Improving BitTorrent

1
ImprovingBitTorrent

• Designing a new BitTorrent Client

2
Overview

• How BitTorrent Works
• Experimental Analyses
• Modeling Gittins Indices
• Our New Algorithms
• Proposed Evaluation
• HELP!

3
File Organization
File
4
2
3
1
Piece256KB
Block16KB
Incomplete Piece
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Initialization
webserver
user

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On the Wire Protocol

• (Over TCP)

Non-keepalive messages
0 choke 1 unchoke 2 interested 3 not
interested 4 have 5 bitfield 6 request 7
piece 8 cancel
ID/Infohash Handshake
BitField
BitField
Local Peer
Remote Peer
Interested 0choked 1
Interested 0choked 1
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Piece Selection

• Pipelining (5 requests)
• Strict Priority
• 3 stages
• Random first piece
• Rarest First
• Endgame mode

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Peer Selection

• Focus on Rate
• Upload to at most 4 peers
• Random Unchoke
• Global rate cap only

8
Analyses

• N. Liogkas et al., Exploiting BitTorrent For Fun
  (But Not Profit), IPTPS 2006.
• A. Legout et al., Rarest First and Choke
  Algorithms Are Enough, June 2006
• A. R. Bharambe et al. Analyzing and Improving
  BitTorrent Performance, Feb 2005.

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Exploitation Liogkas et al.

• PlanetLab Real Torrent
• Exploits
• Download only from seeds
• Median improvement of 7-20
• Download only from fastest peers
• Contradictory results
• good in PlanetLab (22)
• bad in the wild(-1 to -30)
• Advertise false pieces
• 22 better download rates

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Piece Selection Legout et al.
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Piece Selection Legout et al.
a local peer interested in remote
peer b time peers are connected c
remote peer interested in local peer
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Piece Selection Legout et al.

• Limitations
• In 25 of the torrents the local peer is not
  interested.
• Who is interested in me?

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Gittins Indices

• The Two-armed bandit problem
• Same prize
• Unknown winning probabilities
• Known discount factor
• Limitation
• Infinitely discounted or fixed horizon
• Projects remain dormant
• One project at a time
• P. Whittle 1988
• Restless bandits
• m projects on average
• Asymptotically optimal for large m and n

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Piece Selection

• Key ideas
• We dont care about piece selection
• Keep download pipes full
• Peer Queues
• Assume
• We receive pieces at a rate of 1 per second

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Piece Selection

Local Peer
Remote Peer
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Piece Selection
Q1
Peer 1
Q2
Remote Peer
Local Peer
Peer 2
Q3
Peer 3
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Piece Selection

• Cost of empty queue Ci per unit time
• Discount factor d lt1
• Piece consumption rate li (Poison Process)
• Queue has length li(t)

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Piece Selection

• Minimize
• where

We can Solve this with Gittins Indices!
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Piece Selection Algorithm(alpha version)

• Calculate Gittins Index for each queue
• Feed the queue with the highest index.
• Two Problems
• 1 piece goes into many queues
• it takes a while to get the pieces

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Piece Selection Algorithm(beta version)

• Calculate Gittins Index for each queue
• Calculate Gittins index for each piece.
• n(piece) SA
• Get piece with highest index possible.

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Piece Selection Algorithm
highest index
Peer 1
Peer 2
lowest index
Peer 3
time
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Piece Selection Algorithm(Final Release)

• Calculate Gittins Index for each queue.
• Calculate Gittins index for each piece.
• Extrapolate piece download finishing time.
• Order finishing times (earliest first).
• Order pieces (highest index first).
• Match best piece with earliest finishing time.

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Peer Selection

• Key Ideas
• upload rate is a perishable good
• Throttle each connection individually
• Do not limit number of downloaders to 4

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Peer Selection Our Model

• Series of rounds
• Edownload rate f(upload rate)
• Estimate download rates with have.

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Peer Selection Algorithm

• Keep clients with best ratios.
• Throttle to estimated safe bound.
• Solve Knapsack problem
• Total upload bandwidth max weight
• Item value download rate we are getting
• Item weight estimated upload bound
• If download/upload rate ratio is lt 1
• Random unchoke

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Evaluation

• Simultaneously
• Identical machines and network.
• Repeat experiment and swap clients.
• Different torrents

vs.
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HELP!

• What we need
• An awesome python programmer 0)
• Whats in it for you
• We are cool! 0)
• Publications
• Open Source Real Estate

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• Thank you!
• defigued_at_cs.ucdavis.edu