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بخشی از مقاله انگلیسیعنوان انگلیسی:Chord: A Scalable Peer-to-peer Lookup Protocol for Internet Applications~~en~~

Abstract

A fundamental problem that confronts peer-to-peer applications is the efficient location of the node that stores a desired data item. This paper presents Chord, a distributed lookup protocol that addresses this problem. Chord provides support for just one operation: given a key, it maps the key onto a node. Data location can be easily implemented on top of Chord by associating a key with each data item, and storing the key/data pair at the node to which the key maps. Chord adapts efficiently as nodes join and leave the system, and can answer queries even if the system is continuously changing. Results from theoretical analysis and simulations show that Chord is scalable: Communication cost and the state maintained by each node scale logarithmically with the number of Chord nodes.

۱ Introduction

Peer-to-peer systems and applications are distributed systems without any centralized control or hierarchical organization, in which each node runs software with equivalent functionality. A review of the features of recent peer-to-peer applications yields a long list: redundant storage, permanence, selection of nearby servers, anonymity, search, authentication, and hierarchical naming. Despite this rich set of features, the core operation in most peer-to-peer systems is efficient location of data items. The contribution of this paper is a scalable protocol for lookup in a dynamic peer-to-peer system with frequent node arrivals and departures.

The Chord protocol supports just one operation: given a key, it maps the key onto a node. Depending on the application using Chord, that node might be responsible for storing a value associated with the key. Chord uses consistent hashing [12] to assign keys to Chord nodes. Consistent hashing tends to balance load, since each node receives roughly the same number of keys, and requires relatively little movement of keys when nodes join and leave the system.

Previous work on consistent hashing assumes that each node is aware of most of the other nodes in the system, an approach that does not scale well to large numbers of nodes. In contrast, each Chord node needs “routing” information about only a few other nodes. Because the routing table is distributed, a Chord node communicates with other nodes in order to perform a lookup. In the steady state, in an N-node system, each node maintains information about only O(log N) other nodes, and resolves all lookups via O(log N) messages to other nodes. Chord maintainsits routing information as nodesjoin and leave the system.

A Chord node requires information about O(log N) other nodes for efficient routing, but performance degrades gracefully when that information is out of date. This is important in practice because nodes will join and leave arbitrarily, and consistency of even O(log N) state may be hard to maintain. Only one piece of information per node need be correct in order for Chord to guarantee correct (though possibly slow) routing of queries; Chord has a simple algorithm for maintaining this information in a dynamic environment.

The contributions of this paper are the Chord algorithm, the proof of its correctness, and simulation results demonstrating the strength of the algorithm. We also report some initial results on how the Chord routing protocol can be extended to take into account the physical network topology. Readers interested in an application of Chord and how Chord behaves on a small Internet testbed are referred to Dabek et al. [9]. The results reported by Dabek et al. are consistent with the simulation results presented in this paper.

The rest of this paper is structured as follows. Section II compares Chord to related work. Section III presents the system model that motivates the Chord protocol. Section IV presents the Chord protocol and proves several of its properties. Section V presents simulations supporting our claims about Chord’s performance. Finally, we summarize our contributions in Section VII.

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