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/*
* Copyright (c) 2019 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/goog_cc/robust_throughput_estimator.h"
#include <stddef.h>
#include <algorithm>
#include <utility>
#include <vector>
#include "absl/types/optional.h"
#include "api/transport/network_types.h"
#include "api/units/data_rate.h"
#include "api/units/data_size.h"
#include "api/units/time_delta.h"
#include "api/units/timestamp.h"
#include "modules/congestion_controller/goog_cc/acknowledged_bitrate_estimator_interface.h"
#include "rtc_base/checks.h"
#include "rtc_base/logging.h"
namespace webrtc {
RobustThroughputEstimator::RobustThroughputEstimator(
const RobustThroughputEstimatorSettings& settings)
: settings_(settings),
latest_discarded_send_time_(Timestamp::MinusInfinity()) {
RTC_DCHECK(settings.enabled);
}
RobustThroughputEstimator::~RobustThroughputEstimator() {}
bool RobustThroughputEstimator::FirstPacketOutsideWindow() {
if (window_.empty())
return false;
if (window_.size() > settings_.max_window_packets)
return true;
TimeDelta current_window_duration =
window_.back().receive_time - window_.front().receive_time;
if (current_window_duration > settings_.max_window_duration)
return true;
if (window_.size() > settings_.window_packets &&
current_window_duration > settings_.min_window_duration) {
return true;
}
return false;
}
void RobustThroughputEstimator::IncomingPacketFeedbackVector(
const std::vector<PacketResult>& packet_feedback_vector) {
RTC_DCHECK(std::is_sorted(packet_feedback_vector.begin(),
packet_feedback_vector.end(),
PacketResult::ReceiveTimeOrder()));
for (const auto& packet : packet_feedback_vector) {
// Ignore packets without valid send or receive times.
// (This should not happen in production since lost packets are filtered
// out before passing the feedback vector to the throughput estimator.
// However, explicitly handling this case makes the estimator more robust
// and avoids a hard-to-detect bad state.)
if (packet.receive_time.IsInfinite() ||
packet.sent_packet.send_time.IsInfinite()) {
continue;
}
// Insert the new packet.
window_.push_back(packet);
window_.back().sent_packet.prior_unacked_data =
window_.back().sent_packet.prior_unacked_data *
settings_.unacked_weight;
// In most cases, receive timestamps should already be in order, but in the
// rare case where feedback packets have been reordered, we do some swaps to
// ensure that the window is sorted.
for (size_t i = window_.size() - 1;
i > 0 && window_[i].receive_time < window_[i - 1].receive_time; i--) {
std::swap(window_[i], window_[i - 1]);
}
constexpr TimeDelta kMaxReorderingTime = TimeDelta::Seconds(1);
const TimeDelta receive_delta =
(window_.back().receive_time - packet.receive_time);
if (receive_delta > kMaxReorderingTime) {
RTC_LOG(LS_WARNING)
<< "Severe packet re-ordering or timestamps offset changed: "
<< receive_delta;
window_.clear();
latest_discarded_send_time_ = Timestamp::MinusInfinity();
}
}
// Remove old packets.
while (FirstPacketOutsideWindow()) {
latest_discarded_send_time_ = std::max(
latest_discarded_send_time_, window_.front().sent_packet.send_time);
window_.pop_front();
}
}
absl::optional<DataRate> RobustThroughputEstimator::bitrate() const {
if (window_.empty() || window_.size() < settings_.required_packets)
return absl::nullopt;
TimeDelta largest_recv_gap(TimeDelta::Zero());
TimeDelta second_largest_recv_gap(TimeDelta::Zero());
for (size_t i = 1; i < window_.size(); i++) {
// Find receive time gaps.
