Introduction to the network layer (control plane)

The network layer control-plane is the logic responsible for deciding:

• how datagrams are routed
• how network-layer devices and services are configured and managed

Elements linking the data and control planes are:

• a forwarding table (for destination-based forwarding)
• a flow table (for generalized forwarding)

Architectures

There are two main ways to compute and install these tables:

1. traditional per-router control:
   • each router runs its own routing algorithm
   • routers exchange routing information with one another
   • every router contains both a forwarding function (sending packets) and a routing function (computing routes)
   • protocols such as OSPF and BGP use this approach
2. SDN (Software-Defined Networking) control:
   • a logically centralized controller computes routing information for the entire network and distributes it to routers
   • each router contains a control agent (CA) that:
     • communicates with the controller
     • receives configuration updates
     • configures and manages that router's flow table

In traditional routing, routers communicate directly.

In SDN, routers do not communicate with each other to compute routes. Instead, their control agents communicate only with the centralized controller.