Source code that does the tcp6 cksum calculation :
static int tcp6_cksum(const struct ip6_hdr *ip6, const struct tcphdr *tp,
u_int len)
{
size_t i;
register const u_int16_t *sp;
u_int32_t sum;
union {
struct {
struct in6_addr ph_src;
struct in6_addr ph_dst;
u_int32_t ph_len;
u_int8_t ph_zero[3];
u_int8_t ph_nxt;
} ph;
u_int16_t pa[20];
} phu;
/* pseudo-header */
bzero((char *)&phu, sizeof(phu));
phu.ph.ph_src = ip6->ip6_src;
phu.ph.ph_dst = ip6->ip6_dst;
phu.ph.ph_len = htonl(len);
phu.ph.ph_nxt = IPPROTO_TCP;
sum = 0;
for (i = 0; i < sizeof(phu.pa) / sizeof(phu.pa[0]); i++)
sum += phu.pa[i];
sp = (const u_int16_t *)tp;
for (i = 0; i < (len & ~1); i += 2)
sum += *sp++;
if (len & 1)
sum += htons((*(const u_int8_t *)sp) << 8);
while (sum > 0xffff)
sum = (sum & 0xffff) + (sum >> 16);
sum = ~sum & 0xffff;
return (sum);
}
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