path: root/media/cast/sender/congestion_control.cc

// Copyright 2014 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.

// The purpose of this file is determine what bitrate to use for mirroring.
// Ideally this should be as much as possible, without causing any frames to
// arrive late.

// The current algorithm is to measure how much bandwidth we've been using
// recently. We also keep track of how much data has been queued up for sending
// in a virtual "buffer" (this virtual buffer represents all the buffers between
// the sender and the receiver, including retransmissions and so forth.)
// If we estimate that our virtual buffer is mostly empty, we try to use
// more bandwidth than our recent usage, otherwise we use less.

#include "media/cast/sender/congestion_control.h"

#include <deque>

#include "base/logging.h"
#include "media/cast/cast_config.h"
#include "media/cast/cast_defines.h"

namespace media {
namespace cast {

class AdaptiveCongestionControl : public CongestionControl {
 public:
  AdaptiveCongestionControl(base::TickClock* clock,
                            uint32 max_bitrate_configured,
                            uint32 min_bitrate_configured,
                            double max_frame_rate);

  ~AdaptiveCongestionControl() final;

  void UpdateRtt(base::TimeDelta rtt) final;

  void UpdateTargetPlayoutDelay(base::TimeDelta delay) final;

  // Called when an encoded frame is sent to the transport.
  void SendFrameToTransport(uint32 frame_id,
                            size_t frame_size_in_bits,
                            base::TimeTicks when) final;

  // Called when we receive an ACK for a frame.
  void AckFrame(uint32 frame_id, base::TimeTicks when) final;

  // Returns the bitrate we should use for the next frame.
  uint32 GetBitrate(base::TimeTicks playout_time,
                    base::TimeDelta playout_delay) final;

 private:
  struct FrameStats {
    FrameStats();
    // Time this frame was first enqueued for transport.
    base::TimeTicks enqueue_time;
    // Time this frame was acked.
    base::TimeTicks ack_time;
    // Size of encoded frame in bits.
    size_t frame_size_in_bits;
  };

  // Calculate how much "dead air" (idle time) there is between two frames.
  static base::TimeDelta DeadTime(const FrameStats& a, const FrameStats& b);
  // Get the FrameStats for a given |frame_id|.  Never returns nullptr.
  // Note: Older FrameStats will be removed automatically.
  FrameStats* GetFrameStats(uint32 frame_id);
  // Discard old FrameStats.
  void PruneFrameStats();
  // Calculate a safe bitrate. This is based on how much we've been
  // sending in the past.
  double CalculateSafeBitrate();

  // Estimate when the transport will start sending the data for a given frame.
  // |estimated_bitrate| is the current estimated transmit bitrate in bits per
  // second.
  base::TimeTicks EstimatedSendingTime(uint32 frame_id,
                                       double estimated_bitrate);

  base::TickClock* const clock_;  // Not owned by this class.
  const uint32 max_bitrate_configured_;
  const uint32 min_bitrate_configured_;
  const double max_frame_rate_;
  std::deque<FrameStats> frame_stats_;
  uint32 last_frame_stats_;
  uint32 last_acked_frame_;
  uint32 last_enqueued_frame_;
  base::TimeDelta rtt_;
  size_t history_size_;
  size_t acked_bits_in_history_;
  base::TimeDelta dead_time_in_history_;

  DISALLOW_COPY_AND_ASSIGN(AdaptiveCongestionControl);
};

class FixedCongestionControl : public CongestionControl {
 public:
  FixedCongestionControl(uint32 bitrate) : bitrate_(bitrate) {}
  ~FixedCongestionControl() final {}

  void UpdateRtt(base::TimeDelta rtt) final {}

  void UpdateTargetPlayoutDelay(base::TimeDelta delay) final {}

  // Called when an encoded frame is sent to the transport.
  void SendFrameToTransport(uint32 frame_id,
                            size_t frame_size_in_bits,
                            base::TimeTicks when) final {}

  // Called when we receive an ACK for a frame.
  void AckFrame(uint32 frame_id, base::TimeTicks when) final {}

  // Returns the bitrate we should use for the next frame.
  uint32 GetBitrate(base::TimeTicks playout_time,
                    base::TimeDelta playout_delay) final {
    return bitrate_;
  }

 private:
  uint32 bitrate_;
  DISALLOW_COPY_AND_ASSIGN(FixedCongestionControl);
};


CongestionControl* NewAdaptiveCongestionControl(
    base::TickClock* clock,
    uint32 max_bitrate_configured,
    uint32 min_bitrate_configured,
    double max_frame_rate) {
  return new AdaptiveCongestionControl(clock,
                                       max_bitrate_configured,
                                       min_bitrate_configured,
                                       max_frame_rate);
}

CongestionControl* NewFixedCongestionControl(uint32 bitrate) {
  return new FixedCongestionControl(bitrate);
}

// This means that we *try* to keep our buffer 90% empty.
// If it is less full, we increase the bandwidth, if it is more
// we decrease the bandwidth. Making this smaller makes the
// congestion control more aggressive.
static const double kTargetEmptyBufferFraction = 0.9;

