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this manner until it reaches the destination. When the first RDP reaches the destination,
the destination node verifies the signature of the source node and then sends a digitally
signed route reply packet (REP) back to the source. The REP travels the same path as the
RDP, and the same signing procedure is performed by intermediate nodes. Because the
destination must sign the REP message, only the destination is allowed to respond to the
295
10.7 OTHER TECHNIQUES


RDP. Also, because RDP messages are signed at each hop and do not contain a hop count
or a source route, malicious nodes have no opportunity to intentionally redirect traffic.
The Secure Routing Protocol (SRP) [40] is another approach to secure routing that is
based on the assumption of the existence of a security association between the source and
destination node. To initiate communication, the two nodes negotiate a shared secret key.
A message authentication code (MAC) is used to ensure that the reply message is not
modified en route to the source node. Only the destination can respond to route query
messages, and the source is assured that the destination was reached because the shared
key is used as input to the MAC computation.
Like SRP, Ariadne assumes that all pairs of communicating nodes have secret MAC
keys [22]. Each pair of nodes maintains two keys, one for each direction of communica-
tion. Ariadne is utilizes symmetric cryptographic primitives and is based on the DSR rout-
ing protocol and the TESLA broadcast authentication protocol [49]. Ariadne has the prop-
erties that source and destination nodes can authenticate each other due to the secret keys,
and that the source node can authenticate each entry on the path returned in the route re-
ply. In addition, a one-way function is utilized so that no intermediate node can remove a
previous node in the source route contained in route request and reply messages.
The Security Aware Routing (SAR) protocol described in [62] relies on trust levels to
provide security. Nodes form a trust hierarchy whereby each node is assigned a specific
trust level. Designed to run over a reactive routing protocol such as AODV or DSR, route
request and reply messages are assigned a security level by the source node. Only nodes
with at least the indicated level of security can process and forward the control messages.
Hence, SAR discovers routes in which all nodes along the path meet the desired level of
security.
Finally, a mechanism for securing the AODV protocol is presented in [63]. The proto-
col, SAODV utilizes digital signatures and hash chains for securing AODV control mes-
,
sages. The digital signatures are utilized to authenticate the nonmutable fields of the con-
trol messages, whereas the hash chains are used to secure the hop count information. The
approach assumes the nodes have access to a key management system so that the nodes
can obtain the public keys of the other nodes within the network.

10.7.3.2 Intrusion Detection and Monitoring Schemes. Intrusion detection is a
mechanism widely used in wired networks to detect malicious invaders and trigger an ap-
propriate response. Intrusion detection in ad hoc networks is somewhat less straightfor-
ward because membership in the network is open to virtually any user. Hence, it is diffi-
cult to detect when a user is actually a malicious intruder. Nevertheless, intrusion
detection techniques can be employed in ad hoc networks to detect misbehaving nodes,
particularly in networks in which the membership is well defined. Such networks include
military networks and collaborative networks comprised of a team of individuals.
An intrusion detection and response mechanism is presented in [65]. It uses the coop-
erative statistical anomaly detection model to protect against attacks on routing protocols
or other wireless applications and services. Each intrusion detection system (IDS) agent
runs independently and monitors local activities. Intrusions are detected from local traces,
and responses are subsequently initiated. If an anomaly is detected in the local data, or if
the evidence is inconclusive and a broader search is warranted, neighboring IDS agents
cooperate to participate in global intrusion detection actions.
Another approach taken by a handful of protocols is to monitor node behavior to detect
misbehaving nodes. Node misbehavior can come in many forms; however, a common ac-
296 ROUTING APPROACHES IN MOBILE AD HOC NETWORKS


tion to monitor is whether a node en route to a destination forwards data packets that it re-
ceives for that destination. For instance, the monitoring system described in [37] uses nodes
called watchdogs to monitor the forwarding of data packets by intermediate nodes. After a
node transmits a data packet, it promiscuously listens to determine whether the next hop
along the path forwards the data packet to its next hop. For this functionality to work, a node
must know the identity of the node two hops further along the path. In addition to the watch-
dog, network nodes also run a pathrater module to determine the reliability of paths. Nodes
maintain ratings for other network nodes and, hence, select paths with the highest aggregate
rating. The watchdog system is used to maintain the rating for neighboring nodes.
A similar monitoring system is described in [9]. This approach incorporates a trust man-
ager and reputation system with a path manager to detect misbehaving or nonconforming
nodes and exclude them from routing. The difference with this approach is that nodes prop-
agate information about detected misbehaving nodes so that those nodes can be excluded
from participation in the network. When a node receives such a message indicating the mis-
behavior of another node, the node must be able to authenticate the source of that message.
This prevents denial of service attacks by malicious nodes against other, benign nodes.


10.8 CONCLUSION

As has been shown in this chapter, there exists a vast variety of routing protocols designed
specifically for ad hoc mobile networks. These networks create a hostile routing environ-
ment due to the mobility of the nodes and the resulting ephemeral nature of the network
links. However, significant strides have been made toward the development of robust rout-
ing protocols that can deliver high percentages of traffic, even in dynamic environments.
It is likely that there does not exist a single routing protocol that can solve the needs of
every conceivable ad hoc network scenario. Rather, the selection of a routing protocol for
a given network is likely to be dependent upon the dominating characteristics of that net-
work. Hence, certain routing protocols are likely to perform best in networks of one set of
characteristics, while others will perform better in networks with a differing set of charac-
teristics. More work is needed to identify the sets of characteristics that promote the opti-
mum behavior of each individual protocol and class of protocols.


ACKNOWLEDGMENTS

This work is largely based on a tutorial on mobile ad hoc networks created jointly by the
author and Sung-Ju Lee of Hewlett-Packard Laboratories. The author would like to thank
Thomas Clausen, Zygmunt Haas, Yih-Chun Hu, Sung-Ju Lee, Richard Ogier, Marc Pearl-
man, Prince Samar, and Fred Templin for their insightful comments and contributions to
the protocol descriptions in this chapter.


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