Network Services and Service Chains Overview
The terms network service and service chain are sometimes used interchangeably, but they are not the same; you need to understand the difference between them:
A network service is a final product offered to end users with a full description of its functionality and specified performance.
Administrators deploy network services between two locations in a virtual network, so that traffic traveling in a specific direction on that link is subject to action from that service. This term is defined in the ETSI Network Functions Virtualization (NFV) standard.
A service chain refers to the structure of a network service, and consists of a set of linked network functions, which are provided by specific virtualized network functions (VNFs), with a defined direction for traffic flow and defined ingress and egress points. Although not defined in the ETSI NFV standard, this term is regularly used in NFV and software-defined networking (SDN).
You can create a service chain in the Network Service Designer by using:
One VNF instance that provides one or more functions. See Figure 1.
Using one VNF instance instead of multiple instances increases performance.
Multiple instances of the same VNF, each providing certain functions. See Figure 2.
Using multiple instances of the same VNF lowers performance, such as when you want to create differentiated services.
Instances of different VNFs, each providing certain functions. See Figure 2.
You might need to use different VNFs if one VNF cannot fulfill all network functions or if a particular VNF offers an advantage for a network function.