There is described a method for mapping data representing an instance of a service model for a network onto policy information for controlling one or more Software Defined Network (SDN) switches in the network. The instance of a service model comprises a topology of one or more network functions and at least one virtual network function and a data flow through the topology, and the at least one virtual network function defines a dynamic flow control policy in dependence upon which data flow through the virtual network function to or from one or more of the network functions is selectively controlled. The policy information is provided to one or more SDN control entities for controlling the one or more SDN switches in the network so that data flows through the network in accordance with the service model instance.