## 3 Problem Description and Proposed Method  
The overall structure of our experimental evaluation is based on the generic architecture of ONOS/SDN-IP implementation [35] as shown in Figure 1 and generated the experimental output of our research use case using Mininet as shown in Figure 2. All the external networks, that is, external ASs, are supposed to be the legacy networks, directly connected with backbone transit AS, which is SDN based. For routing purposes, the Quagga routing suite [36] was used, in which BGP was implemented. For SDN integration with a legacy network, the ONOS controller was configured to implement BGP and SDN-IP with reactive routing enabled. A complete legacy network was created as shown in Figure 3, where BGP and OSPF were used for routing purposes. Each router is a virtual machine loaded with BGP and OSPF configurations. In Figure 3, r5 is a route reflector router [37] and r4, r3, r6, r7, and r8 are its clients, whereas r4-r2 and r3-r1 are BGP peers. Figure 1: Use case network of SDN integration with legacy networks. Figure 2: Experimental topology of SDN integrated with legacy networks. AS, Autonomous Sysgem, BGP, Boarder Gateway Protocol, SDN, Software-Defined Networking. The SDN integrated legacy network and pure legacy network were chosen in the present study to analyze network performance. For comparison between them (hybrid SDN and legacy network), a distributed internet traffic generator (D-ITG) [38], a Packet InterNet Groper (_PING_), lperf, and statistical rules were applied. A varying number and size of packets were created and transmitted from one host to another in each network, and the corresponding delay, latency and total packet transmission were observed. The _PING_ utility was used to check the network connectivity between hosts. In both networks, first, _PING_ tool was used to ensure total communication in the network; then it was used for finding out RTT. We used _IPERF_ tool to determine the maximum bandwidths offered by the network. The compatibility and performance of SDN integration with the legacy network for multi-domain routing purposes were clearly depicted by the statistical values and comparison techniques required for SDN and the legacy network. Mininet emulated the hosts as a Linux machine. ONOS, running on the same machine, was used as an SDN controller. The experimental setup of this study is depicted in Figure 4. The experiment was executed in a virtual environment for which we used Oracle VirtualBox as the hyper-visor. The guest operating system was allocated with 4 GB of base memory and two processor cores. The experiment was carried out on the Ubuntu 14.10 (64 bit) operating system. The network topology file used Mininet to create routers, hosts and OpenFlow switches. Quagga version 0.99.23 was configured in the routers for BGP routing in SDN, while OSPF and BGP were configured for the legacy network. Routers and hosts received data routed through the OpenFlow switches. The ONOS (version 1.2.1 user) interface provided the visualization of the network. DITG, PING, and IPERF tools were used for the data collection. The collected data were analyzed and visualized using the Python Matplotlib module. A brief descriptions of the APIs enabled over ONOS are follows. * **OpenFlow**: It is the southbound protocol in the SDN control plane. It enables communications between the data plane and the control plane devices so that the control plane deploys flow rules in the OpenFlow table of the data plane and provides a decision for every packet incoming to the data plane devices. * **Config**: This application is activated for loading the network configuration. * **ProxyARP**: It is the process of resolving the address of a host. It responds to address resolution protocol (ARP) or neighbor discovery protocol requests on behalf of a target host if the target host is known to ONOS (i.e., it is present and active in the host store). ProxyARP considers the network to be a single IP network, therefore, if it receives  
Figure 3: Use-case of legacy routing network.