## 1 Introduction  
Operability, maintainability, and network addressing issues in the legacy IPv4 networks are being investigated with the emergence of next-generation networking paradigms like software-defined networking (SDN) and Internet Protocol Version 6 (IPv6) [1]. The network addressing and routing issues are being undertaken by IPv6-enabled networking, while the operational complexities of the vertically integrated legacy network can be minimized by the implementation of SDN. IPv6 and the SDN are not backward-compatible, leading to complexities and challenges in the real-time migration of the legacy IPv4 networking system, which is currently operating world-wide. Network migration is inevitable for service providers, though there have been several challenges with migration from the technical, financial, as well as business perspectives [2]. The higher costs of network migration to replace or upgrade network devices, transformations in software systems and related services, along with a lack of skilled human resources to operate and maintain the newer networking systems are becoming the major hurdles of technology migration in service provider networks. Internet service provider (ISP) networks consist of multiple autonomous systems, and are generally heterogeneous in nature. Hence, multi-domain heterogeneous network migration planning is a challenge. The solution of IPv4 address depletion was devised more than three decades ago in the 1990s with the introduction of IPv6 addressing, also termed as \"IP next generation (IPng)\" [3]. However, the global IPv6 adoption rate is not satisfactory [4]; it has just crossed 25% adoption worldwide, while the adoption rate in developing countries is still below 1% [5]. The early stage of IPv6 network migration and the premature stage of SDN migration [2; 5] in telecommunications (Telcos) and ISP networks indicate that sufficient efforts in the research and development of SDN and IPv6 network migration are needed. IPv6 and SDN are interrelated, because IPv6 deals with routing and addressing in the IP layer, while SDN deals with the controlling of the operations of the networking management layer. IPv6 avoids the IPv4 routing and addressing issues, while SDN avoids the management and controlling issues of legacy networks. These two networking paradigms, which are recognized in the network operation layer, are operated by service providers. Hence, in our previous works, we introduced a joint SDN and IPv6 network as an SDN-enabled IPv6 network called the software-defined IPv6 (SoDIP6) network, with its benefits and challenges for efficient migration planning [1; 2; 6]. Combining the features of IPv6 and SDN creates more robust networks. Those combined features [2] are more attractive for service providers for the timely migration of their legacy networks. For migration cost reduction with optimization of the capital and operational expenditures (CapEX and OpEX), the joint migration planning of SDN and the IPv6 network is beneficial for service providers [7]. Well-defined migration methods, progressive migration scenarios, sufficient applications, and protocol standards are supportive for ISPs to take decisions for IPv6 migration, but there is still a lack of standards and applications towards bringing SDN implementation in ISP networks close to reality. However different transition techniques have been defined; dual-stack IPv6 and hybrid SDN approach [8; 9] are the more appropriate solutions for smooth and gradual transition to IPv6 and SDN. The evaluation of production standards under Hybrid SDN require deep investigation. With the implementation of multi-domain SoDIP6 networks for service providers, the controller placement problem during and after the migration is a major concern. Increasing the number of SDN switches during migration affects the controller capacity. To address the problem of increasing control traffic with the increase of SDN switches, a suitable approach is needed to locate the controller and add other controllers in the network for load balancing.