quic/core/congestion_control/tcp_cubic_sender_bytes.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 "quic/core/congestion_control/tcp_cubic_sender_bytes.h"

#include <algorithm>
#include <cstdint>
#include <string>

#include "quic/core/congestion_control/prr_sender.h"
#include "quic/core/congestion_control/rtt_stats.h"
#include "quic/core/crypto/crypto_protocol.h"
#include "quic/core/quic_constants.h"
#include "quic/platform/api/quic_bug_tracker.h"
#include "quic/platform/api/quic_flags.h"
#include "quic/platform/api/quic_logging.h"

namespace quic {

namespace {
// Constants based on TCP defaults.
const QuicByteCount kMaxBurstBytes = 3 * kDefaultTCPMSS;
const float kRenoBeta = 0.7f;  // Reno backoff factor.
// The minimum cwnd based on RFC 3782 (TCP NewReno) for cwnd reductions on a
// fast retransmission.
const QuicByteCount kDefaultMinimumCongestionWindow = 2 * kDefaultTCPMSS;
}  // namespace

TcpCubicSenderBytes::TcpCubicSenderBytes(
    const QuicClock* clock,
    const RttStats* rtt_stats,
    bool reno,
    QuicPacketCount initial_tcp_congestion_window,
    QuicPacketCount max_congestion_window,
    QuicConnectionStats* stats)
    : rtt_stats_(rtt_stats),
      stats_(stats),
      reno_(reno),
      num_connections_(kDefaultNumConnections),
      min4_mode_(false),
      last_cutback_exited_slowstart_(false),
      slow_start_large_reduction_(false),
      no_prr_(false),
      cubic_(clock),
      num_acked_packets_(0),
      congestion_window_(initial_tcp_congestion_window * kDefaultTCPMSS),
      min_congestion_window_(kDefaultMinimumCongestionWindow),
      max_congestion_window_(max_congestion_window * kDefaultTCPMSS),
      slowstart_threshold_(max_congestion_window * kDefaultTCPMSS),
      initial_tcp_congestion_window_(initial_tcp_congestion_window *
                                     kDefaultTCPMSS),
      initial_max_tcp_congestion_window_(max_congestion_window *
                                         kDefaultTCPMSS),
      min_slow_start_exit_window_(min_congestion_window_) {}

TcpCubicSenderBytes::~TcpCubicSenderBytes() {}

void TcpCubicSenderBytes::SetFromConfig(const QuicConfig& config,
                                        Perspective perspective) {
  if (perspective == Perspective::IS_SERVER) {
    if (!GetQuicReloadableFlag(quic_unified_iw_options)) {
      if (config.HasReceivedConnectionOptions() &&
          ContainsQuicTag(config.ReceivedConnectionOptions(), kIW03)) {
        // Initial window experiment.
        SetInitialCongestionWindowInPackets(3);
      }
      if (config.HasReceivedConnectionOptions() &&
          ContainsQuicTag(config.ReceivedConnectionOptions(), kIW10)) {
        // Initial window experiment.
        SetInitialCongestionWindowInPackets(10);
      }
      if (config.HasReceivedConnectionOptions() &&
          ContainsQuicTag(config.ReceivedConnectionOptions(), kIW20)) {
        // Initial window experiment.
        SetInitialCongestionWindowInPackets(20);
      }
      if (config.HasReceivedConnectionOptions() &&
          ContainsQuicTag(config.ReceivedConnectionOptions(), kIW50)) {
        // Initial window experiment.
        SetInitialCongestionWindowInPackets(50);
      }
      if (config.HasReceivedConnectionOptions() &&
          ContainsQuicTag(config.ReceivedConnectionOptions(), kMIN1)) {
        // Min CWND experiment.
        SetMinCongestionWindowInPackets(1);
      }
    }
    if (config.HasReceivedConnectionOptions() &&
        ContainsQuicTag(config.ReceivedConnectionOptions(), kMIN4)) {
      // Min CWND of 4 experiment.
      min4_mode_ = true;
      SetMinCongestionWindowInPackets(1);
    }
    if (config.HasReceivedConnectionOptions() &&
        ContainsQuicTag(config.ReceivedConnectionOptions(), kSSLR)) {
      // Slow Start Fast Exit experiment.
      slow_start_large_reduction_ = true;
    }
    if (config.HasReceivedConnectionOptions() &&
        ContainsQuicTag(config.ReceivedConnectionOptions(), kNPRR)) {
      // Use unity pacing instead of PRR.
      no_prr_ = true;
    }
  }
}

