## 5 Simulations
### Simulation Results for Different Numbers of Vehicular Nodes  
Here we compare three metric values versus three numbers of vehicular nodes, that is, 3468, 867 and 216, to perform the CCD procedure using SocialCode, SC without DP, RCM + CodeOn and FRAME + CodeOn. In this simulation, the number of the destination nodes that distribute the file is proportional to that of total vehicular nodes. Hence, the larger the number of the destination nodes is, the higher the packet collision probability among nearby destination nodes is [18]. This in turn diminishes the average decoding percentage and raises the average file downloading delay of the destination node for all four schemes, as shown in Figures 12 and 13. For example, average downloading delays of SocialCode versus 3468, 867 and 216 nodes in Figure 13 are about 558, 253 and 188 s, respectively. Further, no matter the total number of nodes is, average decoding percentages of RCM + CodeOn, FRAME + CodeOn and SC without DP are smaller than those of SocialCode, as shown in Figure 12. Oppositely, their average downloading delays are larger than those of SocialCode, as shown in Figure 13. Table 6 lists normalized ratios for average file downloading delays of SocialCode, SC without DP and FRAME + CodeOn with respect to that of RCM + CodeOn under three numbers of nodes. It shows that SocialCode achieves the lowest normalized ratio for the file downloading delays among all of them for three different numbers of nodes.  
RCM + CodeOn and FRAME + CodeOn. As mentioned above, the destination node with RCM + CodeOn exhausts its credits faster than that with FRAME + CodeOn. Oppositely, when the nearby destination or overhearing node receives the innovative content piece broadcast by some destination node, SocialCode and SC without DP will stimulate this destination node by raising its average credits by \(\alpha\), which is paid by the receiving node.  
As mentioned above, when the intermediate node with RCM + CodeOn and FRAME + CodeOn receives a content piece from the destination node, this destination one will pay one unit of its credit to increase that of the receiving intermediate node. With the same reasons mentioned in Section 4.2, SocialCode and SC without DP achieve more steady values on average credits of the intermediate node than RCM + CodeOn and FRAME + CodeOn do under three different numbers of nodes, as shown in Figure 14. Further, because SocialCode assigns dynamic forwarding priorities and backoff delays of all nearby intermediate nodes with Equations (4) and (5) respectively to schedule their sequences on re-broadcasting content pieces, the intermediate node with SocialCode has higher probability to re-broadcast an innovative content piece to its successive destination node than that using SC without DP does. Hence, the average credit of the intermediate node with SocialCode decreases slower than that using SC without DP. As shown in (a-c) of Figure 14, SC without DP gives higher average credits to the destination node but lower ones to the intermediate node than SocialCode does, no matter which number of nodes is used in the simulation. Further, the larger the total number of nodes is, the less stable and lower average credits the intermediate node has but the higher ones the destination node does. As mentioned above, these results also exhibit that SocialCode provides more even forwarding opportunities among intermediate nodes than SC without DP does.  
Finally, average credits of the overhearing node using RCM + CodeOn and FRAME + CodeOn remain unchanged but those of the overhearing node using SocialCode and SC without DP reduce to zero soon. Consequently, SocialCode and SC without DP penalize the overhearing node by preventing it from receiving any successive piece or broadcasting its own ones because of its selfish operation during the CCD procedure.