## 4 Multicast Routing Approaches in DTN
### Geographical Multicast Routing in DTN
#### 4.4.1 Ferry/MUE-Based  
Ferry-Based Interdomain Multicast Routing: Interdomain multicast refers to the delivery of multicast messages between multiple domains. [PERSON] and [PERSON] [67] introduced the Ferry-Based Interdomain Multicast Routing (FBIMR) scheme to support multicast routing for data forwarding across multiple domains. FBIMR deploys one or more ferries to deliver multicast traffic from one domain to another. A source forwards multicast messages to the domain leader for delivery. A ferry helps in transferring the interdomain traffic from its own local group to outside groups by contacting the domain leaders. FBIMR works efficiently when coupled with the Encounter-Based Multicast Routing with redundancy (EBMR2) scheme for intradomain multicast delivery within local groups. Figure 12 depicts the multicast message delivery process of the FBIMR scheme. Initially, the source forwards multicast messages to the group leader. The Message Ferry (MF) travels from Group 1 to visit the Group Leader (GL) of another group. The GL uses the EBMR2 scheme to deliver multicast messages to the destinations within its own domain after receiving the messages from MF. FBIMR constructs a hierarchical multicast to lower the management overhead.  
New Vehicular Delay-Tolerant Networks: [PERSON] et al. [68] proposed a multicast routing protocol in vehicular DTN, namely New Vehicular Delay-Tolerant Networks (NewVDTN), which is a combination of some geographic-based routing features [69], PROPHET [58], and spray-and-wait [70] protocols. A dense scenario with high vehicle density is differentiated from a sparse scenario with low vehicle density according to an estimation of the number of nodes encountered. In dense scenarios, the scheme selects MULE (Mobile Ubiquitous LAN Extensions) nodes carefully by considering their movement directions and the network general storage level estimation, and the scheme then assigns a TTL to the source message to reduce congestion. Messages are scheduled according to their priority and general buffer occupancy estimation. In sparse scenarios, the scheme will forward messages to as many MULEs as possible to achieve a better ratio of delivery to the destinations. The list of encountered nodes is always sorted based on the estimated vehicle density. It is possible to forward messages to the nodes with higher contact probabilities.  
Social Dynamic Features Aware: The authors in Reference [71] examined the social dynamic features of vehicles in macroscopic and microscopic views. The former indicates that messages can be propagated between two areas easily when there is high traffic flow of taxis between them. The latter indicates the movement pattern of individual taxis with transition probability to reach one of the areas in the map based on past trajectory. Combining both macro and micro dynamic features of vehicles, the authors proposed GEOgraphic multicast routing based on the Social Dynamic Features Aware (GeoSDFA) scheme. GeoSDFA is a mobility-aware geocast scheme that calculates a likelihood value for each message, which is the probability of vehicles to forward a message towards a specific area. A node compares its messages' likelihood value with its encountered node and forwards the messages if it finds that the likelihood value of the encountered node is higher. However, there might be chances that a vehicle visits the destination areas frequently, even when the likelihood value is low. If the likelihood value of an encountered node for a message is lower, a node will further compare its contact strength to destination areas with the encountered node. Different from existing geocast schemes, GeoSDFA is not solely based on vehicle probability, but also takes into consideration the realistic scenario that some vehicles with lower probabilities are able to reach some areas.