us_wroff_extra
wroff = connp->conn_ht_iphc_allocated + us->us_wroff_extra;
ulp_hdr_len, data_mp, data_len, us->us_wroff_extra, &cksum, errorp);
mp1 = allocb(alloclen + us->us_wroff_extra, BPRI_MED);
connp->conn_wroff = len + us->us_wroff_extra;