get the total size of all under layers
        :return: number of bytes

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