EN> - (an instance of) the IP protocol processing on a node EN> - a IP service access point (i.e. the point where the EN> transport protocols attach to IP) EN> - a transport service access point EN> - something higher up in the protocol stack
And, yes, the fact that I am not giving you a more concrete response is intentional. I've offered some specific text. I want others to fix it. (Or, rather, I can't come up with anything better...)
I think it will serve us well to _start_ with reference to a local instance of an application, and then derive text back from it. Focusing only on the lower layers causes us to miss what is most interesting and useful, I fear.