## 5 Key Enablers
### Software-Defined Controllers for Edge Cloud
#### 5.3.7 Tungsten Fabric Controller  
Tungsten Fabric (TF) is an open-source multi-cloud, multi-stack SDN controller that is responsible for providing the management, control, and analytics function for virtualized, containerized, or bare-metal networks [72]. The simplified architecture of TF is shown in Figure 13.  
Figure 13: Architecture details of Tungsten Fabric and working principle [72].  
The TF architecture is composed of two basic components, controller and vRouter.  
TF Controller: It is an SDN controller that has three key components--configuration, analytics, and control. Moreover, controllers use northbound REST API to communicate with the orchestrator, GUI, and applications, and southbound BGP, Netconf, and XMPP to communicate with the vRouter and underlying network. The configuration module is responsible for configuring network topology. The configuration module can also be implemented using a policy-based or intent-based approach. The control module is responsible for communicating between vRouters, and underlying network devices. It exchanges routes to ensure the consistency of the network state. It is also responsible for steering the data flow and sending the traffic flow configuration to the vRouter. The analytics component collects real-time network data such as logs, events, flow statistics, status of the network elements, and debugging data, and analyzes and corelates them for trouble shooting and better management of the network.  
vRouter: It is installed in each host that runs the workload to provide integrated routing and forwarding between virtual machines, containers, and external networks. It also enforces network and security policies. Two key components of vRouter are agent and forwarder. The vRouter agent is a user-space process that maintains XMPP sessions with the controller. It exchanges routes and receives forwarding policies with control components and reports logs, statistics, and events to the analytics component of the controller. The vRouter forwarder performs packet processing based on flows received from the agent. It drops the packet or forwards the packet to the local virtual machine or encapsulates it and sends it to another destination.  
Tungsten Fabric (TF) can provide seamless networking between VMs and containers deploying in an OpenStack environment. Moreover, its features such as openness, multi-tenant networking, efficient edge replacement, distributed load balancing, and service chaining could make it a potential framework for edge cloud network deployment [72; 73].  
Several industry examples on edge computing platforms, including AWS Wavelength and Azure Local, incorporate software defined network (SDN) features to support the delivery of low-latency applications at the network edge. Azure Local utilize SDN controllers to create and manage virtual networks, enforce Quality of Service (QoS) policies, and ensure dynamic load balancing [74; 75; 76]. Similarly, AWS Wavelength Zones are deployed within telecommunication environments, where an SDN is employed to facilitate dynamic traffic routing and optimize latency-sensitive communication within localized geographic regions [77; 78]. In the following section, we discussed several challenges and potential future research directions that need to be addressed in an SDN-enabled edge cloud environment.