## 3 Synopsis of KDN Paradigm
### Detailed KDN Architecture
#### 3.2.4 Control Layer  
The control layer, which can consist of multiple SDN controllers determined by the control model paradigm, is in charge of transmitting to the infrastructure layer forwarding rules, authorization rules, rules for prioritizing data flows based on quality of service, etc. Program guidelines and dynamic rules or perceptions derived using the knowledge layer are both used to drive control in the KDN [207], as shown in Figure 7.  
The integrated control layer and application layer may communicate with one another thanks to the northbound interface, which can be implemented using an ad hoc [208], RESTful [209], intent-based [210], or language-based API [211]. On the other hand, using protocols like OpenFlow, ForCES, OpFlex, and others, the southbound interface serves as a bridge between the infrastructure layer and the control layer. It is used to transmit raw facts from the data forwarding components to the controller and to convey flow rules from the control layer to the infrastructure layer equipment [202]. Additionally, in order to have an overall perspective of the network, east-westbound interfaces like ALTO [212], Hyperflow [213], ONOS [210], Onix [214], etc. allow communication between the physically scattered controllers.  
Centralized control, decentralized/distributed control, or mixed/hybrid/combined control models are all possible in KDN. NOX [208], Trema [215], Ryu [216], Meridian [217], and other controllers maintain the conceptually and physically centered controller design, but SMaRtLight [218], HyperFlow [213], ONOS [210], Onix [214], Kandoo [219], Orion [220], and other controllers retain the conceptually centralized and physically scattered architecture. On the other hand, mixed control architectures such as DevoFlow [221], Fibbing [222], HybridFlow [223], etc. incorporate a blend of completely centralized and fully scattered designs. Moreover, controllers like Distributed-SDN Controllers (DISCO) [224], D-SDN [225], Cardigan [226], etc. represent conceptually and physically scattered controller paradigms. Note that either complete or partial consistency exists among the dispersed controllers.  
OpenFlow-enabled packet control provides the maximum level of control resolution. On the other hand, there are benefits to rough-grained flow control, such as decreased costs for control layer communication, that uses packets in the form of a flux of numerous packets, like traffic flows, depending on quality of service.  
Reactive control approaches cause modifications to the network in response to flows or happenings. On the other hand, the controller pre-computes the switching components with a collection of rules when utilizing proactive control to manage all potential streams of traffic prior to the traffic even reaching the switches.  
One of the controller's primary operations is to figure out the ideal route for data streams (path computation) and traffic optimization, which involves improving the traffic fluxes in order to boost the network's efficiency with the help of the gathered data. Additionally, the control layer may gather raw facts such as traffic information, QoS data, regulations, security incidents, protocols used for routing, etc. [227]. Moreover, by implementing a policy engine, the control layer may instruct network gadgets to perform specific tasks when specific requirements are satisfied, executing policies, as shown in Figure 7. These network guidelines are transformed into rules using a policy orchestrator (engine) by taking into account additional information, other rules, and perceptions from knowledge [228].  
Network data are routed via a network service sequence using flexible/agile service chaining, where the controller selects the services to be included in the chain depending on changing network circumstances [229]. Furthermore, the controller may also flexibly build simulated networks, such as private virtual networks, and scale them depending on dynamic network requirements [230].