 radius" (i.e., the magnitude of a given failure) (Miller, 2015) that further emphasize the need for resilient VF designs. There are many other considerations in resiliency engineering that are not covered in depth in this chapter (Lyu, 1996; Musa, 1999). Those fall under the broader topic of software reliability engi- neering and include topics such as software stability testing, fault insertion testing, measurement, and fault detection. Some key principles that are illustrated in this chapter are Single-site availability can be expected to be in the 3-9s to 4-9s range VF availability in a single site cannot exceed the site availability High availability can be achieved through multisite VF design Planned downtime further impacts availability and reinforces the need for multisite design High availability requires mature, fault-tolerant software Accurate fault detection and fast failover are critical to achieving high availability NETWORK CLOUD INFRASTRUCTURE AVAILABILITY 5.1.1 SINGLE-SITE AVAILABILITY Within a single Network Cloud site (Figure 5.1), there are layers of infrastructure that must be bal- anced to achieve a target availability with an effective cost profile. Figure 5.2 shows an example of some of the key contributors to single-site VF availability. Each layer, like a "dial" on A/V equipment, can be tuned to achieve an overall availability profile. Some typical target levels of availability and the corresponding downtimes are represented by the vertical lines and are also shown in Table 5.1. As a first-order approximation, availability can be estimated by adding all of the Building the Network of the Future Downtime 53 min 5 min 30 sec Downtime Availability 3 9's 4 9's 5 9's 6 9's per year Application 53 min Virtualization (VM, Hypervisor) 53 min Bare Metal HW, 53 min Compute/Storage Infrastructure Fabric 30 sec Physical Cloud Infrastructure 53 min Downtime 3.5 hr Overall Availability 99.96% FIGURE 5.2 Key availability contributors within a single site. TABLE 5.1 Availability to Downtime/Year Convers