DatagramSocket setTrafficClass

I was wondering if anyone could provide any details of which
environments anyone has got the setTrafficClass method of the
DatagramSocket class to make alterations to the DSCP field in the IP
header. I realise that the document says that support is optional,
but I am trying to find out in which environments it does actually
work.

I have written the code below and have ran it under windows xp and
vista and also under the latest version of knoppix - but in every
occasion when I use wireshark I get the result:

Differentiated Services Field: 0x00 (DSCP 0x00: Default; ECN: 0x00)

The version of Java on vista is Java version Java(TM) SE Runtime
Environment (build 1.6.0_02-b06), unfortunatly I don't have the
version of Java on knoppix to hand.

Under knoppix I can use iperf and change the -S parameter to
successfully change the DSCP field in the packet, so I am guessing
that I should be able to do the same with Java but I have yet to
achieve this.

All help, suggestions are greatly received.

import java.net.*;
import java.io.*;

class UDP_TOS_TEST {
    public static void main(String[] args){
        new UDP_TOS_TEST();
    }

    public UDP_TOS_TEST(){
        try{
            DatagramSocket dgramSocket = new DatagramSocket();

            int counter = 0;
            byte[] out = new byte[1000];

            DatagramPacket dgram = new DatagramPacket(out, 1000,
InetAddress.getByName("192.168.1.1"), 6000) ;
            dgram.setData(out);

            while(true){
                counter = (counter+1)%256;
                dgramSocket.setTrafficClass(counter);
                dgramSocket.send(dgram);
            }
        } catch (Exception e){
            e.printStackTrace();
        }
    }
}