, then the availability may be much lower and may not meet the desired overall service availability. Figure 5.13 illustrates sensitivity of availability to these key factors. Design decisions for the level of redundancy required within sites and across sites are highly dependent on the combination of failure parameters. Reliability analysis is an important part of the design process to drive optimal solutions. In the virtual Call Session Control Function (vCSCF) example shown in Figure 5.12, there can be multiple subcomponents that make up the whole solu- tion, and the vCSCF must interoperate with other IMS components. Redundancy combined with low failure rates, high failure detection rates, and fast recovery times are essential to achieving high availability. The complexity requires careful reliability design and test validation to ensure the desired targets are met in a cost-effective solution. 5.4.3 EXAMPLE: STATELESS NETWORK FUNCTION WITH MULTISITE DESIGN Finally, consider a stateless service with georedundancy and fast failover, such as vDNS-R (Domain Name System Resolver). DNS is a stateless application that translates user-friendly domain names or URLs to the numerical IP addresses needed for locating/identifying computer services and devices. This is an example of a service that is perfectly suited for virtualization. This service can be hosted across several georedundant sites (see Figure 5.14) utilizing locally redundant VMs on commodity hardware. These types of services fall into the category of "Stateless function with site failover" (5-9s +). vDNS service can achieve very high availability through a number of resiliency features. First, clients access vDNS servers using primary and alternate virtual IP (VIP) addresses, normally pro- vided by the Internet service provider (ISP) and stored at the client. If the primary VIP does not respond to a DNS query after multiple (~5) retries over a short interval of time (<1 min), the client attempts to connect and send queries to the alternate vDNS VIP. Next, e