// This is the size of our history in frames. Larger values makes the
// congestion control adapt slower.
static const size_t kHistorySize = 100;

AdaptiveCongestionControl::FrameStats::FrameStats() : frame_size_in_bits(0) {
}

AdaptiveCongestionControl::AdaptiveCongestionControl(
    base::TickClock* clock,
    uint32 max_bitrate_configured,
    uint32 min_bitrate_configured,
    double max_frame_rate)
    : clock_(clock),
      max_bitrate_configured_(max_bitrate_configured),
      min_bitrate_configured_(min_bitrate_configured),
      max_frame_rate_(max_frame_rate),
      last_frame_stats_(static_cast<uint32>(-1)),
      last_acked_frame_(static_cast<uint32>(-1)),
      last_enqueued_frame_(static_cast<uint32>(-1)),
      history_size_(kHistorySize),
      acked_bits_in_history_(0) {
  DCHECK_GE(max_bitrate_configured, min_bitrate_configured) << "Invalid config";
  frame_stats_.resize(2);
  base::TimeTicks now = clock->NowTicks();
  frame_stats_[0].ack_time = now;
  frame_stats_[0].enqueue_time = now;
  frame_stats_[1].ack_time = now;
  DCHECK(!frame_stats_[0].ack_time.is_null());
}

CongestionControl::~CongestionControl() {}
AdaptiveCongestionControl::~AdaptiveCongestionControl() {}

void AdaptiveCongestionControl::UpdateRtt(base::TimeDelta rtt) {
  rtt_ = (7 * rtt_ + rtt) / 8;
}

void AdaptiveCongestionControl::UpdateTargetPlayoutDelay(
    base::TimeDelta delay) {
  const int max_unacked_frames =
      std::min(kMaxUnackedFrames,
               1 + static_cast<int>(delay * max_frame_rate_ /
                                    base::TimeDelta::FromSeconds(1)));
  DCHECK_GT(max_unacked_frames, 0);
  history_size_ = max_unacked_frames + kHistorySize;
  PruneFrameStats();
}

// Calculate how much "dead air" there is between two frames.
base::TimeDelta AdaptiveCongestionControl::DeadTime(const FrameStats& a,
                                                    const FrameStats& b) {
  if (b.enqueue_time > a.ack_time) {
    return b.enqueue_time - a.ack_time;
  } else {
    return base::TimeDelta();
  }
}

double AdaptiveCongestionControl::CalculateSafeBitrate() {
  double transmit_time =
      (GetFrameStats(last_acked_frame_)->ack_time -
       frame_stats_.front().enqueue_time - dead_time_in_history_).InSecondsF();

  if (acked_bits_in_history_ == 0 || transmit_time <= 0.0) {
    return min_bitrate_configured_;
  }
  return acked_bits_in_history_ / std::max(transmit_time, 1E-3);
}

AdaptiveCongestionControl::FrameStats*
AdaptiveCongestionControl::GetFrameStats(uint32 frame_id) {
  int32 offset = static_cast<int32>(frame_id - last_frame_stats_);
  DCHECK_LT(offset, static_cast<int32>(kHistorySize));
  if (offset > 0) {
    frame_stats_.resize(frame_stats_.size() + offset);
    last_frame_stats_ += offset;
    offset = 0;
  }
  PruneFrameStats();
  offset += frame_stats_.size() - 1;
  // TODO(miu): Change the following to DCHECK once crash fix is confirmed.
  // http://crbug.com/517145
  CHECK(offset >= 0 && offset < static_cast<int32>(frame_stats_.size()));
  return &frame_stats_[offset];
}

void AdaptiveCongestionControl::PruneFrameStats() {
 while (frame_stats_.size() > history_size_) {
    DCHECK_GT(frame_stats_.size(), 1UL);
    DCHECK(!frame_stats_[0].ack_time.is_null());
    acked_bits_in_history_ -= frame_stats_[0].frame_size_in_bits;
    dead_time_in_history_ -= DeadTime(frame_stats_[0], frame_stats_[1]);
    DCHECK_GE(acked_bits_in_history_, 0UL);
    VLOG(2) << "DT: " << dead_time_in_history_.InSecondsF();
    DCHECK_GE(dead_time_in_history_.InSecondsF(), 0.0);
    frame_stats_.pop_front();
  }
}

void AdaptiveCongestionControl::AckFrame(uint32 frame_id,
                                         base::TimeTicks when) {
  FrameStats* frame_stats = GetFrameStats(last_acked_frame_);
  while (IsNewerFrameId(frame_id, last_acked_frame_)) {
    FrameStats* last_frame_stats = frame_stats;
    frame_stats = GetFrameStats(last_acked_frame_ + 1);
    if (frame_stats->enqueue_time.is_null()) {
      // Can't ack a frame that hasn't been sent yet.
      return;
    }
    last_acked_frame_++;
    if (when < frame_stats->enqueue_time)
      when = frame_stats->enqueue_time;

    frame_stats->ack_time = when;
    acked_bits_in_history_ += frame_stats->frame_size_in_bits;
    dead_time_in_history_ += DeadTime(*last_frame_stats, *frame_stats);
  }
}

void AdaptiveCongestionControl::SendFrameToTransport(uint32 frame_id,
                                                     size_t frame_size_in_bits,
                                                     base::TimeTicks when) {
  last_enqueued_frame_ = frame_id;
  FrameStats* frame_stats = GetFrameStats(frame_id);
  frame_stats->enqueue_time = when;
  frame_stats->frame_size_in_bits = frame_size_in_bits;
}

base::TimeTicks AdaptiveCongestionControl::EstimatedSendingTime(
    uint32 frame_id,
    double estimated_bitrate) {
  const base::TimeTicks now = clock_->NowTicks();

