## 4 Evaluation
### Quantifying the Burden on Nodes  
Another issue is the scenario-specific performance associated with inferring burden from buffer information. Depending on movement patterns and encounter opportunities, some nodes are able to free their buffers faster than others. Consequently, buffer occupancy may not always provide a good reflection of the burden on nodes. In fact, instantaneous buffer occupancy depends on various variables that may not correlate with the relative amount of contribution a node has done in forwarding messages (e.g., TTL, the number of destinations that can be directly encountered, the queuing policy in use, and routing protocol conditions for dropping messages). To demonstrate this, we select Node 114 and 177, two neighbouring nodes running on PRoPHET, for comparison under the fluctuating data traffic pattern illustrated in Figure 17. Messages are generated in two phases, one on the second day and the other on the fourth day, with a 1-day rest period between them. In both phases, each source node generates 1 message every hour for the first 6 h, followed by 1 message every 4 h for the rest of the day. Figure 16: Number of failed transmissions (aka. “aborted messages” in the ONE simulator) recorded by PRoPHET, CCAF and FLARoute. Figure 17: Message generation rate at each source node for investigating the impact of fluctuating data traffic on inferring node burden from buffer information. Since we have configured energy to be consumed only when messages are sent or received (cf., Section 4.1.1), the burden on nodes can be studied from their energy consumption. Hence, to observe the impact of the fluctuating data traffic on the resource utilization of the selected nodes, we record the energy consumed on hourly basis in Figure 18. From the figure, it can be observed that Node 114 is the more popular node, as it consumes more energy. Also, the energy consumed on Node 114 is more than that on Node 177 at any point in time. This signifies that the burden on Node 114 remains higher than the burden on Node 177 throughout the simulation. In simulations, energy level can give an insight to node burden since the environment can be controlled to consume energy from only routing related tasks. However, energy level is not a suitable burden measure for designing fair routing solutions intended for real-world implementation. As mentioned earlier in Section 2.1.3, users perform various energy consuming tasks on their devices. Hence, unless the energy consumed due to participation in routing can be determined, energy level is not a valid measure for node burden in real-life scenarios. The existing fair routing solutions infer burden from buffer information. To observe how buffer information responds to the fluctuating data traffic, we record the buffer occupancy of both nodes every hour in Figure 19. Node 177, despite being the less popular node, experiences higher buffer occupancy for extensive periods, as indicated in the figure. This is mainly because Node 114 is able to free its buffer at a much higher rate. This shows that the buffer state favours nodes that free their buffers at a slower rate, hence may not be able to reflect the energy a node has consumed due to routing. Thus, erroneous routing decisions could occur if burden is inferred from buffer information during these periods. For instance, existing proposals for improving fairness would detect a higher burden on Node 177: from 33 to 39 h; for most of the period between 76 h and 87 h; from 95 h to 104 h; and for most of the time afterwards. During these periods, however, energy consumption (cf., Figure 18) indicates that Node 114 remains the more burdened node by a long margin. Figure 19: Buffer occupancy against time for Node 114 and 177. Figure 18: Energy consumption on Node 114 and Node 177. It is fair to say that existing approaches for improving fairness try to balance the buffer occupancy of nodes, rather than achieving fairness in terms of resource utilization (e.g., energy consumed in sending and receiving messages).