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webrtc/modules/congestion_controller/bbr/bbr_network_controller.cc
Sebastian Jansson 9de4ef4b24 Adds signed infinity to timestamps. 
Negative infinity is useful in comparisons as it represents a value
guaranteed to be less than any other timestamp. This avoid using
Timestamp::ms(0) to represent such a value or having to use optional
for the special case of first time execution.

To avoid ambiguity, Timestamp::Infinity is renamed to PlusInfinity.

Bug: webrtc:9709
Change-Id: Ib2debff91036e94d1dc903ec0c6faae30febde4e
Reviewed-on: https://webrtc-review.googlesource.com/79440
Commit-Queue: Sebastian Jansson <srte@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24556}
2018-09-04 17:14:05 +00:00

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/*
* Copyright (c) 2018 The WebRTC project authors. All Rights Reserved.
*
* Use of this source code is governed by a BSD-style license
* that can be found in the LICENSE file in the root of the source
* tree. An additional intellectual property rights grant can be found
* in the file PATENTS. All contributing project authors may
* be found in the AUTHORS file in the root of the source tree.
*/
#include "modules/congestion_controller/bbr/bbr_network_controller.h"
#include <algorithm>
#include <array>
#include <string>
#include <vector>
#include "rtc_base/checks.h"
#include "rtc_base/logging.h"
#include "rtc_base/system/fallthrough.h"
#include "system_wrappers/include/field_trial.h"
namespace webrtc {
namespace bbr {
namespace {
// If greater than zero, mean RTT variation is multiplied by the specified
// factor and added to the congestion window limit.
const double kBbrRttVariationWeight = 0.0f;
// Congestion window gain for QUIC BBR during PROBE_BW phase.
const double kProbeBWCongestionWindowGain = 2.0f;
// The maximum packet size of any QUIC packet, based on ethernet's max size,
// minus the IP and UDP headers. IPv6 has a 40 byte header, UDP adds an
// additional 8 bytes. This is a total overhead of 48 bytes. Ethernet's
// max packet size is 1500 bytes, 1500 - 48 = 1452.
const DataSize kMaxPacketSize = DataSize::bytes(1452);
// Default maximum packet size used in the Linux TCP implementation.
// Used in QUIC for congestion window computations in bytes.
const DataSize kDefaultTCPMSS = DataSize::bytes(1460);
// Constants based on TCP defaults.
const DataSize kMaxSegmentSize = kDefaultTCPMSS;
// The gain used for the slow start, equal to 2/ln(2).
const double kHighGain = 2.885f;
// The gain used in STARTUP after loss has been detected.
// 1.5 is enough to allow for 25% exogenous loss and still observe a 25% growth
// in measured bandwidth.
const double kStartupAfterLossGain = 1.5;
// The gain used to drain the queue after the slow start.
const double kDrainGain = 1.f / kHighGain;
// The length of the gain cycle.
const size_t kGainCycleLength = 8;
// The size of the bandwidth filter window, in round-trips.
const BbrRoundTripCount kBandwidthWindowSize = kGainCycleLength + 2;
// The time after which the current min_rtt value expires.
constexpr int64_t kMinRttExpirySeconds = 10;
// The minimum time the connection can spend in PROBE_RTT mode.
constexpr int64_t kProbeRttTimeMs = 200;
// If the bandwidth does not increase by the factor of |kStartupGrowthTarget|
// within |kRoundTripsWithoutGrowthBeforeExitingStartup| rounds, the connection
// will exit the STARTUP mode.
const double kStartupGrowthTarget = 1.25;
// Coefficient to determine if a new RTT is sufficiently similar to min_rtt that
// we don't need to enter PROBE_RTT.
const double kSimilarMinRttThreshold = 1.125;
constexpr int64_t kInitialBandwidthKbps = 300;
const int64_t kInitialCongestionWindowPackets = 32;
// The minimum CWND to ensure delayed acks don't reduce bandwidth measurements.
