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

Index: net/quic/congestion_control/tcp_cubic_bytes_sender.cc
diff --git a/net/quic/congestion_control/tcp_cubic_bytes_sender.cc b/net/quic/congestion_control/tcp_cubic_bytes_sender.cc
index 7f817ebfb5855ba10f8d275bd6d94f234e89a68f..ef6172a92932f5e4401027ea6d1a2130d03a4dcc 100644
--- a/net/quic/congestion_control/tcp_cubic_bytes_sender.cc
+++ b/net/quic/congestion_control/tcp_cubic_bytes_sender.cc
@@ -52,7 +52,8 @@ TcpCubicBytesSender::TcpCubicBytesSender(
       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() {}
 
@@ -75,6 +76,11 @@ void TcpCubicBytesSender::SetFromConfig(const QuicConfig& config,
       min4_mode_ = true;
       min_congestion_window_ = kDefaultTCPMSS;
     }
+    if (config.HasReceivedConnectionOptions() &&
+        ContainsQuicTag(config.ReceivedConnectionOptions(), kSSLR)) {
+      // Slow Start Fast Exit experiment.
+      slow_start_large_reduction_ = true;
+    }
   }
 }
 
@@ -156,6 +162,12 @@ void TcpCubicBytesSender::OnPacketLost(QuicPacketNumber packet_number,
   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.";
@@ -169,17 +181,21 @@ void TcpCubicBytesSender::OnPacketLost(QuicPacketNumber packet_number,
 
   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.