Today's class was mostly on Prof. Krishnamurthy's talk on routing within a multi-hop wireless network. Multi-hop wireless networks are networks involving multiple static routers connecting wirelessly over some area, and are commonly seen in things like city-wide wireless networks, campus networks, surveillance, and the military. Research on this kind of networks is being carried out in many places, involving Rutgers, MIT, and UCR. What separates these networks from wired networks is the fact that spatial distance and arrangement matter.
Currently, the most popular protocol uses ETX (Expected Transmission Count) as a metric of connection quality. ETX uses number of expected number of packets to send one good packet as weight on a network graph connection. However, most protocols put too much weight on number of hops needed to a destination, and so favors longer, less reliable hops over short, very reliable hops between routers. This forces each router to send more packets, expecting them to not arrive successfully. In addition, ETX is blind to where unreliable connections are located – lost packets toward the end of a path means that a failure message will have to travel all the way back to the sender, causing more network congestion.
Prof. Krishnamurthy proposes ETOP, a protocol that takes into account both the number of hops, reliability, and the location of unreliable connections. The metric uses probe packets to determine where unreliable connections are and places greater cost toward unreliable segments which are farther in the path. Because of this, the protocol generates non-commutative paths (Paths from routers A to B aren't necessarily the same as B to A). Regardless, he has proven that greedy algorithms are usable for determining paths given this metric, and so Dijkstra's algorithm is usable for the protocol.
ETOP on average shows better goodput (useful bandwidth) compared to ETX, especially for multi-hop paths. The protocol interacts somewhat chaotically with TCP, making its congestion window fluctuate wildly, but almost always better goodput nonetheless. ETOP also shows worse round trip time since it may favor paths with more hops.
Prof. Krishnamurthy also talked about security in a wireless network, in particular about safeguards against certain common attacks. A wormhole attack places an attractive link in the network, allowing the attacker to snoop on many of the packets going through the network. Gray hole and black hole attacks expand on this concept by also consuming some or all of the incoming packets. This effectively creates a denial of service attack. Sybil attacks expand on wormholes by spoofing as multiple clients to obtain disproportionately many packets. Colluded attacks involve multiple clients working together to give their fake connections more reliability.
His proposal for network security involve a protocol which detects and uproots attackers. The protocol (separate from ETOP) stops attackers by first looking for suspicious traits for every client on the network. Questionable clients are then interrogated. Challenge packets are sent to the offending clients, which must be replied to in a certain way. Failure rates incompatible with their advertised reliability would expel those clients from the network. This protocol has a very high success rate and low false positive rate, but makes the network significantly less efficient.
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