// Does not inflate the pacing rate.
const int64_t kDefaultMinCongestionWindowPackets = 4;
const int64_t kDefaultMaxCongestionWindowPackets = 2000;
const char kBbrConfigTrial[] = "WebRTC-BweBbrConfig";
} // namespace
BbrNetworkController::BbrControllerConfig::BbrControllerConfig(
std::string field_trial)
: probe_bw_pacing_gain_offset("probe_bw_pacing_gain_offset", 0.25),
encoder_rate_gain("encoder_rate_gain", 1),
encoder_rate_gain_in_probe_rtt("encoder_rate_gain_in_probe_rtt", 1),
exit_startup_rtt_threshold("exit_startup_rtt_threshold",
TimeDelta::PlusInfinity()),
initial_congestion_window(
"initial_cwin",
kInitialCongestionWindowPackets * kDefaultTCPMSS),
min_congestion_window(
"min_cwin",
kDefaultMinCongestionWindowPackets * kDefaultTCPMSS),
max_congestion_window(
"max_cwin",
kDefaultMaxCongestionWindowPackets * kDefaultTCPMSS),
probe_rtt_congestion_window_gain("probe_rtt_cwin_gain", 0.75),
pacing_rate_as_target("pacing_rate_as_target", false),
exit_startup_on_loss("exit_startup_on_loss", true),
num_startup_rtts("num_startup_rtts", 3),
rate_based_recovery("rate_based_recovery", false),
max_aggregation_bytes_multiplier("max_aggregation_bytes_multiplier", 0),
slower_startup("slower_startup", false),
rate_based_startup("rate_based_startup", false),
initial_conservation_in_startup("initial_conservation",
CONSERVATION,
{
{"NOT_IN_RECOVERY", NOT_IN_RECOVERY},
{"CONSERVATION", CONSERVATION},
{"MEDIUM_GROWTH", MEDIUM_GROWTH},
{"GROWTH", GROWTH},
}),
fully_drain_queue("fully_drain_queue", false),
max_ack_height_window_multiplier("max_ack_height_window_multiplier", 1),
probe_rtt_based_on_bdp("probe_rtt_based_on_bdp", false),
probe_rtt_skipped_if_similar_rtt("probe_rtt_skipped_if_similar_rtt",
false),
probe_rtt_disabled_if_app_limited("probe_rtt_disabled_if_app_limited",
false) {
ParseFieldTrial(
{
&exit_startup_on_loss,
&encoder_rate_gain,
&encoder_rate_gain_in_probe_rtt,
&exit_startup_rtt_threshold,
&fully_drain_queue,
&initial_congestion_window,
&initial_conservation_in_startup,
&max_ack_height_window_multiplier,
&max_aggregation_bytes_multiplier,
&max_congestion_window,
&min_congestion_window,
&num_startup_rtts,
&pacing_rate_as_target,
&probe_bw_pacing_gain_offset,
&probe_rtt_based_on_bdp,
&probe_rtt_congestion_window_gain,
&probe_rtt_disabled_if_app_limited,
&probe_rtt_skipped_if_similar_rtt,
&rate_based_recovery,
&rate_based_startup,
&slower_startup,
},
field_trial);
}
BbrNetworkController::BbrControllerConfig::~BbrControllerConfig() = default;
BbrNetworkController::BbrControllerConfig::BbrControllerConfig(
const BbrControllerConfig&) = default;
BbrNetworkController::BbrControllerConfig
BbrNetworkController::BbrControllerConfig::FromTrial() {
return BbrControllerConfig(
webrtc::field_trial::FindFullName(kBbrConfigTrial));
}
BbrNetworkController::DebugState::DebugState(const BbrNetworkController& 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()) {}
BbrNetworkController::DebugState::DebugState(const DebugState& state) = default;
BbrNetworkController::BbrNetworkController(NetworkControllerConfig config)
: config_(BbrControllerConfig::FromTrial()),
rtt_stats_(),
random_(10),
loss_rate_(),
mode_(STARTUP),
sampler_(new BandwidthSampler()),
round_trip_count_(0),
