"Survivable Routing in Multi-hop Wireless Networks"

Today we attended a talk given be Prof. Krishnamurthy on "Survivable Routing in Multi-hop Wireless Networks." We are starting to see multi-hop networks due to the interest from industry leading companies and the ability to obtain cheap hardware in order to implement the network. These networks consists of devices such as, laptops, handhelds, and smartphones, and routers that act as access points to provide interconnection between these terminal devices. These networks are starting to gain popularity because they use cheap hardware, the are easily and quickly expandable, and they are manageable and reliable. These networks are good networks for implementation in homes, campuses, hotels, as well as public transportation systems such as surveillance cameras, and especially military communication. There many experimental networks in use at major universities and cities such as MIT, Rutgers, Houston, and Singapore.

The talk then moved to talking about the routing in these multi-hop wireless networks. Now while these networks have their benefits there are also some issues that have to be ironed out. Theses issues include security, use of shortest path routing, and the lack of dealing with lower level functionalities such as modulation rate. Now while shortest path routing is good for wired communication, in wireless communication it leads to long links of poor quality and as a result poor performance. There is no way ideal way to establish link quality but packet delivery ratio is a reasonable tool but is still not quite the best. That is where the Expected Transmission Count (ETX) comes in. In ETX, probes are sent to every router and every router recieves these probes. Then the ETX value of a node can be computed by the equation 1/p_i where p_i is the ration of total probes received to total probes sent, or the probability that a probe will reach the node. Then the route can be determined by selecting those routers that have the smallest ETX values. This was further extended with Expected Transmission Times (ETT) which takes account for multiple transmission rates by sending probes at multiple rates.

Next we discussed the idea that order matters in these routing algorithms and that both ETX and ETT do not deal with the position of these nodes. ETOP was designed to capture the three factors that affect the cost of a path: number of nodes, quality of links, and relative position of the node.The cost of an ETOP path can be determined by adding the expected number of transmissions with the number of retransmissions required to deliver the packet. ETOP has shown better performance over ETX, and its higher reliability allows TCP to more aggressively ramp up and as a result TCP throughput improves.

The final issue is the issue concerning security. These networks do deal with security issues such as wormholes or corrupted nodes that lead to data compromisation. An attacker can perform attacks by reporting false p_f values, colluding with another attacker which can all cause routes to route through adversaries. Attackers can send more probes in order to indicate a higher quality in order to attract routes. In order to deal with these potential attackers, nodes should detect if probes are being received at a higher frequency than expected or by placing random nonce in a probe and when computing link qualities the random nonces must be reported as well, in an attempt to stop adversaries from taking control of routes.