s that are running on data center infrastructure, which has significantly lower availability. This means that the VNF will need to continue operating in the event of a server failure. In theory, continuous operation can be achieved by running VFs on redundant servers organized as a high-availability cluster. Both VMWare and KVM have capabilities to organize a group of servers as a high-availability cluster. For the purpose of understanding resiliency, we refer to the VNF classification from Section 3.1.3-host VNFs and middle box VNFs. A host VNF is a source or sink of traffic. Transport layer connections that carry end-user sessions will terminate on a host VNF. DNS and RADIUS are examples of host VNFs in an IP network. Middle box VNFs behave like pipes in the middle of a network connection or flow. Packets that enter a middle box VNF almost always exit to be sent to final destination. Packets that The MTBF and availability figures will vary depending upon many factors, including physical and electrical design. In hardware-based networks, there is a close correlation between infrastructure reliability and service availability. The main point of this section is to explain how software-centric networks incorporate the use of COTS hardware and physical design limitations and deliver high availability for services. Network Functions Virtualization transit through a middle box VNF may undergo changes such as header transformation. Proxies often change HTTP header fields to ensure a desirable effect such as making a web page display on a mobile device more readable. Modification of time to live (TTL) or adding/removing an encapsulation header field by an edge router, are other examples of header modification by a VNF. Middle box VNFs have at least two interfaces. Connecting a trusted network (e.g., a private network) to an untrusted network (e.g., the Internet) is an example of the use of two interfaces. Connecting a customer network to the core network is another example. A middle box VNF always resides in between two