void TcpCubicSenderBytes::AdjustNetworkParameters(const NetworkParams& params) {
  if (params.bandwidth.IsZero() || params.rtt.IsZero()) {
    return;
  }
  SetCongestionWindowFromBandwidthAndRtt(params.bandwidth, params.rtt);
}

float TcpCubicSenderBytes::RenoBeta() const {
  // kNConnectionBeta is the backoff factor after loss for our N-connection
  // emulation, which emulates the effective backoff of an ensemble of N
  // TCP-Reno connections on a single loss event. The effective multiplier is
  // computed as:
  return (num_connections_ - 1 + kRenoBeta) / num_connections_;
}

void TcpCubicSenderBytes::OnCongestionEvent(
    bool rtt_updated,
    QuicByteCount prior_in_flight,
    QuicTime event_time,
    const AckedPacketVector& acked_packets,
    const LostPacketVector& lost_packets) {
  if (rtt_updated && InSlowStart() &&
      hybrid_slow_start_.ShouldExitSlowStart(
          rtt_stats_->latest_rtt(), rtt_stats_->min_rtt(),
          GetCongestionWindow() / kDefaultTCPMSS)) {
    ExitSlowstart();
  }
  for (const LostPacket& lost_packet : lost_packets) {
    OnPacketLost(lost_packet.packet_number, lost_packet.bytes_lost,
                 prior_in_flight);
  }
  for (const AckedPacket& acked_packet : acked_packets) {
    OnPacketAcked(acked_packet.packet_number, acked_packet.bytes_acked,
                  prior_in_flight, event_time);
  }
}

void TcpCubicSenderBytes::OnPacketAcked(QuicPacketNumber acked_packet_number,
                                        QuicByteCount acked_bytes,
                                        QuicByteCount prior_in_flight,
                                        QuicTime event_time) {
  largest_acked_packet_number_.UpdateMax(acked_packet_number);
  if (InRecovery()) {
    if (!no_prr_) {
      // PRR is used when in recovery.
      prr_.OnPacketAcked(acked_bytes);
    }
    return;
  }
  MaybeIncreaseCwnd(acked_packet_number, acked_bytes, prior_in_flight,
                    event_time);
  if (InSlowStart()) {
    hybrid_slow_start_.OnPacketAcked(acked_packet_number);
  }
}

void TcpCubicSenderBytes::OnPacketSent(
    QuicTime /*sent_time*/,
    QuicByteCount /*bytes_in_flight*/,
    QuicPacketNumber packet_number,
    QuicByteCount bytes,
    HasRetransmittableData is_retransmittable) {
  if (InSlowStart()) {
    ++(stats_->slowstart_packets_sent);
  }

  if (is_retransmittable != HAS_RETRANSMITTABLE_DATA) {
    return;
  }
  if (InRecovery()) {
    // PRR is used when in recovery.
    prr_.OnPacketSent(bytes);
  }
  QUICHE_DCHECK(!largest_sent_packet_number_.IsInitialized() ||
                largest_sent_packet_number_ < packet_number);
  largest_sent_packet_number_ = packet_number;
  hybrid_slow_start_.OnPacketSent(packet_number);
}

bool TcpCubicSenderBytes::CanSend(QuicByteCount bytes_in_flight) {
  if (!no_prr_ && InRecovery()) {
    // PRR is used when in recovery.
    return prr_.CanSend(GetCongestionWindow(), bytes_in_flight,
                        GetSlowStartThreshold());
  }
  if (GetCongestionWindow() > bytes_in_flight) {
    return true;
  }
  if (min4_mode_ && bytes_in_flight < 4 * kDefaultTCPMSS) {
    return true;
  }
  return false;
}

QuicBandwidth TcpCubicSenderBytes::PacingRate(
    QuicByteCount /* bytes_in_flight */) 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
  // doesn't prevent us from filling the window.
  QuicTime::Delta srtt = rtt_stats_->SmoothedOrInitialRtt();
  const QuicBandwidth bandwidth =
      QuicBandwidth::FromBytesAndTimeDelta(GetCongestionWindow(), srtt);
  return bandwidth * (InSlowStart() ? 2 : (no_prr_ && InRecovery() ? 1 : 1.25));
}

