ronization requirements. In the previrtualized network, high availability for these applications was achieved using pur- pose-built highly available hardware. In the early phases of virtualization, many of these VFs do not yet have local redundancy. Local redundancy with no site failover. These VFs are also typically associated with trans- port and network access functions, where end users physically terminate their access at a single site. However, applications in this category have the ability to replicate and synchro- nize data within a site, enabling local redundancy. Stateful function with site failover. Most of the critical layer 4-7 service VFs fall into this category. Examples are VFs associated with Voice over IP control plane or mobility control plane. While these VFs service end users, the users do not physically terminate access at these sites. The VFs are stateful, maintaining information about call state and customer profiles. The synchronization requirements are not quite as demanding, and service pro- viders for these VFs have a tradition of architecting for georedundancy and local redun- dancy. These applications achieve high availability using established techniques at the VF layer to ensure fast failover. As the Network Cloud matures in its ability to handle network applications, there will be opportunities to use cloud orchestration and ONAP capabilities instead of custom VF capabilities for some of the resiliency needs. Stateless function with site failover. These are the "ideal" VFs for a cloud. Because these applications are stateless, it is relatively straightforward to deploy them in many sites and dynamically distribute load across the sites. This also allows for seamless failure recovery since it can be done by taking one of the failed instances out of service and redistributing the load. An example of this type of VF is distributed Domain Name System (DNS). This categorization is summarized in Table 5.2. The categories represent the types of VFs that exist today. As both the Network Cl