FairTorrent: Bringing Fairness to Peer-to-Peer Systems

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FairTorrent Bringing Fairness to Peer-to-Peer
Systems

• Alex Sherman
• Jason Nieh
• Cliff Stein

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Problem

• Lack of fairness in bandwidth allocation in P2P
  systems
• Users are not incentivized to contributed
  bandwidth
• Proliferation of free-riders
• No QoS guarantee (for file-sharing, live
  streaming)

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No traditional solution

• No central entity that controls resources
• Bandwidth resources are geographically
  distributed
• The amount of resources not know in advance
• Resources may very over time

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Outline

• Background / Related Work
• Proposed algorithm FairTorrent
• Evaluation

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Background

• Free-riding inherent to P2P systems. (e.g. In
  Gnutella 70 of users do not contribute Adar,
  2000)
• BitTorrent introduces tit-for-tat
• File is split into chunks that peers file-share
• Tit-for-tat a peer reciprocates by uploading to
  N-k peers with the best download rate k
  optimistically selected peers
• Eventually fair under some static workloads
  (Legout SIGMETRICS 07, Srikant SIGCOMM 04)

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Background (slide 2)

• BitTorrent tit-for-tat can be exploited
• LargeView , Sirivianos et.al.IPTPS 07
• BitTyrant, Piatek, et.al USENIX 07
• Free-riding, Locher,et.al HotNets 05
• Causes
• Long discovery times of like-peers
• Optimistic unchoking
• Unstable peering relations

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Background (slide 3)

• Reputation-based systems (e.g. Eigentrust)
• Problem with collusion, bootstrapping
• Does not translate to fairness
• Credit-based systems (eg. Dandelion, Karma)
• Heavy management overhead (eg. Credit Service)
• Does not guarantee real-time fairness (e.g. if I
  rack up credit I can free-ride)

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Background (slide 4)

• Block-based TFT (Bharambe INFOCOM 06, Pai
  04, Jun EP2PS 05)
• Upload to any peer as long as
• upload download lt constant threshold
• Bharambe , Pai under-utilization of capacity.
  Fix split peers based on bandwidth (hard to
  achieve)
• Jun many complex tuning parameters

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Proposed Solution FairTorrent

• FairTorrent a packet-based scheduling algorithm
  that achieves fair-bandwidth allocation among
  peers in a file-sharing swarm, while maximizing
  capacity utilization.
• Built on top of BitTorrent

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FairTorrent Algorithm Definition

• Terminology
• Leechers peers that are still downloading data
• Seeds peers that only serve data
• Each Leecher L_i
• Keeps track of a deficit variable DF_ij with each
  leecher L_j
• DF_ij Send_ij Recv_ij
• Schedules to send the next packet to a leeacher
  L_j with the smallest deficit DF_ij
• Each Seed schedules packets using Round-Robin

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FairTorrent Achieves

• Fast Convergence of data exchange rate between a
  pair of leechers
• Fast Convergence of a leechers total upload rate
  to its download rate from other leechers
• Small standard deviation of download rate
• Leads to fairness and predictabled download times
• Maximizes capacity utilization

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Example

• Leechers L_1, L_2, and L_3 with upload capacities
  3, 2 and 2 respectively
• Upload rates under equal-split (left) , and
  fairtorrent (right).

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Unchoking Behavior

• BitTorrent client unchokes k peers at a time
  allowes these peers to request data
• FairTorrent unchokes any peer that may request
  data

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Measure of Fairness

• Instantaneous Service Error (of Leecher L_i)
• Maximum Instantaneous Service Error

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Hypothesis

• Assumptions
• Peers always have data to exchange
• ( upload rate of
  leecher i)
• Hypothesis
• (n of nodes, p
  packet size)
• Proof for the 3-node case. Similations /
  empericals results for the general case.

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FairTorrent Implementation

• Requires
• No apriori knowledge or measurements of peers
  bandwidth
• No centralized management
• No special tuning parameters
• No change to BitTorrent Protocol

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Methodology

• Implemented FairTorrent on top of BitTorrent
  python client
• Instrumented FairTorrent, BitTorrent, Azureus,
  BitTyrant to measure service error, performance
• Ran tests on the PlanetLab to compare FairTorrent
  with other clients

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Test Parameters

• 32 MB file
• 10 seeds upload at 25 KB/s
• 50 leechers
• Uniform Distribution (1-50 KB/s)
• Skewed Distribution (1 high uploader)
• BiModal Distribution (high uploaders,
  free-riders)
• Ran 5 tests for each network (FT, BT, AZ, TY)
• Mixed Networks

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Max Service Error
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FairTorrent Rate Convergence
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Rate Convergence Comparison
BitTorrent
FairTorrent
BitTyrant
Azureus
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FairTorrent Utilization
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Utilization Comparison
BitTorrent
FairTorrent
BitTyrant
Azureus
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Worst Completion Time (seconds)

• FairTorrent 1347, BitTorrent 1892, Azureus
  1849, BitTyrant 2266 (FT completes 37-68 faster)

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Standard Deviation

• FairTorrent exhibits low standard deviation (1.8
  KB/s on average)

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Skewed Distribution

• One high uploading leecher at 50 KB/s
• 49 low uploading leechers at 1-5 KB/s
• Ran 5 tests for each of the 4 networks
• Ran 5 tests replacing the high uploader with
  FairTorrent in BitTorrent, Azureus and BitTyrant

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Max Service Error

• FairTorrent (555KB) BitTorrent (51MB),
  Azureus(31 MB) BitTyrant (113 MB)

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High Uploader Completion Time

• High uploader completes in 3-5 times faster in
  FairTorrent

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Bi-modal Distribution

• 25 high uploaders 40-50 KB/s
• 25 free-riders 0-3 KB/s

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High Uploaders E_max
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High Uploaders Completion Time
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Dynamic Tests

• Test duration 7500 seconds
• Nodes arrive in 5 second intervals
• Upon completion nodes re-join with a clean cache
  fresh identity
• 10 permanent seeds ( 50 KB/s)
• 100 leechers that leave and re-join

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Max Service Error
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Completion Times
FairTorrent
Azureus
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Conclusions

• New algorithm for fair bandwidth allocation

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Evaluation Parameters

• Instrumented all clients to measure service error
  / fairness and performance
• Measured service error, performance, etc
• Parameters
• 50 leechers with various upload capacities (up to
  50KB/s) selected from various distributions
  uniform, bimodal, skewed
• 10 seeds upload at 25 KB/s
• Download of a 32MB file

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Preliminary Results (uniform distribution)

• EM was an order of magnitude smaller in the case
  of FairTorrent (under 1MB)
• Completion time of all clients 40 faster under
  FT
• Bandwidth utilization 95.3 for FairTorrent vs.
  82.8 to 93.7 for other clients

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Proposed Extensions

• Extend FairTorrent to a case where peers
  participate in multiple downloads (or multiple
  swarms)
• Extend BitTorrent to leverage multiple swarms and
  FairTorrent for a more optimal bandwidth
  allocation across swarms