ATPD_SEQ_MASK
#define ATPD_SET_SEQNO(hdrp, atp) ((isphdr_t *)hdrp)->rqs_seqno &= ~ATPD_SEQ_MASK, ((isphdr_t *)hdrp)->rqs_seqno |= (atp)->seqno
#define ATPD_GET_SEQNO(hdrp) (((isphdr_t *)hdrp)->rqs_seqno & ATPD_SEQ_MASK)