performance in decentralized filesharing networks theodore hong freenet project
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Performance in Decentralized Filesharing Networks
Theodore HongFreenet Project
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Styles of collaboration
Centralized model e.g. Napster global index held by central authority
(single point of failure) direct contact between requestors and
providers
Decentralized model e.g. Freenet, Gnutella no global index – local knowledge only
(approximate answers) contact mediated by chain of intermediaries
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Key questions
Does it work? can we find the data? query success rates
length of query paths
Does it scale? logarithmic / linear / polynomial
Is it robust? participants are unreliable different failure modes possible
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An abstract model
Can model the network as a graph:
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Querying the network
Answering a query means finding a path source = requestor destination = provider
A distributed search problem! approximate global solution using local
knowledge same problem as IP routing
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The Freenet algorithm
Graph structure actively evolves over time new links form between nodes files migrate through network adaptive routing
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Initial simulations
Ring topology, 1000 nodes:
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Initial simulations (cont’d)
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Why does it work?
The small-world model Milgram: six degrees of separation Watts: between order and randomness
short-distance clustering + long-distance shortcuts
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Links in the small world
P(n) ~ 1/n 1.5
“Scale-free” link distribution P(n) = 1 /n k
most nodes have only a few connections some have a lot of links
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Small-world links (cont’d)
Real-world examples movie actors (Kevin Bacon game) world-wide web nervous system of worm C. elegans
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The importance of routing
Existence of short paths is not enough – they must be found
Adaptivity helps Freenet find good paths
Compare: a random-routing network
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Scalability
Real-world networks are much larger nearly 400,000 downloads of Freenet 50 million Napster users
How well does Freenet scale?
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Fault-tolerance
Unreliability is normal in peer-to-peer
Two types of failure: random failure targeted attack
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Random failure
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Targeted attack
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To do
Variable disk/bandwidth capacity if you build it, will they come?
Participants leaving and re-entering
File lifetimes “lifetime” is relative relationship between ease of retrieval and
popularity, size impact of splitting and combining
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Conclusions
Local approximations can be good enough
Small-world model provides useful framework
Metrics to consider: query pathlength clustering coefficient link distribution
Issues to consider: scalability fault tolerance under various scenarios
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For more information
“Performance” chapter in Peer-to-Peer
I. Clarke, O. Sandberg, B. Wiley, T.W. Hong, “Freenet: a distributed anonymous information storage and retrieval system,” in Workshop on Design Issues in Anonymity and Unobservability, ed. by H. Federrath. Springer: New York (2001)
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