by carefully employing a cool-down interval-for which no additional auto-scaling is allowed to be executed. 16.10.2 SCALING TRIGGERS There are multiple types of VNF scaling that differ by the triggers for scaling and the ways of issu- ing the scaling requests. The following three models of VNF scaling have been identified by ETSI GS NVF SW 001 [23]: 1. Auto-scaling 2. On-demand 3. Manually triggered scaling In the auto-scaling model, VNF manager and the NFV orchestrator monitor the VNF KPIs and issue a request to trigger scaling; whereas, in the on-demand scaling, the VNF monitors the KPIs of Network Measurements its components (i.e., VNFs) and sends a scaling request to the VNF manager [20]. In manually trig- gered scaling, the operators from OSS/BSS trigger scaling based on their observation/expectation of an increased/decreased load on the VNF. 16.10.3 SCALE OUT: ILLUSTRATIVE EXAMPLE The following describes an illustrative example of scaling out. Imagine a virtual machine running a VNF and a VNF manager monitoring the VNF resource consumption, that is, CPU, and network I/O usage, for auto-scaling of the VNF. The virtual machine is currently using its resources at the following levels: CPU usage = 45%, memory usage = 32%, and network usage = 484 Kbps. The load to the VNF keeps increasing, leading to the following virtual machine resource consumption : CPU usage = 75%, memory usage = 51%, and network usage = 806 Kbps. At this point, a VNF manager observes that the CPU usage of the virtual machine reached the CPU utilization threshold (75%) for scaling out. This makes the VNF manager send out a request to the NFV orchestrator for triggering a VNF scaling out action. Then, the NFV orchestrator and cloud infrastructure manager allocate a new virtual machine and a load balancer to the VNF. After the new virtual machine and the load balancer for the VNF are successfully created and are running, the increased load to the VNF is effectively distributed between the two virtual machines by the load balancer. 16.11 VNF EF