## 2 Related Work  
This section reviews prior research on routing policies toward maximizing the successful message delivery and minimizing message overhead in OppNets. We first discuss routing policies that replicate a message with a limited number of copies, mention policies that find the best candidate nodes to replicate a message, and then briefly describe our proposed policy.  
When spraying a limited number of copies of a message in a network, most policies assume a sufficient number of nodes in a network move at higher speeds and cover a large area. Therefore, they are likely to encounter more nodes and expedite the message delivery to a target node [4; 5; 21; 22; 23]. [PERSON] et al. [22] considered spraying a limited number of copies of a message with the help of relays with diverse characteristics and mobility patterns. For example, nodes that have recently seen the destination, nodes that have high mobility and encounter many nodes, and nodes with unique contacts in different periods can replicate the message. However, [PERSON] et al. [24] pointed out that [PERSON] et al. [22] did not consider the transmission bandwidth between mobile nodes when transferring a message. Similarly, [PERSON] et al. [21] used the past encounter rate as a metric to decide the number of replicas of a given message which would be sent to other nodes. [PERSON] et al. [23] selected a nearest relay to the destination node to receive a message by using the location information of the destination and all the nodes in its vicinity. [PERSON] et al. [4], however, stated that [PERSON] et al. [23] require additional map topology information and to consider the stationary destination. With historical information such as location, speed, and time duration, [PERSON] et al. [4] estimated the movement range of a destination and sprayed a limited number of copies of a message toward that range. However, [PERSON] et al. [4] assumed encounter rates between mobile nodes to be identical and considered a stationary target node.  
Finding better candidate nodes is usually achieved by maintaining some utility or fitness function by nodes. This utility or fitness function is then used as a metric to relay messages [9; 12; 13; 25]. [PERSON] et al. [25] considered meeting time to deduce the conditional meeting probability of nodes. The global centrality, i.e., the ability of a node to communicate with other nodes in a network, and predictability of meeting the destination node were used to select the best candidate node. [PERSON] et al. [13] collected contact information to compute social ties such as the number of common nodes between a node pair and selected relays with strong social ties with the destination node. However, the local maximum problem remains unresolved in [13; 25]. Forwarding a message to a node with a higher delivery chance does not mean that it is moving closer to the destination. In a study by [PERSON] et al. [26], a node with a higher source-to-destination probability or higher global activeness was selected as the next candidate node. Based on the nodes' contact patterns, the activeness and probability of reaching the destination were calculated. Though finding better candidate nodes can improve the message delivery service, a message replica may be transmitted multiple times, loop around the network, and induce more messaging overhead in a network [22]. This situation can be avoided by creating a limited number of copies of a message in a network.  
The preceding literature presents significant efforts for message forwarding in OppNets. Our proposed policy adopts controlled replication and sprays a limited number of copies of a message in the network. Additionally, relays are selected by considering their location information, encounter frequency with the destination, moving direction of the destination, and centrality. Therefore, our policy exploits both the geographic information and contact frequency of relays to enhance the message delivery in OppNets.