 redundancy" (3- to 4-9s). However, higher levels of resiliency (4- to 5-9s) can often be achieved by locating the redundant PER equipment in hardened (5-9s) network DCs. Now suppose that the same services are implemented in a virtualized cloud environment, where a single Network Cloud site offers say 3- to 4-9s of availability. As shown in Figure 5.10, achieving levels of resiliency comparable to the precloud environment for such services requires automated orchestration capabilities to achieve seamless application failover following commodity cloud equipment failures. However, even with fast intrasite failover, service availability is still limited by the availability of a single cloud site itself. Thus, in order to achieve resiliency levels comparable to the precloud services for these stateful access services, a multisite design with dynamic rehoming is typically employed (shown in Figure 5.11). 5 9's Geo Failover Automation Redundancy Local Failover Automation Local Redundancy Designing for Higher Availability FIGURE 5.10 Stateful service design for high availability. Architecting the Network Cloud for High Availability Metro Network Switch Switch Switch Switch Network Fabric Network Fabric Comput Compute Compute Compute Cloud infrastracture Cloud Infrastructure Site A Site B FIGURE 5.11 Multisite design with dynamic rehoming. 5.4.2 EXAMPLE: LAYER 4 STATEFUL CONTROL FUNCTIONS Consider the case of layer 4 virtualized functions that provide control functions for real-time appli- cations. In today's highly mobile communication networks, real-time services such as VoLTE, IP messaging, and video conferencing are provided by SIP-based solutions. The Mobility 3GPP stan- dards organization specifies IP Multimedia System (IMS), standards including functions, inter- faces, and protocols (3GPP, 2016). One of the key IMS functions is the call session control function (CSCF), which provides call control for the entire duration of sessions it handles. The CSCF moni- tors all signaling messages for locally registered users