blob: 266c8250b013ba6ae2e3f02e8ebc1299a535c65f [file] [log] [blame]
// Copyright 2019 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/bbr2_sender.h"
#include <cstddef>
#include "quic/core/congestion_control/bandwidth_sampler.h"
#include "quic/core/congestion_control/bbr2_drain.h"
#include "quic/core/congestion_control/bbr2_misc.h"
#include "quic/core/crypto/crypto_protocol.h"
#include "quic/core/quic_bandwidth.h"
#include "quic/core/quic_tag.h"
#include "quic/core/quic_types.h"
#include "quic/platform/api/quic_flag_utils.h"
#include "quic/platform/api/quic_flags.h"
#include "quic/platform/api/quic_logging.h"
#include "common/print_elements.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;
const float kInitialPacingGain = 2.885f;
const int kMaxModeChangesPerCongestionEvent = 4;
} // namespace
// Call |member_function_call| based on the current Bbr2Mode we are in. e.g.
//
// auto result = BBR2_MODE_DISPATCH(Foo());
//
// is equivalent to:
//
// Bbr2ModeBase& Bbr2Sender::GetCurrentMode() {
// if (mode_ == Bbr2Mode::STARTUP) { return startup_; }
// if (mode_ == Bbr2Mode::DRAIN) { return drain_; }
// ...
// }
// auto result = GetCurrentMode().Foo();
//
// Except that BBR2_MODE_DISPATCH guarantees the call to Foo() is non-virtual.
//
#define BBR2_MODE_DISPATCH(member_function_call) \
(mode_ == Bbr2Mode::STARTUP \
? (startup_.member_function_call) \
: (mode_ == Bbr2Mode::PROBE_BW \
? (probe_bw_.member_function_call) \
: (mode_ == Bbr2Mode::DRAIN \
? (drain_.member_function_call) \
: (probe_rtt_or_die().member_function_call))))
Bbr2Sender::Bbr2Sender(QuicTime now,
const RttStats* rtt_stats,
const QuicUnackedPacketMap* unacked_packets,
QuicPacketCount initial_cwnd_in_packets,
QuicPacketCount max_cwnd_in_packets,
QuicRandom* random,
QuicConnectionStats* stats,
BbrSender* old_sender)
: mode_(Bbr2Mode::STARTUP),
rtt_stats_(rtt_stats),
unacked_packets_(unacked_packets),
random_(random),
connection_stats_(stats),
params_(kDefaultMinimumCongestionWindow,
max_cwnd_in_packets * kDefaultTCPMSS),
model_(&params_,
rtt_stats->SmoothedOrInitialRtt(),
rtt_stats->last_update_time(),
/*cwnd_gain=*/1.0,
/*pacing_gain=*/kInitialPacingGain,
old_sender ? &old_sender->sampler_ : nullptr),
initial_cwnd_(cwnd_limits().ApplyLimits(
(old_sender) ? old_sender->GetCongestionWindow()
: (initial_cwnd_in_packets * kDefaultTCPMSS))),
cwnd_(initial_cwnd_),
pacing_rate_(kInitialPacingGain * QuicBandwidth::FromBytesAndTimeDelta(
cwnd_,
rtt_stats->SmoothedOrInitialRtt())),
startup_(this, &model_, now),
drain_(this, &model_),
probe_bw_(this, &model_),
probe_rtt_(this, &model_),
last_sample_is_app_limited_(false) {
QUIC_DVLOG(2) << this << " Initializing Bbr2Sender. mode:" << mode_
<< ", PacingRate:" << pacing_rate_ << ", Cwnd:" << cwnd_
<< ", CwndLimits:" << cwnd_limits() << " @ " << now;
QUICHE_DCHECK_EQ(mode_, Bbr2Mode::STARTUP);
}
void Bbr2Sender::SetFromConfig(const QuicConfig& config,
Perspective perspective) {
