ers. 3.4 NFV RESILIENCY AND SCALING VNFs enable on-demand deployment and elastic growth of network services. The shift from a physical hardware-appliance-based architecture to a software-based, multivendor, and potentially open-source-based architecture creates many benefits (as outlined in Section 3.1) while also intro- ducing new challenges. Delivering service continuity and predictable performance, on parity with today's carrier-grade network environment, will require a combination of better software engineer- ing and an architecture that leverages a dynamic on-demand infrastructure. It will also require VNF-specific logic in some cases. As we will see in this section, both the Network Cloud platform and the specific VNF will need to be architected to achieve high performance and reliability. Resiliency (Smith, 2011) is the property of a VNF that enables very high availability of the services. VNFs are complex software functions implemented on servers running Linux and an appropriate hypervisor. In the simplest form of implementation, VNF availability will be less than or equal to server availability (this is discussed in more detail in Chapter 5). When a server fails, all software tasks that run on that server will fail and need to be restarted. Servers can fail due to a number of reasons. The most common reasons are hardware failures, os failures, hypervisor fail- ures, and planned events for taking the server out of service for maintenance and upgrades. Server failures are infrequent events and can be modeled using an exponential distribution. Average server availability in a service provider data center is in the range of 99.9%-99.99% (50-500 unavailable minutes in a year). This range is derived from server mean time between failure (MTBF) of approx- imately 50,000 h and assumptions regarding restoration time in the event of failures.* Service providers typically deliver 99.99%-99.999% availability for most network services. The fundamental challenge is to deliver very high availability of network function