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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 "net/third_party/quiche/src/quic/core/packet_number_indexed_queue.h"
#include "net/third_party/quiche/src/quic/core/quic_bandwidth.h"
#include "net/third_party/quiche/src/quic/core/quic_packets.h"
#include "net/third_party/quiche/src/quic/core/quic_time.h"
#include "net/third_party/quiche/src/quic/core/quic_types.h"
#include "net/third_party/quiche/src/quic/platform/api/quic_export.h"
namespace quic {
namespace test {
class BandwidthSamplerPeer;
} // namespace test
// A subset of BandwidthSampler::ConnectionStateOnSentPacket which is returned
// to the caller when the packet is acked or lost.
struct QUIC_EXPORT_PRIVATE SendTimeState {
: is_valid(false),
total_bytes_lost(0) {}
SendTimeState(bool is_app_limited,
QuicByteCount total_bytes_sent,
QuicByteCount total_bytes_acked,
QuicByteCount total_bytes_lost)
: is_valid(true),
total_bytes_lost(total_bytes_lost) {}
SendTimeState(const SendTimeState& other) = default;
// Whether other states in this object is valid.
bool is_valid;
// Whether the sender is app limited at the time the packet was sent.
// App limited bandwidth sample might be artificially low because the sender
// did not have enough data to send in order to saturate the link.
bool is_app_limited;
// Total number of sent bytes at the time the packet was sent.
// Includes the packet itself.
QuicByteCount total_bytes_sent;
// Total number of acked bytes at the time the packet was sent.
QuicByteCount total_bytes_acked;
// Total number of lost bytes at the time the packet was sent.
QuicByteCount total_bytes_lost;
struct QUIC_EXPORT_PRIVATE BandwidthSample {
// The bandwidth at that particular sample. Zero if no valid bandwidth sample
// is available.
QuicBandwidth bandwidth;
// The RTT measurement at this particular sample. Zero if no RTT sample is
// available. Does not correct for delayed ack time.
QuicTime::Delta rtt;
// States captured when the packet was sent.
SendTimeState state_at_send;
: bandwidth(QuicBandwidth::Zero()), rtt(QuicTime::Delta::Zero()) {}
// An interface common to any class that can provide bandwidth samples from the
// information per individual acknowledged packet.
class QUIC_EXPORT_PRIVATE BandwidthSamplerInterface {
virtual ~BandwidthSamplerInterface() {}
// Inputs the sent packet information into the sampler. Assumes that all
// packets are sent in order. The information about the packet will not be
// released from the sampler until it the packet is either acknowledged or
// declared lost.
virtual void OnPacketSent(
QuicTime sent_time,
QuicPacketNumber packet_number,
QuicByteCount bytes,
QuicByteCount bytes_in_flight,
HasRetransmittableData has_retransmittable_data) = 0;
// Notifies the sampler that the |packet_number| is acknowledged. Returns a
// bandwidth sample. If no bandwidth sample is available,
// QuicBandwidth::Zero() is returned.
virtual BandwidthSample OnPacketAcknowledged(
QuicTime ack_time,
QuicPacketNumber packet_number) = 0;
// Informs the sampler that a packet is considered lost and it should no
// longer keep track of it.
virtual SendTimeState OnPacketLost(QuicPacketNumber packet_number) = 0;
// Informs the sampler that the connection is currently app-limited, causing
// the sampler to enter the app-limited phase. The phase will expire by
// itself.
virtual void OnAppLimited() = 0;
// Remove all the packets lower than the specified packet number.
virtual void RemoveObsoletePackets(QuicPacketNumber least_unacked) = 0;
// Total number of bytes sent/acked/lost in the connection.
virtual QuicByteCount total_bytes_sent() const = 0;
virtual QuicByteCount total_bytes_acked() const = 0;
virtual QuicByteCount total_bytes_lost() const = 0;
// Application-limited information exported for debugging.
virtual bool is_app_limited() const = 0;
virtual QuicPacketNumber end_of_app_limited_phase() const = 0;
// BandwidthSampler keeps track of sent and acknowledged packets and outputs a
// bandwidth sample for every packet acknowledged. The samples are taken for
// individual packets, and are not filtered; the consumer has to filter the
// bandwidth samples itself. In certain cases, the sampler will locally severely
// underestimate the bandwidth, hence a maximum filter with a size of at least
// one RTT is recommended.
