A Measurement Study of Peer-to-Peer File Sharing

Systems

Presented by Hakim Weatherspoon

CS294-4: Peer-to-Peer Systems

By Stefan Saroiu, P. Krishna Gummadi, Steven D. GribbleUniversity of Washington

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What is definition of P2P?• Debatable

– Systems that lack dedicated, centralized infrastructure.

– Depend on voluntary participation of peers to contribute resources.

– Membership is ad hoc and dynamic• Challenge

– Organize peers such that they can cooperate to provide a useful service to the community.

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Big Idea• Measure current P2P systems

– Extract common peer and organizational characteristics

• Surprising, no one else thought of measure first.– Current P2P designers building with “P2P Purist”

ideals

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Measurement Methodology•Crawl

– Snapshot of system– Gathered IP address and reported characteristics

•Probe– Several days– Measured properties

• Bottleneck bw: Proportional to capacity of slowest link• Latency: RTT of 40B TCP packet • Lifetime: active, inactive, offline• number of files to share (not measured)

•Flaw – one vantage point– Measurements taken from Seattle, WA.

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Napster and GnutellaArchitecture

• Napster– Send query to centralized server– Contact peer to download– Server store index and peer characteristics

• Gnutella – Contact a gateway (e.g. limewire.com)– Perform a constrained flood to find new peers and

perform queries.

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Crawling the P2P Systems

•Napters– Look all most popular songs– Identified 40-60% of total users

• contributed 85-90% of system resources•Gnutella

– Ping/Pong with large TTL• Discovered 8-10,000 peers (i.e. 25-50% of total

population)

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Measurement Characteristics

•Bottleneck bw– Proportional to capacity of slowest link between two hosts

•Latency– RTT of 40B TCP packet

•Lifetime– active, inactive, offline– Combinations of TCP SYN, TCP RST, Connection Established, Timeout

•Flaw – one vantage point– Measurements taken from Seattle, WA.

• Note: number of files to share (not measured)

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Measurement Questions

•High bandwidth, low latency profile of a server?•High availability profile of a server?•Accurate reported bw?•No sharing, always downloading profile of client?•Number of shared files?•Resilience in face of attach?

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Measurement Results•Server Peers

– High BW, Low Latency, and High availability•78% have downstream bw of at least 100Kpbs•22% have upstream bw of less than 100Kbps•Peers lie so that resources are not over used.

– Identified free users.•Bottomline

– Undesirable and detrimental if download capacity exceeds upload bw of system.

•Gnutella has few low bw peers– Flooding to expensive– Users for technically savvy???

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Key takeaways

• Significant amount of heterogeneity– BW, latency, availability, degree of sharing– Over five orders of magnitude differences

• Client/Server model is reality– Even though systems designed with symmetric

responsibility

• Peers lie!

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Result

• P2P systems must take into account the suitability of a given peer for a specific task.