unsigned short cksum(struct ip *ip, int len){
           long sum = 0;  /* assume 32 bit long, 16 bit short */
 
           while(len > 1){
             sum += *((unsigned short*) ip)++;
             if(sum & 0x80000000)   /* if high order bit set, fold */
               sum = (sum & 0xFFFF) + (sum >> 16);
             len -= 2;
           }
 
           if(len)       /* take care of left over byte */
             sum += (unsigned short) *(unsigned char *)ip;
 
           while(sum>>16)
             sum = (sum & 0xFFFF) + (sum >> 16);
 
           return ~sum;
}
 

return ~sum;