WebRTC Bandwidth Allocation

by walter

Table of Contents

file

Title Bandwidth allocation by REMB

ModuleRtpRtcpImpl->RTCPReceiver: IncomingPacket
RTCPReceiver->RTCPReceiver: TriggerCallbacksFromRtcpPacket
RTCPReceiver->RtpTransportControllerSend:OnReceivedEstimatedBitrate(bitrate)
RtpTransportControllerSend->GoogCcNetworkController: OnRemoteBitrateReport(receiveTime, bitrate)
GoogCcNetworkController->SendSideBandwidthEstimation: UpdateReceiverEstimate
SendSideBandwidthEstimation->SendSideBandwidthEstimation: ApplyTargetLimits
SendSideBandwidthEstimation->SendSideBandwidthEstimation: UpdateTargetBitrate
#SendSideBandwidthEstimation->LinkCapacityTracker:OnRateUpdate
GoogCcNetworkController->Call: OnTargetTransferRate
Call->BitrateAllocator: OnNetworkEstimateChanged
BitrateAllocator->BitrateAllocator: AllocateBitrates

Snippets

  • AllocateBitrates 带宽分配方法
std::map<BitrateAllocatorObserver*, int> AllocateBitrates(
    const std::vector<AllocatableTrack>& allocatable_tracks,
    uint32_t bitrate) {
  if (allocatable_tracks.empty())
    return std::map<BitrateAllocatorObserver*, int>();

  if (bitrate == 0)
    return ZeroRateAllocation(allocatable_tracks);

  uint32_t sum_min_bitrates = 0;
  uint32_t sum_max_bitrates = 0;
  for (const auto& observer_config : allocatable_tracks) {
    sum_min_bitrates += observer_config.config.min_bitrate_bps;
    sum_max_bitrates += observer_config.config.max_bitrate_bps;
  }

  // Not enough for all observers to get an allocation, allocate according to:
  // enforced min bitrate -> allocated bitrate previous round -> restart paused
  // streams.
  if (!EnoughBitrateForAllObservers(allocatable_tracks, bitrate,
                                    sum_min_bitrates))
    return LowRateAllocation(allocatable_tracks, bitrate);

  // All observers will get their min bitrate plus a share of the rest. This
  // share is allocated to each observer based on its bitrate_priority.
  if (bitrate <= sum_max_bitrates)
    return NormalRateAllocation(allocatable_tracks, bitrate, sum_min_bitrates);

  // All observers will get up to transmission_max_bitrate_multiplier_ x max.
  return MaxRateAllocation(allocatable_tracks, bitrate, sum_max_bitrates);
}

  • UpdateControlState

    void RtpTransportControllerSend::UpdateControlState() {
absl::optional<TargetTransferRate> update = control_handler_->GetUpdate();
if (!update)
return;
retransmission_rate_limiter_.SetMaxRate(update->target_rate.bps());
// We won't create control_handler_ until we have an observers.
RTC_DCHECK(observer_ != nullptr);
observer_->OnTargetTransferRate(*update);
}

    EnoughBitrateForAllObservers 带宽足够和话

    bool EnoughBitrateForAllObservers(
const std::vector<AllocatableTrack>& allocatable_tracks,
uint32_t bitrate,
uint32_t sum_min_bitrates) {
if (bitrate < sum_min_bitrates)
return false;

uint32_t extra_bitrate_per_observer =
  (bitrate - sum_min_bitrates) /
  static_cast<uint32_t>(allocatable_tracks.size());
for (const auto& observer_config : allocatable_tracks) {
if (observer_config.config.min_bitrate_bps + extra_bitrate_per_observer <
    observer_config.MinBitrateWithHysteresis()) {
  return false;
}
}
return true;
}

LowRateAllocation 带宽不够的分析策略

// Allocates bitrate to observers when there isn't enough to allocate the
// minimum to all observers.
std::map<BitrateAllocatorObserver*, int> LowRateAllocation(
    const std::vector<AllocatableTrack>& allocatable_tracks,
    uint32_t bitrate) {
  std::map<BitrateAllocatorObserver*, int> allocation;
  // Start by allocating bitrate to observers enforcing a min bitrate, hence
  // remaining_bitrate might turn negative.
  int64_t remaining_bitrate = bitrate;
  for (const auto& observer_config : allocatable_tracks) {
    int32_t allocated_bitrate = 0;
    if (observer_config.config.enforce_min_bitrate)
      allocated_bitrate = observer_config.config.min_bitrate_bps;

    allocation[observer_config.observer] = allocated_bitrate;
    remaining_bitrate -= allocated_bitrate;
  }

  // Allocate bitrate to all previously active streams.
  if (remaining_bitrate > 0) {
    for (const auto& observer_config : allocatable_tracks) {
      if (observer_config.config.enforce_min_bitrate ||
          observer_config.LastAllocatedBitrate() == 0)
        continue;

      uint32_t required_bitrate = observer_config.MinBitrateWithHysteresis();
      if (remaining_bitrate >= required_bitrate) {
        allocation[observer_config.observer] = required_bitrate;
        remaining_bitrate -= required_bitrate;
      }
    }
  }