last_sent_packet_(0),
current_round_trip_end_(0),
max_bandwidth_(kBandwidthWindowSize, DataRate::Zero(), 0),
default_bandwidth_(DataRate::kbps(kInitialBandwidthKbps)),
max_ack_height_(kBandwidthWindowSize, DataSize::Zero(), 0),
aggregation_epoch_start_time_(),
aggregation_epoch_bytes_(DataSize::Zero()),
bytes_acked_since_queue_drained_(DataSize::Zero()),
max_aggregation_bytes_multiplier_(0),
min_rtt_(TimeDelta::Zero()),
last_rtt_(TimeDelta::Zero()),
min_rtt_timestamp_(Timestamp::MinusInfinity()),
congestion_window_(config_.initial_congestion_window),
initial_congestion_window_(config_.initial_congestion_window),
min_congestion_window_(config_.min_congestion_window),
max_congestion_window_(config_.max_congestion_window),
pacing_rate_(DataRate::Zero()),
pacing_gain_(1),
congestion_window_gain_constant_(kProbeBWCongestionWindowGain),
rtt_variance_weight_(kBbrRttVariationWeight),
cycle_current_offset_(0),
last_cycle_start_(Timestamp::MinusInfinity()),
is_at_full_bandwidth_(false),
rounds_without_bandwidth_gain_(0),
bandwidth_at_last_round_(DataRate::Zero()),
exiting_quiescence_(false),
exit_probe_rtt_at_(),
probe_rtt_round_passed_(false),
last_sample_is_app_limited_(false),
recovery_state_(NOT_IN_RECOVERY),
end_recovery_at_(),
recovery_window_(max_congestion_window_),
app_limited_since_last_probe_rtt_(false),
min_rtt_since_last_probe_rtt_(TimeDelta::PlusInfinity()) {
RTC_LOG(LS_INFO) << "Creating BBR controller";
if (config.constraints.starting_rate)
default_bandwidth_ = *config.constraints.starting_rate;
constraints_ = config.constraints;
Reset();
}
BbrNetworkController::~BbrNetworkController() {}
void BbrNetworkController::Reset() {
round_trip_count_ = 0;
rounds_without_bandwidth_gain_ = 0;
if (config_.num_startup_rtts > 0) {
is_at_full_bandwidth_ = false;
EnterStartupMode();
} else {
is_at_full_bandwidth_ = true;
EnterProbeBandwidthMode(constraints_->at_time);
}
}
NetworkControlUpdate BbrNetworkController::CreateRateUpdate(
Timestamp at_time) const {
DataRate bandwidth = BandwidthEstimate();
if (bandwidth.IsZero())
bandwidth = default_bandwidth_;
TimeDelta rtt = GetMinRtt();
DataRate pacing_rate = PacingRate();
DataRate target_rate =
config_.pacing_rate_as_target ? pacing_rate : bandwidth;
if (mode_ == PROBE_RTT)
target_rate = target_rate * config_.encoder_rate_gain_in_probe_rtt;
else
target_rate = target_rate * config_.encoder_rate_gain;
target_rate = std::min(target_rate, pacing_rate);
if (constraints_) {
if (constraints_->max_data_rate) {
target_rate = std::min(target_rate, *constraints_->max_data_rate);
pacing_rate = std::min(pacing_rate, *constraints_->max_data_rate);
}
if (constraints_->min_data_rate) {
target_rate = std::max(target_rate, *constraints_->min_data_rate);
pacing_rate = std::max(pacing_rate, *constraints_->min_data_rate);
}
}
NetworkControlUpdate update;
TargetTransferRate target_rate_msg;
target_rate_msg.network_estimate.at_time = at_time;
target_rate_msg.network_estimate.bandwidth = bandwidth;
target_rate_msg.network_estimate.round_trip_time = rtt;
// TODO(srte): Fill in field below with proper value.
target_rate_msg.network_estimate.loss_rate_ratio = 0;
// In in PROBE_BW, target bandwidth is expected to vary over the cycle period.