// This class bases its samples on the slope of two curves: the number of bytes
// sent over time, and the number of bytes acknowledged as received over time.
// It produces a sample of both slopes for every packet that gets acknowledged,
// based on a slope between two points on each of the corresponding curves. Note
// that due to the packet loss, the number of bytes on each curve might get
// further and further away from each other, meaning that it is not feasible to
// compare byte values coming from different curves with each other.
// The obvious points for measuring slope sample are the ones corresponding to
// the packet that was just acknowledged. Let us denote them as S_1 (point at
// which the current packet was sent) and A_1 (point at which the current packet
// was acknowledged). However, taking a slope requires two points on each line,
// so estimating bandwidth requires picking a packet in the past with respect to
// which the slope is measured.
// For that purpose, BandwidthSampler always keeps track of the most recently
// acknowledged packet, and records it together with every outgoing packet.
// When a packet gets acknowledged (A_1), it has not only information about when
// it itself was sent (S_1), but also the information about the latest
// acknowledged packet right before it was sent (S_0 and A_0).
// Based on that data, send and ack rate are estimated as:
// send_rate = (bytes(S_1) - bytes(S_0)) / (time(S_1) - time(S_0))
// ack_rate = (bytes(A_1) - bytes(A_0)) / (time(A_1) - time(A_0))
// Here, the ack rate is intuitively the rate we want to treat as bandwidth.
// However, in certain cases (e.g. ack compression) the ack rate at a point may
// end up higher than the rate at which the data was originally sent, which is
// not indicative of the real bandwidth. Hence, we use the send rate as an upper
// bound, and the sample value is
// rate_sample = min(send_rate, ack_rate)
// An important edge case handled by the sampler is tracking the app-limited
// samples. There are multiple meaning of "app-limited" used interchangeably,
// hence it is important to understand and to be able to distinguish between
// them.
// Meaning 1: connection state. The connection is said to be app-limited when
// there is no outstanding data to send. This means that certain bandwidth
// samples in the future would not be an accurate indication of the link
// capacity, and it is important to inform consumer about that. Whenever
// connection becomes app-limited, the sampler is notified via OnAppLimited()
// method.
// Meaning 2: a phase in the bandwidth sampler. As soon as the bandwidth
// sampler becomes notified about the connection being app-limited, it enters
// app-limited phase. In that phase, all *sent* packets are marked as
// app-limited. Note that the connection itself does not have to be
// app-limited during the app-limited phase, and in fact it will not be
// (otherwise how would it send packets?). The boolean flag below indicates
// whether the sampler is in that phase.
// Meaning 3: a flag on the sent packet and on the sample. If a sent packet is
// sent during the app-limited phase, the resulting sample related to the
// packet will be marked as app-limited.
// With the terminology issue out of the way, let us consider the question of
// what kind of situation it addresses.
// Consider a scenario where we first send packets 1 to 20 at a regular
// bandwidth, and then immediately run out of data. After a few seconds, we send
// packets 21 to 60, and only receive ack for 21 between sending packets 40 and
// 41. In this case, when we sample bandwidth for packets 21 to 40, the S_0/A_0
// we use to compute the slope is going to be packet 20, a few seconds apart
// from the current packet, hence the resulting estimate would be extremely low
// and not indicative of anything. Only at packet 41 the S_0/A_0 will become 21,
// meaning that the bandwidth sample would exclude the quiescence.
// Based on the analysis of that scenario, we implement the following rule: once
// OnAppLimited() is called, all sent packets will produce app-limited samples
// up until an ack for a packet that was sent after OnAppLimited() was called.
