## 5 Discussion and Future Work  
Virtualization capability of SDN enables powerful APIs, which can be used to control many network functionalities with intelligence. The lack of backward compatibility of SDN with the legacy network enforces research communities to develop a robust system for interoperability between SDN and legacy networks for smooth and low cost migration approaches. Hybrid SDN is the only solution for having a smooth transition to pure SDN for service providers so that customers can receive uninterruptible services during the migration period. SDN-IP is an ONOS application that is used to peer SDN networks with external networks on the Internet using the standard BGP so that service providers can run hybrid SDN in their existing network during the transition period of their network migration. The SDN-IP controlled network acts as a transit AS that interconnects different legacy IP networks, considering each external network as a different AS domain, and interfaces with the SDN network through its BGP-speaking border routers. In this study, an SDN integrated network was created with Mininet and experimental tests were carried out to ensure successful communication between the hosts in different ASs. The output of the tests showed a smooth transmission of data between hosts, thus providing testament to the possibilities of interoperability between two different networking paradigms. A similar legacy network was also created and observed to note the difference in the characteristics of an integrated SDN and legacy network. To give statistical significance to this experiment, correlation coefficients of the QoS parameters were determined. These values can be further utilized with artificial intelligence  
Figure 21: Routes learnt by reflector router (r5) in the legacy network. Figure 20: Routes Learnt by ONOS. technologies or to develop machine learning models or rules about the network. These insights can be useful in wide areas of applications such as traffic classification, routing optimization, quality of service and quality of experience prediction, resource management, security enhancement and many other purposes. The comparison between legacy and SDN integrated networks aimed at verifying the speculations about SDN performing better than legacy networks. By studying both the networks based on key performance indicators, such as bandwidth, packet transmission rate, and round trip time, the performance of the SDN network was observed to be much better than its legacy counterpart. Similarly, the exploratory data analysis of the values of the QoS parameters collected from the network provided us with insights about the network. The scope of this experimental analysis was limited to an IPv4 addressing network only. A routing performance analysis with the networks enabled with IPv6 addresses could further help to establish the significance and prospects of SDN implementation with future networking technologies. Additionally, this study was limited to comparing the performance of hybrid SDN and a legacy network to determine the feasibility of the migration of the legacy networks into SDN. We read some of the literature related to SDN routing approaches [22; 23], already mentioned in Section 2, since we are particularly dealing with routing in hybrid networks. Reference [23] deals with dynamic routing algorithm implementation in a pure SDN environment, while the article OSHI [22] considers hybrid switch networks. However, our scenario of routing implementation does not particularly match with the concepts provided in [22; 23]. OSHI [22] considers hybrid or dual stack switch, that is, legacy routing and OpenFlow implementation, in a single switch. The dual stack switch consumes a higher load as compared with a single stack switch because it has to handle dual packets routing/forwarding at once as well as a separate access control list needing to be maintained.