Service Function in SDN

C.

In Software-Defined Networking (SDN), a service function refers to a specific network function that provides a specific service. It can be implemented as a software function, and it can be virtualized and dynamically chained to achieve the desired service.

Option A, "a virtual server," is not an accurate definition of a service function in SDN. While a service function can be deployed on a virtual server, a virtual server is a generic term for a software-based instance of a server, not a specific network function.

Option B, "a virtual chain," is a closer definition but is not precise. A virtual chain refers to the sequence of virtual network functions that are chained together to provide a specific service. While a service function can be part of a virtual chain, a virtual chain is not the same as a service function.

Option C, "a virtual network function," is a more accurate definition of a service function in SDN. A virtual network function (VNF) is a software-based network function that can be instantiated on-demand and chained together with other VNFs to create a service chain. VNFs can be used to implement a wide range of network functions, such as firewalls, load balancers, and routers, and they can be virtualized to run on commodity hardware.

Option D, "a virtual device," is a broad term that could refer to any virtualized network device, such as a virtual router, switch, or firewall. While a service function could be implemented as a virtual device, not all virtual devices are service functions.

Therefore, the correct answer to the question is Option C: a virtual network function.