> It is kinda fun to work directly at IP level and not use TCP or UDP at all.

Possibly dumb question: if you're not using ports (because you're not using TCP at all), what does routing look like, in terms of this article? How does the kernel decide which traffic goes to application A and which traffic goes to application B? Or is all traffic directly visible to each application, if they aren't bound to a port? Can you bind to a specific IP, or just to an interface?