TimeDelta gap = window_[i].receive_time - window_[i - 1].receive_time;
if (gap > largest_recv_gap) {
second_largest_recv_gap = largest_recv_gap;
largest_recv_gap = gap;
} else if (gap > second_largest_recv_gap) {
second_largest_recv_gap = gap;
}
}
Timestamp first_send_time = Timestamp::PlusInfinity();
Timestamp last_send_time = Timestamp::MinusInfinity();
Timestamp first_recv_time = Timestamp::PlusInfinity();
Timestamp last_recv_time = Timestamp::MinusInfinity();
DataSize recv_size = DataSize::Bytes(0);
DataSize send_size = DataSize::Bytes(0);
DataSize first_recv_size = DataSize::Bytes(0);
DataSize last_send_size = DataSize::Bytes(0);
size_t num_sent_packets_in_window = 0;
for (const auto& packet : window_) {
if (packet.receive_time < first_recv_time) {
first_recv_time = packet.receive_time;
first_recv_size =
packet.sent_packet.size + packet.sent_packet.prior_unacked_data;
}
last_recv_time = std::max(last_recv_time, packet.receive_time);
recv_size += packet.sent_packet.size;
recv_size += packet.sent_packet.prior_unacked_data;
if (packet.sent_packet.send_time < latest_discarded_send_time_) {
// If we have dropped packets from the window that were sent after
// this packet, then this packet was reordered. Ignore it from
// the send rate computation (since the send time may be very far
// in the past, leading to underestimation of the send rate.)
// However, ignoring packets creates a risk that we end up without
// any packets left to compute a send rate.
continue;
}
if (packet.sent_packet.send_time > last_send_time) {
last_send_time = packet.sent_packet.send_time;
last_send_size =
packet.sent_packet.size + packet.sent_packet.prior_unacked_data;
}
first_send_time = std::min(first_send_time, packet.sent_packet.send_time);
send_size += packet.sent_packet.size;
send_size += packet.sent_packet.prior_unacked_data;
++num_sent_packets_in_window;
}
// Suppose a packet of size S is sent every T milliseconds.
// A window of N packets would contain N*S bytes, but the time difference
// between the first and the last packet would only be (N-1)*T. Thus, we
// need to remove the size of one packet to get the correct rate of S/T.
// Which packet to remove (if the packets have varying sizes),
// depends on the network model.
// Suppose that 2 packets with sizes s1 and s2, are received at times t1
// and t2, respectively. If the packets were transmitted back to back over
// a bottleneck with rate capacity r, then we'd expect t2 = t1 + r * s2.
// Thus, r = (t2-t1) / s2, so the size of the first packet doesn't affect
// the difference between t1 and t2.
// Analoguously, if the first packet is sent at time t1 and the sender
// paces the packets at rate r, then the second packet can be sent at time
// t2 = t1 + r * s1. Thus, the send rate estimate r = (t2-t1) / s1 doesn't
// depend on the size of the last packet.
recv_size -= first_recv_size;
send_size -= last_send_size;
// Remove the largest gap by replacing it by the second largest gap.
// This is to ensure that spurious "delay spikes" (i.e. when the
// network stops transmitting packets for a short period, followed
// by a burst of delayed packets), don't cause the estimate to drop.
// This could cause an overestimation, which we guard against by
// never returning an estimate above the send rate.
RTC_DCHECK(first_recv_time.IsFinite());
RTC_DCHECK(last_recv_time.IsFinite());
TimeDelta recv_duration = (last_recv_time - first_recv_time) -
largest_recv_gap + second_largest_recv_gap;
recv_duration = std::max(recv_duration, TimeDelta::Millis(1));
if (num_sent_packets_in_window < settings_.required_packets) {
// Too few send times to calculate a reliable send rate.
return recv_size / recv_duration;
}
RTC_DCHECK(first_send_time.IsFinite());
RTC_DCHECK(last_send_time.IsFinite());
TimeDelta send_duration = last_send_time - first_send_time;
send_duration = std::max(send_duration, TimeDelta::Millis(1));
return std::min(send_size / send_duration, recv_size / recv_duration);
}
} // namespace webrtc
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