bandwidth_sampler.h | Explore in Territory

// 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.

#ifndef QUICHE_QUIC_CORE_CONGESTION_CONTROL_BANDWIDTH_SAMPLER_H_
#define QUICHE_QUIC_CORE_CONGESTION_CONTROL_BANDWIDTH_SAMPLER_H_

#include "quiche/quic/core/congestion_control/send_algorithm_interface.h"
#include "quiche/quic/core/congestion_control/windowed_filter.h"
#include "quiche/quic/core/packet_number_indexed_queue.h"
#include "quiche/quic/core/quic_bandwidth.h"
#include "quiche/quic/core/quic_packet_number.h"
#include "quiche/quic/core/quic_packets.h"
#include "quiche/quic/core/quic_time.h"
#include "quiche/quic/core/quic_types.h"
#include "quiche/quic/core/quic_unacked_packet_map.h"
#include "quiche/quic/platform/api/quic_export.h"
#include "quiche/quic/platform/api/quic_flags.h"
#include "quiche/common/quiche_circular_deque.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 QUICHE_EXPORT SendTimeState { … };

struct QUICHE_EXPORT ExtraAckedEvent { … };

struct QUICHE_EXPORT BandwidthSample { … };

// MaxAckHeightTracker is part of the BandwidthSampler. It is called after every
// ack event to keep track the degree of ack aggregation(a.k.a "ack height").
class QUICHE_EXPORT MaxAckHeightTracker { … };

// An interface common to any class that can provide bandwidth samples from the
// information per individual acknowledged packet.
class QUICHE_EXPORT BandwidthSamplerInterface { … };

// 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 a previously
// acknowledged packet 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 QUICHE_EXPORT BandwidthSampler : public BandwidthSamplerInterface { … };

}  // namespace quic

#endif  // QUICHE_QUIC_CORE_CONGESTION_CONTROL_BANDWIDTH_SAMPLER_H_
chromium/net/third_party/quiche/src/quiche/quic/core/congestion_control/bandwidth_sampler.h