// In other modes the is no given period, therefore the same value as in
// PROBE_BW is used for consistency.
target_rate_msg.network_estimate.bwe_period =
rtt * static_cast<int64_t>(kGainCycleLength);
target_rate_msg.target_rate = target_rate;
target_rate_msg.at_time = at_time;
update.target_rate = target_rate_msg;
PacerConfig pacer_config;
// A small time window ensures an even pacing rate.
pacer_config.time_window = rtt * 0.25;
pacer_config.data_window = pacer_config.time_window * pacing_rate;
if (IsProbingForMoreBandwidth())
pacer_config.pad_window = pacer_config.data_window;
else
pacer_config.pad_window = DataSize::Zero();
pacer_config.at_time = at_time;
update.pacer_config = pacer_config;
update.congestion_window = GetCongestionWindow();
return update;
}
NetworkControlUpdate BbrNetworkController::OnNetworkAvailability(
NetworkAvailability msg) {
Reset();
rtt_stats_.OnConnectionMigration();
return CreateRateUpdate(msg.at_time);
}
NetworkControlUpdate BbrNetworkController::OnNetworkRouteChange(
NetworkRouteChange msg) {
constraints_ = msg.constraints;
Reset();
if (msg.constraints.starting_rate)
default_bandwidth_ = *msg.constraints.starting_rate;
rtt_stats_.OnConnectionMigration();
return CreateRateUpdate(msg.at_time);
}
NetworkControlUpdate BbrNetworkController::OnProcessInterval(
ProcessInterval msg) {
return CreateRateUpdate(msg.at_time);
}
NetworkControlUpdate BbrNetworkController::OnStreamsConfig(StreamsConfig msg) {
return NetworkControlUpdate();
}
NetworkControlUpdate BbrNetworkController::OnTargetRateConstraints(
TargetRateConstraints msg) {
constraints_ = msg;
return CreateRateUpdate(msg.at_time);
}
bool BbrNetworkController::InSlowStart() const {
return mode_ == STARTUP;
}
NetworkControlUpdate BbrNetworkController::OnSentPacket(SentPacket msg) {
last_sent_packet_ = msg.sequence_number;
if (msg.data_in_flight.IsZero() && sampler_->is_app_limited()) {
exiting_quiescence_ = true;
}
if (!aggregation_epoch_start_time_) {
aggregation_epoch_start_time_ = msg.send_time;
}
sampler_->OnPacketSent(msg.send_time, msg.sequence_number, msg.size,
msg.data_in_flight);
return NetworkControlUpdate();
}
bool BbrNetworkController::CanSend(DataSize bytes_in_flight) {
return bytes_in_flight < GetCongestionWindow();
}
DataRate BbrNetworkController::PacingRate() const {
if (pacing_rate_.IsZero()) {
return kHighGain * initial_congestion_window_ / GetMinRtt();
}
return pacing_rate_;
}
DataRate BbrNetworkController::BandwidthEstimate() const {
return max_bandwidth_.GetBest();
}
DataSize BbrNetworkController::GetCongestionWindow() const {
if (mode_ == PROBE_RTT) {
return ProbeRttCongestionWindow();
}
if (InRecovery() && !config_.rate_based_recovery &&
!(config_.rate_based_startup && mode_ == STARTUP)) {
return std::min(congestion_window_, recovery_window_);
}
return congestion_window_;
}
double BbrNetworkController::GetPacingGain(int round_offset) const {
if (round_offset == 0)
return 1 + config_.probe_bw_pacing_gain_offset;
else if (round_offset == 1)
return 1 - config_.probe_bw_pacing_gain_offset;
else
return 1;
}
bool BbrNetworkController::InRecovery() const {
return recovery_state_ != NOT_IN_RECOVERY;
}
bool BbrNetworkController::IsProbingForMoreBandwidth() const {
return (mode_ == PROBE_BW && pacing_gain_ > 1) || mode_ == STARTUP;
}
NetworkControlUpdate BbrNetworkController::OnTransportPacketsFeedback(
TransportPacketsFeedback msg) {
Timestamp feedback_recv_time = msg.feedback_time;
absl::optional<SentPacket> last_sent_packet =
msg.PacketsWithFeedback().back().sent_packet;
if (!last_sent_packet.has_value()) {
RTC_LOG(LS_WARNING) << "Last ack packet not in history, no RTT update";
} else {
Timestamp send_time = last_sent_packet->send_time;
TimeDelta send_delta = feedback_recv_time - send_time;
rtt_stats_.UpdateRtt(send_delta, TimeDelta::Zero(), feedback_recv_time);
}
const DataSize total_data_acked_before = sampler_->total_data_acked();
bool is_round_start = false;
bool min_rtt_expired = false;
std::vector<PacketResult> lost_packets = msg.LostWithSendInfo();
DiscardLostPackets(lost_packets);
std::vector<PacketResult> acked_packets = msg.ReceivedWithSendInfo();
int packets_sent =
static_cast<int>(lost_packets.size() + acked_packets.size());
int packets_lost = static_cast<int>(lost_packets.size());
loss_rate_.UpdateWithLossStatus(msg.feedback_time.ms(), packets_sent,
packets_lost);
// Input the new data into the BBR model of the connection.
