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// 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();
}
}
void BbrSender::AdjustNetworkParameters(const NetworkParams& params) {
const QuicBandwidth& bandwidth = params.bandwidth;
const QuicTime::Delta& rtt = params.rtt;
if (!params.quic_bbr_donot_inject_bandwidth && !bandwidth.IsZero()) {
max_bandwidth_.Update(bandwidth, round_trip_count_);
}
if (!rtt.IsZero() && (min_rtt_ > rtt || min_rtt_.IsZero())) {
min_rtt_ = rtt;
}
if (params.quic_fix_bbr_cwnd_in_bandwidth_resumption && mode_ == STARTUP) {
if (bandwidth.IsZero()) {
// Ignore bad bandwidth samples.
return;
}
auto cwnd_bootstrapping_rtt = params.quic_bbr_donot_inject_bandwidth
? GetMinRtt()
: rtt_stats_->SmoothedOrInitialRtt();
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;
if (params.quic_bbr_fix_pacing_rate) {
// 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;
}
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)) {
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) {
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