. The CSCF decides how to process SIP mes- sages, including routing to application servers and network routing servers. In addition, the CSCF enforces policies specified by the service provider. In IMS-based services, it is critical to provide high availability as well as maintain call state in order to minimize service disruptions during failures. These same needs apply equally to virtual IMS solutions. For the case of a virtual CSCF, local redundancy and georedundancy are needed to ensure high reliability. Solutions may include local redundancy implemented with load sharing across all active VMs (shown in Figure 5.12). In addition, failure detection and recovery times should be less than a second. Accurate call state is critical to proper session processing; hence, the redundancy solution must ensure call state is maintained across local VM instances as well as across sites. Various imple- mentations can be used to achieve call state replication with minimal lag and a control scheme to determine master call state. IP Network Site 1 Site m FIGURE 5.12 Example of multisite design with local redundancy. Building the Network of the Future Failure Rate Sensitivity Failure Detection Sensitivity MTTR Sensitivity Mean Time Between Failures Probability Failure Detection Mean Time to Repair FIGURE 5.13 Availability sensitivity to key parameters. Finally, layer 4 protocols such as SIP and diameter are used to monitor and detect failures and to redirect sessions to appropriate VM instances with minimal disruptions. If a message is not received within the expected time interval, then one or more retry messages are sent within a short time. Typically, failures can be determined within a second and failover to alternate instances within seconds. Depending on the architecture and implementation, it is possible to achieve availability of 99.999% or higher. Conversely, if the failure rates are too high, the ability to successfully detect and recover from failures is too low, or the time to recover following a failure is too long