ICPF_RX_HDR
icp->icp_flags = ICPF_RX_HDR;
MPASS(icp->icp_flags == ICPF_RX_HDR);
MPASS(icp->icp_flags & ICPF_RX_HDR); /* Data is optional. */
icp->icp_flags |= ICPF_RX_HDR;