| // 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 "quic/core/congestion_control/bbr_sender.h" |
| |
| #include <algorithm> |
| #include <sstream> |
| #include <string> |
| |
| #include "absl/base/attributes.h" |
| #include "quic/core/congestion_control/rtt_stats.h" |
| #include "quic/core/crypto/crypto_protocol.h" |
| #include "quic/core/quic_time.h" |
| #include "quic/core/quic_time_accumulator.h" |
| #include "quic/platform/api/quic_bug_tracker.h" |
| #include "quic/platform/api/quic_flag_utils.h" |
| #include "quic/platform/api/quic_flags.h" |
| #include "quic/platform/api/quic_logging.h" |
| |
| namespace quic { |
| |
| namespace { |
| // Constants based on TCP defaults. |
| // The minimum CWND to ensure delayed acks don't reduce bandwidth measurements. |
| // Does not inflate the pacing rate. |
| const QuicByteCount kDefaultMinimumCongestionWindow = 4 * kMaxSegmentSize; |
| |
| // The gain used for the STARTUP, equal to 2/ln(2). |
| const float kDefaultHighGain = 2.885f; |
| // The newly derived gain for STARTUP, equal to 4 * ln(2) |
| const float kDerivedHighGain = 2.773f; |
| // The newly derived CWND gain for STARTUP, 2. |
| const float kDerivedHighCWNDGain = 2.0f; |
| // The cycle of gains used during the PROBE_BW stage. |
| const float kPacingGain[] = {1.25, 0.75, 1, 1, 1, 1, 1, 1}; |
| |
| // The length of the gain cycle. |
| const size_t kGainCycleLength = sizeof(kPacingGain) / sizeof(kPacingGain[0]); |
| // The size of the bandwidth filter window, in round-trips. |
| const QuicRoundTripCount kBandwidthWindowSize = kGainCycleLength + 2; |
| |
| // The time after which the current min_rtt value expires. |
| const QuicTime::Delta kMinRttExpiry = QuicTime::Delta::FromSeconds(10); |
| // The minimum time the connection can spend in PROBE_RTT mode. |
| const QuicTime::Delta kProbeRttTime = QuicTime::Delta::FromMilliseconds(200); |
| // If the bandwidth does not increase by the factor of |kStartupGrowthTarget| |
| // within |kRoundTripsWithoutGrowthBeforeExitingStartup| rounds, the connection |
| // will exit the STARTUP mode. |
| const float kStartupGrowthTarget = 1.25; |
| const QuicRoundTripCount kRoundTripsWithoutGrowthBeforeExitingStartup = 3; |
| } // namespace |
| |
| BbrSender::DebugState::DebugState(const BbrSender& sender) |
| : mode(sender.mode_), |
| max_bandwidth(sender.max_bandwidth_.GetBest()), |
| round_trip_count(sender.round_trip_count_), |
| gain_cycle_index(sender.cycle_current_offset_), |
| congestion_window(sender.congestion_window_), |
| is_at_full_bandwidth(sender.is_at_full_bandwidth_), |
| bandwidth_at_last_round(sender.bandwidth_at_last_round_), |
| rounds_without_bandwidth_gain(sender.rounds_without_bandwidth_gain_), |
| min_rtt(sender.min_rtt_), |
| min_rtt_timestamp(sender.min_rtt_timestamp_), |
| recovery_state(sender.recovery_state_), |
| recovery_window(sender.recovery_window_), |
| last_sample_is_app_limited(sender.last_sample_is_app_limited_), |
| end_of_app_limited_phase(sender.sampler_.end_of_app_limited_phase()) {} |
| |
| BbrSender::DebugState::DebugState(const DebugState& state) = default; |
| |
| BbrSender::BbrSender(QuicTime now, |
| const RttStats* rtt_stats, |
| const QuicUnackedPacketMap* unacked_packets, |
| QuicPacketCount initial_tcp_congestion_window, |
| QuicPacketCount max_tcp_congestion_window, |
| QuicRandom* random, |
| QuicConnectionStats* stats) |
| : rtt_stats_(rtt_stats), |
| unacked_packets_(unacked_packets), |
| random_(random), |
| stats_(stats), |
| mode_(STARTUP), |
| sampler_(unacked_packets, kBandwidthWindowSize), |
| round_trip_count_(0), |
| num_loss_events_in_round_(0), |
| bytes_lost_in_round_(0), |
| max_bandwidth_(kBandwidthWindowSize, QuicBandwidth::Zero(), 0), |
| min_rtt_(QuicTime::Delta::Zero()), |
| min_rtt_timestamp_(QuicTime::Zero()), |
| congestion_window_(initial_tcp_congestion_window * kDefaultTCPMSS), |
| initial_congestion_window_(initial_tcp_congestion_window * |
| kDefaultTCPMSS), |
| max_congestion_window_(max_tcp_congestion_window * kDefaultTCPMSS), |
