## 2 Background and Related Work
### Black-Hole Solutions in DT-IoV
A packet delivery record includes identifying the nodes that exchanged packets, the number of received packets from the encountered node, the number of forwarded packets to the encountered node, and the current time-stamp. Each node has two specific tables in its memory, a receiving record table (RRT) and a self record table (SRT). The RRT maintains the most recent packet records generated by its encountered nodes. The SRT keeps the most recent packet records it generates for each node encounter. The scheme begins when two nodes meet. Each node requests the other node's RRT. With the RRT, a node can calculate if the other node forwards few packets as compared with packets it receives, similarly to the ER scheme of the MDS described above. A threshold is used to mark nodes as suspicious. If the threshold condition is passed, the SRT is used to determine whether the node met drops our packets. When a node is marked as suspicious more than a certain number of times, it is classified as malicious. Otherwise, the nodes exchange packets and generate packet records. This detecting black-holes method has a black-hole detection ratio for different mobility schemes of over 85% and a false-positive ratio of less than 1%. While it can detect packet drops, which [14; 15] could not, it still suffers from the same problems. Furthermore, in [16], no punishment mechanism is described to identify malicious nodes or a reward to well-behaved nodes. Without punishment, malicious nodes do not have an incentive to cooperate in the network. Another scheme, called RCAR [17], does not detect black-hole attackers but limits the effects of their presence. This scheme also presents some interesting ideas. Identically to previously described schemes, nodes maintain a local notion of reputation. In RCAR, messages carry a list, called nlist, with the identifiers of nodes that forwarded it. If the message passes more than once in the same node, it is not added again. To avoid malicious nodes adding or modifying identifiers, the message also carries a list, called slist, of digital signatures to certify the information in nlist. When a node receives a message, it updates the reputation of nodes that forward the message as specified in the lists. When the message is successfully delivered to its destination, the destination sends an acknowledgment (ACK) message to the sender. The ACK starts with the nlist and slist of the message it is acknowledging. Otherwise, it works as a regular message so that it can follow a different path from the one taken by the original message, and different nodes can contribute to its forwarding, being added to the lists. When the ACK reaches the original sender, it uses the lists' information to update the reputation information of nodes that forward the messages. Some problems are found in the scheme. Nodes cannot distinguish a message dropped by a black-hole node from a message dropped, because a node has no buffer space. Furthermore, there is no information about the specific node that dropped the message. To manage this situation, an aging mechanism decreases all nodes' reputation periodically. However, the smaller a node reputation is, the less likely it is that the node is chosen to forward messages. Other problems can be found in RCAR. Namely, not knowing or not having an idea of which node misbehaved, messages from that node can still be received, and the node has no incentive to cooperate. In [18], a cooperative watchdog system (CWS) is proposed. The CWS monitors messages delivered, relayed, and dropped. The CWS of a node exchanges reputation information with other nodes, which is a significant advantage as compared with previously described schemes. However, while the impact on routing performance is assessed, the misbehaving detection ratio is not assessed, and neither are the ratios of correct and incorrect classifications. Other methods exist to try to solve black-hole attacks in delay-tolerant networks. This section only presented some of them and described only the most relevant parts of those schemes.