# A Privacy-Preserving Protocol for Utility-Based Routing in DTNs
## 5 Performance Evaluation
### Performance Comparison
#### 5.2.1 Influence of Buffer Size  
As shown in Figure 3, the delivery ratio increases as the buffer size increases for all the routing protocols. This is because nodes can carry more messages with larger buffer, and subsequently deliver more messages successfully. The delivery ratios of PPUR1, PPUR2, PPUR3, and FaceChange are smaller than PROPHET. This is because in PPUR, only when encountering the AP can nodes deliver encounter record information to TA and get the routing utility value. So, in PPUR, nodes cannot update the routing utility instantly, compared with PROPHET, leading to a slightly smaller delivery ratio. In FaceChange, as the nodes need some time to collect encounter evidence, the routing utility is not updated quickly enough to reflect the changes on meeting frequencies among nodes, leading to inaccurate guidance on routing and degraded success ratio. We also can see that PPUR4 has a similar delivery ratio to PROPHET. This is because when there are more APs, nodes have more chances to deliver encounter record information to the TA and get the routing utility value quickly. When there are 4 APs, the delivery ratio of PPUR is almost the same or even larger than PROPHET if the buffer is more than 10 M. This is because in PROPHET, the routing utility is updated immediately after an encounter happens, which may cause it to deviate from the average utility value due to a burst on encountering nodes, leading to inaccurate message forwarding.  
The impact of buffer size on delay is shown in Figure 4. It can be seen that delivery delay increases as buffer size increases for all routing protocols. This is because when buffer size increases, nodes can store and deliver more messages with longer delay, leading to the increase of average delivery delay. The average delivery delays of PPUR1, PPUR2, PPUR3, and FaceChange are smaller than PROPHET. This is because as seen in Figure 3, nodes in PPUR and FaceChange cannot update the routing utility instantly, so some messages with longer delay are not delivered. PPUR4 has a similar delay to PROPHET, and this is because nodes can update the routing utility value instantly when there are more APs and deliver more messages with larger delay.  
The impact of buffer size on overhead ratio is shown in Figure 5. For all the protocols, the impact of buffer size has two aspects. On the one hand, as buffer size increases, nodes can get more forwarding chances, so the number of forwarding will increase, leading to the increase of overhead ratio. On the other hand, more messages are successfully delivered, leading to the decrease of overhead ratio. The impact of the latter outweighs that of the former, so the overhead ratio decreases. The overhead ratios of PPUR-AP1 and PPUR-AP4 are smaller than those of FaceChange and PROPHET because fewer messages are forwarded. The overhead ratios of PPUR-AP2 and PPUR-AP3 are larger than that of PROPHET because fewer messages are delivered.  
From the above, we know that PPUR can achieve similar performance to PROPHET and achieve larger delivery ratio and smaller overhead ratio compared with FaceChange when having proper numbers of APs.  
Figure 3: Delivery ratio for varying buffer size. PPUR1–4: APs 1–4 of the privacy-preserving protocol for utility-based routing (PPUR).