| min_congestion_window_(kDefaultMinimumCongestionWindow), |
| high_gain_(kDefaultHighGain), |
| high_cwnd_gain_(kDefaultHighGain), |
| drain_gain_(1.f / kDefaultHighGain), |
| pacing_rate_(QuicBandwidth::Zero()), |
| pacing_gain_(1), |
| congestion_window_gain_(1), |
| congestion_window_gain_constant_( |
| static_cast<float>(GetQuicFlag(FLAGS_quic_bbr_cwnd_gain))), |
| num_startup_rtts_(kRoundTripsWithoutGrowthBeforeExitingStartup), |
| cycle_current_offset_(0), |
| 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), |
| has_non_app_limited_sample_(false), |
| recovery_state_(NOT_IN_RECOVERY), |
| recovery_window_(max_congestion_window_), |
| slower_startup_(false), |
| rate_based_startup_(false), |
| enable_ack_aggregation_during_startup_(false), |
| expire_ack_aggregation_in_startup_(false), |
| drain_to_target_(false), |
| detect_overshooting_(false), |
| bytes_lost_while_detecting_overshooting_(0), |
| bytes_lost_multiplier_while_detecting_overshooting_(2), |
| cwnd_to_calculate_min_pacing_rate_(initial_congestion_window_), |
| max_congestion_window_with_network_parameters_adjusted_( |
| kMaxInitialCongestionWindow * kDefaultTCPMSS) { |
| if (stats_) { |
| // Clear some startup stats if |stats_| has been used by another sender, |
| // which happens e.g. when QuicConnection switch send algorithms. |
| stats_->slowstart_count = 0; |
| stats_->slowstart_duration = QuicTimeAccumulator(); |
| } |
| EnterStartupMode(now); |
| set_high_cwnd_gain(kDerivedHighCWNDGain); |
| } |
| |
| BbrSender::~BbrSender() {} |
| |
| void BbrSender::SetInitialCongestionWindowInPackets( |
| QuicPacketCount congestion_window) { |
| if (mode_ == STARTUP) { |
| initial_congestion_window_ = congestion_window * kDefaultTCPMSS; |
| congestion_window_ = congestion_window * kDefaultTCPMSS; |
| cwnd_to_calculate_min_pacing_rate_ = std::min( |
| initial_congestion_window_, cwnd_to_calculate_min_pacing_rate_); |
| } |
| } |
| |
| bool BbrSender::InSlowStart() const { |
| return mode_ == STARTUP; |
| } |
| |
| void BbrSender::OnPacketSent(QuicTime sent_time, |
| QuicByteCount bytes_in_flight, |
| QuicPacketNumber packet_number, |
| QuicByteCount bytes, |
| HasRetransmittableData is_retransmittable) { |
| if (stats_ && InSlowStart()) { |
| ++stats_->slowstart_packets_sent; |
| stats_->slowstart_bytes_sent += bytes; |
| } |
| |
| last_sent_packet_ = packet_number; |
| |
| if (bytes_in_flight == 0 && sampler_.is_app_limited()) { |
| exiting_quiescence_ = true; |
| } |
| |
| sampler_.OnPacketSent(sent_time, packet_number, bytes, bytes_in_flight, |
| is_retransmittable); |
| } |
| |
| void BbrSender::OnPacketNeutered(QuicPacketNumber packet_number) { |
| sampler_.OnPacketNeutered(packet_number); |
| } |
| |
| bool BbrSender::CanSend(QuicByteCount bytes_in_flight) { |
| return bytes_in_flight < GetCongestionWindow(); |
| } |
| |
| QuicBandwidth BbrSender::PacingRate(QuicByteCount /*bytes_in_flight*/) const { |
| if (pacing_rate_.IsZero()) { |
| return high_gain_ * QuicBandwidth::FromBytesAndTimeDelta( |
| initial_congestion_window_, GetMinRtt()); |
| } |
| return pacing_rate_; |
| } |
| |
| QuicBandwidth BbrSender::BandwidthEstimate() const { |
| return max_bandwidth_.GetBest(); |
| } |
| |
| QuicByteCount BbrSender::GetCongestionWindow() const { |
| if (mode_ == PROBE_RTT) { |
| return ProbeRttCongestionWindow(); |
| } |
| |
| if (InRecovery()) { |
| return std::min(congestion_window_, recovery_window_); |
| } |
| |
| return congestion_window_; |
| } |
| |
| QuicByteCount BbrSender::GetSlowStartThreshold() const { |
| return 0; |
| } |
| |
| bool BbrSender::InRecovery() const { |
| return recovery_state_ != NOT_IN_RECOVERY; |
| } |
| |
| bool BbrSender::ShouldSendProbingPacket() const { |
| if (pacing_gain_ <= 1) { |
| return false; |
| } |
| |
| // TODO(b/77975811): If the pipe is highly under-utilized, consider not |
| // sending a probing transmission, because the extra bandwidth is not needed. |
| return true; |
| } |
| |
| void BbrSender::SetFromConfig(const QuicConfig& config, |
| Perspective perspective) { |
