ips_ip_wroff_extra
mp = allocb_tmpl(ipst->ips_ip_wroff_extra + hdr_len, src);
mp->b_rptr += ipst->ips_ip_wroff_extra;
mp->b_wptr += hdr_len + ipst->ips_ip_wroff_extra;
hmp = allocb_tmpl(unfragmentable_len + ipst->ips_ip_wroff_extra, mp);
hmp->b_rptr += ipst->ips_ip_wroff_extra;
ipst->ips_ip_wroff_extra, mp);
hmp->b_rptr += ipst->ips_ip_wroff_extra;