The USB protocol was simple by design, so it could be implemented in small dumb devices like pen drives. More specifically, it used two couples of cables, one couple was for power and the other for data (four wires in total). Having a single half-duplex data line means you need some way of arbitrating who can send data at any time. The easiest way to do it is having a single machine that decides who gets to send data (master), and the easiest way to decide the master is to not do it and have the computer always do the master. This means you couldn’t connect two computers together because they would both try to be the master.

I used the past tense because you may have noticed that micro USB have 5 pins and not 4, that’s because phones are computers and they use the 5th pin to decide how to behave. If it’s grounded they act as a slave (the male micro to male A cable grounds it). If it has a resistor (the otg cable has it) it act as master. And if the devices are connected with a wire on that pin (on some special micro to micro) they negotiate the connection.

When they made usb 3.0 and they realized that not having the 5th wire on the usb-A was stupid, so they put it (along side some extra data lines) that’s why they have an odd number of wires. So with usb 3 you can connect computers together, but you need a special cable that uses the negotiation wire. Also I don’t know what software you need for it to work.

Usb-c is basically two USB 3.0 in the same cable, so you can probably connect computers with that. But often the port on the devices only uses one, so it might not be faster. Originally they put the pins for two connections so you could flip the connector, but later they realized they could use them to get double speed.

We can connect two computers over USB and we do it all the time. E.g. your phone and your PC. One or both of the computers needs to be able to act as a device. Most Android computers can switch between host and device depending on what’s plugged in. It all comes down to implementation.

I found some directions that might help.

Enabling USB-C OTG Device Mode :
Ensure the Linux device has a USB-C port that supports OTG functionality.
In the device tree, set the dr_mode property of the USB OTG controller to “peripheral” or “otg” to enable device mode.
Configure the TUSB320 USB-C controller (or equivalent) to operate in UFP (Upstream Facing Port) mode, which allows the device to act as a USB peripheral.
Configuring USB Gadget Drivers :
Load the appropriate Linux USB gadget driver for the desired functionality, such as g_ether for Ethernet over USB, g_serial for a serial device, etc.
Manually configure the USB network interface, such as assigning an IP address to usb0.
Connecting to a Host :
Use a USB-C to USB-C or USB-C to USB-A cable to connect the Linux device in OTG device mode to a host PC.
The host PC should then detect the Linux device as a USB peripheral, allowing file transfer, network connectivity, or other functionality depending on the configured gadget driver.

Gateworks.com Wiki Linux OTG
Kernel.org Driver-API USB Gadget
Collabora Blog Modern Linux USB Gadget integration with Systemd Part1

A tool : gt
Rust library : usb-gadget
C Library : libusbgx

Google “connecting 2 computers with USB”

If you have two PCs with USB ports, you can connect them to one another using a special type of USB cable called a “bridging” cable. You can also technically connect two Macs via USB, but you’ll need to add a USB-to-Ethernet adapter and Ethernet cable to the mix.

There are also articles about it:
https://hardwaresecrets.com/connecting-two-pcs-using-a-usb-usb-cable/

We do it routinely with our phones (some of us), I admit IDK why it can’t be just as easy between 2 computers?

Because they’re both hosts, you need a host and a client iirc.

With usb-c you should be able to load a driver that allows network connectivity regardless of otg mode. Or was it Thunderbolt?

Update: I thought of thunderbolt-net which works with Thunderbolt 3 and probably USB4

Because USB builds a tree, with the USB-controller as stem.

Isn’t there something like that for ethernet?

In general, because what everyone thinks of when they say USB is a host-device protocol, and you’re trying to connect two hosts together.

But USB is a blanket term for a number of protocols, and there are protocols that allow a host to switch roles and become a device, depending on what it’s connected to.

If you see a PC being advertised with “dual role USB ports” or support for “USB-OTG”, then you can connect it to another PC. Otherwise, you almost certainly can’t.

If all you want to do is transfer files, you can use something like QuickDAV to transfer files over your local network.

You can’t connect a steam deck to your PC?

Universal Serial Bus.

Serial communication between two hosts requires either robust auto negotiation or a crossover cable.

Usb doesn’t have good auto negotiation between two hosts so you gotta use a crossover cable.

heres one

But… there’s no guarantee that the computers usb controllers and operating systems will support it.