p_cksum
u_short sum = p_cksum(outip, (u_short *)outudp, protlen, protlen);
u_short sum = p_cksum(outip, (u_short *)outudp, protlen, 8);
tcp->th_sum = p_cksum(outip, (u_short *)tcp, protlen, protlen);
u_short p_cksum(struct ip *, u_short *, int, int);