if (config.HasClientRequestedIndependentOption(kB2NA, perspective)) {
params_.add_ack_height_to_queueing_threshold = false;
}
if (config.HasClientRequestedIndependentOption(kB2RP, perspective)) {
params_.avoid_unnecessary_probe_rtt = false;
}
if (config.HasClientRequestedIndependentOption(k1RTT, perspective)) {
params_.startup_full_bw_rounds = 1;
}
if (config.HasClientRequestedIndependentOption(k2RTT, perspective)) {
params_.startup_full_bw_rounds = 2;
}
if (config.HasClientRequestedIndependentOption(kB2HR, perspective)) {
params_.inflight_hi_headroom = 0.15;
}
if (config.HasClientRequestedIndependentOption(kICW1, perspective)) {
max_cwnd_when_network_parameters_adjusted_ = 100 * kDefaultTCPMSS;
}
ApplyConnectionOptions(config.ClientRequestedIndependentOptions(perspective));
}
void Bbr2Sender::ApplyConnectionOptions(
const QuicTagVector& connection_options) {
if (GetQuicReloadableFlag(quic_bbr2_extra_acked_window) &&
ContainsQuicTag(connection_options, kBBR4)) {
QUIC_RELOADABLE_FLAG_COUNT_N(quic_bbr2_extra_acked_window, 1, 2);
model_.SetMaxAckHeightTrackerWindowLength(20);
}
if (GetQuicReloadableFlag(quic_bbr2_extra_acked_window) &&
ContainsQuicTag(connection_options, kBBR5)) {
QUIC_RELOADABLE_FLAG_COUNT_N(quic_bbr2_extra_acked_window, 2, 2);
model_.SetMaxAckHeightTrackerWindowLength(40);
}
if (ContainsQuicTag(connection_options, kBBQ2)) {
params_.startup_cwnd_gain = 2.885;
params_.drain_cwnd_gain = 2.885;
model_.set_cwnd_gain(params_.startup_cwnd_gain);
}
if (ContainsQuicTag(connection_options, kB2LO)) {
params_.ignore_inflight_lo = true;
}
if (ContainsQuicTag(connection_options, kB2NE)) {
params_.always_exit_startup_on_excess_loss = true;
}
if (ContainsQuicTag(connection_options, kB2SL)) {
params_.startup_loss_exit_use_max_delivered_for_inflight_hi = false;
}
if (ContainsQuicTag(connection_options, kB2H2)) {
params_.limit_inflight_hi_by_max_delivered = true;
}
if (ContainsQuicTag(connection_options, kB2DL)) {
params_.use_bytes_delivered_for_inflight_hi = true;
}
if (ContainsQuicTag(connection_options, kB2RC)) {
params_.enable_reno_coexistence = false;
}
if (ContainsQuicTag(connection_options, kBSAO)) {
model_.EnableOverestimateAvoidance();
}
if (ContainsQuicTag(connection_options, kBBQ6)) {
params_.decrease_startup_pacing_at_end_of_round = true;
}
if (ContainsQuicTag(connection_options, kBBQ7)) {
params_.bw_lo_mode_ = Bbr2Params::QuicBandwidthLoMode::MIN_RTT_REDUCTION;
}
if (ContainsQuicTag(connection_options, kBBQ8)) {
params_.bw_lo_mode_ = Bbr2Params::QuicBandwidthLoMode::INFLIGHT_REDUCTION;
}
if (ContainsQuicTag(connection_options, kBBQ9)) {
params_.bw_lo_mode_ = Bbr2Params::QuicBandwidthLoMode::CWND_REDUCTION;
}
if (ContainsQuicTag(connection_options, kB201)) {
params_.probe_bw_check_cwnd_limited_before_aggregation_epoch = true;
}
if (GetQuicReloadableFlag(quic_bbr2_no_probe_up_exit_if_no_queue) &&
ContainsQuicTag(connection_options, kB202)) {
params_.probe_up_dont_exit_if_no_queue_ = true;
}