if (!acked_packets.empty()) {
int64_t last_acked_packet =
acked_packets.rbegin()->sent_packet->sequence_number;
is_round_start = UpdateRoundTripCounter(last_acked_packet);
min_rtt_expired =
UpdateBandwidthAndMinRtt(msg.feedback_time, acked_packets);
UpdateRecoveryState(last_acked_packet, !lost_packets.empty(),
is_round_start);
const DataSize data_acked =
sampler_->total_data_acked() - total_data_acked_before;
UpdateAckAggregationBytes(msg.feedback_time, data_acked);
if (max_aggregation_bytes_multiplier_ > 0) {
if (msg.data_in_flight <=
1.25 * GetTargetCongestionWindow(pacing_gain_)) {
bytes_acked_since_queue_drained_ = DataSize::Zero();
} else {
bytes_acked_since_queue_drained_ += data_acked;
}
}
}
// Handle logic specific to PROBE_BW mode.
if (mode_ == PROBE_BW) {
UpdateGainCyclePhase(msg.feedback_time, msg.prior_in_flight,
!lost_packets.empty());
}
// Handle logic specific to STARTUP and DRAIN modes.
if (is_round_start && !is_at_full_bandwidth_) {
CheckIfFullBandwidthReached();
}
MaybeExitStartupOrDrain(msg);
// Handle logic specific to PROBE_RTT.
MaybeEnterOrExitProbeRtt(msg, is_round_start, min_rtt_expired);
// Calculate number of packets acked and lost.
DataSize data_acked = sampler_->total_data_acked() - total_data_acked_before;
DataSize data_lost = DataSize::Zero();
for (const PacketResult& packet : lost_packets) {
data_lost += packet.sent_packet->size;
}
// After the model is updated, recalculate the pacing rate and congestion
// window.
CalculatePacingRate();
CalculateCongestionWindow(data_acked);
CalculateRecoveryWindow(data_acked, data_lost, msg.data_in_flight);
// Cleanup internal state.
if (!acked_packets.empty()) {
sampler_->RemoveObsoletePackets(
acked_packets.back().sent_packet->sequence_number);
}
return CreateRateUpdate(msg.feedback_time);
}
NetworkControlUpdate BbrNetworkController::OnRemoteBitrateReport(
RemoteBitrateReport msg) {
return NetworkControlUpdate();
}
NetworkControlUpdate BbrNetworkController::OnRoundTripTimeUpdate(
RoundTripTimeUpdate msg) {
return NetworkControlUpdate();
}
NetworkControlUpdate BbrNetworkController::OnTransportLossReport(
TransportLossReport msg) {
return NetworkControlUpdate();
}
TimeDelta BbrNetworkController::GetMinRtt() const {
return !min_rtt_.IsZero() ? min_rtt_
: TimeDelta::us(rtt_stats_.initial_rtt_us());
}
DataSize BbrNetworkController::GetTargetCongestionWindow(double gain) const {
DataSize bdp = GetMinRtt() * BandwidthEstimate();
DataSize congestion_window = gain * bdp;
// BDP estimate will be zero if no bandwidth samples are available yet.
