I didn't say I considered http and TCP to be part of the same level, I consider parts of them to be part of the same level (e.g. the basic nature of requests, like the GP was talking about)

Full use of xhr needs some (but not complete) understanding of http and TCP because the abstraction isn't a perfect one, nor is it supposed to be.

However, full understanding of HTTP/TCP can be very helpful, even if not necessary.