CST311 Week 6

The sixth week of CST 311 focused on the network layer’s control plane.

The control plane is one of the two halves of the network layer. The first half, the data plane, is concerned with forwarding datagrams at routers / packet switches. “Forwarding” is the act of moving a datagram from a router’s input interface to a corresponding output interface. The second half of the network layer, the control plane, deals with the overall route a datagram takes between its source endpoint and its destination endpoint.

Every router on an autonomous system (AS) runs the same routing protocol. In the traditional control plane model, routers individually execute the selected routing algorithm, and somehow coordinate between each other to populate the routing and forwarding tables of every router in the AS. Each router then individually uses its forwarding table to make forwarding decisions.

An alternative to this monolithic control plane is a software defined network (SDN). In essence, SDNs impose a clear separation between the data plane and the control plane: routers simply forward datagrams based on the rules in their forwarding table – they do not populate the forwarding tables themselves. Instead, a separate machine on the network, called an SDN controller, is responsible for populating each router’s forwarding table. Decoupling routing policy from the mechanism of forwarding allows for tremendous flexibility. Routing decisions in an SDN are made by software on the controller, instead of the traditional, rigidly defined routing protocols. It is evident that providing software control over an otherwise low-level mechanism results in solutions that are more nuanced and effective at fulfilling specific network needs.