u_short ip_sum_calc( u_short len_ip_header, u_short * buff )
{
    u_short word16;
    u_int sum = 0;
    u_short i;
    // make 16 bit words out of every two adjacent 8 bit words in the packet
    // and add them up
    for( i = 0; i < len_ip_header; i = i+2 )
    {
        word16 = ( ( buff[i]<<8) & 0xFF00 )+( buff[i+1] & 0xFF );
        sum = sum + (u_int) word16;
    }
    // take only 16 bits out of the 32 bit sum and add up the carries
    while( sum >> 16 )
        sum = ( sum & 0xFFFF ) + ( sum >> 16 );
    // one's complement the result
    sum = ~sum;

    return ((u_short) sum);
}