if (congestion_window.IsZero()) {
congestion_window = gain * initial_congestion_window_;
}
return std::max(congestion_window, min_congestion_window_);
}
DataSize BbrNetworkController::ProbeRttCongestionWindow() const {
if (config_.probe_rtt_based_on_bdp) {
return GetTargetCongestionWindow(config_.probe_rtt_congestion_window_gain);
}
return min_congestion_window_;
}
void BbrNetworkController::EnterStartupMode() {
mode_ = STARTUP;
pacing_gain_ = kHighGain;
congestion_window_gain_ = kHighGain;
}
void BbrNetworkController::EnterProbeBandwidthMode(Timestamp 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_.Rand(kGainCycleLength - 2);
if (cycle_current_offset_ >= 1) {
cycle_current_offset_ += 1;
}
last_cycle_start_ = now;
pacing_gain_ = GetPacingGain(cycle_current_offset_);
}
void BbrNetworkController::DiscardLostPackets(
const std::vector<PacketResult>& lost_packets) {
for (const PacketResult& packet : lost_packets) {
sampler_->OnPacketLost(packet.sent_packet->sequence_number);
}
}
bool BbrNetworkController::UpdateRoundTripCounter(int64_t last_acked_packet) {
if (last_acked_packet > current_round_trip_end_) {
round_trip_count_++;
current_round_trip_end_ = last_sent_packet_;
return true;
}
return false;
}
bool BbrNetworkController::UpdateBandwidthAndMinRtt(
Timestamp now,
const std::vector<PacketResult>& acked_packets) {
TimeDelta sample_rtt = TimeDelta::PlusInfinity();
for (const auto& packet : acked_packets) {
BandwidthSample bandwidth_sample = sampler_->OnPacketAcknowledged(
now, packet.sent_packet->sequence_number);
last_sample_is_app_limited_ = bandwidth_sample.is_app_limited;
if (!bandwidth_sample.rtt.IsZero()) {
sample_rtt = std::min(sample_rtt, bandwidth_sample.rtt);
}
if (!bandwidth_sample.is_app_limited ||
bandwidth_sample.bandwidth > BandwidthEstimate()) {
max_bandwidth_.Update(bandwidth_sample.bandwidth, round_trip_count_);
}
}
// If none of the RTT samples are valid, return immediately.
if (sample_rtt.IsInfinite()) {
return false;
}
last_rtt_ = sample_rtt;
min_rtt_since_last_probe_rtt_ =
std::min(min_rtt_since_last_probe_rtt_, sample_rtt);
const TimeDelta kMinRttExpiry = TimeDelta::seconds(kMinRttExpirySeconds);
// 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_rtt < min_rtt_ || min_rtt_.IsZero()) {
if (ShouldExtendMinRttExpiry()) {
min_rtt_expired = false;
} else {
min_rtt_ = sample_rtt;
}
min_rtt_timestamp_ = now;
// Reset since_last_probe_rtt fields.
min_rtt_since_last_probe_rtt_ = TimeDelta::PlusInfinity();
app_limited_since_last_probe_rtt_ = false;
}
return min_rtt_expired;
}
bool BbrNetworkController::ShouldExtendMinRttExpiry() const {
if (config_.probe_rtt_disabled_if_app_limited &&
app_limited_since_last_probe_rtt_) {
// Extend the current min_rtt if we've been app limited recently.
return true;
}
const bool min_rtt_increased_since_last_probe =
min_rtt_since_last_probe_rtt_ > min_rtt_ * kSimilarMinRttThreshold;
if (config_.probe_rtt_skipped_if_similar_rtt &&
app_limited_since_last_probe_rtt_ &&
!min_rtt_increased_since_last_probe) {
// Extend the current min_rtt if we've been app limited recently and an rtt
// has been measured in that time that's less than 12.5% more than the
// current min_rtt.
