Update from chromium https://crrev.com/302282

net/quic/congestion_control/prr_sender.cc

+// Copyright (c) 2014 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/prr_sender.h"
+
+#include "net/quic/quic_protocol.h"
+
+namespace net {
+
+namespace {
+// Constant based on TCP defaults.
+const QuicByteCount kMaxSegmentSize = kDefaultTCPMSS;
+}  // namespace
+
+PrrSender::PrrSender()
+    : bytes_sent_since_loss_(0),
+      bytes_delivered_since_loss_(0),
+      ack_count_since_loss_(0),
+      bytes_in_flight_before_loss_(0) {
+}
+
+void PrrSender::OnPacketSent(QuicByteCount sent_bytes) {
+    bytes_sent_since_loss_ += sent_bytes;
+}
+
+void PrrSender::OnPacketLost(QuicByteCount bytes_in_flight) {
+  bytes_sent_since_loss_ = 0;
+  bytes_in_flight_before_loss_ = bytes_in_flight;
+  bytes_delivered_since_loss_ = 0;
+  ack_count_since_loss_ = 0;
+}
+
+void PrrSender::OnPacketAcked(QuicByteCount acked_bytes) {
+  bytes_delivered_since_loss_ += acked_bytes;
+  ++ack_count_since_loss_;
+}
+
+QuicTime::Delta PrrSender::TimeUntilSend(
+    QuicByteCount congestion_window,
+    QuicByteCount bytes_in_flight,
+    QuicPacketCount slowstart_threshold) const {
+  // Return QuicTime::Zero In order to ensure limited transmit always works.
+  if (bytes_sent_since_loss_ == 0 || bytes_in_flight < kMaxSegmentSize) {
+    return QuicTime::Delta::Zero();
+  }
+  if (congestion_window > bytes_in_flight) {
+    // During PRR-SSRB, limit outgoing packets to 1 extra MSS per ack, instead
+    // of sending the entire available window. This prevents burst retransmits
+    // when more packets are lost than the CWND reduction.
+    //   limit = MAX(prr_delivered - prr_out, DeliveredData) + MSS
+    if (bytes_delivered_since_loss_ + ack_count_since_loss_ * kMaxSegmentSize <=
+            bytes_sent_since_loss_) {
+      return QuicTime::Delta::Infinite();
+    }
+    return QuicTime::Delta::Zero();
+  }
+  // Implement Proportional Rate Reduction (RFC6937).
+  // Checks a simplified version of the PRR formula that doesn't use division:
+  // AvailableSendWindow =
+  //   CEIL(prr_delivered * ssthresh / BytesInFlightAtLoss) - prr_sent
+  if (bytes_delivered_since_loss_ * slowstart_threshold * kMaxSegmentSize >
+          bytes_sent_since_loss_ * bytes_in_flight_before_loss_) {
+    return QuicTime::Delta::Zero();
+  }
+  return QuicTime::Delta::Infinite();
+}
+
+}  // namespace net