In our discussion thus far and in Figure 4.6, we’ve implicitly assumed that the routing control plane fully resides and executes in a routing processor within the router. The network-wide routing control plane is thus decentralized—with different pieces (e.g., of a routing algorithm) executing at different routers and interacting by sending control messages to each other. Indeed, today’s Internet routers and the routing algorithms we’ll study in Section 4.6 operate in exactly this manner. Additionally, router and switch vendors bundle their hardware data plane and software control plane together into closed (but inter-operable) platforms in a vertically integrated product.