QuicBandwidth TcpCubicSenderBytes::BandwidthEstimate() const {
  QuicTime::Delta srtt = rtt_stats_->smoothed_rtt();
  if (srtt.IsZero()) {
    // If we haven't measured an rtt, the bandwidth estimate is unknown.
    return QuicBandwidth::Zero();
  }
  return QuicBandwidth::FromBytesAndTimeDelta(GetCongestionWindow(), srtt);
}

bool TcpCubicSenderBytes::InSlowStart() const {
  return GetCongestionWindow() < GetSlowStartThreshold();
}

bool TcpCubicSenderBytes::IsCwndLimited(QuicByteCount bytes_in_flight) const {
  const QuicByteCount congestion_window = GetCongestionWindow();
  if (bytes_in_flight >= congestion_window) {
    return true;
  }
  const QuicByteCount available_bytes = congestion_window - bytes_in_flight;
  const bool slow_start_limited =
      InSlowStart() && bytes_in_flight > congestion_window / 2;
  return slow_start_limited || available_bytes <= kMaxBurstBytes;
}

bool TcpCubicSenderBytes::InRecovery() const {
  return largest_acked_packet_number_.IsInitialized() &&
         largest_sent_at_last_cutback_.IsInitialized() &&
         largest_acked_packet_number_ <= largest_sent_at_last_cutback_;
}

bool TcpCubicSenderBytes::ShouldSendProbingPacket() const {
  return false;
}

void TcpCubicSenderBytes::OnRetransmissionTimeout(bool packets_retransmitted) {
  largest_sent_at_last_cutback_.Clear();
  if (!packets_retransmitted) {
    return;
  }
  hybrid_slow_start_.Restart();
  HandleRetransmissionTimeout();
}

std::string TcpCubicSenderBytes::GetDebugState() const {
  return "";
}

void TcpCubicSenderBytes::OnApplicationLimited(
    QuicByteCount /*bytes_in_flight*/) {}

void TcpCubicSenderBytes::SetCongestionWindowFromBandwidthAndRtt(
    QuicBandwidth bandwidth,
    QuicTime::Delta rtt) {
  QuicByteCount new_congestion_window = bandwidth.ToBytesPerPeriod(rtt);
  // Limit new CWND if needed.
  congestion_window_ =
      std::max(min_congestion_window_,
               std::min(new_congestion_window,
                        kMaxResumptionCongestionWindow * kDefaultTCPMSS));
}

void TcpCubicSenderBytes::SetInitialCongestionWindowInPackets(
    QuicPacketCount congestion_window) {
  congestion_window_ = congestion_window * kDefaultTCPMSS;
}

void TcpCubicSenderBytes::SetMinCongestionWindowInPackets(
    QuicPacketCount congestion_window) {
  min_congestion_window_ = congestion_window * kDefaultTCPMSS;
}

void TcpCubicSenderBytes::SetNumEmulatedConnections(int num_connections) {
  num_connections_ = std::max(1, num_connections);
  cubic_.SetNumConnections(num_connections_);
}

void TcpCubicSenderBytes::ExitSlowstart() {
  slowstart_threshold_ = congestion_window_;
}

void TcpCubicSenderBytes::OnPacketLost(QuicPacketNumber packet_number,
                                       QuicByteCount lost_bytes,
                                       QuicByteCount prior_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 (largest_sent_at_last_cutback_.IsInitialized() &&
      packet_number <= largest_sent_at_last_cutback_) {
    if (last_cutback_exited_slowstart_) {
      ++stats_->slowstart_packets_lost;
      stats_->slowstart_bytes_lost += lost_bytes;
      if (slow_start_large_reduction_) {
        // Reduce congestion window by lost_bytes for every loss.
        congestion_window_ = std::max(congestion_window_ - lost_bytes,
                                      min_slow_start_exit_window_);
        slowstart_threshold_ = congestion_window_;
      }
    }
    QUIC_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;
  }

