Yep, and TCP is L4. So TCP/IP is L4/L3. The usual criticism of the OSI model is that there are grey areas and dependencies, where a lower layers bleed into hig

Yep, and TCP is L4. So TCP/IP is L4/L3. The usual criticism of the OSI model is that there are grey areas and dependencies, where a lower layers bleed into higher layers (e.g. L2 switching which can work a lot like routing in modern hardware), and higher layers that are tightly bound to lower layers, making the distinctions unclear. The purist in me wants to agree, but I think the model is too useful to disregard casually. The value comes from the abstraction, and like Newtonian physics, it is a great model -- until it isn't.