## 2 Background and Motivation
They concluded by identifying QoS, scalability, security, and management as some of the challenges in SFC traffic steering. Although their survey is comprehensive in terms of SDN-based traffic steering in SFC, we focus our work not only on traffic steering in SFC, but also on the implementation frameworks used to achieve a scalable, resilient, and high-performance SFC. [PERSON] et al. [26] presented a review of integrated NFV/SDN architectures, where they considered implementation frameworks that combined NFV and SDN. They identified vNF scheduling and placement, improving network programmability, and the possibility of deploying multiple SDN controllers to achieve scalability, security, and standardisation of SDN/NFV solutions as some of the open challenges. We extend their contributions by considering proposals that implement NFV/SFC and the challenges that require further research. As one of the early attempts in this domain, the survey by [PERSON] et al. [34] explores the challenges faced in mobile and wireless network (MWN) environments. Their work carefully describes how software-defined wireless networks (SDWNs) and wireless network virtualization (WNV) can be used in addressing the challenges of MWN networks. A survey on SFC was presented by [PERSON] et al. [1] and [PERSON] et al. [33], where the authors focused on resource allocation approaches in the literature. [PERSON] et al. [29] presented open challenges in service function chaining for next-generation networks. A taxonomy of prior work was presented, where they classified it into data -and control-plane SFC solutions. Our survey considers a broader scope by creating a taxonomy that is beyond control and data-plane solutions, as well as presenting state-of-the-art technologies and open research challenges. [PERSON] et al. [32] presented a detailed survey of SFC, where the authors identified optimal resource allocation, dynamic service mapping, and policy enforcement, as some of the challenges in these environments. [PERSON] et al. [7] and [PERSON] et al. [4] focused on the placement of resources in SFC environments, where the former presented a survey of network function placement in SFC, and the latter focused on the placement of virtual resources. In their survey, [PERSON] et al. [22] explored the traffic load balancing approaches used in SDN network environments. Our work presents the relationship between SDN, NFV, and SFC, as well as a novel taxonomy of implementation frameworks. [PERSON] et al. [28] presented a survey of SFC and NFV implementation efforts. Some of the future research directions presented by [28] include SLA and QoS approaches, online chaining of service functions, availability and resilience of chains, security, and energy efficiency. Our survey provides a more technical look into the implementation frameworks by first classifying the state-of-the-art frameworks and the proposed solutions. The comprehensive survey presented by [PERSON] et al. [30] focus on SDN technologies employed in network environments such as the data centre, edge, access, and core networks. They present their findings in relation to IoT use cases. They identified open research challenges such as platform independence, policy enforcement, mobility management, and the fully practical implementations of SDN-based solutions in IoT environments. In their comprehensive survey, [PERSON] et al. [24] classified SDN/NFV approaches into availability, placement, and load-balancing solutions. They identified the ordering of SFCs, resiliency, security, topology configuration, and service placement, as some of the challenges related to current SFC implementations. We extend this work by (1) creating a taxonomy that captures state-of-the-art frameworks; (2) presenting frameworks that also capture the chaining of vNFs, which is key to the design of next-generation networks;and (3) presenting more open challenges that are key to achieving scalability of network functions, resilience, and high performance in NFV/SFC network environments. The survey carried out by [PERSON] et al.