return true;
}
return false;
}
void BbrNetworkController::UpdateGainCyclePhase(Timestamp now,
DataSize prior_in_flight,
bool has_losses) {
// 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 && prior_in_flight <= GetTargetCongestionWindow(1)) {
should_advance_gain_cycling = true;
}
if (should_advance_gain_cycling) {
cycle_current_offset_ = (cycle_current_offset_ + 1) % kGainCycleLength;
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 (config_.fully_drain_queue && pacing_gain_ < 1 &&
GetPacingGain(cycle_current_offset_) == 1 &&
prior_in_flight > GetTargetCongestionWindow(1)) {
return;
}
pacing_gain_ = GetPacingGain(cycle_current_offset_);
}
}
void BbrNetworkController::CheckIfFullBandwidthReached() {
if (last_sample_is_app_limited_) {
return;
}
DataRate target = bandwidth_at_last_round_ * kStartupGrowthTarget;
if (BandwidthEstimate() >= target) {
bandwidth_at_last_round_ = BandwidthEstimate();
rounds_without_bandwidth_gain_ = 0;
return;
}
rounds_without_bandwidth_gain_++;
if ((rounds_without_bandwidth_gain_ >= config_.num_startup_rtts) ||
(config_.exit_startup_on_loss && InRecovery())) {
is_at_full_bandwidth_ = true;
}
}
void BbrNetworkController::MaybeExitStartupOrDrain(
const TransportPacketsFeedback& msg) {
TimeDelta exit_threshold = config_.exit_startup_rtt_threshold;
TimeDelta rtt_delta = last_rtt_ - min_rtt_;
if (mode_ == STARTUP &&
(is_at_full_bandwidth_ || rtt_delta > exit_threshold)) {
if (rtt_delta > exit_threshold)
RTC_LOG(LS_INFO) << "Exiting startup due to rtt increase from: "
<< ToString(min_rtt_) << " to:" << ToString(last_rtt_)
<< " > " << ToString(min_rtt_ + exit_threshold);
mode_ = DRAIN;
pacing_gain_ = kDrainGain;
congestion_window_gain_ = kHighGain;
}
if (mode_ == DRAIN && msg.data_in_flight <= GetTargetCongestionWindow(1)) {
EnterProbeBandwidthMode(msg.feedback_time);
}
}
void BbrNetworkController::MaybeEnterOrExitProbeRtt(
const TransportPacketsFeedback& msg,
bool is_round_start,
bool min_rtt_expired) {
if (min_rtt_expired && !exiting_quiescence_ && mode_ != PROBE_RTT) {
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_.reset();
}
if (mode_ == PROBE_RTT) {
sampler_->OnAppLimited();
if (!exit_probe_rtt_at_) {
// 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 (msg.data_in_flight < ProbeRttCongestionWindow() + kMaxPacketSize) {
exit_probe_rtt_at_ = msg.feedback_time + TimeDelta::ms(kProbeRttTimeMs);
probe_rtt_round_passed_ = false;
}
} else {
if (is_round_start) {
probe_rtt_round_passed_ = true;
}
if (msg.feedback_time >= *exit_probe_rtt_at_ && probe_rtt_round_passed_) {
min_rtt_timestamp_ = msg.feedback_time;
if (!is_at_full_bandwidth_) {
EnterStartupMode();
} else {
EnterProbeBandwidthMode(msg.feedback_time);
}
}
}
}
exiting_quiescence_ = false;
}
void BbrNetworkController::UpdateRecoveryState(int64_t last_acked_packet,
bool has_losses,
bool is_round_start) {
// 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;
if (mode_ == STARTUP) {
recovery_state_ = config_.initial_conservation_in_startup;
}
// This will cause the |recovery_window_| to be set to the correct
// value in CalculateRecoveryWindow().
recovery_window_ = DataSize::Zero();
// 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:
case MEDIUM_GROWTH:
if (is_round_start) {
recovery_state_ = GROWTH;
}
RTC_FALLTHROUGH();
case GROWTH:
// Exit recovery if appropriate.
if (!has_losses &&
(!end_recovery_at_ || last_acked_packet > *end_recovery_at_)) {
recovery_state_ = NOT_IN_RECOVERY;
}
break;
}
}
void BbrNetworkController::UpdateAckAggregationBytes(
Timestamp ack_time,
DataSize newly_acked_bytes) {
if (!aggregation_epoch_start_time_) {
RTC_LOG(LS_ERROR)
<< "Received feedback before information about sent packets.";
RTC_DCHECK(aggregation_epoch_start_time_.has_value());
return;
}
// Compute how many bytes are expected to be delivered, assuming max bandwidth
// is correct.