if (GetQuicReloadableFlag(quic_bbr2_ignore_inflight_hi_in_probe_up) &&
ContainsQuicTag(connection_options, kB203)) {
QUIC_RELOADABLE_FLAG_COUNT(quic_bbr2_ignore_inflight_hi_in_probe_up);
params_.probe_up_ignore_inflight_hi = true;
}
if (GetQuicReloadableFlag(quic_bbr2_startup_extra_acked) &&
ContainsQuicTag(connection_options, kB204)) {
QUIC_RELOADABLE_FLAG_COUNT_N(quic_bbr2_startup_extra_acked, 1, 2);
model_.SetReduceExtraAckedOnBandwidthIncrease(true);
}
if (GetQuicReloadableFlag(quic_bbr2_startup_extra_acked) &&
ContainsQuicTag(connection_options, kB205)) {
QUIC_RELOADABLE_FLAG_COUNT_N(quic_bbr2_startup_extra_acked, 2, 2);
params_.startup_include_extra_acked = true;
}
if (GetQuicReloadableFlag(quic_bbr2_exit_startup_on_persistent_queue2) &&
ContainsQuicTag(connection_options, kB207)) {
params_.exit_startup_on_persistent_queue = true;
}
if (ContainsQuicTag(connection_options, kBBRA)) {
model_.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, 2, 2);
model_.SetLimitMaxAckHeightTrackerBySendRate(true);
}
if (GetQuicReloadableFlag(
quic_bbr2_add_bytes_acked_after_inflight_hi_limited) &&
ContainsQuicTag(connection_options, kBBQ0)) {
params_.probe_up_includes_acks_after_cwnd_limited = true;
}
if (GetQuicReloadableFlag(quic_bbr2_startup_probe_up_loss_events) &&
ContainsQuicTag(connection_options, kB206)) {
QUIC_RELOADABLE_FLAG_COUNT(quic_bbr2_startup_probe_up_loss_events);
params_.startup_full_loss_count = params_.probe_bw_full_loss_count;
}
}
Limits<QuicByteCount> Bbr2Sender::GetCwndLimitsByMode() const {
switch (mode_) {
case Bbr2Mode::STARTUP:
return startup_.GetCwndLimits();
case Bbr2Mode::PROBE_BW:
return probe_bw_.GetCwndLimits();
case Bbr2Mode::DRAIN:
return drain_.GetCwndLimits();
case Bbr2Mode::PROBE_RTT:
return probe_rtt_.GetCwndLimits();
default:
QUIC_NOTREACHED();
return Unlimited<QuicByteCount>();
}
}
const Limits<QuicByteCount>& Bbr2Sender::cwnd_limits() const {
return params().cwnd_limits;
}
void Bbr2Sender::AdjustNetworkParameters(const NetworkParams& params) {
model_.UpdateNetworkParameters(params.rtt);
if (mode_ == Bbr2Mode::STARTUP) {
const QuicByteCount prior_cwnd = cwnd_;
QuicBandwidth effective_bandwidth =
std::max(params.bandwidth, model_.BandwidthEstimate());
connection_stats_->cwnd_bootstrapping_rtt_us =
model_.MinRtt().ToMicroseconds();
if (params.max_initial_congestion_window > 0) {
max_cwnd_when_network_parameters_adjusted_ =
params.max_initial_congestion_window * kDefaultTCPMSS;
}
cwnd_ = cwnd_limits().ApplyLimits(
std::min(max_cwnd_when_network_parameters_adjusted_,
model_.BDP(effective_bandwidth)));
if (!params.allow_cwnd_to_decrease) {
cwnd_ = std::max(cwnd_, prior_cwnd);
}
pacing_rate_ = std::max(pacing_rate_, QuicBandwidth::FromBytesAndTimeDelta(
cwnd_, model_.MinRtt()));
}
}
void Bbr2Sender::SetInitialCongestionWindowInPackets(
QuicPacketCount congestion_window) {
if (mode_ == Bbr2Mode::STARTUP) {
// The cwnd limits is unchanged and still applies to the new cwnd.
