We present a functional approach, based on the ACL2 logic, for the specification of system on a chip communication architectures. The functional style allows the method to be compositional for both system definition and validation. When performed in the context of the ACL2 logic, all the definitions and theorems are not only reusable, but also constitute an executable and proven valid specification for the system. We illustrate the approach on a state of the art network on chip: the Octagon. We prove that messages travel over this network without being modified and eventually reach their expected destination.