Why am I unable to prioritize TCP traffic using ToS fields?

I am trying to prioritize TCP traffic using ToS field in IP header. I am saturating the interface(ethernet) by sending 1GB data through iperf with ToS field set to 0x10 (Minimize-Delay). I then start another TCP client with default ToS (0). Expectation : My TCP client should not send data till iperf completes sending its data. Result: The data from my client is sent even tough iperf is sending packets with higher priority. I also tried to create the same scenario by creating 2 separate clients and allocating 0x10 and 0x08 ToS to respective clients using iptables. I used : iptables -A PREROUTING -t mangle -p tcp --sport 5000 -j TOS --set-tos Minimize-Delay I am still not able to prioritize one client over other. Altough I can see the packets marked with ToS in wireshark. I am using Ubuntu (14.04) with iptables version 1.4.21