Trouble setting IPv6 traffic class (QoS) bits with iperf
 

I am running iperf 2.0.5 on 2.6.23.1-42.fc8. I want to use the -S option to set the IPv6 traffic class (QoS) bits.

I used the following command;

iperf -c fec0::4:f5d2:c695:40e3:cab6 -b 10k -t 1000000 -S 0x38 -p 5111 -l 64 -V

With tcpdump, I can see that the traffic class is still 0x0.

I know the -S option works with IPv4. Does it work with IPv6? If so, what am I doing wrong?