10 * kDefaultTCPMSS;
     }
     if (config.HasReceivedConnectionOptions() &&
         ContainsQuicTag(config.ReceivedConnectionOptions(), kMIN1)) {
       // Min CWND experiment.
       min_congestion_window_ = kDefaultTCPMSS;
     }
     if (config.HasReceivedConnectionOptions() &&
         ContainsQuicTag(config.ReceivedConnectionOptions(), kMIN4)) {
       // Min CWND of 4 experiment.
       min4_mode_ = true;
       min_congestion_window_ = kDefaultTCPMSS;
     }
+    if (config.HasReceivedConnectionOptions() &&
+        ContainsQuicTag(config.ReceivedConnectionOptions(), kSSLR)) {
+      // Slow Start Fast Exit experiment.
+      slow_start_large_reduction_ = true;
+    }
   }
 }
 
 void TcpCubicBytesSender::ResumeConnectionState(
     const CachedNetworkParameters& cached_network_params,
     bool max_bandwidth_resumption) {
   QuicBandwidth bandwidth = QuicBandwidth::FromBytesPerSecond(
       max_bandwidth_resumption
           ? cached_network_params.max_bandwidth_estimate_bytes_per_second()
           : cached_network_params.bandwidth_estimate_bytes_per_second());
@@ skipping 61 matching lines @@
   hybrid_slow_start_.OnPacketAcked(acked_packet_number, InSlowStart());
 }
 
 void TcpCubicBytesSender::OnPacketLost(QuicPacketNumber packet_number,
                                        QuicByteCount bytes_in_flight) {
   // TCP NewReno (RFC6582) says that once a loss occurs, any losses in packets
   // already sent should be treated as a single loss event, since it's expected.
   if (packet_number <= largest_sent_at_last_cutback_) {
     if (last_cutback_exited_slowstart_) {
       ++stats_->slowstart_packets_lost;
+      if (slow_start_large_reduction_) {
+        // Reduce congestion window by 1 MSS for every loss.
+        congestion_window_ =
+            max(congestion_window_ - kDefaultTCPMSS, min_congestion_window_);
+        slowstart_threshold_ = congestion_window_;
+      }
     }
     DVLOG(1) << "Ignoring loss for largest_missing:" << packet_number
              << " because it was sent prior to the last CWND cutback.";
     return;
   }
   ++stats_->tcp_loss_events;
   last_cutback_exited_slowstart_ = InSlowStart();
   if (InSlowStart()) {
     ++stats_->slowstart_packets_lost;
   }
 
   prr_.OnPacketLost(bytes_in_flight);
 
-  if (reno_) {
+  // TODO(jri): Separate out all of slow start into a separate class.
+  if (slow_start_large_reduction_) {
+    DCHECK_LT(kDefaultTCPMSS, congestion_window_);
+    congestion_window_ = congestion_window_ - kDefaultTCPMSS;
+  } else if (reno_) {
     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_ < min_congestion_window_) {
     congestion_window_ = min_congestion_window_;
   }
+  slowstart_threshold_ = congestion_window_;
   largest_sent_at_last_cutback_ = largest_sent_packet_number_;
   // Reset packet count from congestion avoidance mode. We start counting again
   // when we're out of recovery.
   num_acked_packets_ = 0;
   DVLOG(1) << "Incoming loss; congestion window: " << congestion_window_
            << " slowstart threshold: " << slowstart_threshold_;
 }
 
 bool TcpCubicBytesSender::OnPacketSent(
     QuicTime /*sent_time*/,
@@ skipping 170 matching lines @@
   largest_sent_packet_number_ = 0;
   largest_acked_packet_number_ = 0;
   largest_sent_at_last_cutback_ = 0;
   congestion_window_ = initial_tcp_congestion_window_;
   max_congestion_window_ = initial_max_tcp_congestion_window_;
   slowstart_threshold_ = initial_max_tcp_congestion_window_;
   last_cutback_exited_slowstart_ = false;
 }
 
 }  // namespace net