ication Network services abstraction layer Service interface SDN control layer Orchestration Application Control plane Management support plane Control layer Services Services management Control layer support and services orchestration Control Management Resource abstraction layer abstraction layer abstraction Control plane interface Management plane interface Network slicing SDN resource layer Resource layer Device and resource abstraction layer Resource layer control support management support Forwarding Operational transport processing plane plane Network device ITU's SDN architecture SDN architecture by IETF App - SDN application IETF - Internet engineering task force ITU - International telecommunication union SDN - Software defined networking Figure 3.14 SDN layered architecture. Overall, SDN spans multiple planes [29]. The forwarding plane is responsible for handling the packets based on instructions from the control plane. The control plane includes all signaling messages, which are pushed to the network devices with the aim of implementing decisions about how the packets should be forwarded (in the routers, switches) in the forwarding plane. While the control plane focuses mainly on the forwarding plane of network nodes, the management plane focuses on operational aspects of the network (i.e. on the operational plane), which include monitoring, configuring, and maintaining various SDN-capable network devices. For example, one of the most popular protocols in the SDN research phase, the OpenFlow, is intended for use in the control and management planes [29]. Finally, the application plane is where SDN applications and services reside, where such SDN applications/services directly or indirectly define network behavior. Such applications can be developed in modular manner and can span multiple planes in the SDN architecture. Generally, SDN is based on separation of user plane (i.e. data plane) and control plane in the network. That also provides the possibility to enhance QoS provision in the net- work via 