Once the CoMb servers on the routing path are discovered MIDAS assigns required

Once the comb servers on the routing path are

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multiple NFPs. Once the CoMb servers on the routing path are discovered, MIDAS assigns required NFs to CoMb servers hosted by multiple NFPs in two stages. The first stage is to compute each NFP’s responsibility considering utilization balancing across NFPs and location dependency. The heuristic for the second stage is used to select CoMb servers within the scope of a single NFPs while respecting the order. B. VM-based Framework Isolation is an indispensable property for both performance and security, especially in the context of multi-tenant clouds, where the resources are shared among cloud tenants. Not for isolation, the busy VNFs may consume much more re- sources than other co-residing VNFs, leading to starvation and violation of the stringent SLO. Moreover, isolation is perceived as the key to cloud security. As a result, most NFV framworks contain NFs in virtual machines (VMs) for isolation. A running VM reserves a fixed amount of hardware resources (including CPU, memory, hard disk, etc.) regardless of its workload. Consequently, to the best of my knowledge, it is assumed in all VM-based frameworks that one instance of VNF consumes fixed resources and possesses fixed processing capacity. The heavyweight VM prohibits installing all the required VNFs on the routing path for every traffic flow for two main reasons: The VMs consumes substantial resources. The VM booting time is prohibitively long (tens of seconds). VM installation happens at a much coarser time-scale than traffic dynamics. Instead, all these VM-based frameworks steer traffic flows between VNFs to fulfill service chaining. On the other hand, these VM-based frameworks are different from hardware NFs in that they can decide where to place VNFs in runtime, hence more freedom to optimize the performance&cost. To achieve the best performance benefits from smart NF placements, they would better be augmented by other mecha- nisms to incorporate traffic dynamics, such as flow distribution (small time-scale load balancing), dynamic scaling in/out (resource efficiency), etc. We briefly discuss this topic in Sec. III-B4. Flowstream [11] is the first VM-based framework, which was published as early as 2009. The authors of Flowstream noticed the narrowed performance gap between commodity switches/servers and customized high-end switches/hardware NFs, and successfully predicted the rise of commodity hard- ware to process traffic. NFs are consolidated to servers in the form of VMs. The traffic flow routes depend on the fined- grained control of SDN. Nevertheless, Flowstream is just a high-level design without concrete implementation. 1) Path Loosely Controlled Placement: There are existing placement algorithms, resolving the problem of mapping VMs onto physical machines in the cloud. Some VM-based frame- works directly rely on these placement algorithms where VNFs are viewed as ordinary VMs.
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