How layering protocol software violates separation of concerns

We show how layering in communication software violates the principle of separating concerns. Examples illustrate that
_ layering couples receiver-and sender functions,
_ sharing message headers among layers creates dependencies between layers that largely compromise re-usability,
_ QoS management is scattered over all layers,
and layers are tightly coupled with the sub-system that executes it.
Layering hence leads often to large and hardly re-usable modules, tangles possibly independent parts of code, and makes implemented protocols difficult and error-prone to extend.