[L4s-discuss] Configuring a L4S test plant

[Matteo Guarna S303434 <[e-mail]>]

Greetings everyone,

I hope the question isn't too off-topic, please forgive me in advance if 
it is so.

I am still trying to perform some fairness measurements with both L4S 
and classic flow, although now on a physical test plant instead of a 
virtualized one. I'm relying on the L4STeam Github project for the 
deployment of the L4S architecture and I am looking for someone who's 
familiar with the project and might be willing to help me: in fact I 
seem not to be able to achieve the correct configuration.

My setup is very simple: I have four servers (two senders and two 
receivers) exchanging two traffic flows through one server acting as a 
router. One client-server pair uses Prague as CC, while the other uses 
Cubic. All servers have the patched kernel provided in the 
https://github.com/L4STeam/linux/ repository branch.

If I trigger a congestion on the router by generating both the Prague 
and the Cubic flows (let's say the flows measure 100 Mbit/s each, and 
they come though a L2 switch both on the same router's input interface 
on a 1Gb Ethernet link; only a 100M link though is in place on the 
output interface towards the receivers) I see the L4S flow having higher 
delay, higher jitter and a smaller (and more variable) bandwidth share. 
The Prague share is 1/4 of the Cubic share. I am sending an attachment 
with a graphical representation of the scenario here described.

I configured my L4S endpoints as follows:

- I set the CC as tcp Prague (sysctl -w 
net.ipv4.tcp_congestion_control=prague)
- I set the AccEcn, even if it's not necessary apparently (sysctl -w 
net.ipv4.tcp_ecn=3)
- I disabled the required offloading capabilities on the endpoints (sudo 
ethtool -K $NETIF tso off gso off gro off lro off)
- I configured the fair queue on the endpoints (sudo tc qdisc replace 
dev $NETIF root fq)

I configured my router as follows:

- I enabled forwarding through these interfaces to obtain the routing 
capabilities (sudo sysctl -w net.ipv4.ip_forward=1)
- I set the dualpi2 on both interfaces (sudo tc qdisc replace dev $NETIF 
root dualpi2)

I then applied the fair queue and disabled the offloading capabilities 
on both my classic endpoints to ensure that the classic and l4s flows 
act as fairly as possible, but to no avail (even without these 
precautions the results remain roughly the same).

I am sure I am missing some important details in the setup, and I would 
really appreciate some help.

Regards,

Matteo

P.s.
I just want to point out that by looking at the packet traces everything 
seems fine: Prague carries the ECN=1, the dualpi2 marks packets with 
ECN=3, the AccEcn control signals on the ACE fields are coherent, and no 
losses occur in the Prague flow, while they do happen with the Cubic 
flow. It looks like Prague is underperforming for whatever reason. 
Furthermore, if I switch back to two Cubic flows I measure perfect 
share, equal delay and equal jitter, so it looks to me like there are no 
physical impairments on the testbed.