## 2 Cloud-Native Network Architecture in Future Networks
### Service-Based Architecture for the 5G Network  
The Service-Based Architecture (SBA) developed by 3 GPP for the 5G core network is a representative case of cloud-native design in network architecture. The 5G SBA is based on network virtualization and softwarization: Network Functions (NFs) on the data plane and control plane are separated, and the NFs defined in the architecture may be realized as software instances deployed on a virtual infrastructure. On top of virtualization and softwarization, the 5G SBA adopts the service-oriented principle through the notion of NF services exposed by control plane NFs and the Service-Based Interface (SBI) for interactions among NFs services.  
As depicted in Figure 1, the 5G SBA decouples the data plane and the control plane. The data plane comprises User Equipment (UE), the Radio Access Network (RAN), the User Plane Function (UPF), and the Data Network (DN). The interfaces between the data plane functions currently are still defined as reference points (N1-N6 in Figure 1). The 5G control plane comprises a set of NFs including the Access and Mobility Management Function (AMF), the Authentication Server Function (AUSF), the Session Management Function (SMF), the Policy Control Function (PCF), the Network Slice Selection Function (NSSF), Unified Data Management (UDM), the Unified Data Repository (UDR), the Network Repository Function (NRF), the Network Exposure Function (NEF), and the Application Function (AF) [22].  
Each control plane NF may expose one or multiple NF services as a service provider and also access the services exposed by other NFs as a service consumer. All the NF services are accessed through the SBI. With the SBI, the 5G SBA shifts from a point-to-point reference point-based interface toward web-based communications among NF services. The SBI may be realized by means of REST API calls using HTTP/2 on top of TLS/TCP as the transport protocol. The emerging transport protocol Quick UDP Internet Connections (QUIC) is paving the way toward HTTP/3 for supporting SBI communications [23].  
The 5G SBA allows each service consumer to discover a suitable producer of a service instance. Service discovery in the SBA is supported by the NRF, which keeps a repository of all available NF instances and the services they expose. Each NF instance is required to register its profile at the NRF, and the NF profile contains relevant data about the NF including the services it provides and the binding information for accessing the services. The NEF may expose NF services provided inside the network to authorized external consumers such as AFs. The 3 GPP introduced the service framework support function, referred to as Service Communication Proxy (SCP) in Release 16, which aims to extract the common NF  
Figure 1: The Service-Based Architecture (SBA) for the 5G network.  
processes related to service registration, discovery, selection, and communication binding into a unified platform [24; 25].  
The 5G SBA supports two modes of interaction between the NF service producer and consumer. In the request-response model, an NF service consumer sends a request message to the target service provided by a producer and receives the corresponding response message back from the producer. In the subscribe-notify model, an NF service consumer subscribes to an NF service for a (set of) certain event(s) and receives a notification message from the service producer whenever a subscribed event occurs.  
Although the current 3 GPP specification limits the SBA to the control plane of the 5G core network, the cloud-native design principle is expected to be applied in a broader scope in the future 6G network. A natural evolution beyond the current 5G SBA would be an integration of the SBA framework for the core network into an end-to-end architecture that comprises multiple planes, including the data, control, and application planes as defined in the SDN architecture, and across multiple domains including the access, transport, and core networks.