DataSize expected_bytes_acked =
max_bandwidth_.GetBest() * (ack_time - *aggregation_epoch_start_time_);
// Reset the current aggregation epoch as soon as the ack arrival rate is less
// than or equal to the max bandwidth.
if (aggregation_epoch_bytes_ <= expected_bytes_acked) {
// Reset to start measuring a new aggregation epoch.
aggregation_epoch_bytes_ = newly_acked_bytes;
aggregation_epoch_start_time_ = ack_time;
return;
}
// Compute how many extra bytes were delivered vs max bandwidth.
// Include the bytes most recently acknowledged to account for stretch acks.
aggregation_epoch_bytes_ += newly_acked_bytes;
max_ack_height_.Update(aggregation_epoch_bytes_ - expected_bytes_acked,
round_trip_count_);
}
void BbrNetworkController::CalculatePacingRate() {
if (BandwidthEstimate().IsZero()) {
return;
}
DataRate target_rate = pacing_gain_ * BandwidthEstimate();
if (config_.rate_based_recovery && InRecovery()) {
pacing_rate_ = pacing_gain_ * max_bandwidth_.GetThirdBest();
}
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_ = initial_congestion_window_ / rtt_stats_.min_rtt();
return;
}
// Slow the pacing rate in STARTUP once loss has ever been detected.
const bool has_ever_detected_loss = end_recovery_at_.has_value();
if (config_.slower_startup && has_ever_detected_loss) {
pacing_rate_ = kStartupAfterLossGain * BandwidthEstimate();
return;
}
// Do not decrease the pacing rate during the startup.
pacing_rate_ = std::max(pacing_rate_, target_rate);
}
void BbrNetworkController::CalculateCongestionWindow(DataSize bytes_acked) {
if (mode_ == PROBE_RTT) {
return;
}
DataSize target_window = GetTargetCongestionWindow(congestion_window_gain_);
if (rtt_variance_weight_ > 0.f && !BandwidthEstimate().IsZero()) {
target_window += rtt_variance_weight_ * rtt_stats_.mean_deviation() *
BandwidthEstimate();
} else if (max_aggregation_bytes_multiplier_ > 0 && is_at_full_bandwidth_) {
// Subtracting only half the bytes_acked_since_queue_drained ensures sending
// doesn't completely stop for a long period of time if the queue hasn't
// been drained recently.
if (max_aggregation_bytes_multiplier_ * max_ack_height_.GetBest() >
bytes_acked_since_queue_drained_ / 2) {
target_window +=
max_aggregation_bytes_multiplier_ * max_ack_height_.GetBest() -
bytes_acked_since_queue_drained_ / 2;
}
} else if (is_at_full_bandwidth_) {
target_window += max_ack_height_.GetBest();
}
// 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_data_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 BbrNetworkController::CalculateRecoveryWindow(DataSize bytes_acked,
DataSize bytes_lost,
DataSize bytes_in_flight) {
if (config_.rate_based_recovery ||
(config_.rate_based_startup && mode_ == STARTUP)) {
return;
}
if (recovery_state_ == NOT_IN_RECOVERY) {
return;
}
// Set up the initial recovery window.
if (recovery_window_.IsZero()) {
recovery_window_ = 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.
// In MEDIUM_GROWTH, release |bytes_acked| / 2 to split the difference.
if (recovery_state_ == GROWTH) {
recovery_window_ += bytes_acked;
} else if (recovery_state_ == MEDIUM_GROWTH) {
recovery_window_ += bytes_acked / 2;
}
// Sanity checks. Ensure that we always allow to send at least
// |bytes_acked| in response.
recovery_window_ = std::max(recovery_window_, bytes_in_flight + bytes_acked);
recovery_window_ = std::max(min_congestion_window_, recovery_window_);
}
void BbrNetworkController::OnApplicationLimited(DataSize bytes_in_flight) {
if (bytes_in_flight >= GetCongestionWindow()) {
return;
}
app_limited_since_last_probe_rtt_ = true;
sampler_->OnAppLimited();
RTC_LOG(LS_INFO) << "Becoming application limited. Last sent packet: "
<< last_sent_packet_
<< ", CWND: " << ToString(GetCongestionWindow());
}
} // namespace bbr
} // namespace webrtc
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