## 1 Introduction
Specifically, the contribution proposed in this paper, which extends our previous work [12], can be summarized as follows:  
* The detailed description of the conceptual approach through which ArTIC-MS is specified;
* The complete specification of the ArTIC-MS functional view, by identifying the building blocks composing the conceptual architecture and introducing the relevant capabilities they provide;
* The specification of the ArTIC-MS operational view, which is outlined throughout the description of actors and relevant use cases. Regarding the conceptual approach at the basis of ArTIC-MS specification, this paper introduces a _MSaaS reference architecture (RA)_ that aims at identifying the abstract components and related capabilities required for addressing the development of MSaaS applications in a federated infrastructure. Then, the proposed RA derives the conceptual architecture. ArTIC-MS exploits TOSCA (topology and orchestration specification for cloud applications) [13] as an enabling standard to effectively ease the development of M&S applications by integrating simulation components provided by different partners, and deployed onto heterogeneous cloud infrastructure, in order to maximize the reuse of existing components. Regarding the functional specification of ArTIC-MS, it should be underlined that, from a general perspective, its capabilities allow simulation developers to cope with both the infrastructure and application orchestration. The latter has been investigated in previous contributions, where approaches, tools, and standards in the MDE field have been introduced to support the generation of executable simulations from abstract orchestrations, such as [14]. Thus, this aspect is not further discussed in this work, which focuses on infrastructure orchestration. From a concrete point of view, this work also discusses an example application where a MSaaS prototype implementation based on ArTIC-MS was developed to assess how the proposed approach might effectively contribute to satisfying the FAIR principles of a MSaaS application in a federation of heterogeneous cloud infrastructure. The experimentation was carried out within the scope of the _MSaaS Architectures and Services for training and experimentation (MASTER)_ project, a research effort carried out under the National Program for Defense Research. The MASTER project aims at identifying innovative solutions for developing M&S applications in the cloud. Specifically, the addressed case focuses on the investigation of approaches to promote the interoperability of M&S resources. In this respect, the proposed example deals with the integration of the ArTIC-MS prototype with the open cloud ecosystem application (OCEAN) [15], a MSaaS platform based on the open-source OpenStack cloud infrastructure [16]. The rest of this paper is structured as follows. Section 2 revises the existing literature and clarifies the novelty of the proposed contribution, Section 3 provides the concepts at the basis of this work by briefly outlining the TOSCA standard and the OpenStack IaaS cloud implementation, Section 4 outlines the adopted architectural approach and introduces the MSaaS RA, Section 5 illustrates the ArTIC-MS conceptual architecture, Section 6 provides an operational description of ArTIC-MS throughout the specification of relevant use cases, Section 7 discusses the experimentation. Finally, Section 8 provides concluding remarks.