LDAF: Low-Bandwidth Distributed Applications Framework in a Use Case of Blockchain-Enabled IoT Devices  
[PERSON]  
1 Faculty of Electrical Engineering, University of Ljubljana, Trzaska 25, SI-1000 Ljubljana, Slovenia;  
[EMAIL_ADDRESS] (D.D. layer protocols for IoT messaging (e.g., Constrained Application Protocol (CoAP) [4] and Message Queuing Telemetry Transport (MQTT) [5]), to dedicated low power personal areas (e.g., Bluetooth Low Energy (BLE) [6] and IEEE 802.15.4 [7]), low-power long-range networks (e.g., Low Power, Wide Area networking protocol (LoRaWAN) [8] and Narrow-band Internet of Things (NB-IoT) [9]), and sensor and peer-aware (e.g., Wi-Fi Direct [10] and IEEE 802.15.8 [11]) communication networks. Gateways have long been an essential part of heterogeneous and distributed information, service and communication infrastructures (e.g., signaling and media gateways in VoIP systems, gateway GPRS support node (GGSN) in 2/3G, serving (SGW) and packet data network (PGW) gateways in EPC, etc.). A communication gateway in IoT makes possible the interoperability of heterogeneous communication systems, but can also provide additional communication services, as for example security. An application gateway is finely tuned based on the given application's requirements. If it is data driven, gateways focus on message and protocol adaptations and optimizations. They can, e.g., provide API harmonization. Application-driven ones assist mediating services [12] and adapt application protocol operations. In the field of IoT, there have been examples of application-specific gateways reported for smart homes, e-health, e-mobility, and including gateways in blockchain--based IoT infrastructures [13]. Gateways may not be independent entities: they can be part of a broader middleware solution. Fog computing led to additional importance for gateways and growth in their functionality. In fog--based IoT solutions, they bring a part of the initial analytics close to the data source and thus facilitate the decentralization of application deployment. Such gateways are no longer just communication and application mediators, but now contribute to the scalability, security, programmability, and real-time features of fog systems [14]. Fog and related mobile edge computing provide architectural approaches that are complementary to traditional cloud-centric systems [15]. Thus computing, storage, networking, and control in modern information and communication systems can be allocated at various points in the system to meet the requirements of emerging applications and challenges imposed by the IoT [14]. These can be stringent latency requirements, network bandwidth constraints, and resource constraints of end devices, intermittent connectivity, or security. We are still searching for the best architectural principles to support IoT by fog [16]. A strong and solid architectural foundation must consider application requirements and be able to balance between these requirements and networking and computing resources. These challenges are additionally pointed out with novel combinations of technologies, such as the IoT supported by the blockchain [17]. Improvements that IoT and blockchain integration can result in decentralization and scalability; device management and authorization; trustworthiness in data; trusted sharing service; data monetization; autonomous machine-to-machine transactions; including smart contract--based decentralized applications; and micropayments. Application opportunities are being sought in areas as smart cities, smart homes, mobility, health, logistic, and food traceability [18]. There are still a great number of open issues that have to be studied in order to seamlessly use IoT and blockchain technologies together. [PERSON] et al. [19] point out several challenges and future research directions for effective implementation of security for IoT devices, including BC. These include resource limitations, device heterogeneity, interoperability of (security) protocols, and scalability and latency of BC networks.