## 4 Multicast Routing Approaches in DTN
### Social-Aware Multicast Routing in DTN
#### 4.1.3 Discussion  
Mobile devices, such as smartphones, carried by individuals possess specific social characteristics due to the behavior and interest of their owners. Those who share similar interests can comprise Social Wireless Networks (SWNs), where the interaction patterns can be leveraged to achieve efficient data delivery. For example, the smartphone carried by an individual collects traffic data along the way when driving from home to work and shares the information with other road users who have similar interests. Positive social characteristics are common metrics exploited by researchers to design a social-driven solution to increase message delivery in DTN. Table 1 shows the performance comparison of social-aware multicast routing in DTN. Most of the schemes are compared with one of the common benchmark algorithms, which are epidemic routing in terms of delivery ratio, cost, and latency. The epidemic protocol usually stands out from others in terms of delivery ratio due to its flooding-based nature which can achieve a high rate in delivery. SDM, MDM [34], Cloud-based Multicasting [35], and TLMR [36] focus on social centrality and community to select the most suitable forwarding node based on the contact probability of a node. SDM and MDM are cost-effective because a minimum number of relays in multicast data forwarding are utilized, and hence they have a lower delivery cost. The performance of the cloud-based multicast scheme is highly dependent on the size of the destination cloud. Its delivery delay is lower when cloud size is small, with a tradeoff of higher delivery cost due to the increased number of forwarding nodes. However, the First In First Out (FIFO) message dropping policy might cause some multicast messages to be discarded before they reach the destination cloud. Reference [36] has a higher delivery cost due to the longer forwarding path that involves more relays, but the chosen route has lower latency. The routing performance is not promising if the individual has low social centrality and community characteristics. The SDMR [37] introduces two social metrics to compute the differences between nodes instead of social similarity, which is beneficial in terms of expanding forwarding coverage and relieving high centrality nodes from exhaustion. The latency for SDMR is high due to the extension of the coverage area, and a moving node will carry the message for a longer period. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  
For the social-aware schemes that are based on interest profile, QGR [38] considers the interest level of a user on specific content to implement a coupon dissemination mechanism using multicast routing strategies. QGR sacrifices latency to minimize the forwarding cost. Polymorphic epidemic multicast routing [39], Multi-CSDO, and Reference [40] only consider offline social features in constructing the user interest profile. Furthermore, all the social-aware multicast routing schemes did not take energy metric into account in the relay selection, which is a significant element in social-based networks that are comprised of highly mobile nodes. Multiple social characteristic metrics can be considered jointly when designing a multicast routing algorithm to improve the predictability of mobility patterns, since human behaviors are not dependent solely on contact opportunities. Furthermore, the combination of social metrics and opportunistic metrics may improve the routing performance, but inappropriate combinations could result in the degradation of overall network performance, and the complexity of the routing strategies increases proportionally with the number of metrics taken into consideration. Some nodes are selfish in nature and they are reluctant to participate in message relaying. The authors in Reference [41] proposed a Congestion Aware and Selfishness Aware Social Routing protocol (CAASSR) that considers energy level as the reason for a node to behave selfishly. [PERSON] et al.