I was looking back over the spec and it does specifically mention Ethernet. Anyway, I'm thinking either Ethernet TCP/IP or Modbus TCP.
I'd like the information to be available to the R2 and the S7 via the scratchpad. Can you access scratchpad information via Modbus TCP? I've been able to read digital and analog I/O via Modbus without any special software. That would be the easiest solution. I would assign a scratchpad register for incoming data and another for outgoing. Then I would just read and write to those registers without dealing with the communications details.