In an SDN, what is the controller's function?

In Software-Defined Networking (SDN), the controller acts as the "brain" of the network. Its primary function is to centralize control and intelligence by setting packet-handling policies. This means it dictates how network devices (like switches and routers) should forward traffic based on defined rules and network-wide objectives. Forwarding packets is the role of the data plane (network devices), not the controller. Multicast replication at the hardware level is also a data plane function. Fragmenting and reassembling packets is typically handled by network devices or hosts, not the SDN controller. The controller focuses on the control plane, making decisions and programming the data plane devices accordingly.