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packet do not have to estimate the cash that needs to be loaded on the packet The disadvantage of the Packet Trade Model is that the source does not need to expend any cash to originate a packet, leaving the scheme open to denial of service attacks from the source In the most promising variation of this scheme [97], the tamper-resistant hardware module maintains a nuglet counter Whenever the node sends its own packets, the counter is decreased Whenever the node forwards packets destined for other nodes, the counter is increased This encourages nodes to participate in the MANET routing process This is because, if nodes want to originate traf c, their counter needs to remain positive, which requires them to forward packets from other nodes Under simulations this scheme also seems to provide good throughput for the network, even when there is a signi cant number of misbehaving nodes The limitation of the scheme is that it does not propagate knowledge of node misbehavior throughout the network, ostensibly to improve overall performance It also relies on tamper-resistant hardware, which may not be available (or realistic) in certain applications Having considered the types of architectures possible as well as the implications of the choices made, we next look at another important factor This is the data or evidence that nodes collect Analysis and decisions are based on the collected evidence
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Detections of intrusions depend very much on the quality of the data on which these decisions are based Each node has access to data collected using a variety of ways Potential types of evidence depend on the nature of collection and include:
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local evidence available on the node; traf c observed on the broadcast wireless medium through promiscuous monitoring; evidence made available by other nodes
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One of the simplifying assumptions we make here is that the data that is required for detecting an attack is not encrypted This is usually the case for routing messages, IP header information, and possibly content of messages If a large portion of the network traf c is encrypted, the IDS s ability to analyze intrusion detection information may be signi cantly limited Note that the traf c can still be collected A potential solution to this problem is to make keys available at IDS nodes that can be used for decrypting the relevant data Distribution of such keys may not always be possible and may potentially introduce additional vulnerabilities into the security scheme Note also as mentioned earlier that, even when traf c is encrypted at a particular layer, the intrusion detection analysis could operate at a different layer We next look at the types of evidence 551 Local Evidence
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Local evidence is the most reliable information for a node to use when performing intrusion detection There are several sources of data available, including audit logs of host activity and failed logins as well as network traf c received or relayed by the node The traf c that can be monitored by a node not only includes traf c that it originates for
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other nodes and the traf c that is destined for the node but also traf c relayed by this node as part of the routing process 552 Promiscuous Monitoring
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In wireless networks, radios have the ability to observe all the traf c that is transmitted by every node near them provided the signal is strong enough The process of promiscuous monitoring itself is not dif cult A wireless node receives all the packets that have strong enough signal for them to be interpreted correctly at the physical and medium access control layer Under normal conditions the medium access control layer lters out all the packets whose destination layer 2 address (MAC address) does not match the address of the node in question, but when a node is operating in the promiscuous mode, the MAC layer does not lter any packet that it receives but delivers all these packets to the higher layer This results in the node being able to receive all packets being transmitted in its neighborhood Note that this includes packets that will not be relayed by the node The number of neighbors that a node can hear depends on the power levels used by the neighbors For example, if the power level of the transmitting node is high, then the strength of the signal is high and a single node may be able to hear a large number of its neighbors over a fairly wide area Operation in promiscuous mode increases the number and types of attack that a node can detect This is because a node can observe a signi cant portion of the traf c sent and received by its neighbors and therefore the node can determine whether any of those neighbors are misbehaving Therefore, several techniques that take advantage of promiscuous monitoring have been proposed Several detection techniques discussed in this chapter depend on promiscuous monitoring Promiscuous monitoring also has signi cant limitations, though [84] There are several radios that do not support promiscuous monitoring since the corresponding drivers at the MAC layer might not allow the ltering to be turned off The promiscuous monitoring mode of operation also increases the power consumption for the radio because it requires radios to monitor transmissions not destined for them Intrusion detection techniques based on promiscuous mode monitoring may generate a number of false positives (and missed detections) because promiscuous monitoring may not provide an accurate view of successful transmissions of neighboring nodes A node using promiscuous monitoring may not see all the packets that are received or transmitted by its neighbor For example, a temporary obstruction or link loss may block some of its neighbor s traf c from being observable In other cases, a node may move in and out of range from the node monitoring its traf c promiscuously, resulting in an inaccurate view of the traf c There is another subtle case that limits the effectiveness of promiscuous monitoring called the hidden terminal effect (see Figure 510) Let us assume that node C is monitoring its neighbor, node B As shown in the gure, node C is within B s transmission range and therefore node C can use promiscuous monitoring to determine whether node B is forwarding packets as expected (and therefore not launching a packet dropping attack) Let us assume that node C has sent a packet to node B that is destined for node A and is now waiting to see whether node B will forward the packet to node A At the same time node C is within node D s transmission range If node D transmits some traf c at the same time node B is forwarding node C s packet to node A, then node C will see a collision of node B s and D s transmission and will therefore not be able to see node B s transmission Node B s transmission to A might actually have been successful
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