void net_checksum_calculate(uint8_t *data, int length)

{

    int hlen, plen, proto, csum_offset;

    uint16_t csum;



    if ((data[14] & 0xf0) != 0x40)

	return; /* not IPv4 */

    hlen  = (data[14] & 0x0f) * 4;

    plen  = (data[16] << 8 | data[17]) - hlen;

    proto = data[23];



    switch (proto) {

    case PROTO_TCP:

	csum_offset = 16;

	break;

    case PROTO_UDP:

	csum_offset = 6;

	break;

    default:

	return;

    }



    if (plen < csum_offset+2)

	return;



    data[14+hlen+csum_offset]   = 0;

    data[14+hlen+csum_offset+1] = 0;

    csum = net_checksum_tcpudp(plen, proto, data+14+12, data+14+hlen);

    data[14+hlen+csum_offset]   = csum >> 8;

    data[14+hlen+csum_offset+1] = csum & 0xff;

}
