modules/congestion_controller/bbr/bandwidth_sampler.h

/*
 *  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.
 */
// Based on the Quic implementation in Chromium.

#ifndef MODULES_CONGESTION_CONTROLLER_BBR_BANDWIDTH_SAMPLER_H_
#define MODULES_CONGESTION_CONTROLLER_BBR_BANDWIDTH_SAMPLER_H_

#include "absl/types/optional.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/bbr/packet_number_indexed_queue.h"

namespace webrtc {
namespace bbr {

namespace test {
class BandwidthSamplerPeer;
}  // namespace test

struct BandwidthSample {
  // The bandwidth at that particular sample. Zero if no valid bandwidth sample
  // is available.
  DataRate bandwidth;

  // The RTT measurement at this particular sample.  Zero if no RTT sample is
  // available.  Does not correct for delayed ack time.
  TimeDelta rtt;

  // Indicates whether the 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;

  BandwidthSample()
      : bandwidth(DataRate::Zero()),
        rtt(TimeDelta::Zero()),
        is_app_limited(false) {}
};

// 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
// data_size sent over time, and the number of data_size 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
// data_size 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 = (data_size(S_1) - data_size(S_0)) / (time(S_1) - time(S_0))
//   ack_rate = (data_size(A_1) - data_size(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 BandwidthSampler {
 public:
  BandwidthSampler();
  ~BandwidthSampler();
  // 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 the packet is either acknowledged or
  // declared lost.
  void OnPacketSent(Timestamp sent_time,
                    int64_t packet_number,
                    DataSize data_size,
                    DataSize data_in_flight);

  // Notifies the sampler that the |packet_number| is acknowledged. Returns a
  // bandwidth sample. If no bandwidth sample is available, bandwidth is set to
  // DataRate::Zero().
  BandwidthSample OnPacketAcknowledged(Timestamp ack_time,
                                       int64_t packet_number);

  // Informs the sampler that a packet is considered lost and it should no
  // longer keep track of it.
  void OnPacketLost(int64_t packet_number);

  // 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.
  void OnAppLimited();

  // Remove all the packets lower than the specified packet number.
  void RemoveObsoletePackets(int64_t least_unacked);

  // Total number of data_size currently acknowledged by the receiver.
  DataSize total_data_acked() const;

  // Application-limited information exported for debugging.
  bool is_app_limited() const;
  int64_t end_of_app_limited_phase() const;

 private:
  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.
    Timestamp sent_time;

    // Size of the packet.
    DataSize size;

    // The value of |total_data_sent_| at the time the packet was sent.
    // Includes the packet itself.
    DataSize total_data_sent;

    // The value of |total_data_sent_at_last_acked_packet_| at the time the
    // packet was sent.
    DataSize total_data_sent_at_last_acked_packet;

    // The value of |last_acked_packet_sent_time_| at the time the packet was
    // sent.
    absl::optional<Timestamp> last_acked_packet_sent_time;

    // The value of |last_acked_packet_ack_time_| at the time the packet was
    // sent.
    absl::optional<Timestamp> last_acked_packet_ack_time;

    // The value of |total_data_acked_| at the time the packet was
    // sent.
    DataSize total_data_acked_at_the_last_acked_packet;

    // The value of |is_app_limited_| at the time the packet was
    // sent.
    bool is_app_limited;

    // Snapshot constructor. Records the current state of the bandwidth
    // sampler.
    ConnectionStateOnSentPacket(Timestamp sent_time,
                                DataSize size,
                                const BandwidthSampler& sampler);

    // Default constructor.  Required to put this structure into
    // PacketNumberIndexedQueue.
    ConnectionStateOnSentPacket();
    ~ConnectionStateOnSentPacket();
  };

  // The total number of congestion controlled data_size sent during the
  // connection.
  DataSize total_data_sent_;

  // The total number of congestion controlled data_size which were
  // acknowledged.
  DataSize total_data_acked_;

  // The value of |total_data_sent_| at the time the last acknowledged packet
  // was sent. Valid only when |last_acked_packet_sent_time_| is valid.
  DataSize total_data_sent_at_last_acked_packet_;

  // The time at which the last acknowledged packet was sent. Set to
  // Timestamp::Zero() if no valid timestamp is available.
  absl::optional<Timestamp> last_acked_packet_sent_time_;

  // The time at which the most recent packet was acknowledged.
  absl::optional<Timestamp> last_acked_packet_ack_time_;

  // The most recently sent packet.
  int64_t 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.
  int64_t 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_;

  // Handles the actual bandwidth calculations, whereas the outer method handles
  // retrieving and removing |sent_packet|.
  BandwidthSample OnPacketAcknowledgedInner(
      Timestamp ack_time,
      int64_t packet_number,
      const ConnectionStateOnSentPacket& sent_packet);
};

}  // namespace bbr
}  // namespace webrtc

#endif  // MODULES_CONGESTION_CONTROLLER_BBR_BANDWIDTH_SAMPLER_H_