n- tication and authorization of access to network elements. The perimeter is set up by carefully con- figuring the network elements, and auditing tools are used to periodically check that the proper protections are in place. Since setting up the perimeter requires changes to many different physical components from many different vendors, maintenance of the perimeter is a complex and time- consuming operational job (Figure 9.10). The perimeter remains a useful concept in the cloud-based architecture, but the nature of the perimeter changes [3]. Rather than one perimeter that encloses all the critical resources, the cloud- based architecture allows critical resources to be distributed, which enables finer-grained protec- tion rules to be applied. Where the appliance-based perimeter divided the world into "Management Team" and "Everyone Else," the cloud-based architecture allows us to partition critical resources into smaller groups and allows them to be in their own networks. By controlling access to those net- works, we enforce least privilege and make it harder for a single attacker to compromise substantial parts of the network. The same security best practices apply to the SDN controller. The SDN controller (or the VMs and servers hosting the SDN controller) will be protected with the same controls that protect the network from various types of intrusion and vulnerabilities. These security mechanisms include but are not limited to software scanning, security audit, secured access based on roles and responsibili- ties, and active DDoS protection. A network cloud elastic infrastructure will enable dynamic security control for SDN controllers. A benefit of deploying SDN controllers in a cloud platform is that if compromised, the SDN con- troller will be isolated, shutdown, quarantined, and replaced by another instance of a secured SDN controller. This ability to dynamically instantiate a new VNF to maintain SDN controller function also improves the failure recovery period. Consider two scenarios where an SDN cont