  // Allocate bitrate to previously paused streams.
  if (remaining_bitrate > 0) {
    for (const auto& observer_config : allocatable_tracks) {
      if (observer_config.LastAllocatedBitrate() != 0)
        continue;

      // Add a hysteresis to avoid toggling.
      uint32_t required_bitrate = observer_config.MinBitrateWithHysteresis();
      if (remaining_bitrate >= required_bitrate) {
        allocation[observer_config.observer] = required_bitrate;
        remaining_bitrate -= required_bitrate;
      }
    }
  }

  // Split a possible remainder evenly on all streams with an allocation.
  if (remaining_bitrate > 0)
    DistributeBitrateEvenly(allocatable_tracks, remaining_bitrate, false, 1,
                            &allocation);

  RTC_DCHECK_EQ(allocation.size(), allocatable_tracks.size());
  return allocation;
}

NormalRateAllocation 按优先级的带宽分配策略

// Allocates the bitrate based on the bitrate priority of each observer. This
// bitrate priority defines the priority for bitrate to be allocated to that
// observer in relation to other observers. For example with two observers, if
// observer 1 had a bitrate_priority = 1.0, and observer 2 has a
// bitrate_priority = 2.0, the expected behavior is that observer 2 will be
// allocated twice the bitrate as observer 1 above the each observer's
// min_bitrate_bps values, until one of the observers hits its max_bitrate_bps.
std::map<BitrateAllocatorObserver*, int> NormalRateAllocation(
    const std::vector<AllocatableTrack>& allocatable_tracks,
    uint32_t bitrate,
    uint32_t sum_min_bitrates) {
  std::map<BitrateAllocatorObserver*, int> allocation;
  std::map<BitrateAllocatorObserver*, int> observers_capacities;
  for (const auto& observer_config : allocatable_tracks) {
    allocation[observer_config.observer] =
        observer_config.config.min_bitrate_bps;
    observers_capacities[observer_config.observer] =
        observer_config.config.max_bitrate_bps -
        observer_config.config.min_bitrate_bps;
  }

  bitrate -= sum_min_bitrates;

  // TODO(srte): Implement fair sharing between prioritized streams, currently
  // they are treated on a first come first serve basis.
  for (const auto& observer_config : allocatable_tracks) {
    int64_t priority_margin = observer_config.config.priority_bitrate_bps -
                              allocation[observer_config.observer];
    if (priority_margin > 0 && bitrate > 0) {
      int64_t extra_bitrate = std::min<int64_t>(priority_margin, bitrate);
      allocation[observer_config.observer] +=
          rtc::dchecked_cast<int>(extra_bitrate);
      observers_capacities[observer_config.observer] -= extra_bitrate;
      bitrate -= extra_bitrate;
    }
  }

  // From the remaining bitrate, allocate a proportional amount to each observer
  // above the min bitrate already allocated.
  if (bitrate > 0)
    DistributeBitrateRelatively(allocatable_tracks, bitrate,
                                observers_capacities, &allocation);

  return allocation;
}

MaxRateAllocation 最大带宽分配策略

// Allocates bitrate to observers when there is enough available bandwidth
// for all observers to be allocated their max bitrate.
std::map<BitrateAllocatorObserver*, int> MaxRateAllocation(
    const std::vector<AllocatableTrack>& allocatable_tracks,
    uint32_t bitrate,
    uint32_t sum_max_bitrates) {
  std::map<BitrateAllocatorObserver*, int> allocation;

  for (const auto& observer_config : allocatable_tracks) {
    allocation[observer_config.observer] =
        observer_config.config.max_bitrate_bps;
    bitrate -= observer_config.config.max_bitrate_bps;
  }
  DistributeBitrateEvenly(allocatable_tracks, bitrate, true,
                          kTransmissionMaxBitrateMultiplier, &allocation);
  return allocation;
}

ZeroRateAllocation 零带宽分配策略

// Allocates zero bitrate to all observers.
std::map<BitrateAllocatorObserver*, int> ZeroRateAllocation(
    const std::vector<AllocatableTrack>& allocatable_tracks) {
  std::map<BitrateAllocatorObserver*, int> allocation;
  for (const auto& observer_config : allocatable_tracks)
    allocation[observer_config.observer] = 0;
  return allocation;
}

Log keyword

  • Current BWE

    void BitrateAllocator::OnNetworkEstimateChanged(TargetTransferRate msg) {

//...
// Periodically log the incoming BWE.
int64_t now = msg.at_time.ms();
if (now > last_bwe_log_time_ + kBweLogIntervalMs) {
RTC_LOG(LS_INFO) << "Current BWE " << last_target_bps_;
last_bwe_log_time_ = now;
}

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