## 4 Performance Evaluation
### Buffer Management Evaluation  
In this subsection, we evaluate the different buffer management policies for different buffer sizes, in order to select the best combination. Several simulations were executed for all the 16 possible combinations using the epidemic protocol. Each forwarding and dropping combination was tested with a message TTL of 12 or 24 h, and buffer sizes of 50 MB, 100 MB, 200 MB and 1 GB. These simulation parameters are summarised in Table 1.  
Figures 8a,b show the relation between delivery probability and average message latency for each of the simulations. This plot uses a point for each buffer management policy combination with the same symbol for a particular buffer size. It shows that there is a clear correlation between delivery probability and latency and that, as the buffer size increases, the variability of the delivery probability decreases. This last fact is even more evident when increasing the TTL for the messages, where a minimum of a 200 MB buffer is required to stabilise this metric. The main factor for a good delivery probability is how long messages are available in the network (TTL), for larger times, the probability of arrival increases. The buffer size also plays an important role, as the buffer size increases the buffer management policies have a smaller impact on the message delivery. When the buffer size is large enough to keep almost all messages, the dropping policy is no longer relevant. Tables 2 and 3 show the delivery probability for each policy combination, where the best values are for the combination of _smallest_ forwarding and _largest_ dropping policies. In this case, the buffer management policies are giving priority to the small messages, which also have a better transmission chance in a short contact time. \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Values** \\ \hline Buffer Size & 50 MB, 100 MB, 200 MB, 1 GB \\ Time to Live & 12 h, 24 h \\ Forward policy & Random, FIFO, Oldest, Smallest \\ Discard policy & Random, FIFO, Oldest, Largest \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Simulation parameters varied to evaluate queue management policies. Figure 7: Average speed and number of contacts for all nodes at each hour of the movement trace. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Buffer Size**} & \multirow{2}{*}{**Drop Policy**} & \multicolumn{4}{c}{**Forwarding Policy**} \\ \cline{3-6}  & & **Random** & **FIFO** & **TTL** & **Smallest** \\ \hline \multirow{4}{*}{50 MB} & Random & 0.394 & 0.386 & 0.380 & 0.415 \\  & FIFO & 0.403 & 0.398 & 0.395 & 0.415 \\  & TTL & 0.393 & 0.377 & 0.366 & 0.417 \\  & Largest & 0.433 & 0.425 & 0.421 & 0.440 \\ \hline \multirow{4}{*}{100 MB} & Random & 0.426 & 0.419 & 0.415 & 0.439 \\  & FIFO & 0.427 & 0.420 & 0.417 & 0.438 \\ \cline{1-1}  & TTL & 0.426 & 0.418 & 0.415 & 0.439 \\ \cline{1-1}  & Largest & 0.433 & 0.424 & 0.420 & 0.442 \\ \hline
200 MB & Any & 0.434 & 0.427 & 0.423 & 0.444 \\ \hline
1 GB & Any & 0.434 & 0.427 & 0.423 & 0.444 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Delivery probability for a 12 h TTL (Time to Live) with different buffer sizes and queue policies.