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Lewis & Clark College Portlan d, OR Slide 1 of 18 Request Algorithms in Freenet-style Peer-to-Peer Systems Jeff Lesh Lewis & Clark College Portland, OR USA Dr. Jens Mache, Jason Guchereau, Melanie Gilbert, Felix Ramli, Matt Willkensin
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Page 1: Request Algorithms In Freenet Style Peer To Peer Systems

Lewis & Clark College Portland, OR

Slide 1 of 18

Request Algorithms in Freenet-style Peer-to-Peer Systems

Jeff Lesh

Lewis & Clark College

Portland, OR USADr. Jens Mache, Jason Guchereau, Melanie Gilbert,

Felix Ramli, Matt Willkensin

Page 2: Request Algorithms In Freenet Style Peer To Peer Systems

Lewis & Clark College Portland, OR

Slide 2 of 18

Overview

Introduction to Freenet Performance issues

Modified Request Algorithms Results

Conclusions

Page 3: Request Algorithms In Freenet Style Peer To Peer Systems

Lewis & Clark College Portland, OR

Slide 3 of 18

What is Freenet?

completely decentralized forwards requests to one neighbor dynamically changes the overlay network and

replicates files on demand. Adapts to network usage Improves when used

Global storage space

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Lewis & Clark College Portland, OR

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=Data Request =Data Return

Node AReferences Data Store225 - B

932 -M

940-R

925

930

931

Node DReferences Data Store100 - G

101 - H

113 - J

099

111

113

Node BReferences Data Store025 - X

115 - C

230 - F

224

225

233

Node CReferences Data Store111 - D

112 - E

119 - L

115

118

120

111 - D

225 - B

932 -M

940-R

025 - X

115 - C

230 - F

111 - D

112 - E

119 - L

099

111

113

025 - X

111 - D

115 - C

230 - F

225 - B

932 -M

940-R

Request AlgorithmRequest Algorithm

•These changes in the overlay network improves the performance of subsequent queries

•Some caching as well.

Page 5: Request Algorithms In Freenet Style Peer To Peer Systems

Lewis & Clark College Portland, OR

Slide 5 of 18

Freenet’s design goals

Privacy Privacy for information producers, holders, and

consumers; Resistance to information censorship;

Performance High availability and reliability through decentralization;

and Efficient, scalable, and adaptive storage and routing.

Page 6: Request Algorithms In Freenet Style Peer To Peer Systems

Lewis & Clark College Portland, OR

Slide 6 of 18

This talk focuses on efficiency.

Our measure of efficiency is pathlength. Important for:

Users - delay Chance of slow links

Network – bandwidth usage

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Lewis & Clark College Portland, OR

Slide 7 of 18

Our Questions

How good are the existing algorithms? “As the network is used, pathlength decreases”

[Hong’01]. But, this assumes 50% inserts and 50% requests.

Can we improve it?

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Lewis & Clark College Portland, OR

Slide 8 of 18

Figure 2 50:50 vs. 99:1

Page 9: Request Algorithms In Freenet Style Peer To Peer Systems

Lewis & Clark College Portland, OR

Slide 9 of 18

Why does this happen?

Insert and request algorithms creates specialized references. Insert does more.

Requests don’t change the overlay network if they fail (and they do a lot at first).

We want to change these weaknesses.

Page 10: Request Algorithms In Freenet Style Peer To Peer Systems

Lewis & Clark College Portland, OR

Slide 10 of 18

Small-world effect

The overlay network created by the routing tables is an example of a small-world network.

Specialization as well as references to nodes that have far different specialization is key.

Real-world example. Also common in:

Social networks - film actor collaboration Neural Networks - C. Elegans (worm) Power Grid [Duncan J. Watt et.al. Nature ‘98]

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Lewis & Clark College Portland, OR

Slide 11 of 18

The Experiments

Aurora Simulator – written by Theodor Hong It simulates nodes interacting with other nodes Inserting and requesting existing documents and

recording the number of hops it takes to find them.

The data we present is an average from 100 simulation runs.

Page 12: Request Algorithms In Freenet Style Peer To Peer Systems

Lewis & Clark College Portland, OR

Slide 12 of 18

Modified Request Algorithms

Learning from failed requests. Announcing successful requests.

Breadth-random fashion Breadth-neighbor fashion

Page 13: Request Algorithms In Freenet Style Peer To Peer Systems

Lewis & Clark College Portland, OR

Slide 13 of 18

Failed requests

As is they don’t add any short-cuts. Our goal was to take advantage of the work

already done up to the expiration of the HTL. Short-cut from the requesting node to the last

node’s data item with key closest to requested one.

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Lewis & Clark College Portland, OR

Slide 14 of 18

Example

Page 15: Request Algorithms In Freenet Style Peer To Peer Systems

Lewis & Clark College Portland, OR

Slide 15 of 18

Successful Requests

The main idea is to add extra reference pointing to the fulfiller for each remaining HTL.

Breadth-random Breadth-neighbor approach worked best.

Here is announces itself to its neighbors with key close to the one it fulfilled.

This promotes specialization.

Page 16: Request Algorithms In Freenet Style Peer To Peer Systems

Lewis & Clark College Portland, OR

Slide 16 of 18

Example - Successful Requests.

Reference Table155 Z202 B901 C912 H939 V

Who to announce to?

Data Store

900

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Lewis & Clark College Portland, OR

Slide 17 of 18

Results

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Lewis & Clark College Portland, OR

Slide 18 of 18

Conclusions

The original request algorithm is not very effective at changing the overlay network.

They way to improve the performance of Freenet is to enhance the small-world properties that it already has.

Our combined algorithm was able,in simulation, to lower the pathlength under 99 to 1 conditions from 225 to 25.

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Lewis & Clark College Portland, OR

Slide 19 of 18

Backup Slides

Page 20: Request Algorithms In Freenet Style Peer To Peer Systems

Lewis & Clark College Portland, OR

Slide 20 of 18

Differences from distributed hash table. Data is placed deterministically

Items located within bounded number of hops, but Securing against attack, load balancing, and (for some)

exploiting network proximity becomes more difficult.

Open source We can modify, learn.

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Lewis & Clark College Portland, OR

Slide 21 of 18

50,000 Actions (99:1)

Original median pathlength: 2.94 Recycled

100% : 6.25 25% : 2.88