Function TcpChecksum

freebsd/netinet/libalias/alias_util.c:107–150

}

u_short
TcpChecksum(struct ip *pip)
{
	u_short *ptr;
	struct tcphdr *tc;
	int nhdr, ntcp, nbytes;
	int sum, oddbyte;

	nhdr = pip->ip_hl << 2;
	ntcp = ntohs(pip->ip_len) - nhdr;

	tc = (struct tcphdr *)ip_next(pip);
	ptr = (u_short *) tc;

/* Add up TCP header and data */
	nbytes = ntcp;
	sum = 0;
	while (nbytes > 1) {
		sum += *ptr++;
		nbytes -= 2;
	}
	if (nbytes == 1) {
		oddbyte = 0;
		((u_char *) & oddbyte)[0] = *(u_char *) ptr;
		((u_char *) & oddbyte)[1] = 0;
		sum += oddbyte;
	}
/* "Pseudo-header" data */
	ptr = (void *)&pip->ip_dst;
	sum += *ptr++;
	sum += *ptr;
	ptr = (void *)&pip->ip_src;
	sum += *ptr++;
	sum += *ptr;
	sum += htons((u_short) ntcp);
	sum += htons((u_short) pip->ip_p);

/* Roll over carry bits */
	sum = (sum >> 16) + (sum & 0xffff);
	sum += (sum >> 16);

/* Return checksum */
	return ((u_short) ~ sum);
}
#endif	/* not _KERNEL */

void