// Note that while the scenario above is not the only scenario when the
// connection is app-limited, the approach works in other cases too.
class QUIC_EXPORT_PRIVATE BandwidthSampler : public BandwidthSamplerInterface {
~BandwidthSampler() override;
void OnPacketSent(QuicTime sent_time,
QuicPacketNumber packet_number,
QuicByteCount bytes,
QuicByteCount bytes_in_flight,
HasRetransmittableData has_retransmittable_data) override;
BandwidthSample OnPacketAcknowledged(QuicTime ack_time,
QuicPacketNumber packet_number) override;
SendTimeState OnPacketLost(QuicPacketNumber packet_number) override;
void OnAppLimited() override;
void RemoveObsoletePackets(QuicPacketNumber least_unacked) override;
QuicByteCount total_bytes_sent() const override;
QuicByteCount total_bytes_acked() const override;
QuicByteCount total_bytes_lost() const override;
bool is_app_limited() const override;
QuicPacketNumber end_of_app_limited_phase() const override;
friend class test::BandwidthSamplerPeer;
// ConnectionStateOnSentPacket represents the information about a sent packet
// and the state of the connection at the moment the packet was sent,
// specifically the information about the most recently acknowledged packet at
// that moment.
struct ConnectionStateOnSentPacket {
// Time at which the packet is sent.
QuicTime sent_time;
// Size of the packet.
QuicByteCount size;
// The value of |total_bytes_sent_at_last_acked_packet_| at the time the
// packet was sent.
QuicByteCount total_bytes_sent_at_last_acked_packet;
// The value of |last_acked_packet_sent_time_| at the time the packet was
// sent.
QuicTime last_acked_packet_sent_time;
// The value of |last_acked_packet_ack_time_| at the time the packet was
// sent.
QuicTime last_acked_packet_ack_time;
// Send time states that are returned to the congestion controller when the
// packet is acked or lost.
SendTimeState send_time_state;
// Snapshot constructor. Records the current state of the bandwidth
// sampler.
ConnectionStateOnSentPacket(QuicTime sent_time,
QuicByteCount size,
const BandwidthSampler& sampler)
: sent_time(sent_time),
sampler.total_bytes_lost_) {}
// Default constructor. Required to put this structure into
// PacketNumberIndexedQueue.
: sent_time(QuicTime::Zero()),
last_acked_packet_ack_time(QuicTime::Zero()) {}
// Copy a subset of the (private) ConnectionStateOnSentPacket to the (public)
// SendTimeState. Always set send_time_state->is_valid to true.
void SentPacketToSendTimeState(const ConnectionStateOnSentPacket& sent_packet,
SendTimeState* send_time_state) const;
// The total number of congestion controlled bytes sent during the connection.
QuicByteCount total_bytes_sent_;
// The total number of congestion controlled bytes which were acknowledged.
QuicByteCount total_bytes_acked_;
// The total number of congestion controlled bytes which were lost.
QuicByteCount total_bytes_lost_;
// The value of |total_bytes_sent_| at the time the last acknowledged packet
// was sent. Valid only when |last_acked_packet_sent_time_| is valid.
QuicByteCount total_bytes_sent_at_last_acked_packet_;
// The time at which the last acknowledged packet was sent. Set to
// QuicTime::Zero() if no valid timestamp is available.
QuicTime last_acked_packet_sent_time_;
// The time at which the most recent packet was acknowledged.
QuicTime last_acked_packet_ack_time_;
// The most recently sent packet.
QuicPacketNumber last_sent_packet_;
// Indicates whether the bandwidth sampler is currently in an app-limited
// phase.
bool is_app_limited_;
// The packet that will be acknowledged after this one will cause the sampler
// to exit the app-limited phase.
QuicPacketNumber end_of_app_limited_phase_;
// Record of the connection state at the point where each packet in flight was
// sent, indexed by the packet number.
PacketNumberIndexedQueue<ConnectionStateOnSentPacket> connection_state_map_;
// Maximum number of tracked packets.
const QuicPacketCount max_tracked_packets_;
// Handles the actual bandwidth calculations, whereas the outer method handles
// retrieving and removing |sent_packet|.
BandwidthSample OnPacketAcknowledgedInner(
QuicTime ack_time,
QuicPacketNumber packet_number,
const ConnectionStateOnSentPacket& sent_packet);
} // namespace quic