webrtc中的FEC配置

void VideoSendStreamImpl::ConfigureProtection() {
  RTC_DCHECK_RUN_ON(worker_queue_);

  // Consistency of FlexFEC parameters is checked in MaybeCreateFlexfecSender.
  const bool flexfec_enabled = (flexfec_sender_ != nullptr);

  // Consistency of NACK and RED+ULPFEC parameters is checked in this function.
  const bool nack_enabled = config_->rtp.nack.rtp_history_ms > 0;
  int red_payload_type = config_->rtp.ulpfec.red_payload_type;
  int ulpfec_payload_type = config_->rtp.ulpfec.ulpfec_payload_type;

  // Shorthands.
  auto IsRedEnabled = [&]() { return red_payload_type >= 0; };
  auto DisableRed = [&]() { red_payload_type = -1; };
  auto IsUlpfecEnabled = [&]() { return ulpfec_payload_type >= 0; };
  auto DisableUlpfec = [&]() { ulpfec_payload_type = -1; };

  // If enabled, FlexFEC takes priority over RED+ULPFEC.
  if (flexfec_enabled) {
    // We