## 6 Future Work and Conclusion
### Future Work: A Cross Layer Approach to Increase the Robustness of a MP2P System  
While most traditional security mechanisms for P2P systems are efficient when an underlay with sufficient bandwidth is available, they fail when resources are limited. Therefore, we discuss a cross-layered approach in this section. Cross-layering can improve the robustness of MP2P networks without strongly increasing the required bandwidth. In promiscuous mode, the neighbors of malicious nodes are able to detect dropped messages, as proposed for the _Watchdog_. Further, most DHT algorithms as Pastry or Chord forward requests closer to the destination with each hop in the P2P overlay. Therefore, malicious nodes that redirect messages can be detected whenever a packet is forwarded to a node that is further away regarding the logical distance to the object-ID. In the case of dropped or redirected lookup requests, the monitoring neighbor node initiates a new lookup and forwards the request to the determined next intermediate overlay node. As there may be more than a single node in transmission range, benign neighbor nodes have to coordinate themselves before flooding the network with several redundant lookup requests. In the best case, the node that has forwarded the request before a misbehaving node was reached restarts the lookup process. Several countermeasure were proposed to provide secure and robust MANET routing. In the following, we assume a benign behavior of each node regarding the underlay routing and message forwarding and only consider malicious behavior in the overlay. We assume that both source and destination nodes behave benignly. As the source node benefits from a correct routing, a malicious behavior regarding a routing attack is very unlikely. Whenever a destination node behaves maliciously, replicas can be used to increase the availability of objects stored at malicious nodes. Therefore, we consider monitoring intermediate overlay nodes only. While in a P2P system using a recursive lookup algorithm on top of a wired underlay, malicious nodes can most probably drop messages unnoticed, in wireless networks neighbors can detect dropped or redirected messages. For this, each node in transmission range of sender \(r_{s}\) and malicious receiver \(r_{r}\) (please note that we refer to a receiver regarding a one-hop physical wireless transmission, not a destination node of a multi hop route) monitors the packets sent by both nodes. If a malicious or selfish node receives a packet but neither forwards it nor responds with a reply message to the source node (as shown in Figure 8(b)), all neighbors of both nodes (\(r_{s}\cap r_{r}\)) including the sender can detect this attack. Figure 8: Examples for an attack on an MP2P network and the reaction of the cross-layer countermeasure. Selfish intermediate overlay nodes affect the delivery rate of lookup requests as they drop received requests instead of forwarding them in order to reduce the energy consumption. This behavior can be monitored by neighbors which, in turn, can forward the packet on their own as each node in the network maintains a lookup table for the P2P overlay. In order to prevent loops, unique request IDs and a time to live field have to be used. Also, when a malicious node redirects lookup request, neighbor nodes could forward the request to the correct next hop node or initiate a new lookup process. Due to this, colluding nodes may not generate a routing loop. Regarding the cross-layer approach, several factors that affect the efficiency of security measures have to be analyzed, including node mobility and physical node density in the underlay. As an example, an intermediate node could leave reception range of a monitoring neighbor before a packet is forwarded correctly. This way, false positives could be generated, _i.e._, the monitoring neighbor detects a packet loss where none occurred. Therefore, only neighbors that are in close proximity (regarding transmission/reception range) to the monitored node should be considered for performing monitoring tasks.