CSUM_OFFLOAD
ifp->if_hwassist = CSUM_OFFLOAD | CSUM_TSO;
if ((mp->m_pkthdr.csum_flags & CSUM_OFFLOAD) == 0)
ifp->if_hwassist |= CSUM_OFFLOAD;
ifp->if_hwassist &= ~CSUM_OFFLOAD;
if ((mbuf->m_pkthdr.csum_flags & CSUM_OFFLOAD) != 0)