# Evolution of Access Network Sharing and Its Role in 5G Networks
## 4 Software Defined Network (SDN)
### Multi-Tenancy in SDN Controllers  
The first attempt to enable some level of multi-tenancy on an OpenFlow-based control plane was the FlowVisor implementation [54]. FlowVisor is a transparent Proxy Controller that slices resources and provides data forwarding programmability to the tenants. FlowVisor, however, did not enable network topology abstractions, e.g., for path splitting and path migration functions [55]. Thus, it only partially supported network virtualization. [PERSON] et al. [56] have proposed and analyzed three control plane slicing methods (switch, port-wide and domain) implemented by the management plane, which safeguards control plane isolation among tenant virtual networks. They have introduced a Flowspace Slicing Policy (FSP) rule engine, which is an automated mechanism for translating substrate management plane policiesto virtual network mapping control plane rules. Their work complements that of the FlowN [57] technique by introducing slicing methods that are sufficiently generic to permit enforcement of Flowspace isolation policy for large-scale topologies. Figure 4 shows that Flowspace delegation in SDN can be executed using an intermediate control plane slicing layer. The Flowspace slicing policy is designed according to the control plane slicing method, which is an algorithm that guarantees the creation of non-overlapping Flowspace rules. A Proxy controller (e.g., FlowVisor) routes control messages transparently. The other option can be based on the network hypervisor. In this architecture, the control plane consists of a slicing layer, a virtualization layer and a tenant layer. The virtualization layer incorporates virtual network mapping algorithms. In contrast, in the proxy controller architecture, it is the tenant layer of the hypervisor that co-hosts the forwarding logic of the tenants. One of the practical problems in multi-tenant switching architectures is that of packet classification. Shared infrastructure controllers typically use L2 or L3 header fields as slice identifiers. However, using packet IDs to classify data-plane Flowspace slices can fail if the number of slices is above the range of packet ID values. To tackle this scalability problem, [PERSON] et al. [56] have considered logical separators that are also linked to data-plane resources. They have experimented with the domain, switch and port-wide slicing techniques. Their experiments focused only on two architectural approaches for control plane virtualization in OpenFlow enabled multi-tenant SDN domains. Their experiments demonstrate that port-wide slicing technique is most efficient in terms of tenant request acceptance ratio within acceptable memory consumption and control-plane delays. Another approach targeting scalability in hypervisor design is that presented by [PERSON] et al. [58], introducing a virtualization layer that automates the deployment and operation of SDN slices on top of a shared network infrastructure. They have named their technique as AutoSlice (Automated and Scalable Slicing for SDN). The proposed hypervisor consists of the management module and multiple controller proxies. Scalability is ensured through the development of auxiliary software data paths (ASD) in the substrate network. The physical substrate is segmented into multiple SDN domains and has assigned a dedicated controller proxy. Each controller proxy transparently manages access to corresponding domain switches. The work from [PERSON] et al. [59] has further extended the concept of network and control plane virtualisation, by moving the SDN controller to the cloud for dynamically deploying independent SDN  
Figure 4: Proxy Controller-based architecture. controller instances. This process takes a few minutes to deploy a new virtual optical network instance. This represents a considerable advantage compared to the several days that it would otherwise take to manually install and configure a SDN controller on a dedicated server.