## 2 Problem Background
### Existing OppNet Routing Protocols for Sensed Data Collection
#### 2.2.1 Overview of OppNet Routing Techniques  
Many OppNet routing protocols have been proposed so far, ranging from those that require little or no knowledge about the network to more complex ones that utilize prediction approaches to improve overall network performance [30]. In order to cope with intermittent connectivity, the Epidemic [31] approach forwards messages to any encountered node that does not have a copy. Although relatively easy to implement, the performance of Epidemic depends on the availability of resources, which is limited in portable handheld user devices. Direct Transmission [32], on the other hand, keeps messages at the source until the destination is directly encountered. This approach is also relatively easy to implement, but is unsuitable for collecting sensed data to remote gateways. First Contact [33], another routing approach that is relatively easy to implement, forwards messages to the first available relay node, or randomly selects one from a group nodes within transmission range. Due to the criteria for selecting relays, messages are likely to remain buffered for long periods and may never progress towards the destination. To find a balance between Epidemic and Direct Transmission, Spray and Wait [34] (SnW) allows only \(L\) copies of the message in the network. The source forwards copies of the message to any encountered node that does not have a copy, and retains the last copy until the destination is encountered. The Resource Allocation Protocol for Intentional DTN (RAPID) [35] extends Epidemic by obtaining knowledge of the global state of resources with an in-band control channel. These solutions may benefit OppNets due to their simplicity and ease of deployment. However, their major drawback is the lack of knowledge about the forwarding ability of encountered nodes. Furthermore, the performance of SnW may be scenario-specific under more realistic movement patterns, as it depends on a careful selection of \(L\), while the highly dynamic nature of the network may cause RAPID to incur high costs from utilizing an in-band control channel. The need to guarantee delivery without excessive resource consumption or utilizing in-band control channels brought about prediction techniques. This led to various means of obtaining knowledge from locally available information for predicting the progress nodes can make towards delivering messages [36]. Basically, messages are forwarded to relay nodes with higher forwarding utilities--a property that quantifies the progress a node can make towards the destination. Some routing techniques compute forwarding utilities from the number of encounters between nodes: the Probabilistic Routing Protocol using History of Encounters and Transitivity (PRoPHET) [37] forwards messages to nodes that often encounter the destination; the Improved Probabilistic Routing Algorithm (IPRA) [38] considers the encounter history of the node's neighbours and the destination as well; and the Improved Spray and Wait (ISW) [39] forwards messages to nodes that have remained in contact with the destination for longer time slots. Forwarding utilities can also be computed from how recently the destination was encountered: the Time Message System (TMS) [40] selects a relay if it had a more recent encounter with the destination; and the Optimal Opportunistic Forwarding (OPF) [41] derives this knowledge from the inter-meeting time with other nodes in the network. There are also strategies that predict future encounters between two nodes from the similarity of their movement patterns. MobySpace [42] forwards the message to a relay that has a more similar movement pattern with the destination. Other routing techniques derive forwarding utilities from social network characteristics or contextual information. PeopleRank [43] selects relays that have a higher social rank. dLife [44] forwards messages to nodes that have a history of better social engagement with the destination. Bubble Rap [45] computes forwarding utility from node centrality between and within communities.