ip4 =
(ip4_header_t *) u8_ptr_add (ip6, sizeof (*ip6) - sizeof (*ip4));
icmp = (icmp46_header_t *) u8_ptr_add (ip4, sizeof (*ip4));
ip4 =
(ip4_header_t *) u8_ptr_add (ip6, sizeof (*ip6) - sizeof (*ip4));
icmp = (icmp46_header_t *) u8_ptr_add (ip4, sizeof (*ip4));