cwnd_ = cwnd_limits().ApplyLimits(congestion_window * kDefaultTCPMSS);
}
}
void Bbr2Sender::OnCongestionEvent(bool /*rtt_updated*/,
QuicByteCount prior_in_flight,
QuicTime event_time,
const AckedPacketVector& acked_packets,
const LostPacketVector& lost_packets) {
QUIC_DVLOG(3) << this
<< " OnCongestionEvent. prior_in_flight:" << prior_in_flight
<< " prior_cwnd:" << cwnd_ << " @ " << event_time;
Bbr2CongestionEvent congestion_event;
congestion_event.prior_cwnd = cwnd_;
congestion_event.prior_bytes_in_flight = prior_in_flight;
congestion_event.is_probing_for_bandwidth =
BBR2_MODE_DISPATCH(IsProbingForBandwidth());
model_.OnCongestionEventStart(event_time, acked_packets, lost_packets,
&congestion_event);
if (InSlowStart()) {
if (!lost_packets.empty()) {
connection_stats_->slowstart_packets_lost += lost_packets.size();
connection_stats_->slowstart_bytes_lost += congestion_event.bytes_lost;
}
if (congestion_event.end_of_round_trip) {
++connection_stats_->slowstart_num_rtts;
}
}
// Number of mode changes allowed for this congestion event.
int mode_changes_allowed = kMaxModeChangesPerCongestionEvent;
while (true) {
Bbr2Mode next_mode = BBR2_MODE_DISPATCH(
OnCongestionEvent(prior_in_flight, event_time, acked_packets,
lost_packets, congestion_event));
if (next_mode == mode_) {
break;
}
QUIC_DVLOG(2) << this << " Mode change: " << mode_ << " ==> " << next_mode
<< " @ " << event_time;
BBR2_MODE_DISPATCH(Leave(event_time, &congestion_event));
mode_ = next_mode;
BBR2_MODE_DISPATCH(Enter(event_time, &congestion_event));
--mode_changes_allowed;
if (mode_changes_allowed < 0) {
QUIC_BUG(quic_bug_10443_1)
<< "Exceeded max number of mode changes per congestion event.";
break;
}
}
UpdatePacingRate(congestion_event.bytes_acked);
QUIC_BUG_IF(quic_bug_10443_2, pacing_rate_.IsZero())
<< "Pacing rate must not be zero!";
UpdateCongestionWindow(congestion_event.bytes_acked);
QUIC_BUG_IF(quic_bug_10443_3, cwnd_ == 0u)
<< "Congestion window must not be zero!";
model_.OnCongestionEventFinish(unacked_packets_->GetLeastUnacked(),
congestion_event);
last_sample_is_app_limited_ =
congestion_event.last_packet_send_state.is_app_limited;
if (congestion_event.bytes_in_flight == 0 &&
params().avoid_unnecessary_probe_rtt) {
OnEnterQuiescence(event_time);
}
QUIC_DVLOG(3)
<< this
<< " END CongestionEvent(acked:" << quiche::PrintElements(acked_packets)
<< ", lost:" << lost_packets.size() << ") "
<< ", Mode:" << mode_ << ", RttCount:" << model_.RoundTripCount()
<< ", BytesInFlight:" << congestion_event.bytes_in_flight
<< ", PacingRate:" << PacingRate(0) << ", CWND:" << GetCongestionWindow()
<< ", PacingGain:" << model_.pacing_gain()
<< ", CwndGain:" << model_.cwnd_gain()
<< ", BandwidthEstimate(kbps):" << BandwidthEstimate().ToKBitsPerSecond()
<< ", MinRTT(us):" << model_.MinRtt().ToMicroseconds()
<< ", BDP:" << model_.BDP(BandwidthEstimate())
<< ", BandwidthLatest(kbps):"
<< model_.bandwidth_latest().ToKBitsPerSecond()
<< ", BandwidthLow(kbps):" << model_.bandwidth_lo().ToKBitsPerSecond()
<< ", BandwidthHigh(kbps):" << model_.MaxBandwidth().ToKBitsPerSecond()
<< ", InflightLatest:" << model_.inflight_latest()
<< ", InflightLow:" << model_.inflight_lo()
<< ", InflightHigh:" << model_.inflight_hi()
<< ", TotalAcked:" << model_.total_bytes_acked()
<< ", TotalLost:" << model_.total_bytes_lost()
<< ", TotalSent:" << model_.total_bytes_sent() << " @ " << event_time;
}
void Bbr2Sender::UpdatePacingRate(QuicByteCount bytes_acked) {
if (BandwidthEstimate().IsZero()) {
return;
}
if (model_.total_bytes_acked() == bytes_acked) {
// After the first ACK, cwnd_ is still the initial congestion window.