| if (config.HasClientRequestedIndependentOption(k1RTT, perspective)) { |
| num_startup_rtts_ = 1; |
| } |
| if (config.HasClientRequestedIndependentOption(k2RTT, perspective)) { |
| num_startup_rtts_ = 2; |
| } |
| if (config.HasClientRequestedIndependentOption(kBBR3, perspective)) { |
| drain_to_target_ = true; |
| } |
| if (config.HasClientRequestedIndependentOption(kBWM3, perspective)) { |
| bytes_lost_multiplier_while_detecting_overshooting_ = 3; |
| } |
| if (config.HasClientRequestedIndependentOption(kBWM4, perspective)) { |
| bytes_lost_multiplier_while_detecting_overshooting_ = 4; |
| } |
| if (config.HasClientRequestedIndependentOption(kBBR4, perspective)) { |
| sampler_.SetMaxAckHeightTrackerWindowLength(2 * kBandwidthWindowSize); |
| } |
| if (config.HasClientRequestedIndependentOption(kBBR5, perspective)) { |
| sampler_.SetMaxAckHeightTrackerWindowLength(4 * kBandwidthWindowSize); |
| } |
| if (config.HasClientRequestedIndependentOption(kBBQ1, perspective)) { |
| set_high_gain(kDerivedHighGain); |
| set_high_cwnd_gain(kDerivedHighGain); |
| set_drain_gain(1.f / kDerivedHighGain); |
| } |
| if (config.HasClientRequestedIndependentOption(kBBQ3, perspective)) { |
| enable_ack_aggregation_during_startup_ = true; |
| } |
| if (config.HasClientRequestedIndependentOption(kBBQ5, perspective)) { |
| expire_ack_aggregation_in_startup_ = true; |
| } |
| if (config.HasClientRequestedIndependentOption(kMIN1, perspective)) { |
| min_congestion_window_ = kMaxSegmentSize; |
| } |
| if (config.HasClientRequestedIndependentOption(kICW1, perspective)) { |
| max_congestion_window_with_network_parameters_adjusted_ = |
| 100 * kDefaultTCPMSS; |
| } |
| if (config.HasClientRequestedIndependentOption(kDTOS, perspective)) { |
| detect_overshooting_ = true; |
| // DTOS would allow pacing rate drop to IW 10 / min_rtt if overshooting is |
| // detected. |
| cwnd_to_calculate_min_pacing_rate_ = |
| std::min(initial_congestion_window_, 10 * kDefaultTCPMSS); |
| } |
| |
| ApplyConnectionOptions(config.ClientRequestedIndependentOptions(perspective)); |
| } |
| |
| void BbrSender::ApplyConnectionOptions( |
| const QuicTagVector& connection_options) { |
| if (ContainsQuicTag(connection_options, kBSAO)) { |
| sampler_.EnableOverestimateAvoidance(); |
| } |
| if (GetQuicReloadableFlag( |
| quic_bbr_start_new_aggregation_epoch_after_a_full_round) && |
| ContainsQuicTag(connection_options, kBBRA)) { |
| sampler_.SetStartNewAggregationEpochAfterFullRound(true); |
| } |
| if (GetQuicReloadableFlag(quic_bbr_use_send_rate_in_max_ack_height_tracker) && |
| ContainsQuicTag(connection_options, kBBRB)) { |
| QUIC_RELOADABLE_FLAG_COUNT_N( |
| quic_bbr_use_send_rate_in_max_ack_height_tracker, 1, 2); |
| sampler_.SetLimitMaxAckHeightTrackerBySendRate(true); |
| } |
| } |
| |
| void BbrSender::AdjustNetworkParameters(const NetworkParams& params) { |
| const QuicBandwidth& bandwidth = params.bandwidth; |
| const QuicTime::Delta& rtt = params.rtt; |
| |
| if (!rtt.IsZero() && (min_rtt_ > rtt || min_rtt_.IsZero())) { |
| min_rtt_ = rtt; |
| } |
| |
| if (mode_ == STARTUP) { |
| if (bandwidth.IsZero()) { |
| // Ignore bad bandwidth samples. |
| return; |
| } |
| |
| auto cwnd_bootstrapping_rtt = GetMinRtt(); |
| if (params.max_initial_congestion_window > 0) { |
| max_congestion_window_with_network_parameters_adjusted_ = |
| params.max_initial_congestion_window * kDefaultTCPMSS; |
| } |
| const QuicByteCount new_cwnd = std::max( |
| kMinInitialCongestionWindow * kDefaultTCPMSS, |
| std::min(max_congestion_window_with_network_parameters_adjusted_, |
| bandwidth * cwnd_bootstrapping_rtt)); |
| |
| stats_->cwnd_bootstrapping_rtt_us = cwnd_bootstrapping_rtt.ToMicroseconds(); |
| if (!rtt_stats_->smoothed_rtt().IsZero()) { |
| QUIC_CODE_COUNT(quic_smoothed_rtt_available); |
| } else if (rtt_stats_->initial_rtt() != |
| QuicTime::Delta::FromMilliseconds(kInitialRttMs)) { |
| QUIC_CODE_COUNT(quic_client_initial_rtt_available); |
| } else { |
| QUIC_CODE_COUNT(quic_default_initial_rtt); |
| } |
