Mobile wi-fi enabled devices are becoming less expensive and more common all the time. The field is ripe for DHTs like Chord. Given the highly scalable nature of DHTs, the ability to efficiently tap the storage resources of so many devices is very attractive. Unfortunately DHTs have been having a difficult time finding a foothold on mobile devices due to poor performance, both in terms of network overhead and lookup failure. Much of the research that has been done attributes this poor performance to the extra hops packets must make due to the mismatch between the physical topology and the DHT overlay. This research has attempted to bring the logical overlay in line with the physical network to reduce the number of unnecessary hops each packet must make. A large amount of sophisticated and fascinating research has been done to match the physical and logical layers to alleviate this problem. While that mismatch is certainly important, another oft-ignored aspect of the problem is churn caused by changing IP addresses. This paper will present a survey of potential changes to the Chord algorithm (and eventually the network stack) to ameliorate the disruption that changing IP addresses have on the overlay. The first change this paper presents is simply to use the MAC address of the node as a unique identifier instead of the IP address as suggested in the original Chord paper. This allows the node to transition to a new IP address without fear of its key space changing. The next change is to the access point handoff process, where a callback was inserted into the MAC layer of the network stack to allow the Chord application to be notified of a pending access point transition and what its new address will be (termed softish handoff for the purposes of this paper). Finally several transition scenarios are detailed and tested in this paper including: traditional Chord with the IP address used as the identifier (used as a baseline); using the MAC address of there wireless interface as the Chord identifier; broadcasting the new address to associated nodes; removing and reinserting the node on IP change; an implementation of IPMN to keep the TCP connections alive even in the face of rapidly changing IP addresses (this strategy also broadcasts the new IP address to associated nodes that do not have open TCP connections). It is important to note that the focus of this thesis is to address is the disruption that changing IP addresses have on the application level overlay. Softish handoffs were implemented to facilitate this but are not intended to be a replacement of or improvement over soft handoffs provided by CDMA enabled cell networks. The unique id, new address broadcasting and IPMN based socket transitions should have similar effects in an environment with real soft handoffs, assuming there is some notification of a new access point being available. It is also not the purpose of this thesis to present a more efficient implementation of Chord, only to compare strategies for dealing with IP address transition.
Library of Congress Subject Headings
Hashing (Computer science); Peer-to-peer architecture (Computer networks); Internet addresses; Mobile computing
Department, Program, or Center
Computer Science (GCCIS)
Ecklund, Timothy, "A Survey of transition strategies to improve the performance of Chord in the face of rapidly changing IP addresses" (2010). Thesis. Rochester Institute of Technology. Accessed from
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