docs: def _get_underlayers_size(self):
          """
          get the total size of all under layers
          :return: number of bytes
          """
  
          under_layer = self.underlayer
  
          under_layers_size = 0
  
          while under_layer and isinstance(under_layer, Dot1Q):
              under_layers_size += 4
              under_layer = under_layer.underlayer
  
          if under_layer and isinstance(under_layer, Ether):
              # ether header len + FCS len
              under_layers_size += 14 + 4
  
          return under_layers_size