## 4 Multicast Routing Approaches in DTN
### Energy-Aware Multicast Routing in DTN  
Energy is a significant metric in DTN performance since IoT devices are usually limited in battery capacity. They are expected to operate continuously without the need for frequent recharging. Nodes with low residual energy impact the performance of the network, and they do not perform data forwarding efficiently. Multicast induces more transmission overhead, which results in high energy consumption. The residual energy of the nodes can be considered as a metric to design a DTN multicast routing protocol. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Scheme**} & **Key** & **Delivery** & **Delivery** & **Delivery** & **Energy** & **Mobility** & **Compared** \\  & **Solution** & **Ratio** & **Cost** & **Delay** & **Delay** & **Algorithm** \\ \hline \multirow{2}{*}{SCBMR} & Social & \multirow{2}{*}{Higher} & Slightly & \multirow{2}{*}{High} & \multirow{2}{*}{-} & \multirow{2}{*}{High} & Epidemic \\  & Centrality & & & Lower & & & \\ \hline \multirow{2}{*}{QBMR} & Quota & \multirow{2}{*}{High} & \multirow{2}{*}{Low} & \multirow{2}{*}{Moderate} & \multirow{2}{*}{-} & Real Trace & Epidemic \\  & Replication & & & & & & Multicast \\ \hline \multirow{2}{*}{RMDA} & Flooding & \multirow{2}{*}{Higher} & \multirow{2}{*}{High} & \multirow{2}{*}{-} & \multirow{2}{*}{-} & Random & \multirow{2}{*}{ECAM} \\  & Flooding & & & & & & & \\ \hline \multirow{2}{*}{E-GBDS} & Epidemic & \multirow{2}{*}{High} & \multirow{2}{*}{High} & \multirow{2}{*}{-} & \multirow{2}{*}{-} & RWP and & FIFO, \\  & (flooding) & & & & & Real Trace & GBSD \\ \hline \multirow{2}{*}{Non-replication} & Dynamic & \multirow{2}{*}{-} & \multirow{2}{*}{Moderate} & \multirow{2}{*}{High} & \multirow{2}{*}{-} & Real & \multirow{2}{*}{-} \\  & Multicast & Tree & & & & & Traces \\ \hline \multirow{2}{*}{SMTBR} & Importance & \multirow{2}{*}{-} & \multirow{2}{*}{Low} & \multirow{2}{*}{High} & \multirow{2}{*}{-} & SLAW & DTBR \\  & and & Tree-based & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Performance comparison of buffer-efficient multicast routing in DTN. Multicast Routing Protocol using Transfer AIM-listed Probability And Energy: The Multicast Routing Protocol using Transfer AIM-listed Probability And Energy (MRPTPAE) [57] selects the next forwarding node based on the weighted average of the Energy Probability (EP), which is computed from the residual energy and transmission probability of a node. Transmission probability is based on the delivery probability of the Probabilistic Routing Protocol using History of Encounters and Transitivity (PRoPHET) [58], and when combined with EP, the optimal next hop can be determined. The source node will transfer the message to the chosen next hop if the EP of the next hop is greater than the source node. MRPTPAE also introduced a congestion control strategy to compare message forwarding factors during network congestion and drop messages with the lowest forwarding factors. MRPTPAE helps to control the number of messages in the network and leads to lower overall network energy consumption. Energy-Aware Single-Data Multicast: Energy-Aware Single-Data Multicast (EASDM) is a socially-aware multicast routing scheme with energy consumption optimization features. This scheme is extended from the Single-Data Multicast (SDM) that was proposed by [PERSON] et al. [33]. SDM is a social-based multicast routing approach that exploits the social centrality metric for relay selection. SDM is extended to provision energy awareness in multiple multicast sessions in Reference [59]. Participating nodes monitor their residual energy and exchange this information with any encountered node. Relay nodes are selected based on the centrality metric and the remaining energy of the node. As shown in Figure 11, the multicast source groups the nodes with Pi values less than 0.5 into the high-centrality set and gathers the rest into a low-centrality set.