## 2 Related Work  
A DTMSN is fundamentally an opportunistic network in which the topology frequently changes and communication links exist with certain probability, therefore, replication-based efficient data delivery scheme has its rationale. In epidemic routing [6], a sensor will duplicate its data messages to any neighbor in communication range only if the neighbor does not carry a copy of the data message. Obviously, network resources, such as bandwidth, buffer, battery, supply and so on, have a great influence on the performance of epidemic protocols. Given enough network resources, epidemic routing can achieve very high data delivery rate at the cost of huge traffic overhead. However, the bandwidth, buffer and nodes' energy are limited in DTMSNs, thus the epidemic protocol is not applicable for them.  
Spray-and-Wait [10] is a representative quota-based scheme, in which every message is delivered and forwarded with a same upper bound on the amount of replicas. Spray-and-Wait routing includes two phases: (a) spray phase, where data message replicas are generated and injected into the network; (b) wait phase, where data messages with single-quota will not be forwarded until a direct encounter with the respective destinations occurs. Spray-and-Focus [11] is very similar to Spray-and-Wait, and the difference between the two protocols is that Spray-and-Focus allows forwarding of data messages with single-quota in order to improve performance. Although both protocols succeed in limiting some of the traffic overhead compared with flooding-based protocols, their network performances still suffer.  
[PERSON] and [PERSON] [12] presented a replication-based efficient data delivery called RED, which consists of two components for data delivery and message management. First, data delivery uses a history-based method like ZebraNet to calculate the delivery probabilities of sensor nodes. Second, the message management algorithm decides the optimal erasure coding parameters based on sensor's current delivery probability to improve the data delivery ratio. However, as indicated in [3], the optimization of erasure coding parameters is usually inaccurate, especially when the source is very far away from the sinks. In [13], [PERSON] and [PERSON] [12] also proposed a FAD protocol to increase the data delivery ratio in DTMSNs. Besides using the same delivery probability calculation method as RED, FAD further discusses how to constrain the number of data replications in the sensor network by usinga fault tolerance value associated to each data message. However, that protocol still has a quite high transmission overhead.  
Recently, several new routing protocols such as MaxProp [7], PREP [8], RAPID [9] and OPF [14] have been proposed to achieve the desired performance. OPF assumes that all nodes have full routing information, that is, the mean inter-meeting times between all pairs of nodes. Though the authors discuss how to release the assumption from full routing information to partial routing information, the assumption is still strong. MaxProp, PREP and RAPID are new examples of flooding-based protocols. They all attempt to mitigate the inherent resource burden from the basic flooding protocol. However, their traffic overheads are still considerable high, and thus they cannot be considered ideal schemes for DTMSNs.