| if (new_cwnd < congestion_window_ && !params.allow_cwnd_to_decrease) { |
| // Only decrease cwnd if allow_cwnd_to_decrease is true. |
| return; |
| } |
| if (GetQuicReloadableFlag(quic_conservative_cwnd_and_pacing_gains)) { |
| // Decreases cwnd gain and pacing gain. Please note, if pacing_rate_ has |
| // been calculated, it cannot decrease in STARTUP phase. |
| QUIC_RELOADABLE_FLAG_COUNT(quic_conservative_cwnd_and_pacing_gains); |
| set_high_gain(kDerivedHighCWNDGain); |
| set_high_cwnd_gain(kDerivedHighCWNDGain); |
| } |
| congestion_window_ = new_cwnd; |
| |
| // Pace at the rate of new_cwnd / RTT. |
| QuicBandwidth new_pacing_rate = |
| QuicBandwidth::FromBytesAndTimeDelta(congestion_window_, GetMinRtt()); |
| pacing_rate_ = std::max(pacing_rate_, new_pacing_rate); |
| detect_overshooting_ = true; |
| } |
| } |
| |
| void BbrSender::OnCongestionEvent(bool /*rtt_updated*/, |
| QuicByteCount prior_in_flight, |
| QuicTime event_time, |
| const AckedPacketVector& acked_packets, |
| const LostPacketVector& lost_packets) { |
| const QuicByteCount total_bytes_acked_before = sampler_.total_bytes_acked(); |
| const QuicByteCount total_bytes_lost_before = sampler_.total_bytes_lost(); |
| |
| bool is_round_start = false; |
| bool min_rtt_expired = false; |
| QuicByteCount excess_acked = 0; |
| QuicByteCount bytes_lost = 0; |
| |
| // The send state of the largest packet in acked_packets, unless it is |
| // empty. If acked_packets is empty, it's the send state of the largest |
| // packet in lost_packets. |
| SendTimeState last_packet_send_state; |
| |
| if (!acked_packets.empty()) { |
| QuicPacketNumber last_acked_packet = acked_packets.rbegin()->packet_number; |
| is_round_start = UpdateRoundTripCounter(last_acked_packet); |
| UpdateRecoveryState(last_acked_packet, !lost_packets.empty(), |
| is_round_start); |
| } |
| |
| BandwidthSamplerInterface::CongestionEventSample sample = |
| sampler_.OnCongestionEvent(event_time, acked_packets, lost_packets, |
| max_bandwidth_.GetBest(), |
| QuicBandwidth::Infinite(), round_trip_count_); |
| if (sample.last_packet_send_state.is_valid) { |
| last_sample_is_app_limited_ = sample.last_packet_send_state.is_app_limited; |
| has_non_app_limited_sample_ |= !last_sample_is_app_limited_; |
| if (stats_) { |
| stats_->has_non_app_limited_sample = has_non_app_limited_sample_; |
| } |
| } |
| // Avoid updating |max_bandwidth_| if a) this is a loss-only event, or b) all |
| // packets in |acked_packets| did not generate valid samples. (e.g. ack of |
| // ack-only packets). In both cases, sampler_.total_bytes_acked() will not |
| // change. |
| if (total_bytes_acked_before != sampler_.total_bytes_acked()) { |
| QUIC_LOG_IF(WARNING, sample.sample_max_bandwidth.IsZero()) |
| << sampler_.total_bytes_acked() - total_bytes_acked_before |
| << " bytes from " << acked_packets.size() |
| << " packets have been acked, but sample_max_bandwidth is zero."; |
| if (!sample.sample_is_app_limited || |
| sample.sample_max_bandwidth > max_bandwidth_.GetBest()) { |
| max_bandwidth_.Update(sample.sample_max_bandwidth, round_trip_count_); |
| } |
| } |
| |
| if (!sample.sample_rtt.IsInfinite()) { |
| min_rtt_expired = MaybeUpdateMinRtt(event_time, sample.sample_rtt); |
| } |
| bytes_lost = sampler_.total_bytes_lost() - total_bytes_lost_before; |
| if (mode_ == STARTUP) { |
| if (stats_) { |
| stats_->slowstart_packets_lost += lost_packets.size(); |
| stats_->slowstart_bytes_lost += bytes_lost; |
| } |
| } |
| excess_acked = sample.extra_acked; |
| last_packet_send_state = sample.last_packet_send_state; |
| |
| if (!lost_packets.empty()) { |
| ++num_loss_events_in_round_; |
| bytes_lost_in_round_ += bytes_lost; |
| } |
| |
| // Handle logic specific to PROBE_BW mode. |
| if (mode_ == PROBE_BW) { |
| UpdateGainCyclePhase(event_time, prior_in_flight, !lost_packets.empty()); |
| } |
| |
| // Handle logic specific to STARTUP and DRAIN modes. |
| if (is_round_start && !is_at_full_bandwidth_) { |
| CheckIfFullBandwidthReached(last_packet_send_state); |
| } |
| MaybeExitStartupOrDrain(event_time); |
