## 2 Message Replication in OppNets for Sensed Data Collection
### Impact of High Node Population
Increased metadata overhead  
Most of the variable-quota replication techniques neglect control overhead that results from metadata dissemination [69], as they frequently transmit metadata in order to update perceived information that may become stale too quickly. Metadata transmission is thus traded for the overhead incurred in data transmissions, which may still consume the limited resources on nodes. Metadata transmissions may also interfere with the process of data transmission, due to limited bandwidth, highly dynamic user movement and intermittent connectivity. The metadata overhead incurred by VACCINE's anti-packets increases with node population. The delay in propagating them increases as well, since it depends on the rate of node encounters. Anti-packets may therefore fail to fulfil their purpose by the time they are received, as replicas of delivered messages could have been dropped due to TTL exhaustion or buffer overflow. This may result in unnecessary transmission overhead. Further influencing the amount of transmission overhead incurred in the process is the amount of TTL allocated to anti-packets. Metadata dissemination may also increase the number of failed message transmissions. Besides the issue of limited bandwidth, encounter duration is shared between metadata and data  
Figure 5: Transmission overhead under different node populations in the small scenario with PRoPHET (using no replication technique). transmission [69]. The metadata overhead incurred by some replication techniques increases in higher node population. For example, the performance of [PERSON] et al. 's [55] replication technique depends on a mechanism for deleting delivered messages. This requires nodes to exchange metadata in the form of lists containing IDs of delivered messages and expiration times. Another example is [PERSON] et al. 's [49] proposal, which requires information about each encountered node in order to maintain and update a neighbour table. The neighbour table contains node ID, buffer occupancy, a list of messages in the buffer, a list of neighbouring nodes, and a list of time stamps indicating encounter start and end times. [PERSON] et al. [58] require encountered nodes to update their community table, community graph and gateway graph. The information exchanged includes node ID with the corresponding community ID and node ID of gateway nodes for communities. In high node population, frequently transmitting such metadata (alongside existing ones such as summary vectors) may require more time [70] to successfully exchange messages. This contributes to increased contention due to the short-lived nature of node encounters in OppNets, and may eventually result to throughput degradation [71]. Figure 6 shows that the implementation of summary vectors alone reduces PRoPHET's throughput by about 14%. Poor adaptability to increased network dynamicity  
Increased network dynamicity (i.e., more rapid and significant changes in network conditions such as topology, node density and encounter rates) in higher node population may also prevent some replication techniques from performing as expected, especially in the absence of global knowledge. [PERSON] et al. [55] focus on scenarios in which the number of nodes in the network is known (e.g., battlefield scenarios). However, this is not applicable to OppNets for sensed data collection in which the number of nodes increases without prior notification. [PERSON] et al. 's [59] dynamic replication is based on the average hop count of existing routes to the destination. Due to the numerous portable handheld devices and highly dynamic network topology, it is difficult to determine the total number of routes between a node pair in real-world implementation. In the case of n-Epidemic, the choice of \(n\) may need to adapt to different network conditions in order to be suitable for implementation in realistic scenarios.