zyd_tx_header
struct zyd_tx_header *buf_hdr;
m = allocb(msgdsize(mp) + sizeof (struct zyd_tx_header) + 32,
m->b_rptr += sizeof (struct zyd_tx_header);
m->b_rptr -= sizeof (struct zyd_tx_header);
buf_hdr = (struct zyd_tx_header *)m->b_rptr;
additional_size = sizeof (struct zyd_tx_header) + 10;