## 4 Multicast Routing Approaches in DTN
### Discussion  
Energy efficient routing in DTN receives much attention from the research community. Energy deficiency in DTN nodes is always an indisputable challenge that many researchers intend to resolve. Node mobility, frequent contact, and multicast message routing consume much energy which depletes the node's battery power more quickly. Depleted batteries increase the network disruption and impact on the delivery ratio. Table 3 shows the performance comparison of energy-aware multicast routing in DTN. We notice that the energy consumption for mobile nodes in energy-aware schemes is very low compared to other schemes. MRPTPAE [57], EASDM, and EAMDM [60] take into account node residual energy to make relay selections, and this causes the computation overhead to be higher. There are limited studies in the literature addressing provisioning energy-aware routing mechanisms in DTN multicasting services. Nonetheless, energy efficient algorithms can be incorporated into multicast routing schemes, since they are generic. Hence, we discuss some of the proposed energy-related algorithms for unicast and broadcast in DTNs. [PERSON] et al. [62] studied epidemic routing in an energy-constrained DTN and implemented an optimal energy-aware relay selection based on the node's remaining energy and message's age. Meanwhile, the nodes in the Energy Efficient Routing Protocol based on Fuzzy Ant colonies (EERPFant) [63] are able to estimate the remaining energy levels of each potential relay at the time when they meet with their destination. In Reference [64], the Distance-based Energy-Efficient Opportunistic Forwarding (DEEOF) framework estimates the node distance, tolerant delay, and the number of forward \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Scheme**} & \multirow{2}{*}{**Key Solution**} & **Delivery** & **Delivery** & **Delivery** & **Energy** & **Mobility** & **Compared** \\  & & **Ratio** & **Cost** & **Delay** & **Energy** & **Algorithm** \\ \hline \multirow{4}{*}{MRPTPAE} & Transmission & & & & & & \\  & Probability & & & & & & \\  & and Energy & & & & & & \\  & Level & & & & & & \\ \hline \multirow{2}{*}{EASDM} & Social & & & & & & \\  & Centrality and & & & & & & \\  & Energy Level & & & & & & \\ \hline \multirow{2}{*}{EAMDM} & Messages & & & & & & \\  & Priority and & & & & & & \\  & Energy Level & & & & & & \\ \hline \multirow{2}{*}{Ge-PRoPHET} & Encounter & & & & & & \\  & History & High & Low & Low & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Performance comparison of energy-aware multicast routing in DTN. DTN. The nearer the forwarders are or the earlier the time of delivery is, the less energy is consumed for message delivery. EASDM [59] is one of the few protocols that consider energy level in the design of multicast routing algorithm. Many IoT applications have an energy constraint requirement to ensure overall network availability, such as in post-disaster communication scenarios. Conventional energy saving schemes schedule nodes to sleep, reducing contact probabilities and further degrading the overall performance. Routing algorithms that solely replicate an unnecessary number of messages to increase delivery ratios are inefficient, and they cause high overheads due to redundant message replication, transmission, and reception. It is a significant energy waste, especially in delay tolerant and resource-constrained networks. These algorithms are not feasible in resource-constrained IoT environments, since energy is a vital resource to ensure network availability. [PERSON] and [PERSON] [65] devised an energy-aware routing protocol based on PRoPHET in delay-tolerant WSNs. They considered a disaster-related scenario consisting of pedestrians, cars, and trams as mobile nodes to store-carry-forward emergency messages and self-generated sensor data to a static sink node. The combination of the node's residual energy, delivery probability, and the node's category decides the message forwarding on each encounter.