# A Trustworthy Communication Hub for Cyber-Physical Systems
## 2 Related Works
XMPP utilizes a decentralized client-server architecture to keep the clients simple, and pushes most of the complexity into the servers. The architecture is different from the World Wide Web (WWW) in the sense that it supports inter-domain connections called federations. In addition, the email network uses multiple hops between servers to deliver messages, but the XMPP architecture uses direct connections, which helps the creation of real-time messaging and simple clients. SIP has been applied for instant messaging, event notification, presence, and control of networked appliance communication. The capability to support exchanging of heterogeneous information content between heterogeneous resources so that the communication is efficient and communicating parties are capable to understand the meaning of the information is an essential requirement in CPS systems. For example, the IETF CoRE (constrained RESTful environments) working group has specified the constrained application protocol (CoAP) with the goal of supporting REST-like applications in constrained environments in a more efficient way. SIP has been developed for controlling multimedia sessions over the Internet, and after such a control process, the real streams can go directly between the endpoints of communications. Respective solutions are needed in CPS systems; for example, energy domain processes require application of specific information models to enable understanding the message content between communicating parties. Thus, it is very essential to have some \"identities\" for the information so that the communicating parties know what information the other entities are publishing. MQ telemetry transport (MQTT) provides a possible way for such an identification by defining the concept of \"topic\" name, which could be applied as the name for the shared information. However, MQTT seems not to have proper capabilities to define the owners and their physical resources in a simple way. The last, but not the least capability is related to the needs of each owner to control who can access use of his/her/its physical resources (devices) and exposed information, as well as how, when, and in which situations this can happen. The dynamic maintenance of the owner relationship using contact \"Buddy\" list feature of XMPP is possible. However, this is not enough, because there is need to define more specific rules and conditions for the access of CPS resources/devices concerning such aspects as, for example, privacy in a distributed ecosystem. Needs towards more centralized management of access rights have been detected in the work of [19]. A review of the access control solutions that could be applicable for the Internet of Things (IoT) has been analyzed in the work of [20]. On the basis of the detailed analysis, the main challenges of applying access control mechanisms for IoT were detected, and especially the viewpoint of constrained environment is discussed. A dynamic, scalable, and IoT-ready model based on OAuth 2.0 protocol that allows the complete delegation of authorization, so that an access control mechanism as a service is enabled [21]. The challenges of data sharing in multi-cloud environments are analyzed and an extensible access control markup language (XACML) [22] based architecture, called SAFAX, has been provided [23]. SAFAX's architecture with prototype realization allows users to deploy their access control policies in a standard format, in a single location, and augment policy evaluation with information from user selectable external trust services. The policy based access control as a service could enable owners to define and manage their access control policies to protect their resources for multiple stakeholders. Summarizing, the analysis of the requirements arising from our grand challenge proved that there are helpful features in some of the existing technologies; however, none of them is not capable to fulfill them all.