## 2 Related Work  
The NFV resource allocation and routing problem has been studied in [16] by formulating it as a multi-commodity-chain network design problem on a cloud-augmented graph. The paper investigates a static traffic scenario and adopts a simple model in which the sharing of VNFI among service functions and their maximum resource availability are not considered. Some solutions have been proposing bandwidth and cloud resource allocation in dynamic traffic scenarios [17; 18; 19]. [PERSON] et al. [20; 21] design online algorithms to optimize placement of VNF instances in response to on demand workload, considering the trade-off between bandwidth and cloud resource consumption. [PERSON] et al. [1] introduce VM migration policies to save the server power consumption and by limiting the QoS degradation or the energy consumption occurring during the migrations. [PERSON] et al. [22] propose a procedure to evaluate the number of VNFIs over time with the objective to minimize the cloud resources used; their study does not consider the cost of the bandwidth resources. [PERSON] et al. [23] study the problem of reallocation of VNFIs in a dynamic traffic scenario by constraining the number of reallocations so as to limit the deployment costs; they determine the number and the location of the VNFIs that are executed in Virtual Machines (VM) equipped with a pre-determined number of cores dependent on the type of VNF supported by the VNFI. [PERSON] et al. [10] study cloud and bandwidth resource allocation problems in geographically-distributed data centers interconnected by elastic optical networks; the scenario considered suits the business model of the NFV ecosystem very well. [PERSON] et al. [24] and [PERSON] et al. [25] achieve good performance for the SFC routing and VNFI resource allocation problems by modeling the network as a multi-layer graph. In the case of ETSI NFV architecture [26], the algorithms for the cloud and bandwidth resource allocation are implemented in the Network Function Virtualization Orchestrator (NFVO). The NFVO receives from the Virtual Infrastructure Managers (VIM) of the Network Function Virtualization Infrastructure-Point of Presence (NFVI-PoPs) information about the available cloud and bandwidth resources and the NFVI-PoP network topologies. Most of the solutions proposed in literature assume that the VIMs expose a complete description of the NFVI-PoP network topologies with the consequence of drastically increasing the computational complexity of the SRCBA algorithm. Only a few papers propose more scalable solutions [23] in which the NFVO has a limited and simplified visibility of the NFVI-PoP network topology. However, none of these works propose an NFV orchestration architectural solution able to support the scalable algorithms proposed. In this paper, we employ one of the architectural options proposed within the ETSI [11] to support a solution in which: (i) each NFVI-PoP domain is equipped with a Resource Orchestration (RO) whose role is to abstract the NFVI-PoP network topology exposed by the VIM; and (ii) a Network Service Orchestrator (NSO) has the role of deciding where to allocate the resources and to route the SFCs. In addition, we also illustrate how the application of a Segment Routing technology allows for a reduction in the addressing overhead of the VNFIs. Several research works proposed the use of Segment Routing in the context of the Service Function Chaining, but none of them considered SR as an enabler for the implementation of a scalable orchestrator architecture. Ref. [27] exploits the network programming feature of SR to implement a load balancer able to distribute the computation load over a set of servers, by defining a new SR function (named service hunting) to be performed at the gateway.