Message router overview of concrete cases

Continuing the discussion from Message Routing Design:

@tg-x could you see yourself making an overview like this and would you have time? With all the concrete cases? It would help us out a lot in implementing the MR. As it is now it costs me a lot of cognitive effort when I’m reading the prose to keep track of all the cases and srcs and dsts and vias and encapsulated messages and so on.