## 2 Background  
IoT device ecosystems impose changed and shifted priorities for end devices. IoT devices often require long battery life, low deployment and maintenance costs, long operation lifetime, and careful conservation of devices and network resources [26]. These affect the communication networks and protocol stacks in IoT systems. For example, new low-power short (LP-PAN) and long-range (LP-WAN) technologies were developed. Networks like Sigfox, LoRaWAN [8], or NB-IoT [9] are especially dedicated to low bitrate and low power long-range connectivity, which was not achievable in traditional mobile or wireless networks. New radio transmission schemes and physical channels are applied. IoT devices can remain in sleep mode for the majority of their lifetimes to prolong energy autonomy. These networks are being deployed around the globe and readily provide free or commercial IoT communication services. Device connectivity can be provided with short-range technologies, too. In this case devices connect to a gateway that has a persistent internet connection. LP-PAN includes, e.g., Bluetooth (BT) [6; 27] or IEEE 802.15.4 [7]. The latter specifies the physical layer and media access control for low data-rate, low-power, and low-complexity, short-range radio frequency transmissions in wireless personal area networks. It can be further complemented by 6 LoWPAN [28], which defines a binding for the IPv6 WPANs. Additional advancements in wireless networking can be found in peer-aware communications (PAC). IEEE 802.15.8 PAC [11] is WPAN technology optimized for peer-to-peer and infrastructure-less communications with fully distributed coordination. IEEE 802.15.8 PAC features include discovery for peer information without association, a discovery signaling rate typically greater than 100 kb/s, detection of the number of devices in the network, and scalable data transmission rates. Wi-Fi Peer-to-Peer v1.7 [10] is the specification for the Wi-Fi Alliance Wi-Fi CERTIFI Bi-Fi Direct(r) program, which allows Wi-Fi client devices to connect directly with each other by setting up ad-hoc networks, without going through a wireless access point or hotspot. In hybrid IoT-BC systems, PAC and Wi-Fi Direct can complement the IoT-BC interactions where low-latency or lager peer communication bandwidth is required. The IoT protocol stack demonstrates new or adapted transport and application protocols. The TCP/IP header and protocol redundancy can become an issue if low-bandwidth communications are sought. At the transport layer the simpler UDP can be applied instead of TCP. However, as it only provides a best-effort transport service, an adapted security protocol DTLS was needed to operate over UDP. DTLS [29] implements its own sequencing and simple packet retransmission, which would otherwise be a part of the TCP. In an extreme case, IoT stack can renounce the TCP and IP and apply optimized non-IP networking and transport solutions, as for example the non-IP data delivery (NIDD) [30] over NB-IoT (a part of 3 GPP Release 13 Features) or in device-to-gateway communication through LoRaWAN MAC. The application layers Constrained Application Protocol (CoAP) [4] and Message Queuing Telemetry Transport (MQTT) [5] provide low(er) bitrate alternatives to the established HTTP(S) [31; 32] and WebSockets [33]. CoAP is a one-to-one request-response protocol for transferring state information between client and server. It is a simpler alternative to HTTP, which results in simpler hardware requirements for CoAP smart objects, as well as in the protocol's lower communication overhead and therefore of the resulting reduced power consumption [34]. MQTT is a publish-subscribe protocol: a broker decides where to copy and route messages published by the clients. This enables a many-to-many bus for live data communication. Clients mostly apply a TCP connection to the broker MQTT is not optimized for low-bitrate communications; however, the MQTT for Sensor Networks (MQTT-SN) extends the MQTT for low-power and low-cost devices.