###### Abstract  
Network Function Virtualization is a new technology allowing for a elastic cloud and bandwidth resource allocation. The technology requires an orchestrator whose role is the service and resource orchestration. It receives service requests, each one characterized by a Service Function Chain, which is a set of service functions to be executed according to a given order. It implements an algorithm for deciding where both to allocate the cloud and bandwidth resources and to route the SFCs. In a traditional orchestration algorithm, the orchestrator has a detailed knowledge of the cloud and network infrastructures and that can lead to high computational complexity of the SFC Routing and Cloud and Bandwidth resource Allocation (SRCBA) algorithm. In this paper, we propose and evaluate the effectiveness of a scalable orchestration architecture inherited by the one proposed within the European Telecommunications Standards Institute (ETSI) and based on the functional separation of an NFV orchestrator in Resource Orchestrator (RO) and Network Service Orchestrator (NSO). Each cloud domain is equipped with an RO whose task is to provide a simple and abstract representation of the cloud infrastructure. These representations are notified of the NSO that can apply a simplified and less complex SRCBA algorithm. In addition, we show how the segment routing technology can help to simplify the SFC routing by means of an effective addressing of the service functions. The scalable orchestration solution has been investigated and compared to the one of a traditional orchestrator in some network scenarios and varying the number of cloud domains. We have verified that the execution time of the SRCBA algorithm can be drastically reduced without degrading the performance in terms of cloud and bandwidth resource costs. network function virtualization; scalable orchestrator; segment routing; software defined network +
Footnote †: journal:  
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computing resources for the execution of Virtual Network Function Instances (VNFI). The performance indexes used to characterize an SRCBA algorithm are the number of SFCs accepted, the server power consumption [2], the bandwidth used to interconnect the VNFIs [3], the cloud resource amount needed to activate the VNFIs and so on [4; 5; 6]. All of these solutions assume that the NFV orchestrator, responsible for implementing the SRCBA algorithm, knows in detail the network topology and the available resources in the datacenter in which the VNFIs have to be instantiated. This leads to a high computational complexity of the SRCBA algorithm increasing versus the number of servers and switches located in the datacenters [7]. Furthermore, this traditional solution may not be applicable in multi-provider NFV environments that implement business models with the following two players [6; 8; 9; 10]: the Cloud Infrastructure Provider (CInP) and the Internet Service Provider (ISP). The function of an CInP is to deploy and manage the physical resources on which the virtual resources (i.e., Virtual Machine) may be provisioned. The function of an ISP is to rent the physical resources from one or more than one CInP to execute the VNFIs. It provides interconnecting VNFIs through bandwidth resources to create services to the end users. It also manages the NFV Orchestrator that acquires from the CInP through the Virtual Infrastructure Manager (VIM), information about the network topology and the available resources (bandwidth, processing, disk memory) of the Cloud Infrastructure. When CInP and ISP are two different players, the CInP could need to hire from the ISP some of this information and only a partial description could be sent to the NFVO. For this reason, the application of classical SRCBA algorithms is not possible in a Multi-Provider NFV environment.