IP checksum


struct ip {
        u_int          ip_v:4,          /* version */
                       ip_hl:4;         /* header length */
        u_int8_t       ip_tos;          /* type of service */
        u_int16_t      ip_len;          /* total length */
        u_int16_t      ip_id;           /* identification */
        u_int16_t      ip_off;          /* fragment offset field */
        u_int8_t       ip_ttl;          /* time to live */
        u_int8_t       ip_p;            /* protocol */
        u_int16_t      ip_sum;          /* checksum */
        struct in_addr ip_src, ip_dst;  /* source and dest address */
};

entire header covered

ttl decremented by each router
have to update the cksum as well