  if (!no_prr_) {
    prr_.OnPacketLost(prior_in_flight);
  }

  // TODO(b/77268641): Separate out all of slow start into a separate class.
  if (slow_start_large_reduction_ && InSlowStart()) {
    QUICHE_DCHECK_LT(kDefaultTCPMSS, congestion_window_);
    if (congestion_window_ >= 2 * initial_tcp_congestion_window_) {
      min_slow_start_exit_window_ = congestion_window_ / 2;
    }
    congestion_window_ = congestion_window_ - kDefaultTCPMSS;
  } else if (reno_) {
    congestion_window_ = congestion_window_ * RenoBeta();
  } else {
    congestion_window_ =
        cubic_.CongestionWindowAfterPacketLoss(congestion_window_);
  }
  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;
  QUIC_DVLOG(1) << "Incoming loss; congestion window: " << congestion_window_
                << " slowstart threshold: " << slowstart_threshold_;
}

QuicByteCount TcpCubicSenderBytes::GetCongestionWindow() const {
  return congestion_window_;
}

QuicByteCount TcpCubicSenderBytes::GetSlowStartThreshold() const {
  return slowstart_threshold_;
}

// Called when we receive an ack. Normal TCP tracks how many packets one ack
// represents, but quic has a separate ack for each packet.
void TcpCubicSenderBytes::MaybeIncreaseCwnd(
    QuicPacketNumber /*acked_packet_number*/,
    QuicByteCount acked_bytes,
    QuicByteCount prior_in_flight,
    QuicTime event_time) {
  QUIC_BUG_IF(quic_bug_10439_1, InRecovery())
      << "Never increase the CWND during recovery.";
  // Do not increase the congestion window unless the sender is close to using
  // the current window.
  if (!IsCwndLimited(prior_in_flight)) {
    cubic_.OnApplicationLimited();
    return;
  }
  if (congestion_window_ >= max_congestion_window_) {
    return;
  }
  if (InSlowStart()) {
    // TCP slow start, exponential growth, increase by one for each ACK.
    congestion_window_ += kDefaultTCPMSS;
    QUIC_DVLOG(1) << "Slow start; congestion window: " << congestion_window_
                  << " slowstart threshold: " << slowstart_threshold_;
    return;
  }
  // Congestion avoidance.
  if (reno_) {
    // Classic Reno congestion avoidance.
    ++num_acked_packets_;
    // Divide by num_connections to smoothly increase the CWND at a faster rate
    // than conventional Reno.
    if (num_acked_packets_ * num_connections_ >=
        congestion_window_ / kDefaultTCPMSS) {
      congestion_window_ += kDefaultTCPMSS;
      num_acked_packets_ = 0;
    }

    QUIC_DVLOG(1) << "Reno; congestion window: " << congestion_window_
                  << " slowstart threshold: " << slowstart_threshold_
                  << " congestion window count: " << num_acked_packets_;
  } else {
    congestion_window_ = std::min(
        max_congestion_window_,
        cubic_.CongestionWindowAfterAck(acked_bytes, congestion_window_,
                                        rtt_stats_->min_rtt(), event_time));
    QUIC_DVLOG(1) << "Cubic; congestion window: " << congestion_window_
                  << " slowstart threshold: " << slowstart_threshold_;
  }
}

void TcpCubicSenderBytes::HandleRetransmissionTimeout() {
  cubic_.ResetCubicState();
  slowstart_threshold_ = congestion_window_ / 2;
  congestion_window_ = min_congestion_window_;
}

void TcpCubicSenderBytes::OnConnectionMigration() {
  hybrid_slow_start_.Restart();
  prr_ = PrrSender();
  largest_sent_packet_number_.Clear();
  largest_acked_packet_number_.Clear();
  largest_sent_at_last_cutback_.Clear();
  last_cutback_exited_slowstart_ = false;
  cubic_.ResetCubicState();
  num_acked_packets_ = 0;
  congestion_window_ = initial_tcp_congestion_window_;
  max_congestion_window_ = initial_max_tcp_congestion_window_;
  slowstart_threshold_ = initial_max_tcp_congestion_window_;
}

CongestionControlType TcpCubicSenderBytes::GetCongestionControlType() const {
  return reno_ ? kRenoBytes : kCubicBytes;
}

}  // namespace quic