Hello,
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

Can someone kindly help me solve the issue?

Thanks
Varun

How are you enforcing the ToS values?

Setting the ToS field by itself does nothing unless you also configure traffic shaping and map ToS values to queues with different priorities.

Hi Ser Olmy,
Thanks for the suggestion.
I am not enforcing the ToS.
I am under the understanding that the Linux kernel by itself uses the HTB to prioritize the packets if marked with ToS.
Can you kindly clarify if I am wrong.

Also I tried using : sudo tc qdisc add dev eth0 root handle 1:0 htb
but there was still no effect.

Thanks
Varun

Well, let us also never quite forget that: "CPUs work with nanoseconds, while networks still work with "milliseconds, at best."

"What? This packet arrived one millisecond later than that one?" Good grief! "Why, that's millions of nanoseconds!"

And so it goes.

The bottom line is that (I think ...) your objective cannot(!) be achieved through traffic shaping of any kind: At least to my way of thinking, the only possible way to achieve any such objective is to have the two programs in question actually talk to one another. They both must, first of all, be aware of each other. Your "TCP client" must be waiting for the receipt of some message from "iperf" ... a message that "iperf" knows to send after it has, in its judgment, "completed sending its data."

This entire interaction will occur across a timeframe of milliseconds ... or seconds ... or minutes ... or ...