| |
| // Handle logic specific to PROBE_RTT. |
| MaybeEnterOrExitProbeRtt(event_time, is_round_start, min_rtt_expired); |
| |
| // Calculate number of packets acked and lost. |
| QuicByteCount bytes_acked = |
| sampler_.total_bytes_acked() - total_bytes_acked_before; |
| |
| // After the model is updated, recalculate the pacing rate and congestion |
| // window. |
| CalculatePacingRate(bytes_lost); |
| CalculateCongestionWindow(bytes_acked, excess_acked); |
| CalculateRecoveryWindow(bytes_acked, bytes_lost); |
| |
| // Cleanup internal state. |
| sampler_.RemoveObsoletePackets(unacked_packets_->GetLeastUnacked()); |
| if (is_round_start) { |
| num_loss_events_in_round_ = 0; |
| bytes_lost_in_round_ = 0; |
| } |
| } |
| |
| CongestionControlType BbrSender::GetCongestionControlType() const { |
| return kBBR; |
| } |
| |
| QuicTime::Delta BbrSender::GetMinRtt() const { |
| if (!min_rtt_.IsZero()) { |
| return min_rtt_; |
| } |
| // min_rtt could be available if the handshake packet gets neutered then |
| // gets acknowledged. This could only happen for QUIC crypto where we do not |
| // drop keys. |
| return rtt_stats_->MinOrInitialRtt(); |
| } |
| |
| QuicByteCount BbrSender::GetTargetCongestionWindow(float gain) const { |
| QuicByteCount bdp = GetMinRtt() * BandwidthEstimate(); |
| QuicByteCount congestion_window = gain * bdp; |
| |
| // BDP estimate will be zero if no bandwidth samples are available yet. |
| if (congestion_window == 0) { |
| congestion_window = gain * initial_congestion_window_; |
| } |
| |
| return std::max(congestion_window, min_congestion_window_); |
| } |
| |
| QuicByteCount BbrSender::ProbeRttCongestionWindow() const { |
| return min_congestion_window_; |
| } |
| |
| void BbrSender::EnterStartupMode(QuicTime now) { |
| if (stats_) { |
| ++stats_->slowstart_count; |
| stats_->slowstart_duration.Start(now); |
| } |
| mode_ = STARTUP; |
| pacing_gain_ = high_gain_; |
| congestion_window_gain_ = high_cwnd_gain_; |
| } |
| |
| void BbrSender::EnterProbeBandwidthMode(QuicTime now) { |
| mode_ = PROBE_BW; |
| congestion_window_gain_ = congestion_window_gain_constant_; |
| |
| // Pick a random offset for the gain cycle out of {0, 2..7} range. 1 is |
| // excluded because in that case increased gain and decreased gain would not |
| // follow each other. |
| cycle_current_offset_ = random_->RandUint64() % (kGainCycleLength - 1); |
| if (cycle_current_offset_ >= 1) { |
| cycle_current_offset_ += 1; |
| } |
| |
| last_cycle_start_ = now; |
| pacing_gain_ = kPacingGain[cycle_current_offset_]; |
| } |
| |
| bool BbrSender::UpdateRoundTripCounter(QuicPacketNumber last_acked_packet) { |
| if (!current_round_trip_end_.IsInitialized() || |
| last_acked_packet > current_round_trip_end_) { |
| round_trip_count_++; |
| current_round_trip_end_ = last_sent_packet_; |
| if (stats_ && InSlowStart()) { |
| ++stats_->slowstart_num_rtts; |
| } |
| return true; |
| } |
| |
| return false; |
| } |
| |
| bool BbrSender::MaybeUpdateMinRtt(QuicTime now, |
| QuicTime::Delta sample_min_rtt) { |
| // Do not expire min_rtt if none was ever available. |
| bool min_rtt_expired = |
| !min_rtt_.IsZero() && (now > (min_rtt_timestamp_ + kMinRttExpiry)); |
| |
| if (min_rtt_expired || sample_min_rtt < min_rtt_ || min_rtt_.IsZero()) { |
| QUIC_DVLOG(2) << "Min RTT updated, old value: " << min_rtt_ |
| << ", new value: " << sample_min_rtt |
| << ", current time: " << now.ToDebuggingValue(); |
| |
| min_rtt_ = sample_min_rtt; |
| min_rtt_timestamp_ = now; |
| } |
| QUICHE_DCHECK(!min_rtt_.IsZero()); |
| |
| return min_rtt_expired; |
| } |
| |
| void BbrSender::UpdateGainCyclePhase(QuicTime now, |
| QuicByteCount prior_in_flight, |
| bool has_losses) { |
| const QuicByteCount bytes_in_flight = unacked_packets_->bytes_in_flight(); |
| // In most cases, the cycle is advanced after an RTT passes. |
| bool should_advance_gain_cycling = now - last_cycle_start_ > GetMinRtt(); |
| |
| // If the pacing gain is above 1.0, the connection is trying to probe the |
| // bandwidth by increasing the number of bytes in flight to at least |
