## 2 Cloud-Native Network Architecture in Future Networks
### Cloud-Native Network Design  
The current specifications for realizing network virtualization and softwarization, including the NFV and SDN architectures, mainly change how network functions are realized and deployed (as software instances hosted on VMs and/or containers), but do not change much how network functions are designed. The state-of-the-art NFV implementations often replace monolithic hardware-based network functions with their monolithic software VNF counterparts. Such a monolithic VNF design may introduce a large number of common functionalities repeated across different VNFs, which causes some negative consequences that lead to sub-optimal resource usage and hinder network agility. In addition, the NFV and SDN architectures both comprise a set of predefined function blocks that are interconnected via point-to-point interfaces (reference points), which require standardization of a new set of reference points whenever a new function block is added into the architecture. Such monolithic and tight-coupling architectural features still introduce ossification, which limits the flexibility and agility of networking systems for service provisioning [7]. A promising strategy for addressing the ossification issue associated with the current NFV and SDN architecture is to enable finer granularity for network functions in the architecture and a common interface for loose-coupling interaction among network functions. The Service-Oriented Architecture (SOA) with its latest development as the Micro-Service Architecture (MSA) offers an effective approach to achieving this objective. In general, the SOA principle advocates decomposing a large system into a collection of smaller units called services, which essentially are self-contained and platform-independent system modules that can be described, published, accessed, composed, and programmed through a standard interface and messaging protocol [15]. The MSA can be viewed as the second iteration of the SOA that aims to strip away unnecessary levels of complexity in service design in order to focus on the programming of simple services that effectively implement a single functionality [16]. Service instances in the SOA typically run on VMs, and the lightweight container-based virtualization offers an efficient platform for hosting microservices. The SOA and virtualization have been two key pillars of the foundation for cloud technologies [17], and the MSA is an enabler of the latest developments in cloud computing, including the emerging edge computing paradigm. The service-oriented architectural principle has also been adopted in network design, which enables a Network-as-a-Service (NaaS) paradigm that abstracts various network functions and resources as services [18]. It is worth noting that the term \"service\" has a different meaning in the networking field than the \"service\" concept in the SOA/MSA context. In the networking context, a service typically refers to the data transport capability offered to a customer by an ordered set of network functions; while the service concept in the SOA/MSA emphasizes encapsulation of a system module into a self-contained component. On the other hand, the evolution of network design has been influenced by the SOA principle in the past decade [19], and the SOA service concept has been gradually adopted in the recent development of the network architecture with the NaaS paradigm [20]. In the ETSI NFV specifications, while a network service refers to an ordered set of (virtual) network functions specified by a service description (VNF forwarding graph), the SOA principle has been embraced by the NFV architecture at multiple levels. For example, NFV supports NFV Infrastructure-as-a-Service (NFVlaaS), Virtual Network Function-as-a-Service (VNFaaS), and Network Slice-as-a-Service (NSaaS), which all adopt the SOA service concept. In the 3 GPP specifications for the 5G service-based architecture, usage of the term \"service\" becomes more aligned with the SOA service concept than its conventional definition in the networking field.