# A Systematic Literature Review of Reliable Provisioning for Virtual Network Function Chaining
## 1 Introduction  
Network function virtualization (NFV) is a relatively new technology that separates network functions from the hardware [1]. In contrast to traditional deployment, Virtual Network Functions (VNF), such as firewalls or load balancers, can be placed on virtualization software instead of dedicated hardware [2; 3]. The VNF provides the same function as the relevant specific hardware network function, which is augmented by the ability to adapt to network requirements [4]. A sequence of these VNFs in the network can provide a service in the network that is called a Service Function Chain (SFC) [5]. Overall, FNV, VNFs, and SFCs are discarding the expensive network equipment and replacing them with software running on low-cost hardware [6], which can significantly reduce Capital Expenditure (CAPEX) and Operating Expenses (OPEX) [7]. Moreover, adopting the NFV technology in a network enhances its management and flexibility [8]. As a result, VNFs are increasingly popular due to their convenience and flexibility.  
Although NFV architecture introduces new opportunities for service providers to provide services with minimum cost, there are drawbacks to these advantages that require resolving multiple performance, availability, security, and survivability problems [9]. For each VNF, there is one software that is running on physical hardware providing one of the hardware's functions that used to be offered by individual physical hardware. Therefore, in addition to physical hardware failure, VNFs are prone to software faults [10]. Moreover, despite hardware that is subjected to rigorous testing and validation processes, the software is notoriously unreliable, which is why we expect VNFs to fail more often in comparison to traditional middleboxes. Moreover, the failure of a VNF or a link may cause the whole SFC to fail, including the failure of the associated VNFs [11]. Therefore, Virtual NetworkFunction Infrastructure (NFVI) must meet resiliency and geo-redundancy requirements to be able to provide the requested performance, availability, security, and survivability for a continuous service [12]. Ensuring the reliability and resiliency in an NFV is more complex than hardware solutions. A reliable VNF network often requires dedicated mechanisms to ensure high availability while keeping costs low.  
There have been various studies in the NVF provisioning field, which mainly focused on VNF placement and SFC mapping [13; 14; 15; 16]. However, studies discussing reliable provisioning and failure recovery and comparing the different approaches are limited. There are various methods to ensure the service availability of the NFV, including resilience against single or multiple link failures, single or multiple node failures, or providing backup in which the evaluation methods and simulation setup could vary. Therefore, a comprehensive study to evaluate and discuss the differences is a necessity. The goal of this paper is to compare the existing reliable provisioning algorithms and identify the key parameters for the reliable provisioning for virtual network function chaining. The advantages and disadvantages of each algorithm alongside important features of the algorithm are discussed in detail through the sections.  
The rest of the paper is organized as follows. In Section 2, recent reliable provisioning algorithms are discussed. In Section 3, we define the research method by explaining the research questions, search process, and study selection criteria. In Section 4, the result of our systematic literature is discussed. The result includes detailed answers to our research questions through different sub-sections. We present some guidelines for future research and discuss the existing problems of handling failure in an NFV environment in Section 5. Finally, we conclude the paper in Section 6.