ICMP (eg: ping), but not TCP. I set it up so it will load as much as it can, then provide an error if it cant load the next module/protocol (rather than a hard crash).
I spent the rest of the weekend trying to get a simple interface programmed for the application. Unfortunately, MPW versions 3.0 to 3.5 do not seem to be able to generate menubar or window resources that a Macintosh with 68K ROMs can use. The menus are there, but they just show up as white blocks (see screen shot above). I don't think I can figure out a solution in time, but at least command-q actually quits the application now. Good enough.
Mini vMac has been great for testing just to make sure the program basically works. I will spend the last few days testing on my real Macs and getting a download together with all of the required licensing info for PPP and lwip.