Mastodawn

I've had this goofy problem with my #unifi kit (original #UDM/white cylinder) where my speed drops from ~900MB to ~80MB while connected to it. I thought it was an #xfinity issue, but if I connect directly to their modem, I see the speeds I'm expecting. Sometimes rebooting the router will clear it up for a little while. Currently connecting via wifi, but I'd previously seen the same behavior on wired connections, too.

I've done some searching, but have yet to figure out the cause or a fix. Throwing this out to the nerdosphere to see if anyone has any ideas (i.e. boosts are appreciated!).

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David Nelson Feb 28

@jackbrewster Could be a bad Ethernet cable that causes the link to intermittently drop from gigabit to 100M. I’d start by replacing the one that connects the UDM to the modem.

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Jack Brewster Feb 28

@dmnelson Already done. 😕

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dch

Feb 28

@jackbrewster wot does tcpdump show?

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Jack Brewster Mar 5

@dch Thanks for asking. My speeds dropped again so I ran the command, but I have no idea how to interpret it 😄. I read a quick intro, but it assumed a level of networking experience I don't have. Do you have any pointers for what I should look for?

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dch

Mar 5

@jackbrewster ok so let's start with something a little less intimidating. Use a wired PC in preference. We're poking around for a bunch of things:

- dropped packets
- duplicated packets
- size mismatches
- buffering problems

We can capture a lot of this information with wireshark, and then stop it and review the data.

Many issues simply show up in wireshark usually as red or black highlighted lines after capture.

Wireshark also has a menu item Analyse > Expert Information that summarises common problems for us.

The final buffering problems are often called "buffer bloat" see the 3rd link for an excellent explanation.

Try the simple browser-based testing link below. If that comes up with a C or D this is probably the root cause of the flakey performance. You can try looking into Ubiquiti Smart Queues, and reading up about Cake and CoDeL. After tuning my system internet worked a lot better, going from a C to A+, and most importantly, stable internet performance when people are doing video calls and playing games while I try to do "legit work".

It may also be possible to configure this on your xfinity modem too, you'd need to ask your ISP about that.

wireshark: https://www.wireshark.org/learn & https://www.youtube.com/@WireSharkFest or https://wireshark.com/how-to-use-wireshark/ esp section 7.2
bufferbloat: https://dgroshev.com/blog/bufferbloat/
testing: https://www.waveform.com/tools/bufferbloat
unifi smart queues: https://www.unihosted.com/blog/the-ultimate-guide-to-setting-up-unifi-smart-queues