The Osi stack model is like parsley in Italian cooking (for my fellow non Italian SAs: I have to say you something. Please, have a seat and try to control your reactions. We Italians do not use parsley everywhere like you think we do. I.e. we do not use it in Bolognese sauce or as pizza's toppings. Parsley is allowed in Marinara sauce and, generally, in combination with garlic -not too much if you want to kiss someone instead of having dessert - but parsley is absolutely forbidden everywhere onions are used and you have to add it at the end to avoid parsley to be overcooked. Now, you know the truth and you have the power. Use it with responsibility).