If the application layer is required to take notice of what is going on at
the lower levels its because the layering was botched.
no, it just means that all lower-level network services weren't designed
to provide exactly the same set of services and semantics to higher
layers.
in the IP stack, we expect link layers to be diverse, we expect IP
service to be fairly uniform, precisely so that things layered on top of
IP don't have to worry about it. expecting all applications to support
multiple transport protocols adds complexity where it is almost never
needed.
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