code: def _dot1q_headers_size(layer):
          
  
          vlan_headers_size = 0
          under_layer = layer
  
          while under_layer and isinstance(under_layer, Dot1Q):
              vlan_headers_size += LLDPDU.DOT1Q_HEADER_LEN
              under_layer = under_layer.underlayer
  
          return vlan_headers_size, under_layer
docs: calculate size of lower dot1q layers (if present)
          :param layer: the layer to start at
          :return: size of vlan headers, layer below lowest vlan header