I'm not sure what protocol would be running over the USB2 interface - maybe
PPTP - equivalent to some of the dial-up protocols.
A small note: PPTP works on top of IP, it is a VPN protocol.
It encapsulates PPP protocol (which I believe you were talking about).
PPP can run IP over point-to-point connections like VPNs or serial/com ports (connected either directly or with dial up modems).
So, it should be possible to run PPP-over-usb with a usb serial converter.
There are ethernet adapters for USB, but not of all of them are supported by Linux