## 3 Motivation and IoT Applications  
The emerging IoT paradigm tries to connect almost every device to the Internet. However, this is not always possible when the challenging environment results in connectivity disruption, especially in remote areas with limited network connectivity. Although wireless sensor networks (WSN) are one of the key enablers in IoT applications, the lack of continuous electrical supply and imperfect telecommunication infrastructures hamper information exchange. For example, battery-operated sensors are often deployed in outskirt areas to collect environmental data, but the collected data are unable to reach the sink without proper communication. Therefore, DTN is designed to help bridge the gap between intermittently connected WSNs and extend wireless connectivity over opportunistic environments. For example, drones can be deployed in plantations to collect and store temperature and soil humidity data, as shown in Figure 3, and can forward the data to the control center. DTN-enabled IoT devices enable better communications among devices despite connectivity disruptions. The following section discusses some DTN-enabled IoT applications. The Internet of Bikes-DTN (IoB-DTN) [25] protocol exploits a bike sharing system to collect urban data with data aggregation mechanisms by combining multiple data from the same source or different sources into a single DTN bundle. Low cost and low power operated IoT devices (i.e., integrated sensors and communication devices) are installed on the bike to generate data and store the data as a bundle. The bundle is either relayed to encountered bikes or forwarded to the sinks. [PERSON] et al. [26] proposed BoAP, an implementation of the Constrained Application Protocol (CoAP) [27] over BP that replaces UDP with BP in the transport layer to allow CoAP communication in DTN, hence enabling a delay tolerant IoT. With this IoT-over-DTN architecture, the IoT application encapsulates the BoAP message into a BP bundle and the IBR-DTN [28] BP daemon stores the bundle and forwards it to other BP daemons in the DTN. The authors listed multicast as one of the possible optimizations to realize a more efficient BoAP implementation. They suggested that the non-singleton Endpoint Identifier (EID) properties of the BP can replace the IP multicast group address in CoAP and proposed combining multiple payloads addressed to the same destination (i.e., group EID) into a single BP bundle. NanoDTN [29] is a super-lightweight BP implementation for resource-constrained IoT devices, such as sensors. It only retains a few basic functionalities and interoperability with \(\mu\)DTN [30] due to the memory constraint of a sensor. However, nanoDTN does not fit into the scenario or application with the reliability requirement since the authors only use one MAC layer without carrier sensing, which does not ensure reliable delivery. IoT-cum-DTN [31] is an Internet of Hybrid Opportunistic Things architecture framework to interoperate existing DTN and IoT standards with an MQTT Convergence Layer Adapter (CLA). IoT-cum-DTN outperformed References [26; 29] in IoT publish/subscribe semantics using MQTT-SN and IBR-DTN. The IoT-cum-DTN node communicates with normal DTN nodes and IoT publisher/subscriber nodes as peers by encapsulating IoT messages as  
Figure 3: Using drone in DTN for precision agriculture in a palm oil plantation. DTN bundles. A comprehensive survey on using DTN solutions in IoT applications has been presented in Reference [32], and the authors discussed the adoption of DTN approaches in the IoT context. IoT applications require multicasting support for efficient routing and better resource utilization. Although there are some prominent works in the literature addressing multicasting support in DTN, such as References [22; 33], and others to be discussed in the following section, none of them discusses DTN multicast routing in the IoT context. Multicast routing in delay tolerant IoT application imposes more stringent requirements due to the resource-constrained nature of IoT devices.