Ensure that BBR does not drop to exceedingly low pacing rates during the STARTUP phase. Protected …

net/quic/core/congestion_control/bbr_sender.cc

 // Copyright 2016 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/bbr_sender.h"
 
 #include <algorithm>
 #include <sstream>
 
 #include "net/quic/core/congestion_control/rtt_stats.h"
@@ skipping 81 matching lines @@
       last_cycle_start_(QuicTime::Zero()),
       is_at_full_bandwidth_(false),
       rounds_without_bandwidth_gain_(0),
       bandwidth_at_last_round_(QuicBandwidth::Zero()),
       exiting_quiescence_(false),
       exit_probe_rtt_at_(QuicTime::Zero()),
       probe_rtt_round_passed_(false),
       last_sample_is_app_limited_(false),
       recovery_state_(NOT_IN_RECOVERY),
       end_recovery_at_(0),
-      recovery_window_(max_congestion_window_) {
+      recovery_window_(max_congestion_window_),
+      enforce_startup_pacing_rate_increase_(
+          FLAGS_quic_bbr_faster_startup) {
   EnterStartupMode();
 }
 
 BbrSender::~BbrSender() {}
 
 bool BbrSender::InSlowStart() const {
   return mode_ == STARTUP;
 }
 
 bool BbrSender::OnPacketSent(QuicTime sent_time,
@@ skipping 334 matching lines @@
       }
       break;
   }
 }
 
 void BbrSender::CalculatePacingRate() {
   if (BandwidthEstimate().IsZero()) {
     return;
   }
 
-  pacing_rate_ = pacing_gain_ * BandwidthEstimate();
+  QuicBandwidth target_rate = pacing_gain_ * BandwidthEstimate();
+
+  // Ensure that the pacing rate does not drop too low during the startup.
+  if (!is_at_full_bandwidth_ && enforce_startup_pacing_rate_increase_) {
+    // Pace at the rate of initial_window / RTT as soon as RTT measurements are
+    // available.
+    if (pacing_rate_.IsZero() && !rtt_stats_->min_rtt().IsZero()) {
+      pacing_rate_ = QuicBandwidth::FromBytesAndTimeDelta(
+          initial_congestion_window_, rtt_stats_->min_rtt());
+      return;
+    }
+
+    // Do not decrease the pacing rate during the startup.
+    pacing_rate_ = std::max(pacing_rate_, target_rate);
+    return;
+  }
+
+  pacing_rate_ = target_rate;
 }
 
 void BbrSender::CalculateCongestionWindow(QuicByteCount bytes_acked) {
   if (mode_ == PROBE_RTT) {
     return;
   }
 
   QuicByteCount target_window =
       GetTargetCongestionWindow(congestion_window_gain_);
 
@@ skipping 88 matching lines @@
   os << "Minimum RTT timestamp: " << state.min_rtt_timestamp.ToDebuggingValue()
      << std::endl;
 
   os << "Last sample is app-limited: "
      << (state.last_sample_is_app_limited ? "yes" : "no");
 
   return os;
 }
 
 }  // namespace net