pacing_rate_ = QuicBandwidth::FromBytesAndTimeDelta(cwnd_, model_.MinRtt());
return;
}
QuicBandwidth target_rate = model_.pacing_gain() * model_.BandwidthEstimate();
if (model_.full_bandwidth_reached()) {
pacing_rate_ = target_rate;
return;
}
if (params_.decrease_startup_pacing_at_end_of_round &&
model_.pacing_gain() < Params().startup_pacing_gain) {
pacing_rate_ = target_rate;
return;
}
if (params_.bw_lo_mode_ != Bbr2Params::DEFAULT &&
model_.loss_events_in_round() > 0) {
pacing_rate_ = target_rate;
return;
}
// By default, the pacing rate never decreases in STARTUP.
if (target_rate > pacing_rate_) {
pacing_rate_ = target_rate;
}
}
void Bbr2Sender::UpdateCongestionWindow(QuicByteCount bytes_acked) {
QuicByteCount target_cwnd = GetTargetCongestionWindow(model_.cwnd_gain());
const QuicByteCount prior_cwnd = cwnd_;
if (model_.full_bandwidth_reached() || Params().startup_include_extra_acked) {
target_cwnd += model_.MaxAckHeight();
cwnd_ = std::min(prior_cwnd + bytes_acked, target_cwnd);
} else if (prior_cwnd < target_cwnd || prior_cwnd < 2 * initial_cwnd_) {
cwnd_ = prior_cwnd + bytes_acked;
}
const QuicByteCount desired_cwnd = cwnd_;
cwnd_ = GetCwndLimitsByMode().ApplyLimits(cwnd_);
const QuicByteCount model_limited_cwnd = cwnd_;
cwnd_ = cwnd_limits().ApplyLimits(cwnd_);
QUIC_DVLOG(3) << this << " Updating CWND. target_cwnd:" << target_cwnd
<< ", max_ack_height:" << model_.MaxAckHeight()
<< ", full_bw:" << model_.full_bandwidth_reached()
<< ", bytes_acked:" << bytes_acked
<< ", inflight_lo:" << model_.inflight_lo()
<< ", inflight_hi:" << model_.inflight_hi() << ". (prior_cwnd) "
<< prior_cwnd << " => (desired_cwnd) " << desired_cwnd
<< " => (model_limited_cwnd) " << model_limited_cwnd
<< " => (final_cwnd) " << cwnd_;
}
QuicByteCount Bbr2Sender::GetTargetCongestionWindow(float gain) const {
return std::max(model_.BDP(model_.BandwidthEstimate(), gain),
cwnd_limits().Min());
}
void Bbr2Sender::OnPacketSent(QuicTime sent_time,
QuicByteCount bytes_in_flight,
QuicPacketNumber packet_number,
QuicByteCount bytes,
HasRetransmittableData is_retransmittable) {
QUIC_DVLOG(3) << this << " OnPacketSent: pkn:" << packet_number
<< ", bytes:" << bytes << ", cwnd:" << cwnd_
<< ", inflight:" << bytes_in_flight + bytes
<< ", total_sent:" << model_.total_bytes_sent() + bytes
<< ", total_acked:" << model_.total_bytes_acked()
<< ", total_lost:" << model_.total_bytes_lost() << " @ "
<< sent_time;
if (InSlowStart()) {
++connection_stats_->slowstart_packets_sent;
connection_stats_->slowstart_bytes_sent += bytes;
}
if (bytes_in_flight == 0 && params().avoid_unnecessary_probe_rtt) {
OnExitQuiescence(sent_time);
}
model_.OnPacketSent(sent_time, bytes_in_flight, packet_number, bytes,
is_retransmittable);
}
void Bbr2Sender::OnPacketNeutered(QuicPacketNumber packet_number) {
model_.OnPacketNeutered(packet_number);
}
bool Bbr2Sender::CanSend(QuicByteCount bytes_in_flight) {
const bool result = bytes_in_flight < GetCongestionWindow();
return result;
}
QuicByteCount Bbr2Sender::GetCongestionWindow() const {
// TODO(wub): Implement Recovery?
