Refactor Cubic congestion control to separate out PRR into a distinct

net/quic/congestion_control/tcp_cubic_sender.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/congestion_control/tcp_cubic_sender.h"
 
 #include <algorithm>
 
 #include "base/metrics/histogram.h"
+#include "net/quic/congestion_control/prr_sender.h"
 #include "net/quic/congestion_control/rtt_stats.h"
 #include "net/quic/crypto/crypto_protocol.h"
 
 using std::max;
 using std::min;
 
 namespace net {
 
 namespace {
 // Constants based on TCP defaults.
 // The minimum cwnd based on RFC 3782 (TCP NewReno) for cwnd reductions on a
 // fast retransmission.  The cwnd after a timeout is still 1.
 const QuicPacketCount kMinimumCongestionWindow = 2;
 const QuicByteCount kMaxSegmentSize = kDefaultTCPMSS;
 const int64 kInitialCongestionWindow = 10;
 const int kMaxBurstLength = 3;
-};  // namespace
+}  // namespace
 
 TcpCubicSender::TcpCubicSender(
     const QuicClock* clock,
     const RttStats* rtt_stats,
     bool reno,
     QuicPacketCount max_tcp_congestion_window,
     QuicConnectionStats* stats)
     : hybrid_slow_start_(clock),
       cubic_(clock, stats),
       rtt_stats_(rtt_stats),
       stats_(stats),
       reno_(reno),
       num_connections_(2),
       congestion_window_count_(0),
-      prr_out_(0),
-      prr_delivered_(0),
-      ack_count_since_loss_(0),
-      bytes_in_flight_before_loss_(0),
       largest_sent_sequence_number_(0),
       largest_acked_sequence_number_(0),
       largest_sent_at_last_cutback_(0),
       congestion_window_(kInitialCongestionWindow),
       previous_congestion_window_(0),
       slowstart_threshold_(max_tcp_congestion_window),
       previous_slowstart_threshold_(0),
       last_cutback_exited_slowstart_(false),
       max_tcp_congestion_window_(max_tcp_congestion_window) {
 }
@@ skipping 44 matching lines @@
   }
 }
 
 void TcpCubicSender::OnPacketAcked(
     QuicPacketSequenceNumber acked_sequence_number,
     QuicByteCount acked_bytes,
     QuicByteCount bytes_in_flight) {
   largest_acked_sequence_number_ = max(acked_sequence_number,
                                        largest_acked_sequence_number_);
   if (InRecovery()) {
-    PrrOnPacketAcked(acked_bytes);
+    // PRR is used when in recovery.
+    prr_.OnPacketAcked(acked_bytes);
     return;
   }
   MaybeIncreaseCwnd(acked_sequence_number, bytes_in_flight);
   // TODO(ianswett): Should this even be called when not in slow start?
   hybrid_slow_start_.OnPacketAcked(acked_sequence_number, InSlowStart());
 }
 
 void TcpCubicSender::OnPacketLost(QuicPacketSequenceNumber sequence_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 (sequence_number <= largest_sent_at_last_cutback_) {
     if (last_cutback_exited_slowstart_) {
       ++stats_->slowstart_packets_lost;
     }
     DVLOG(1) << "Ignoring loss for largest_missing:" << sequence_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;
   }
-  PrrOnPacketLost(bytes_in_flight);
+
+  prr_.OnPacketLost(bytes_in_flight);
 
   if (reno_) {
     congestion_window_ = congestion_window_ >> 1;
   } else {
     congestion_window_ =
         cubic_.CongestionWindowAfterPacketLoss(congestion_window_);
   }
   slowstart_threshold_ = congestion_window_;
   // Enforce TCP's minimum congestion window of 2*MSS.
   if (congestion_window_ < kMinimumCongestionWindow) {
     congestion_window_ = kMinimumCongestionWindow;
   }
   largest_sent_at_last_cutback_ = largest_sent_sequence_number_;
   // reset packet count from congestion avoidance mode. We start
   // counting again when we're out of recovery.
   congestion_window_count_ = 0;
   DVLOG(1) << "Incoming loss; congestion window: " << congestion_window_
            << " slowstart threshold: " << slowstart_threshold_;
 }
 
