I recently read Invariants: A New Design Methodology for Network Architectures. This paper defines an "invariant" as a property of a design that limits backwards compatibility, and it contrasts "explicit" invariants, which are designed interntionally, from "implicit" invariants, which are unintentional.
The idea of an implicit invariant is especially interesting. When a design fails to address a need from its users, they will use the system in ways that the designers did not intend. For example, port numbers were intended for the simple purpose of multiplexing connections; however, well-known port numbers are now built in to the logic of firewalls and routers.
As the authors acknowledged, the approach is fairly early, but it definitely seems like an interesting way to think about architecture.
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