  // Starting with the time of the latest acknowledgement, extrapolate forward
  // to determine an estimated sending time for |frame_id|.
  //
  // |estimated_sending_time| will contain the estimated sending time for each
  // frame after the last ACK'ed frame.  It is possible for multiple frames to
  // be in-flight; and therefore it is common for the |estimated_sending_time|
  // for those frames to be before |now|.
  base::TimeTicks estimated_sending_time;
  for (uint32 f = last_acked_frame_; IsNewerFrameId(frame_id, f); ++f) {
    FrameStats* const stats = GetFrameStats(f);

    // |estimated_ack_time| is the local time when the sender receives the ACK,
    // and not the time when the ACK left the receiver.
    base::TimeTicks estimated_ack_time = stats->ack_time;

    // If |estimated_ack_time| is not null, then we already have the actual ACK
    // time, so we'll just use it.  Otherwise, we need to estimate when the ACK
    // will arrive.
    if (estimated_ack_time.is_null()) {
      // Model: The |estimated_sending_time| is the time at which the first byte
      // of the encoded frame is transmitted.  Then, assume the transmission of
      // the remaining bytes is paced such that the last byte has just left the
      // sender at |frame_transmit_time| later.  This last byte then takes
      // ~RTT/2 amount of time to travel to the receiver.  Finally, the ACK from
      // the receiver is sent and this takes another ~RTT/2 amount of time to
      // reach the sender.
      const base::TimeDelta frame_transmit_time =
          base::TimeDelta::FromSecondsD(stats->frame_size_in_bits /
                                            estimated_bitrate);
      estimated_ack_time =
          std::max(estimated_sending_time, stats->enqueue_time) +
              frame_transmit_time + rtt_;

      if (estimated_ack_time < now) {
        // The current frame has not yet been ACK'ed and the yet the computed
        // |estimated_ack_time| is before |now|.  This contradiction must be
        // resolved.
        //
        // The solution below is a little counter-intuitive, but it seems to
        // work.  Basically, when we estimate that the ACK should have already
        // happened, we figure out how long ago it should have happened and
        // guess that the ACK will happen half of that time in the future.  This
        // will cause some over-estimation when acks are late, which is actually
        // the desired behavior.
        estimated_ack_time = now + (now - estimated_ack_time) / 2;
      }
    }

    // Since we [in the common case] do not wait for an ACK before we start
    // sending the next frame, estimate the next frame's sending time as the
    // time just after the last byte of the current frame left the sender (see
    // Model comment above).
    estimated_sending_time =
        std::max(estimated_sending_time, estimated_ack_time - rtt_);
  }

  FrameStats* const frame_stats = GetFrameStats(frame_id);
  if (frame_stats->enqueue_time.is_null()) {
    // The frame has not yet been enqueued for transport.  Since it cannot be
    // enqueued in the past, ensure the result is lower-bounded by |now|.
    estimated_sending_time = std::max(estimated_sending_time, now);
  } else {
    // |frame_stats->enqueue_time| is the time the frame was enqueued for
    // transport.  The frame may not actually start being sent until a
    // point-in-time after that, because the transport is waiting for prior
    // frames to be acknowledged.
    estimated_sending_time =
        std::max(estimated_sending_time, frame_stats->enqueue_time);
  }

  return estimated_sending_time;
}

uint32 AdaptiveCongestionControl::GetBitrate(base::TimeTicks playout_time,
                                             base::TimeDelta playout_delay) {
  double safe_bitrate = CalculateSafeBitrate();
  // Estimate when we might start sending the next frame.
  base::TimeDelta time_to_catch_up =
      playout_time -
      EstimatedSendingTime(last_enqueued_frame_ + 1, safe_bitrate);

  double empty_buffer_fraction =
      time_to_catch_up.InSecondsF() / playout_delay.InSecondsF();
  empty_buffer_fraction = std::min(empty_buffer_fraction, 1.0);
  empty_buffer_fraction = std::max(empty_buffer_fraction, 0.0);

  uint32 bits_per_second = static_cast<uint32>(
      safe_bitrate * empty_buffer_fraction / kTargetEmptyBufferFraction);
  VLOG(3) << " FBR:" << (bits_per_second / 1E6)
          << " EBF:" << empty_buffer_fraction
          << " SBR:" << (safe_bitrate / 1E6);
  bits_per_second = std::max(bits_per_second, min_bitrate_configured_);
  bits_per_second = std::min(bits_per_second, max_bitrate_configured_);
  return bits_per_second;
}

}  // namespace cast
}  // namespace media