WebRTC重传包判断

调用处

bool StreamStatisticianImpl::UpdateOutOfOrder(const RtpPacketReceived& packet,
                                              int64_t sequence_number,
                                              int64_t now_ms) {
  // Check if |packet| is second packet of a stream restart.
  if (received_seq_out_of_order_) {
    // Count the previous packet as a received; it was postponed below.
    --cumulative_loss_;

    uint16_t expected_sequence_number = *received_seq_out_of_order_ + 1;
    received_seq_out_of_order_ = absl::nullopt;
    if (packet.SequenceNumber() == expected_sequence_number) {
      // Ignore sequence number gap caused by stream restart for packet loss
      // calculation, by setting received_seq_max_ to the sequence number just
      // before the out-of-order seqno. This gives a net zero change of
      // |cumulative_loss_|, for the two packets interpreted as a stream reset.
      //
      // Fraction loss for the next report may get a bit off, since we don't
      // update last_report_seq_max_ and last_report_cumulative_loss_ in a
      // consistent way.
      last_report_seq_max_ = sequence_number - 2;
      received_seq_max_ = sequence_number - 2;
      return false;
    }
  }

  if (std::abs(sequence_number - received_seq_max_) >
      max_reordering_threshold_) {
    // Sequence number gap looks too large, wait until next packet to check
    // for a stream restart.
    received_seq_out_of_order_ = packet.SequenceNumber();
    // Postpone counting this as a received packet until we know how to update
    // |received_seq_max_|, otherwise we temporarily decrement
    // |cumulative_loss_|. The
    // ReceiveStatisticsTest.StreamRestartDoesntCountAsLoss test expects
    // |cumulative_loss_| to be unchanged by the reception of the first packet
    // after stream reset.
    ++cumulative_loss_;
    return true;
  }

  if (sequence_number > received_seq_max_)
    return false;

  // Old out of order packet, may be retransmit.
  if (enable_retransmit_detection_ && IsRetransmitOfOldPacket(packet, now_ms))
    receive_counters_.retransmitted.AddPacket(packet);
  return true;
}		

重传包判断逻辑

bool StreamStatisticianImpl::IsRetransmitOfOldPacket(
    const RtpPacketReceived& packet,
    int64_t now_ms) const {
  uint32_t frequency_khz = packet.payload_type_frequency() / 1000;
  RTC_DCHECK_GT(frequency_khz, 0);

  int64_t time_diff_ms = now_ms - last_receive_time_ms_;

  // Diff in time stamp since last received in order.
  uint32_t timestamp_diff = packet.Timestamp() - last_received_timestamp_;
  uint32_t rtp_time_stamp_diff_ms = timestamp_diff / frequency_khz;

  int64_t max_delay_ms = 0;

  // Jitter standard deviation in samples.
  float jitter_std = std::sqrt(static_cast<float>(jitter_q4_ >> 4));

  // 2 times the standard deviation => 95% confidence.
  // And transform to milliseconds by dividing by the frequency in kHz.
  max_delay_ms = static_cast<int64_t>((2 * jitter_std) / frequency_khz);

  // Min max_delay_ms is 1.
  if (max_delay_ms == 0) {
    max_delay_ms = 1;
  }
  return time_diff_ms > rtp_time_stamp_diff_ms + max_delay_ms;
}