## 3 Virtual Networking for Microservice Platforms: L2S-M
### Problem Statement
Furthermore, this model allows application developers to pass over the inner networking being performed in an infrastructure since it is completely transparent to the applications themselves. In other words, developers can assume that other modules of an application will always have connectivity, and that no further configurations should be required in the modules to be used in the infrastructure. Despite its advantages, this model might not be suitable for all the services that could be provided through cloud-native platforms. In a previous work, we realised the potential of microservice platforms [34] as enablers for the deployment network and vertical services in the context of the 5 th and 6 th generations of mobile networks (5G/6G) in cloud-native environments (particularly, microservice platforms, which are a subset of functions in the cloud-native model). Microservice platforms, like Kubernetes (K8s), usually employ container technology to build applications in their managed infrastructures. Due to their lightweight nature, in some environments (e.g., resource constrained scenarios), these platforms have multiple benefits over traditional virtualisation solutions like OpenStack, which usually require more resources for the management and deployment of NSs. However, due to the intrinsic nature of NSs, it is necessary to ensure that communication at the lower layers is available (and not just at the application layer) for the functions that build the service. One prominent example of the necessities of virtual networking can be seen in the implementation of a router functionality. A traditional router must analyse incoming packets at the IP level from one of its network interfaces in order to check the destination that will be forwarded. However, this functionality can only be performed if each one of its interfaces is located in a different LAN. The same situation is present when dealing with virtual networks: in virtual networks, each function is located at different LANs (despite their geographical location) and a router functionality would have one interface in each virtual network, enabling the analysis of the incoming/outgoing packets to be forwarded to the corresponding functions located in different (isolated) virtual networks. In consequence, functions are isolated between each other at the network level and can only communicate through the corresponding router, which enables the secure deployment and isolation of network functionalities, as well as their chaining (necessary for NS implementations). However, this behaviour is impossible to achieve in flat networking approaches: if all the functionalities are located in the same LAN (like they are in the flat networking approach), then there is no isolation between functions since they are in the same network domain, so the router cannot \"decide\" the routing/forwarding of the incoming/outgoing packets to each function. Hence, this problem heavily hinders the implementation of networking services in cloud native to host NSes in one, or several, clouds. To further illustrate the issue that microservice platforms have for the implementation of NSes in cloud environments, Figure 1 showcases a logic implementation of a multimedia content delivery service, in particular, a simple content delivery network (CDN) service. This CDN has two HTTP proxies: the first one is utilised to cache the content sent from a multimedia server, which allows to make the content closer to the end users (reducing download times and bandwidth used in the network) while the second one is a firewall that filters undesired requests from a function outside the CDN. Ideally, a service of such characteristics must enable sending the requests from/to the corresponding proxies and servers involved in the CDN, following the schema depicted in the upper side of the figure. Furthermore, the HTTP proxy must be able to analyse incoming packets at the network layer to appropriately filter undesired requests.