Here’s one you might not have heard of before:

I went out this afternoon and bought a USB-C to Gigabit Ethernet dongle. It has two USB-A ports too, which works great for me (I use an external keyboard and mouse with my work Mac). I plug it into my Mac at home, use it all afternoon without any trouble.

In the evening, I head out to get some coffee. When I return, everyone’s complaining that the internet’s not working. Devices can connect to the access points, but they don’t get an IP address from the DSL router.

I checked the DSL router, and everything looked okay (DSL router’s the DHCP server, too). Restarted the router just in case. Turned my desktop on to look at Unifi’s console, and it showed devices connected/connecting. Maybe restarting the router fixed things, I thought.

Later in the evening, I put my laptop to sleep and went to bed, thought I’d read a book on the kindle. And the kindle shows… no internet! It can connect to the access point, but not get an IP address. Curiously enough, it can get one from the DSL modem’s WiFi (same IP network, but different WiFi network).

After a few experiments, I find out that when the Mac goes to sleep, the dongle floods the switch with MPCP, Opcode Pause, length 46 packets.

This question is about Ubuntu, but if you expand the comments, the last one mentions a Chinese Gigabit adapter: https://askubuntu.com/questions/994466/ubuntu-server-flooding-network-with-mpcp-opcode-pause-packets

See also: https://github.com/nwholloway/mpcp

The office network would occasionally have periods when latency across the LAN would degrade hideously (several seconds), and then all connectivity would be lost.

The cause of the failure turned out to be computers that hung during shutdown, and the network card would fire out a barrage of MPCP pause frames.

Every device on the LAN receives the packets, and obligingly suspends transmission. One device would even remain inactive even after the source of MPCP pause frames had stopped.