Unbloating the buffers

Here is a square.

Wait, it's not just a square.

There's more of those.

They feel lonely. Can we help them?

Yes! See, they are now connected and can talk to each other. Much better!

Except… Oh no.

As you awoke one morning from uneasy dreams you found yourself transformed in your bed into a software engineer. A calamity that I find all too familiar.

Now it's on us to figure out how that network should work.

With time and effort, you and I can come up with a protocol that will let bytes go through that cable. We can even make it decently quick and send pictures of exotic tropical fish to each other. It's relatively easy: both sides of the cable are under our control, so we can agree on the speed of exchange, making sure that the sending computer does not overload the receiving one. All's good, and the sun is shining 🕶️

Except… Oh no.

I have more than one cable plugged into my computer. You have an ongoing amphibian emergency that can only be helped by receiving frog pictures. I don't have any, but someone in my network has them.

Computers are now interconnected into a large network, and all links are different. A family of magpies made a nest in Ben's network cabinet, so every time the adult magpies bring food back, the link between me and Ben slows down.

Ben's a smart guy. He knows that electrons are fickle, and the connection speed varies, so he adds a buffer to his computer. If his link to me is slow, he can still receive more from Li, filling the buffer, and transmit it further as fast as he can. This way, the bandwidth is utilised as efficiently as possible, and transient spikes of traffic or slowdowns are buffered.

Let's zoom in.

Look closely: magpie parents are working hard, and the buffer is full. Packets trickle through, but they can only get into the buffer as fast as it is emptied on the other side. Ben's computer asks Li's to slow down.

Now imagine that there is more than one type of packet. Some of the packets belong to a voice call that is happening over the same link!

Voice packets need to get through quickly, but instead they wait in the queue that doesn't go anywhere. The buffer doesn't help throughput, it's not transient anymore, it just sits there. The buffer is actively harmful — it adds a delay! The entire link from Li to me is not only at maximum throughput, it's also slow to react. The latency gets higher for no benefit.

This is called bufferbloat.

Bufferbloat is so common that we don't even think about it. Of course video calls drop out and glitch when you download something, duh! The internet is working hard, it's only natural that it's a bit slow to react.

It doesn't have to be this way¹.

How bad is bufferbloat, really?

I tested my home connection with Flent²:

Here is what happens on this graph:

• Flent starts pinging the remote server (both via ICMP and UDP)
• five seconds into the test, Flent starts uploading and downloading in parallel
• Flent runs multiple TCP streams with different priorities in parallel. BE/BK/CS5/EF are different priorities
• both bandwidth and ping are charted five times a second

Some conclusions:

• the download is pretty stable at 75Mbit per stream
• upload is all over the place, floating from 10Mbit to 5Mbit per stream
• ping raises from a few ms to almost 300ms — 0.3s just for one packet to go there and back again! That makes video calls unusable
• priorities are ignored

I also tested pure, non-parallel bandwidth to the same server with iperf3. On my connection, iperf3 shows total speed of 305Mbit down/48Mbit up. Compared with the numbers above, total download matches Flent precisely, but upload slows down 2–4 times because of the parallel download.

This is not ideal.

What can I do about it?

Let's look again at what happens when I'm trying to download something from Li:

The reason why packets bunch is that there is a bandwidth bottleneck on the way. If I could just sign up for a faster contract, the problem would go away, right? To an extent this is true: faster connection makes it harder to saturate the network.

However, my provider still has a much faster network on their side, so the problem is still there, it's just less likely to occur.

If only there was a way to adjust those queues dynamically, shrinking them if the network is saturated!

This is precisely what Active Queue Management (AQM) does³.

CAKE is real

There are many AQM implementations, but the most modern, performant, non-fiddly one is CAKE.^{a very tortured backronym}

Better internet is one configuration option away⁴:

I traded about 10% of bandwidth (263Mbit down/41Mbit up per iperf3) for:

• constant average bandwidth on both upload and download
• no impact of download on upload
• network load has no visible impact on latency
• effective traffic prioritisation

I believe I can be less conservative with the bandwidth and fiddle with CAKE settings more, but I am happy with this trade as it is. Here's the entirery of VyOs⁵ config changes ⁶:

interfaces {
    input ifb0 {
    }
    pppoe pppoe0 {
        ...
        redirect ifb0
    }
}
 
qos {
    interface ifb0 {
        egress CAKE-WAN-IN
    }
    interface pppoe0 {
        egress CAKE-WAN-OUT
    }
    policy {
        cake CAKE-WAN-IN {
            bandwidth 280mbit
            flow-isolation {
                nat
            }
            /* This should be above 98% of observed physical
            round trip times. If your network is slower, 
            set it higher */            
            rtt 70
        }
        cake CAKE-WAN-OUT {
            bandwidth 45mbit
            flow-isolation {
                nat
            }
            /* Same here */
            rtt 70
        } 
    }
}

No more frog picture delays!

UPDATE: it was an incredible surprise to find a very positive email from Dave Täht, one of the people behind CAKE and bufferbloat.net, in my mailbox! He pointed out a problem in the configuration above:

rtt 30 is REALLY low for most networks. 100ms the default is designed to scale to about 280ms physical in the real world. In general the recommendation is 98% of your typical observed real world rtts. In most cases today I use 70ms.

…[It'd be better to] comment in your rtt setting, so that people in Africa do not copy/paste - at low rates that RTT setting will hurt, and at longer rtts common elsewhere, it hurt too.

…In Africa (about 80% the rtts > 300) we have been recommending 200ms for it, but it makes it really slow to react. See [this google doc] for some thoughts toward improving cake in Africa, on Starlink etc.

And on HN, where this post sparked a lively discussion:

…there is one thing that bothers me about his default config. He set CAKE rtt to 30ms which is OK, so long as 98% or so of the sites he wants to reach are within 30ms. Otherwise the default of 100ms is scaled to be able to manage bandwidth efficiently to sites around the world, and has been shown to work pretty well to real RTTs of 280ms. In general I have been recommending the rtt be set lower with the world consisting of CDNs, but not to 30ms.

I amended the post (and my config) accordingly.

Dave also pointed out that the rrul test I was using has its limitations. It would be interesting to look closer at TCP behaviours in the future.

Thank you Dave⁷!

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