No, I meant the IP and TCP headers. Those are end-end; the Ethernet stuff is just a local wrapping, and can be substituted. Yes. There was a stage before A/B/C. See RFC-760. Basically, yes. The ARPANET supported several different kinds of interfaces between the IMPs (the switching nodes in the ARPANET) and hosts, but the 'usual' one was either 'Local Host' (LH) or 'Distant Host' (DH) which were _basically_ identical except at the very lowest level - LH was TTL, and DH was differential pair. Those interfaces were a custom bit-serial thing with a handshake (with "there's-your-bit", "ready-for-next-bit" lines, etc); see BBN Report #1822: http://www.bitsavers.org/pdf/bbn/imp/BBN1822_Jan1976.pdf So the "ARPANET interface" in the host is a piece of custom hardware (some were DMA; I also used one which was interrupt per byte) which went on the host, which talked 1822 (as it was called), of either the DH or LH physical form. (There was also an Host/IMP interface called VDH, but that used a modem, and a _lot_of software; see here: https://en.wikipedia.org/wiki/Talk:Network_Control_Program#Layer_locations for a bit more about it.) Dunno, but they shouldn't be too hard to add. The real problem is going to be 'what do you hook the simulated ARPANET interfaces up to, and how'? I know they have IMP code running in simulators: http://mailman.trailing-edge.com/pipermail/simh/2013-November/007672.html but I dunno how one would hook _that_ simulation up to a simulated host running a simulated ARPANET interface. Easier, to get this old TCP/IP running, might be to write a Unix V6 driver for an Ethernet card (one the simulators do support - I know Ersatz-11 does the Interlan NI1010A/2010A, which is nice and simple) and write an Ethernet network interface module for that TCP, which talks to said driver; i.e. just replace the ARPANET interface stuff completely. Noel