ar_spa
u_char ar_spa[]; /* sender protocol address */
memcpy(ar_tpa(ah), ar_spa(ah), ah->ar_pln);
memcpy(ar_spa(ah), &taddr, ah->ar_pln);
memcpy(ar_spa(ah), sip, ah->ar_pln);
memcpy(&isaddr, ar_spa(ah), sizeof isaddr);