###### Abstract
In this paper, we propose and investigate a scalable NFV orchestration architecture inherited by ETSI [11]. The functions of the NFVO are split into two functional entities: (i) the Resource Orchestrator (RO) whose role is to orchestrate the resources; and (ii) the Network Service Orchestrator (NSO) whose role is to orchestrate the network service. In the proposed solution, one RO is placed in each Cloud domain and its role is to simplify and abstract the Cloud Infrastructure of which the the VIM notifies him. The RO only notifies the NSO--information about the amount of available cloud resources without providing any detail on the network topology of the datacenter and the available number of servers. This abstraction process implemented by the ROs allows for a simplification of the input data of the SRCBA algorithm and a drastic decrease in its computational complexity. A further contribution of the paper is to propose the use of Segment Routing (SR) [12; 13; 14] to implement the data plane of our architecture. SR is a novel technology allowing the implementation of advanced TE features with a reduced complexity. In this work, we exploit the features of SRv6, i.e., SR over IPv6, to simplify the management of the proposed NFV orchestration solution. In more detail, we make use of the Binding Segment Identifier (SID) tool to hide the complexity of datacenter routing without requiring the use of different technologies, such as NSH or SDN. The main innovative contributions of the paper are the following:  
* the proposal of a scalable and ETSI compliant orchestration architecture in Multi-Provider NFV environments;
* the definition of a SRv6-based data plane for the SFC routing applicable for the proposed scalable orchestration;
* the proposal of an ad hoc algorithm for the proposed orchestration architecture and the investigation of its computational complexity;
* the comparison in terms of computational complexity of the proposed solution with respect to the one of a traditional orchestration. This paper is an extension of [15] in which only some preliminary results are presented. Conversely, in this paper, we add the following contributions: (i) a description of ETSI compliant NFV orchestration architecture is illustrated with the introduction of a architectural solution in which the Network Function Virtualization Orchestrator (NFVO) is split in RO and NSO; Segment Routing is extended to the Network Function Virtualization Infrastructure- Point-of-Presence (NFVI-PoP) domain with the introduction of the Binding Segment IDentifier (BSID) to hide the routing management to the RO, without requiring the use of Software Defined Networks (SDN); (ii) the SRCBA algorithms for the traditional and proposed orchestration solutions are highlighted and their computational complexity is evaluated; (iii) the comparison results between traditional and proposed orchestration solutions are also reported for a long distance network such as NSFNET. The paper is organized as follows: the Related Work is reported in Section 2. We illustrate the scalable orchestration solution in Section 3. A data plane based on Segment Routing technology and a SRCBA algorithm for the proposed NFV orchestration architecture are shown in Sections 4 and 5, respectively. The advantages in terms of reduction in computational complexity are evaluated in Section 6. Finally, conclusions and future research items are described in Section 7.