Reducing speed when downloading.

Hi,

I’m trying to solve/understand what’s going wrong with my new FTTP connection (900/112) from Plusnet.

In a nutshell the problem is that when I download a file that takes a while to download I get the high speed I would expect for a short while followed a very specific lower speed of 41.2MB/s for another short while before it goes back up to the expected higher speed, constantly and predictably lowering and raising the speed at regular intervals and at a set capped speed when going lower. (Please see bellow image, taken while using ubuntu 22 with a direct PPOE connection to the ONT).

Image: https://imgur.com/a/zFga3iD

After a lot of messing about trying to solve the problem with the Plusnet router attached, an opnsense router attached, an edge router X attached and now finally just plugging a ubuntu 22 computer directly into the ONT and configuring a PPOE connection directly to it I feel like the problem lies with the ONT and beyond or an inherent problem with PPOE rather than anything my side as they all produce the same results using different computers, OS’s and network cards (Intel - both onboard and dedicated nic, realtek and atheros)

It’s not just the tele2 file test I’ve tried downloading by the way. It happens with any large file I try to download from multiple file mirrors for many Linux distribution ISO’s and other places I could find that had large files I could download. It also happens with torrents as well and manages to lower the speed to the exact same reduced speed of 41.2MB/s.

The computers I’m downloading too also have crucial and Samsung nvme drives in them so I would like to think it’s not them that are slowing the download down. Going computer to computer across my LAN lets me download a file at full 1000mbps speeds so everything works fine in that regard and proves the system/network/drives are more than capable of handling the those kind of speeds at a consistent high rate.

Having tried to read and understand other posts with similar issues both on these forms and elsewhere would I be right in thinking it’s a PPOE issue or maybe this thing called ‘bufferbloat’?

I feel like I can’t do much more than using an ubuntu 22 computer with direct PPOE connection to the ONT. It’s an I5-3570 with 16gb ram. I feel like it should be fast enough to cope with the speed. I can try using ubuntu 22 and a direct PPOE connect to the ONT with an i9-10900k if people think it might help but that system would require a more involved dismantling of what I’ve already got it setup as and I felt like the i5-3570 should be perfectly capable?

As far as I’ve discovered, so far, everything else is fine and standard speed tests come back great. It’s just the real world application of downloading something at a decent consistent speed, which is what I would want/expect a 900mbps connection to be able to do.

For reference: https://www.thinkbroadband.com/speedtest/16877970415...

So my question is, is there something I’m missing or something else I can try to solve this problem or is this just a limitation of the ONT and the technology used by Plusnet to make the connection and is considered ‘normal’?

If there is nothing I can do I could live with that. I just need to know for sure first as i don’t want to pay for a 900mbps connection only to receive around 353mbps half of the time I’m downloading something when there actually is a solution to stop this happening.

Any help appreciated.

Were you with Plusnet before the move to FTTP ?

Do they still hold those speed profiles for their customers which are on their system ?

Your old FTTC give you a 41meg connection perchance ?

Just a random guess …

I was with Plusnet before I switched to their FTTP connection.

How would I find this out about the speed profiles?

I used to connect at around 61mbps on my FTTC connection giving around 7MB/s download speeds.

I believe it is/was shown on some online ‘customer profile’ or portal type thing.

My theory is scotched though as the speeds you report aren’t a match for those previously received via FTTC.

Someone else will have other suggestions shortly I’m sure.

Not sure what is doing it but I am afraid your theory was squashed from the first image posted. It states "41.2MiB" - that is 329.6Mb/s and so speeds that aren't possible on FTTC.

It is an odd and consistent pattern that suggests that PlusNet may be doing shaping for sustained high speed downloads but if they are then it would seem unnecessary and unfair unless they have capacity issues.

That is an interesting one. Suggestions:

- do a traceroute to the target host, during the "fast" periods and the "slow" periods, and see if there's a difference
- using tcpdump, look at the TTL of the returned packets, during the "fast" periods and the "slow" periods, and see if it changes

Those would be indications of routes changing within Plusnet's network.

Next:

- capture traffic to a file using tcpdump (tcpdump -i pppoe0 -nn -s0 -w ppp.pcap) and analyse it using tcptrace. This isn't for the faint-hearted, but gives you low level details about packet loss, retransmissions, out-of-order packet delivery etc.
- to rule out the issue with writing to SSD (and I *have* seen SSDs which slow down when they heat up), you could run an iperf3 test, although that will also require an endpoint in the cloud somewhere, like a VPS. There is also a speedtest CLI you could try: but you'd need to see if you can configure it to make it run over a longer period, like 60 seconds, to reproduce the behaviour you're seeing.
- install netdata to see instantaneous stats on your Linux box, including network interface traffic. (Netdata is a monitoring tool which collects data at 1-second intervals by default. These days, they try quite hard to get you to signup for Netdata Cloud. Ignore this; netdata works just fine standalone. Point your browser at http://127.0.0.1:19999 to see all the graphs)

Maybe after all that, you will have some more clues.

Have you checked your MTU is correct?
As you have access to various routers that support it, have you tried using SQM e.g Cake or fq_codel? You will lose a little of the speed but should result in a much more consistent connection if it is bufferbloat related.

Do you have a BQM setup (mine is in the sig) and if so what does the connection look like?

Where are you downloading the file to? Is this an SSD cache behaviour issue?

Not sure what is doing it but I am afraid your theory was squashed from the first image posted. It states "41.2MiB" - that is 329.6Mb/s and so speeds that aren't possible on FTTC.

Fair enough

In reply to a post by Zarjaz:

Not sure what is doing it but I am afraid your theory was squashed from the first image posted. It states "41.2MiB" - that is 329.6Mb/s and so speeds that aren't possible on FTTC.

Fair enough