owever, VPN is less capable of supporting hard QoS isolation because QoS isolation on the user and control planes requires better coordination with the management plane. The flexibility behind the network slicing approach needs to address QoS guarantees in the transport networks and at the same time enable transport network openness because future network design is targeted to provide end-to-end QoS guarantees per network slice. 3.7.2 Software-Defined Networking and QoS According to the ITU [27] and also the IETF [28], SDN is a set of techniques that enables the direct programming, orchestration, control, and management of network resources which facilitate the design, delivery, and operation of network services in a deterministic, dynamic, and scalable manner. SDN is a framework which already has a defined layering framework (Figure 3.14) [27] - convenient for interpretation and standardization of different SDN techniques that exist today and those that will appear in the future. According to the SDN layered architecture shown in Figure 3.14, there are three main layers: SDN resource layer. This includes data transport and data processing by network devices [28], and hence includes forwarding and operational planes [29]. SDN control layer. This does resources abstraction, control layer services, and appli- cation layer services, including management support and orchestrations in this SDN control layer [28]. The control abstraction is done in the control plane while the man- agement abstraction layer belongs to the management plane [29]. SDN application layer. This includes all SDN applications, where the term "SDN application" refers to applications that control and manage network resources and not applications or services used by end-users. One example of an SDN application is user application-specific routing in the SDN-enabled network. 3 QoS in NGN and Future Networks SDN application layer Application plane SDN applications Application layer Services application (App) management support and orchestration appl