## 2 Related Work  
Various contributions can be found that demonstrate how simulation-based techniques have been successfully adopted for years in many application domains to support different operational needs [17; 18; 19; 20; 21]. As an example, the roles of M&S technologies and the MSaaS paradigm as key enablers to develop innovative training capabilities, support system analysis, and decision-making were underlined in [22]. Regarding the investigation of abstract architectures which specifically aim at addressing the development of MSaaS infrastructure, relevant contributions can be found in [10; 12; 23]. In [10; 23] the outcomes of the NATO Modeling and Simulation Groups MSG-136 have been presented. Such contributions identify the preliminary requirements of a possible MSaaS reference architecture for supporting the NATO operational needs, specifically with regard to interoperability issues. In this respect, this paper shares the common objective of identifying a reference architecture for MSaaS applications. On the other hand, this work goes beyond: along with the description of a conceptual framework, the TOSCA standard is also introduced for ensuring interoperability and supporting the development of MSaaS applications in federated cloud infrastructure. Moreover, unlike the above-mentioned contributions, ArTIC-MS is not tied to any application domain and can be effectively adopted in various operational contexts. Regarding the development of MSaaS platforms, several contributions have been proposed that address such an issue from different perspectives [17; 24; 25; 26]. In [17; 24], interoperability and composability have been identified as two of the most relevant challenges for M&S systems. While interoperability is defined as the ability to exchange data among simulation components, composability emphasizes the need for a conceptual alignment of data among the components part of a simulation system. In this regard, the main objective of the proposed architecture is two-fold, namely the provision of an approach for the specification of the simulation system conceptual model, to identify the available existing M&S components, and the use of the TOSCA standard for describing both the M&S components and the composed simulation, in order to address interoperability issues. In [25], it is argued that MSaaS-based applications should be easily composed by integrating loosely coupled shared components (in other words, simulation services), in a cloud-based environment. The pillars of the MSaaS ecosystem have been discussed in [26]. According to the proposed architecture, a MSaaS system should be constituted by _M&S services_, which are the building blocks of simulation applications, _registries_, and _registries_, containing M&S services descriptions and implementations, respectively, _processes_, which define how services are discovered, composed, deployed and executed, an _infrastructure_, which describes the simulation environment and, finally, a _portal_, which constitutes the entry point to start the MSaaS process. Such contributions have inspired the essential building blocks at the basis of the ArTIC-MS's architectural design, which also introduces a step forward by addressing interoperability in the case of inter-cloud service composition. Finally, existing literature also provides relevant examples of already available MSaaS platforms, such as CloudSME [27], Simulation Platform [28], and OCEAN [15]. Due to their architecture and rationale, CloudSME and the simulation platform are largely different from ArTIC-MS. CloudSME [27] is a multi-cloud platform for developing and executing commercial cloud-based simulations and its primary target audience includes commercial software vendors and consultant companies in the IT domain, as well as small and medium-sized enterprises (SMEs). The simulation platform [28] consists of a cloud of virtual machines (VMs) upon which a GNU/Linux OS runs.