Today an application developer using a distributed hash table (DHT) with n nodes must choose a DHT protocol from the spectrum between O(1) lookup protocols [9, 18] and O(log n) protocols [20–23,25,26]. O(1) protocols achieve low latency lookups on small or low-churn networks because lookups take only a few hops, but incur high maintenance traffic on large or high-churn networks. O(log n) protocols incur less maintenance traffic on large or highchurn networks but require more lookup hops in small networks. Accordion is a new routing protocol that does not force the developer to make this choice: Accordion adjusts itself to provide the best performance across a range of network sizes and churn rates while staying within a bounded bandwidth budget. The key challenges in the design of Accordion are the algorithms that choose the routing table’s size and content. Each Accordion node learns of new neighbors opportunistically, in a way that causes the density of its neighbors to be inversely proportional to their distance in ID space from the node. This distribution allows Accordion to vary the table size along a continuum while still guaranteeing at most O(log n) lookup hops. The user-specified bandwidth budget controls the rate at which a node learns about new neighbors. Each node limits its routing table size by evicting neighbors that it judges likely to have failed. High churn (i.e., short node lifetimes) leads to a high eviction rate. The equilibrium between the learning and eviction processes determines the table size. Simulations show that Accordion maintains an efficient lookup latency versus bandwidth tradeoff over a wider range of operating conditions than existing DHTs.

During recent years, Distributed Hash Tables (DHTs) have been extensively studied through simulation and analysis. However, due to their limited deployment, it has not been possible to observe the behavior of a widely-deployed DHT in practice. Recently, the popular eMule file-sharing software incorporated a Kademlia-based DHT, called Kad, which currently has around one million simultaneous users. In this paper, we empirically study the performance of the key DHT operation, lookup, over Kad. First, we analytically derive the benefits of different ways to increase the richness of routing tables in Kademlia-based DHTs. Second, we empirically characterize two aspects of the accuracy of routing tables in Kad, namely completeness and freshness, and characterize their impact on Kad’s lookup performance. Finally, we investigate how the efficiency and consistency of lookup in Kad can be improved by performing parallel lookup and maintaining multiple replicas, respectively. Our results pinpoint the best operating point for the degree of lookup parallelism and the degree of replication for Kad.

Distributed Hash Tables (DHTs) are the basic indexing mechanism for decentralized peer-to-peer systems. How to obtain best performance in a largescale wide area context for DHT operations is an important question. Here we introduce parallelization of overlay and DHT operations using native multidestination multicasting, resulting in significant message traffic reduction for both overlay maintenance and lookup operations. We show through simulation savings of up to 30 % message reduction for the 1-hop EpiChord peer-to-peer overlay. 1.

We propose the integration of peer-to-peer network overlays with underlay networking in which multidestination multicast routing is available. Network overlay operations are parallelized by using multidestination multicast messages in the underlying network in place of same-source unicast messages. This mechanism is generally applicable to structured overlays including one-hop, multi-hop, and variablehop, and unstructured overlays. The main result is significant message reduction, which varies according to the overlay algorithm. 1.

Distributed Hash Tables (DHTs) are useful tools for building large scale distributed systems. DHTs provide a hash-table-like interface to applications by routing a key to its responsible node among the current set of participating nodes. DHT deployments are characterized by churn, a continuous process of nodes joining and leaving the network. Lookup latency is important to applications that use DHTs to locate data. In order to achieve low latency lookups, each node needs to consume bandwidth to keep its routing tables up to date under churn. A robust DHT should use bandwidth sparingly and avoid overloading the network when the the deployment scenario deviates from design assumptions. Ultimately, DHT designers are interested in obtaining best latency lookups using a bounded amount of bandwidth across a wide range of operating environments. This thesis presents a new DHT protocol, Accordion, that achieves this goal. Accordion bounds its overhead traffic according to a user specified bandwidth

Abstract—Distributed Hash Tables (DHT) provide a lookup service in peer-to-peer overlay networks. Many valuable applications have been recently built on top of several available DHTs. However, they function poorly when no direct IP connectivity is available to some nodes (e.g., located behind a NAT or firewall) or in the presence of overloaded or malicious nodes. In this paper, we propose a new method for DHT-based routing called cyclic routing. It generalizes existing single-hop look-ahead approach (also known as “Know thy neighbor’s neighbor”) and supports multipath routing. The method provides a systematic way for collecting stable and efficient overlay paths. Cyclic routing has the same theoretical dependability and efficiency upper bounds as basic DHT routing but it is more resilient when IP connectivity is limited or when the overlay suffers from overloaded nodes. Keywords: Distributed hash tables, Peer-to-Peer routing, Lookup availability I.

are becoming increasingly popular. Multi-hop systems achieve a successful lookup in O(log N) hops, whereas one-hop systems approach O(1) hops. Both approaches, but especially one-hop overlays suffer from a high number of identical messages being sent to a number of nodes on the overlay. Previous work showed that P2P networks benefit from the integration of the overlay network with the underlay network in which multi-destination multicast routing is available. This allows combining identical messages from the same source into joint multi-destination multicast messages to significantly reduce the number of messages. Our experimentation has centered around the one-hop EpiChord overlay. Here the problem is described using a Markov Model for more advanced analysis. The Markov Model is believed to be novel in two aspects: it is the first to investigate one-hop overlays and it is the first to study the performance of multi-destination multicast including the consideration of retransmissions of requests. I.

Traditional backup methods are error-prone, cumbersome, and expensive. Distributed backup applications have emerged as promising tools able to avoid these disadvantages, by exploiting unused disk space of remote computers. In this thesis we propose MyriadStore, a peer-to-peer backup system in which nodes store their backed up data by using other nodes ’ storage capacities. MyriadStore makes use of a trading scheme that ensures that a user has as much available storage space in the system as he/she contributes to others. In order to verify that nodes are storing data they have been entrusted with, a challenge mechanism is in place. A node can challenge the nodes that are responsible for holding its data by requesting from them parts of the data. In case they fail to respond correctly to these challenges, they are punished according to a reputation-based punishment model. With this model, the nodes that fail challenges are punished according to the reputation they have in the system. MyriadStore minimizes bandwidth requirements and migration costs by treat-ing separately the storage of the system’s meta-data and the storage of the backed up data. Meta-data is stored on a distributed hash table (DHT) which is provided by the DKS system, while data is stored outside the DHT. This ap-proach also offers flexibility on the placement of the backed up data, a property that facilitates the deployment of the trading scheme. ii

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Abstract—Distributed Hash Tables (DHTs) provide a useful key-to-value lookup service for many Internet applications. However, without additional mechanisms DHTs are vulnerable to attacks. In particular, previous research showed that Chord is not well resistant to malicious nodes that joined the DHT. We introduce the cyclic routing algorithm as an extension of Chord (CR-Chord). Using simulations we compare the lookup availability of Chord and CR-Chord. The results suggest that CR-Chord improves the lookup availability on the average by 1.4 times. When the number of malicious nodes is small, such as 5%, CR-Chord has almost twice lower lookup failure rate.