L10-chord.ppt

# L10-chord.ppt - Chord A Scalable Peer-to-Peer Lookup...

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Chord: A Scalable Peer-to-Peer Lookup Service for Internet Applications * CS587x Lecture Department of Computer Science Iowa State University *I. Stoica, et al., published in SIGCOMM’ 01

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Freenet Highlights Each file is identified by a binary key Each node maintains a routing table, a list of item ( host , key ) Query is sent to the neighbor with the nearest key Files are replication over their retrieval path Problems Host transience Keyword search No guarantee on when a file can be found Awful in requesting for a file that does not exist A B C D E
Research challenge Given a file, can we find the node that stores the file immediately? If there is not such file, the query should be answered within a limited steps

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Motivation example N 0 N 2 N 4 N 6 N 7 N 1 N 3 N 5 Example 1. Assume there are 8 nodes and we can arrange them into a ring 2. Given a file, we can hash it and store it in the node whose id is equal to the hash value
Motivation example N 0 N 2 N 4 N 6 N 7 N 1 N 3 N 5 Example 1. Assume there are 8 nodes and we can arrange them into a ring 2. Given a file, we can hash it and store it in the node whose id is equal to the hash value Problems 1. The number of nodes is not known ahead Dynamically changed 2. Different files must have different hash values The hash domain should be large enough

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Chord at 30,000 feet high Hash node and file Each node can be identified by a hash value, e.g., H(IP) This value is called the node’ s identifier Each file can be identified by a hash value, e.g., H(file) This value is called the file’ s key Node identifier and file key have the same domain space Both are m bits Mapping between nodes and files A file with key K is stored in the node identified by successor(k) If node K exists, this is node K If node K does not exist, this is the next available node
Chord Ring N 0 N 6 N 1 N 3 N 5 Chord Ring Each node is identified by a hash value These nodes can be organized into a ring, although some positions may be empty Node-File mapping K is stored on node whose identifier is successor(K) successor(K) = K if node K exists Otherwise, it is the next available node successor(3) successor(4)

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Notion of Successor(K) N 0 N 6 N 1 N 3 N 5 o File K is stored on node whose identifier is successor(K) successor(K) = K if node K exists Otherwise, it is the next available node successor(3) successor(4) successor(0) 0 successor(1) 1 successor(2) 3 successor(3) 3 successor(4) 5 successor(5) 5 successor(6) 6 successor(7) 0
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