It’s not new, but the subject came up recently, especially at Michael Tsai: USB-C hubs that incorporate an Ethernet port can have the annoying tendency to block the entire local network when they are powered, but not connected to a computer. This is a phenomenon that I myself experienced during my tests of USB-C hubs for iPad Pro, at the beginning of 2019. Of the nine models tested, two hubs had given me problems.
Comparison: nine USB-C hubs for the iPad Pro
How can a USB-C hub block an entire local network? It’s a combination of small factors which, taken separately, are not bothersome, but which can lead to this major bug. Only USB-C hubs that rely on the Power Delivery standard to transmit the energy supplied by an AC adapter are affected, because they remain active at all times, even when they are not plugged into a device. Those that incorporate an Ethernet socket can also leave it permanently active.
Since a USB connection is not always 100% reliable, all hubs and adapters should have a mechanism in place to avoid losing data in the event of a brief connection interruption. Technically, the best option is to add a ” buffer », a buffer memory that takes over during these cuts, but this adds complexity and cost to the product. This is why hub manufacturers often opt for other methods, among which the pause mechanism for Ethernet.
To put it simply, the Ethernet specification provides a “pause” command which interrupts the reception or sending of new data for a short time. USB-C hubs can use it to avoid data loss on the Ethernet side: when the link with the computer is broken, a pause is requested and data will no longer be transmitted by mistake as long as it is the case. But while it works for intermittent blackouts, it’s more troublesome when the hub is still powered without being tethered to a computer.
In this case, the USB-C hubs constantly send the pause command to the local network, which is still not enough to block it entirely. For this, we must add a final factor: the DHCP router, the one that assigns IPs to all local devices – the ADSL or fiber box by default – is also involved. Not all models correctly respect the Ethernet specification and in particular, some models can transmit the pause command to all the devices present on the local network, whether they are connected by wire or Wi-Fi. The entire network is then paused and all devices are inaccessible as long as the hub remains powered without being connected to a computer.
This unfortunate situation is not recent and it is still relevant. There are still many products that are affected by this bug and it is difficult to know in advance if this will be the case for a particular model, especially since even recognized brands are affected.