webrtc视频接收端带宽预测（一）---参数输入

收包到触发远端估算流程：

->bool UdpSocket2WorkerWindows::Process()
->void UdpSocket2Windows::IOCompleted
->void UdpTransportImpl::IncomingRTPCallback
->void UdpTransportImpl::IncomingRTPFunction
->void VideoChannelTransport::IncomingRTPPacket
->int ViENetworkImpl::ReceivedRTPPacket
->int32_t ViEChannel::ReceivedRTPPacket
->int ViEReceiver::ReceivedRTPPacket
->int ViEReceiver::InsertRTPPacket
  ->void IncomingPacket
  ->bool ViEReceiver::ReceivePacket//这一步是对媒体处理，即插入jitterbuffer
->void RemoteBitrateEstimatorImpl::IncomingPacket

inter_arrival.cc中函数分析如下：

timestamp：rtp包中的时间戳
first_timestamp：当前帧第一个包中的时间戳
size：当前帧累计的包大小
complete_time_ms：包到达的时间
calculated_deltas：表示计算了一些delta，用来做返回值

bool InterArrival::ComputeDeltas(uint32_t timestamp,
                                 int64_t arrival_time_ms,
                                 size_t packet_size,
                                 uint32_t* timestamp_delta,
                                 int64_t* arrival_time_delta_ms,
                                 int* packet_size_delta) {
  assert(timestamp_delta != NULL);
  assert(arrival_time_delta_ms != NULL);
  assert(packet_size_delta != NULL);
  bool calculated_deltas = false;
  if (current_timestamp_group_.IsFirstPacket()) {//开始阶段
    // We don't have enough data to update the filter, so we store it until we
    // have two frames of data to process.
    current_timestamp_group_.timestamp = timestamp;
    current_timestamp_group_.first_timestamp = timestamp;
  } else if (!PacketInOrder(timestamp)) {//不乱序
    return false;
  } else if (NewTimestampGroup(arrival_time_ms, timestamp)) {//新的一帧
    // First packet of a later frame, the previous frame sample is ready.
    if (prev_timestamp_group_.complete_time_ms >= 0) {//prev_timestamp_group_这里是上上一个
      *timestamp_delta = current_timestamp_group_.timestamp -
                         prev_timestamp_group_.timestamp;//上一帧和上上一帧的时间戳之差
      *arrival_time_delta_ms = current_timestamp_group_.complete_time_ms -
                               prev_timestamp_group_.complete_time_ms;
      assert(*arrival_time_delta_ms >= 0);
      *packet_size_delta = static_cast<int>(current_timestamp_group_.size) -
          static_cast<int>(prev_timestamp_group_.size);
      calculated_deltas = true;
    }
    prev_timestamp_group_ = current_timestamp_group_;
    // The new timestamp is now the current frame.
    current_timestamp_group_.first_timestamp = timestamp;
    current_timestamp_group_.timestamp = timestamp;
    current_timestamp_group_.size = 0;
  }
  else {
    current_timestamp_group_.timestamp = LatestTimestamp(
        current_timestamp_group_.timestamp, timestamp);
//同一个帧，这里会根据有没有时间戳的拓展包头，决定timestamp有没有变化
  }
  // Accumulate the frame size.
  current_timestamp_group_.size += packet_size;
  current_timestamp_group_.complete_time_ms = arrival_time_ms;

  return calculated_deltas;
}

BelongsToBurst大概意思是两个包之间收到的间隔很小或者传输时延小于0

bool InterArrival::BelongsToBurst(int64_t arrival_time_ms,
                                  uint32_t timestamp) const {
  if (!burst_grouping_) {
    return false;
  }
  assert(current_timestamp_group_.complete_time_ms >= 0);
  int64_t arrival_time_delta_ms = arrival_time_ms -
      current_timestamp_group_.complete_time_ms;
  uint32_t timestamp_diff = timestamp - current_timestamp_group_.timestamp;
  int64_t ts_delta_ms = timestamp_to_ms_coeff_ * timestamp_diff + 0.5;
  if (ts_delta_ms == 0)
    return true;
  int propagation_delta_ms = arrival_time_delta_ms - ts_delta_ms;
  return propagation_delta_ms < 0 &&
      arrival_time_delta_ms <= kBurstDeltaThresholdMs;
}