IXGBE_MTU_HDR
sc->max_frame_size = mtu + IXGBE_MTU_HDR;
sc->max_frame_size = if_getmtu(ifp) + IXGBE_MTU_HDR;
if (mtu > IXGBE_MAX_FRAME_SIZE - IXGBE_MTU_HDR) {
sc->max_frame_size = if_getmtu(ifp) + IXGBE_MTU_HDR;
#define IXGBE_MAX_MTU (IXGBE_MAX_FRAME_SIZE - IXGBE_MTU_HDR)