Performance Analysis of Stealth DHT with Mobile Nodes Andrew MacQuire Andrew Brampton Idris A. Rai Laurent Mathy Computing Department Lancaster University {macquire,brampton,rai,laurent}@comp.lancs.ac.uk Abstract The advances in wireless networking and the consequent emergence of new applications that wireless networks in- creasingly support inevitably leads to low capability mobile nodes connecting to peer-to-peer networks. However, the characteristics of mobile nodes and limitations of access point coverage often cause mobile nodes to lose connectiv- ity, which can cause many mobile nodes to simultaneously rejoin the network. Continuous departures and joins due to the mobility of nodes leads to mobility churn, which can often degrade the performance of the underlying peer-to- peer network significantly. In this paper, we use simula- tions to demonstrate that the Stealth Distributed Hash Ta- ble (Stealth DHT) algorithm is ideally suited for networks with mobile nodes. By avoiding storing state in unreli- able nodes, a Stealth DHT prevents mobile nodes from be- ing used by other nodes to provide services. Consequently, Stealth DHTs eliminate the mobility churn effect and sig- nificantly reduce the amount of overhead as compared to a generic DHT. This paper demonstrates this using simula- tion results that compare the performance of Stealth DHTs to a generic DHT, Pastry. 1. Introduction Previously proposed Distributed Hash Table (DHT) al- gorithms [10][12][14] have commonly assumed connecting an overlay of autonomous and homogeneous nodes together. The autonomicity arises in the sense that nodes may join or leave the network at any time, as well as send and receive data as they wish. The homogeneity arises from the fact that devices on the network are assumed to have similar ca- pabilities in terms of processing power, storage space and network access. Mobile devices, however, are likely to be heterogeneous. They are also often battery powered and prone to moving in and out of signal range, both of which commonly cause loss of network connectivity. In a DHT, it is often the case that a mobile node migrating between access points has to re- join, as both its own state about other nodes and their state about it may have been invalidated. This causes what is termed mobility churn [3]. Mobility churn, just like normal churn [11], is caused when nodes in a peer-to-peer network continually join and leave in an unpredictable fashion. This results in more traffic on the DHT due to both nodes repeat- ing the join procedure, and stale state information having to be detected and discarded by existing nodes. Unfortunately, this degrades routing efficiency and increases end to end de- lay. Many DHT systems have been shown to simply break down under high levels of churn [6][11]. To make mat- ters worse, it has been shown that severe levels of churn are likely for peer-to-peer networks with mobile nodes [3][9]. Mobile devices are also likely to have slow network con- nections relative to stationary devices. The GSM standard, for example, is limited to only 14.4 kbps data transfer. GPRS offers some improvement, averaging at around 40 kbps [7]. Even with newer third generation (3G) devices, transfer rates are still likely to be far lower than many wired devices. In the case of such low bandwidth networks, there is the danger of DHT signalling consuming all the available bandwidth, thus blocking user traffic. We believe that the Stealth DHT algorithm proposed in [1] is an elegant solution which adapts existing DHT al- gorithms themselves to help resolve these problems. A Stealth DHT is a distributed hash table that addresses the problem of heterogeneous capabilities by maintaining two distinct sets of nodes on the network. One set, referred to as stealth nodes, are made effectively “invisible” to all routing operations, meaning that they will never receive any queries, be requested to forward messages, or asked to store keys. Consequently, they cannot intercept nor reply to any messages on the DHT. Ideally, less capable nodes on the network should be designated as stealth nodes, as their lack of responsibilities means they have little effect upon over- all routing performance. The remaining set of nodes on a Stealth DHT are called service nodes, which can execute all the operations supported in a generic DHT. For optimal performance, service nodes should be highly stable and ca-