| // pacing_gain * BDP. Make sure that it actually reaches the target, as long |
| // as there are no losses suggesting that the buffers are not able to hold |
| // that much. |
| if (pacing_gain_ > 1.0 && !has_losses && |
| prior_in_flight < GetTargetCongestionWindow(pacing_gain_)) { |
| should_advance_gain_cycling = false; |
| } |
| |
| // If pacing gain is below 1.0, the connection is trying to drain the extra |
| // queue which could have been incurred by probing prior to it. If the number |
| // of bytes in flight falls down to the estimated BDP value earlier, conclude |
| // that the queue has been successfully drained and exit this cycle early. |
| if (pacing_gain_ < 1.0 && bytes_in_flight <= GetTargetCongestionWindow(1)) { |
| should_advance_gain_cycling = true; |
| } |
| |
| if (should_advance_gain_cycling) { |
| cycle_current_offset_ = (cycle_current_offset_ + 1) % kGainCycleLength; |
| if (cycle_current_offset_ == 0) { |
| ++stats_->bbr_num_cycles; |
| } |
| last_cycle_start_ = now; |
| // Stay in low gain mode until the target BDP is hit. |
| // Low gain mode will be exited immediately when the target BDP is achieved. |
| if (drain_to_target_ && pacing_gain_ < 1 && |
| kPacingGain[cycle_current_offset_] == 1 && |
| bytes_in_flight > GetTargetCongestionWindow(1)) { |
| return; |
| } |
| pacing_gain_ = kPacingGain[cycle_current_offset_]; |
| } |
| } |
| |
| void BbrSender::CheckIfFullBandwidthReached( |
| const SendTimeState& last_packet_send_state) { |
| if (last_sample_is_app_limited_) { |
| return; |
| } |
| |
| QuicBandwidth target = bandwidth_at_last_round_ * kStartupGrowthTarget; |
| if (BandwidthEstimate() >= target) { |
| bandwidth_at_last_round_ = BandwidthEstimate(); |
| rounds_without_bandwidth_gain_ = 0; |
| if (expire_ack_aggregation_in_startup_) { |
| // Expire old excess delivery measurements now that bandwidth increased. |
| sampler_.ResetMaxAckHeightTracker(0, round_trip_count_); |
| } |
| return; |
| } |
| |
| rounds_without_bandwidth_gain_++; |
| if ((rounds_without_bandwidth_gain_ >= num_startup_rtts_) || |
| ShouldExitStartupDueToLoss(last_packet_send_state)) { |
| QUICHE_DCHECK(has_non_app_limited_sample_); |
| is_at_full_bandwidth_ = true; |
| } |
| } |
| |
| void BbrSender::MaybeExitStartupOrDrain(QuicTime now) { |
| if (mode_ == STARTUP && is_at_full_bandwidth_) { |
| OnExitStartup(now); |
| mode_ = DRAIN; |
| pacing_gain_ = drain_gain_; |
| congestion_window_gain_ = high_cwnd_gain_; |
| } |
| if (mode_ == DRAIN && |
| unacked_packets_->bytes_in_flight() <= GetTargetCongestionWindow(1)) { |
| EnterProbeBandwidthMode(now); |
| } |
| } |
| |
| void BbrSender::OnExitStartup(QuicTime now) { |
| QUICHE_DCHECK_EQ(mode_, STARTUP); |
| if (stats_) { |
| stats_->slowstart_duration.Stop(now); |
| } |
| } |
| |
| bool BbrSender::ShouldExitStartupDueToLoss( |
| const SendTimeState& last_packet_send_state) const { |
| if (num_loss_events_in_round_ < |
| GetQuicFlag(FLAGS_quic_bbr2_default_startup_full_loss_count) || |
| !last_packet_send_state.is_valid) { |
| return false; |
| } |
| |
| const QuicByteCount inflight_at_send = last_packet_send_state.bytes_in_flight; |
| |
| if (inflight_at_send > 0 && bytes_lost_in_round_ > 0) { |
| if (bytes_lost_in_round_ > |
| inflight_at_send * |
| GetQuicFlag(FLAGS_quic_bbr2_default_loss_threshold)) { |
| stats_->bbr_exit_startup_due_to_loss = true; |
| return true; |
| } |
| return false; |
| } |
| |
| return false; |
| } |
| |
| void BbrSender::MaybeEnterOrExitProbeRtt(QuicTime now, |
| bool is_round_start, |
| bool min_rtt_expired) { |
| if (min_rtt_expired && !exiting_quiescence_ && mode_ != PROBE_RTT) { |
| if (InSlowStart()) { |
| OnExitStartup(now); |
| } |
| mode_ = PROBE_RTT; |
| pacing_gain_ = 1; |
| // Do not decide on the time to exit PROBE_RTT until the |bytes_in_flight| |
| // is at the target small value. |
| exit_probe_rtt_at_ = QuicTime::Zero(); |
| } |
| |
| if (mode_ == PROBE_RTT) { |
| sampler_.OnAppLimited(); |
| |
| if (exit_probe_rtt_at_ == QuicTime::Zero()) { |