 bool TcpCubicSender::OnPacketSent(QuicTime /*sent_time*/,
                                   QuicByteCount /*bytes_in_flight*/,
                                   QuicPacketSequenceNumber sequence_number,
                                   QuicByteCount bytes,
                                   HasRetransmittableData is_retransmittable) {
   // Only update bytes_in_flight_ for data packets.
   if (is_retransmittable != HAS_RETRANSMITTABLE_DATA) {
     return false;
   }
-
-  prr_out_ += bytes;
+  if (InRecovery()) {
+    // PRR is used when in recovery.
+    prr_.OnPacketSent(bytes);
+  }
   DCHECK_LT(largest_sent_sequence_number_, sequence_number);
   largest_sent_sequence_number_ = sequence_number;
   hybrid_slow_start_.OnPacketSent(sequence_number);
   return true;
 }
 
 QuicTime::Delta TcpCubicSender::TimeUntilSend(
     QuicTime /* now */,
     QuicByteCount bytes_in_flight,
     HasRetransmittableData has_retransmittable_data) const {
   if (has_retransmittable_data == NO_RETRANSMITTABLE_DATA) {
     // For TCP we can always send an ACK immediately.
     return QuicTime::Delta::Zero();
   }
   if (InRecovery()) {
-    return PrrTimeUntilSend(bytes_in_flight);
+    // PRR is used when in recovery.
+    return prr_.TimeUntilSend(GetCongestionWindow(), bytes_in_flight,
+                              slowstart_threshold_);
   }
   if (GetCongestionWindow() > bytes_in_flight) {
     return QuicTime::Delta::Zero();
   }
   return QuicTime::Delta::Infinite();
 }
 
 QuicBandwidth TcpCubicSender::PacingRate() const {
   // We pace at twice the rate of the underlying sender's bandwidth estimate
   // during slow start and 1.25x during congestion avoidance to ensure pacing
@@ skipping 116 matching lines @@
 void TcpCubicSender::RevertRetransmissionTimeout() {
   if (previous_congestion_window_ == 0) {
     LOG(DFATAL) << "No previous congestion window to revert to.";
     return;
   }
   congestion_window_ = previous_congestion_window_;
   slowstart_threshold_ = previous_slowstart_threshold_;
   previous_congestion_window_ = 0;
 }
 
-void TcpCubicSender::PrrOnPacketLost(QuicByteCount bytes_in_flight) {
-  prr_out_ = 0;
-  bytes_in_flight_before_loss_ = bytes_in_flight;
-  prr_delivered_ = 0;
-  ack_count_since_loss_ = 0;
-}
-
-void TcpCubicSender::PrrOnPacketAcked(QuicByteCount acked_bytes) {
-  prr_delivered_ += acked_bytes;
-  ++ack_count_since_loss_;
-}
-
-QuicTime::Delta TcpCubicSender::PrrTimeUntilSend(
-    QuicByteCount bytes_in_flight) const {
-  DCHECK(InRecovery());
-  // Return QuicTime::Zero In order to ensure limited transmit always works.
-  if (prr_out_ == 0 || bytes_in_flight < kMaxSegmentSize) {
-    return QuicTime::Delta::Zero();
-  }
-  if (GetCongestionWindow() > 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 (prr_delivered_ + ack_count_since_loss_ * kMaxSegmentSize <= prr_out_) {
-      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 (prr_delivered_ * slowstart_threshold_ * kMaxSegmentSize >
-          prr_out_ * bytes_in_flight_before_loss_) {
-    return QuicTime::Delta::Zero();
-  }
-  return QuicTime::Delta::Infinite();
-}
-
 CongestionControlType TcpCubicSender::GetCongestionControlType() const {
   return reno_ ? kReno : kCubic;
 }
 
 }  // namespace net