## 2 Related Work  
The higher costs of CapEX and OpEX investments with technical human resources (HR) development are the major hindrance for ISPs trying to implement technology migration. There are also certain implementation challenges with respect to management, availability of technological standards, and user interface provision while providing new technology-based services to customers during and after the network migration [1; 10]. The replacement of existing networking devices or their hardware/firmware upgrades are the only options for network migration. For the optimization of migration cost, the upgrade of network devices is economically feasible compared to replacement. We presented an approach for network migration planning with migration cost optimization via simulations and empirical analysis in our previous works [2; 7; 11], where we focused on skilled HR management, technical issues like hardware/software standards and protocols, and managerial issues like proper scheduling and migration planning for network upgrades. SDN migration in Telcos/ISP networks is a central research focus; however, there are few contributory works attempting to provide the paths for the real-time transition of legacy networks into SDN. The article [10], however, is not concerned with the legacy network migration to SDN; an example use case is well-discussed regarding cost-effective virtual private network service provisioning over an SDN-based federated network with challenges and solutions in providing services over an SDN environment. HARMLESS [8] is an approach that simply patches the OpenFlow soft-switch over the conventional Ethernet switching devices. It is encouraged for the cost-effective deployment of SDN in an enterprise network. For the fairly sustained service providers, those legacy switches that are identified as capable based on their performance for high-speed communication can be upgraded to SDN switches using the HARMLESS concept. This supports incremental deployment towards an SDN mixed network as a part of hybrid/SDN deployment. However, from the overall network operation perspective during migration, the interoperability between legacy IP routing devices and upgraded SDN switches has not been ensured. OSHI [12] is a prototype developed to integrate legacy IP routers with SDN switches in ISP backbone networks. This concept emerged with the operation of dual-stack SDN devices like those of dual-stack IPv6 networking, in which the hybrid IP/SDN node performs legacy routing as well as data plane forwarding. Panopticon [13] is an another proposed approach for the incremental deployment of SDN in the legacy networks. It makes it possible to have logical SDN control ports in the legacy switch. Conceptually, the SDN switch residing in the legacy networks can act as a VLAN gateway to bridge communication between legacy devices or create tunnels between legacy switches through the SDN switches in the path. This appears to have a separate protocol that the author has conceptually realized in order to manage this implementation in data plane devices. The implementation of this approach in carrier-grade ISP networks is fairly complex, and its real implementation in production networks is not known. RouteFlow [14] is a concept that simply mimics the legacy routing operations to ease the gradual migration of legacy ISP networks into SDN by implementing a RouteFlow (RF) server above the SDN controller. A mapping of an SDN network is done in the virtual environment and emulates legacy routing. The RF server provides the routing information to the SDN controller to install flow rulesin the SDN network for proper routing. RouteFlow is still not close to a real-life implementation, which needs additional measures to fix the issues related to performance and operational complexities. Fibbing [15] is an approach that attempts to abstract the distributed routing into centralized control in order to implement the features defined by SDN.