return cwnd_;
}
QuicBandwidth Bbr2Sender::PacingRate(QuicByteCount /*bytes_in_flight*/) const {
return pacing_rate_;
}
void Bbr2Sender::OnApplicationLimited(QuicByteCount bytes_in_flight) {
if (bytes_in_flight >= GetCongestionWindow()) {
return;
}
model_.OnApplicationLimited();
QUIC_DVLOG(2) << this << " Becoming application limited. Last sent packet: "
<< model_.last_sent_packet()
<< ", CWND: " << GetCongestionWindow();
}
QuicByteCount Bbr2Sender::GetTargetBytesInflight() const {
QuicByteCount bdp = model_.BDP(model_.BandwidthEstimate());
return std::min(bdp, GetCongestionWindow());
}
void Bbr2Sender::PopulateConnectionStats(QuicConnectionStats* stats) const {
stats->num_ack_aggregation_epochs = model_.num_ack_aggregation_epochs();
}
void Bbr2Sender::OnEnterQuiescence(QuicTime now) {
last_quiescence_start_ = now;
}
void Bbr2Sender::OnExitQuiescence(QuicTime now) {
if (last_quiescence_start_ != QuicTime::Zero()) {
Bbr2Mode next_mode = BBR2_MODE_DISPATCH(
OnExitQuiescence(now, std::min(now, last_quiescence_start_)));
if (next_mode != mode_) {
BBR2_MODE_DISPATCH(Leave(now, nullptr));
mode_ = next_mode;
BBR2_MODE_DISPATCH(Enter(now, nullptr));
}
last_quiescence_start_ = QuicTime::Zero();
}
}
bool Bbr2Sender::ShouldSendProbingPacket() const {
// TODO(wub): Implement ShouldSendProbingPacket properly.
return BBR2_MODE_DISPATCH(IsProbingForBandwidth());
}
std::string Bbr2Sender::GetDebugState() const {
std::ostringstream stream;
stream << ExportDebugState();
return stream.str();
}
Bbr2Sender::DebugState Bbr2Sender::ExportDebugState() const {
DebugState s;
s.mode = mode_;
s.round_trip_count = model_.RoundTripCount();
s.bandwidth_hi = model_.MaxBandwidth();
s.bandwidth_lo = model_.bandwidth_lo();
s.bandwidth_est = BandwidthEstimate();
s.inflight_hi = model_.inflight_hi();
s.inflight_lo = model_.inflight_lo();
s.max_ack_height = model_.MaxAckHeight();
s.min_rtt = model_.MinRtt();
s.min_rtt_timestamp = model_.MinRttTimestamp();
s.congestion_window = cwnd_;
s.pacing_rate = pacing_rate_;
s.last_sample_is_app_limited = last_sample_is_app_limited_;
s.end_of_app_limited_phase = model_.end_of_app_limited_phase();
s.startup = startup_.ExportDebugState();
s.drain = drain_.ExportDebugState();
s.probe_bw = probe_bw_.ExportDebugState();
s.probe_rtt = probe_rtt_.ExportDebugState();
return s;
}
std::ostream& operator<<(std::ostream& os, const Bbr2Sender::DebugState& s) {
os << "mode: " << s.mode << "\n";
os << "round_trip_count: " << s.round_trip_count << "\n";
os << "bandwidth_hi ~ lo ~ est: " << s.bandwidth_hi << " ~ " << s.bandwidth_lo
<< " ~ " << s.bandwidth_est << "\n";
os << "min_rtt: " << s.min_rtt << "\n";
os << "min_rtt_timestamp: " << s.min_rtt_timestamp << "\n";
os << "congestion_window: " << s.congestion_window << "\n";
os << "pacing_rate: " << s.pacing_rate << "\n";
os << "last_sample_is_app_limited: " << s.last_sample_is_app_limited << "\n";
if (s.mode == Bbr2Mode::STARTUP) {
os << s.startup;
}
if (s.mode == Bbr2Mode::DRAIN) {
os << s.drain;
}
if (s.mode == Bbr2Mode::PROBE_BW) {
os << s.probe_bw;
}
if (s.mode == Bbr2Mode::PROBE_RTT) {
os << s.probe_rtt;
}
return os;
}
} // namespace quic