earchdatacenter.techtarget.com/photostory/4500255104/Data-cente terminology-that-will-get-you-hired/3/Define-continuous-integration-and-delivery-as-Dev-meets-Op 32. n.d. Retrieved from hhttp://searchitoperations.techtarget.com/definition/continuous-delivery-Cl 33. n.d. Retrieved from https://www.sdxcentral.com/wp-content/uploads/2014/09/opnfv-nfv-open-source- k-functions-virtualization-diagram.jpg 34. n.d. Retrieved from http://stackalytics.com/ Architecting the Network Cloud for High Availability Kathleen Meier-Hellstern, Kenichi Futamura, Carolyn Johnson, and Paul Reeser CONTENTS Network Cloud Infrastructure Availability 5.1.1 Single-Site Availability 5.1.2 Availability to Cost Tradeoff (Bottleneck Analysis) The Impact of Planned Downtime and Georedundancy 5.2.1 Illustrative Example of the Impacts of Planned Downtime 5.2.2 Best Design Practices for Minimizing the Impacts of Planned Downtime VF Software Design 5.3.1 Fault Tolerant VM Designs 5.3.1.1 Hot VM Replication 5.3.1.2 Warm VM Replication 5.3.1.3 Passive VM Replication 5.3.2 Low Software Failure Rates and Accurate Fault Detection 5.3.2.1 VF Protection from External Services 5.3.2.2 VF Error Handling 5.3.2.3 VF Software Fault Detection 5.3.2.4 VF Failure Detection and Alerting 5.3.2.5 VF Fast Recovery 5.3.2.6 VF Software Stability 5.3.3 Software Resiliency Engineering Putting It All Together: VF Classification and Examples 5.4.1 Example: Stateful Network Access Services 5.4.2 Example: Layer 4 Stateful Control Functions 5.4.3 Example: Stateless Network Function with Multisite Design Areas for Further Study Acknowledgments References Driven by the need to maintain the reliability of the nation's critical communications infrastructure, the telecommunications industry has a long tradition of designing for high availability, sometimes referred to as "carrier grade availability" or loosely quantified as "five-nines availability" (Lancaster, 1986). Significant service disruptions must be reported to the FCC (Healy, 2016) by wireline, wireless, paging, cable, sate