esptail
sizeof(struct esptail);
sizeof(struct esptail) + ah_authsiz(NULL);
struct esptail *esptail;
padlen = ((blks - ((rlen + sizeof(struct esptail)) % blks)) % blks);
tlen = padlen + sizeof(struct esptail) + alen;
esptail = (struct esptail *)&tail[padlen];
esptail->esp_padlen = padlen;
m_copydata(m, protoff, sizeof(uint8_t), &esptail->esp_nxt);