Starting at Layer 7 is a fundamental mistake, because it perpetuates the execrable and obsolete OSI 7-layer model. Since IP protocols DO NOT map cleanly to the OSI model, trying to shoehorn IP into it causes more misunderstanding than anything else I've seen in networking. Seriously - IGNORE the 7-layer model if you want to understand real-world protocols and their design, especially anything IP-flavored.

(BTW, David Clark, one of the inventors of IP, told me over beers at a long ago Interop that the only reason there are 7 layers is because ISO arbitrarily set up 7 subcommittees to study the problem, and when they couldn't agree on where to draw the lines between the layers when they reconvened, they just made the diplomatic compromise of sticking with the subcommittee boundaries. That's why the OSI model so poorly reflects real world protocol implementations. AFAIK, other than the OSI protocols themselves (MAP/TOP, etc.), which unwisely used the model as an implementation guide, the only protocol to cleanly map to the OSI model is X.25. Does anyone actually use any OSI protocols anymore?)

> Does anyone actually use any OSI protocols anymore?

OK, here's my conspiracy theory:

At a certain point, the OSI people knew they were losing. However, they were the people in charge of the big institutions, so they made a push to ensure that history books would be re-written such that, well, of course we all use the OSI Model, of course the Internet people implemented OSI, there was never a debate, don't be silly! Eastasia and Oceania have always been at peace, and that's why, here in Airstrip One, you need seven layers to describe how anything which moves over a network works!

... and the fact the OSI Model was attached to actual protocols is silently forgotten, as is the fact the ARPANet/Internet people were mildly opposed to "layers" as a conceptual model and never took the OSI Seven-Layer Model as a design document!