| // If the window has reached the appropriate size, schedule exiting |
| // PROBE_RTT. The CWND during PROBE_RTT is kMinimumCongestionWindow, but |
| // we allow an extra packet since QUIC checks CWND before sending a |
| // packet. |
| if (unacked_packets_->bytes_in_flight() < |
| ProbeRttCongestionWindow() + kMaxOutgoingPacketSize) { |
| exit_probe_rtt_at_ = now + kProbeRttTime; |
| probe_rtt_round_passed_ = false; |
| } |
| } else { |
| if (is_round_start) { |
| probe_rtt_round_passed_ = true; |
| } |
| if (now >= exit_probe_rtt_at_ && probe_rtt_round_passed_) { |
| min_rtt_timestamp_ = now; |
| if (!is_at_full_bandwidth_) { |
| EnterStartupMode(now); |
| } else { |
| EnterProbeBandwidthMode(now); |
| } |
| } |
| } |
| } |
| |
| exiting_quiescence_ = false; |
| } |
| |
| void BbrSender::UpdateRecoveryState(QuicPacketNumber last_acked_packet, |
| bool has_losses, |
| bool is_round_start) { |
| // Disable recovery in startup, if loss-based exit is enabled. |
| if (!is_at_full_bandwidth_) { |
| return; |
| } |
| |
| // Exit recovery when there are no losses for a round. |
| if (has_losses) { |
| end_recovery_at_ = last_sent_packet_; |
| } |
| |
| switch (recovery_state_) { |
| case NOT_IN_RECOVERY: |
| // Enter conservation on the first loss. |
| if (has_losses) { |
| recovery_state_ = CONSERVATION; |
| // This will cause the |recovery_window_| to be set to the correct |
| // value in CalculateRecoveryWindow(). |
| recovery_window_ = 0; |
| // Since the conservation phase is meant to be lasting for a whole |
| // round, extend the current round as if it were started right now. |
| current_round_trip_end_ = last_sent_packet_; |
| } |
| break; |
| |
| case CONSERVATION: |
| if (is_round_start) { |
| recovery_state_ = GROWTH; |
| } |
| ABSL_FALLTHROUGH_INTENDED; |
| |
| case GROWTH: |
| // Exit recovery if appropriate. |
| if (!has_losses && last_acked_packet > end_recovery_at_) { |
| recovery_state_ = NOT_IN_RECOVERY; |
| } |
| |
| break; |
| } |
| } |
| |
| void BbrSender::CalculatePacingRate(QuicByteCount bytes_lost) { |
| if (BandwidthEstimate().IsZero()) { |
| return; |
| } |
| |
| QuicBandwidth target_rate = pacing_gain_ * BandwidthEstimate(); |
| if (is_at_full_bandwidth_) { |
| pacing_rate_ = target_rate; |
| return; |
| } |
| |
| // 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; |
| } |
| |
| if (detect_overshooting_) { |
| bytes_lost_while_detecting_overshooting_ += bytes_lost; |
| // Check for overshooting with network parameters adjusted when pacing rate |
| // > target_rate and loss has been detected. |
| if (pacing_rate_ > target_rate && |
| bytes_lost_while_detecting_overshooting_ > 0) { |
| if (has_non_app_limited_sample_ || |
| bytes_lost_while_detecting_overshooting_ * |
| bytes_lost_multiplier_while_detecting_overshooting_ > |
| initial_congestion_window_) { |
| // We are fairly sure overshoot happens if 1) there is at least one |
| // non app-limited bw sample or 2) half of IW gets lost. Slow pacing |
| // rate. |
| pacing_rate_ = std::max( |
| target_rate, QuicBandwidth::FromBytesAndTimeDelta( |
| cwnd_to_calculate_min_pacing_rate_, GetMinRtt())); |
| if (stats_) { |
| stats_->overshooting_detected_with_network_parameters_adjusted = true; |
| } |
| bytes_lost_while_detecting_overshooting_ = 0; |
| detect_overshooting_ = false; |
| } |
| } |
| } |
| |
| // Do not decrease the pacing rate during startup. |
| pacing_rate_ = std::max(pacing_rate_, target_rate); |
| } |
| |
| void BbrSender::CalculateCongestionWindow(QuicByteCount bytes_acked, |
| QuicByteCount excess_acked) { |
| if (mode_ == PROBE_RTT) { |
| return; |
| } |
| |
| QuicByteCount target_window = |
| GetTargetCongestionWindow(congestion_window_gain_); |
| if (is_at_full_bandwidth_) { |
| // Add the max recently measured ack aggregation to CWND. |
| target_window += sampler_.max_ack_height(); |
| } else if (enable_ack_aggregation_during_startup_) { |
| // Add the most recent excess acked. Because CWND never decreases in |
| // STARTUP, this will automatically create a very localized max filter. |
| target_window += excess_acked; |
| } |
| |
| // Instead of immediately setting the target CWND as the new one, BBR grows |
| // the CWND towards |target_window| by only increasing it |bytes_acked| at a |
| // time. |
| if (is_at_full_bandwidth_) { |
| congestion_window_ = |
| std::min(target_window, congestion_window_ + bytes_acked); |
| } else if (congestion_window_ < target_window || |
| sampler_.total_bytes_acked() < initial_congestion_window_) { |
| // If the connection is not yet out of startup phase, do not decrease the |
| // window. |
| congestion_window_ = congestion_window_ + bytes_acked; |
| } |
| |
| // Enforce the limits on the congestion window. |
| congestion_window_ = std::max(congestion_window_, min_congestion_window_); |
| congestion_window_ = std::min(congestion_window_, max_congestion_window_); |
| } |
| |
| void BbrSender::CalculateRecoveryWindow(QuicByteCount bytes_acked, |
| QuicByteCount bytes_lost) { |
| if (recovery_state_ == NOT_IN_RECOVERY) { |
| return; |
| } |
| |
| // Set up the initial recovery window. |
| if (recovery_window_ == 0) { |
| recovery_window_ = unacked_packets_->bytes_in_flight() + bytes_acked; |
| recovery_window_ = std::max(min_congestion_window_, recovery_window_); |
| return; |
| } |
| |
| // Remove losses from the recovery window, while accounting for a potential |
| // integer underflow. |
| recovery_window_ = recovery_window_ >= bytes_lost |
| ? recovery_window_ - bytes_lost |
| : kMaxSegmentSize; |
| |
| // In CONSERVATION mode, just subtracting losses is sufficient. In GROWTH, |
| // release additional |bytes_acked| to achieve a slow-start-like behavior. |
| if (recovery_state_ == GROWTH) { |
| recovery_window_ += bytes_acked; |
| } |
| |
| // Always allow sending at least |bytes_acked| in response. |
| recovery_window_ = std::max( |
| recovery_window_, unacked_packets_->bytes_in_flight() + bytes_acked); |
| recovery_window_ = std::max(min_congestion_window_, recovery_window_); |
| } |
| |
| std::string BbrSender::GetDebugState() const { |
| std::ostringstream stream; |
| stream << ExportDebugState(); |
| return stream.str(); |
| } |
| |
| void BbrSender::OnApplicationLimited(QuicByteCount bytes_in_flight) { |
| if (bytes_in_flight >= GetCongestionWindow()) { |
| return; |
| } |
| |
| sampler_.OnAppLimited(); |
| QUIC_DVLOG(2) << "Becoming application limited. Last sent packet: " |
| << last_sent_packet_ << ", CWND: " << GetCongestionWindow(); |
| } |
| |
| void BbrSender::PopulateConnectionStats(QuicConnectionStats* stats) const { |
| stats->num_ack_aggregation_epochs = sampler_.num_ack_aggregation_epochs(); |
| } |
| |
| BbrSender::DebugState BbrSender::ExportDebugState() const { |
| return DebugState(*this); |
| } |
| |
| static std::string ModeToString(BbrSender::Mode mode) { |
| switch (mode) { |
| case BbrSender::STARTUP: |
| return "STARTUP"; |
| case BbrSender::DRAIN: |
| return "DRAIN"; |
| case BbrSender::PROBE_BW: |
| return "PROBE_BW"; |
| case BbrSender::PROBE_RTT: |
| return "PROBE_RTT"; |
| } |
| return "???"; |
| } |
| |
| std::ostream& operator<<(std::ostream& os, const BbrSender::Mode& mode) { |
| os << ModeToString(mode); |
| return os; |
| } |
| |
| std::ostream& operator<<(std::ostream& os, const BbrSender::DebugState& state) { |
| os << "Mode: " << ModeToString(state.mode) << std::endl; |
| os << "Maximum bandwidth: " << state.max_bandwidth << std::endl; |
| os << "Round trip counter: " << state.round_trip_count << std::endl; |
| os << "Gain cycle index: " << static_cast<int>(state.gain_cycle_index) |
| << std::endl; |
| os << "Congestion window: " << state.congestion_window << " bytes" |
| << std::endl; |
| |
| if (state.mode == BbrSender::STARTUP) { |
| os << "(startup) Bandwidth at last round: " << state.bandwidth_at_last_round |
| << std::endl; |
| os << "(startup) Rounds without gain: " |
| << state.rounds_without_bandwidth_gain << std::endl; |
| } |
| |
| os << "Minimum RTT: " << state.min_rtt << std::endl; |
| 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 quic |