Incentives Build Robustness in BitTorrent

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Incentives Build Robustnessin BitTorrent

• Bram Cohen

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Motivation
Single server

• Scalability issues for client/server systems
• Servers workload grows linearly with number of
  clients
• Flash crowd problem

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An Ideal Solution IP Multicast

• Same stream is shared by all clients receiving
  same data
• Requires infrastructure-level changes
• Security issues
• No widely accepted transport protocol on IP
  multicast layer

Single server
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Current Approaches

• Server farms replace central server
• Scalability problem continues
• Expensive to maintain
• Content delivery networks (CDNs)
• Akamai,
• Still expensive
• Peer-to-peer systems (P2P)
• Newer approach

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P2P Systems (I)

• Let clients, now called peers, share the server
  workload
• Peers forward all the data they receive to other
  peers

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P2P Systems (II)
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Advantages

• P2P solutions are
• Scalable
• Downloading bandwidth grows with number of peers
• Easy to deploy
• No additional hardware
• No change to network infrastructure
• Cheap

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Issues

• Organizing data transfers
• Figuring which peers have which chunks of data
• Deciding where to send these chunks
• Dealing with churning
• Peers come and go
• Enforcing fairness
• Some peers do not upload as many data as they
  download

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BitTorrent

• Unstructured P2P System
• Peers have no parent peers or child peers
• Centralized tracker
• Collects information on peers
• Responds to requests for that information
• Built-in fairness incentive
• Rechoking favors cooperative peers
• Simple user interface

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Importance
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Deployment (I)

• Decision to use BitTorrent is made by publisher
  of file
• Users join BitTorrent to get a file they want
• Most users stops uploading once they have
  downloaded the file
• Standard implementation keeps uploading until
  the BT window closes

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Deployment (II)

• In a typical deployment
• Number of downloaders having parts of the file
  (leeches) increases very fast then peaks at a
  maximum before decreasing exponentially
• Number of downloaders having the whole file
  (seeds) increases more slowly then peaks at a
  maximum before decreasing exponentially

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Evolution
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Starting a BT (I)

• To start a BT, publisher puts on a web server a
  static file with information about
• The file
• Its length
• Its name,
• Hashing information
• The URL of a tracker

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The tracker

• Helps downloaders find each other
• Uses a simple protocol layered on top of HTTP
• New downloader sends information about
• What file its downloading
• What port its listening on
• Tracker replies with a list of peers that are
  downloading the same file

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Starting a BT (II)

• Next step is starting a downloader having the
  whole contents of the file
• The seed
• Web server and tracker havevery low bandwidth
  requirements
• Seed must upload at least once whole file contents

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Connecting with peers

• Standard tracker algorithm returns random lists
  of peers
• Random graphs are very robust
• BitTorrent cuts files into pieces of fixed size,
  typically a quarter megabyte
• Each downloader
• Reports to its peers what pieces it has
• Starts exchanging these pieces with them

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Ensuring data integrity

• Torrent file on web server has SHA1 hashes of all
  the pieces
• Peers dont report that they have a piece until
  theyve checked its hash
• Could have used erasure codes

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Pipelining

• Must avoid delays between pieces being sent
• BT breaks pieces further into sub-pieces
  (typically 16KB in size)
• Always keeps several requests pipelined at once
• Sends a new request each time a sub-piece arrives

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Piece Selection (I)

• Downloaders requesting pieces follow four
  policies
• Strict Priority
• Finish first downloading pieces of which
  downloader has one or more sub-pieces
• Gets complete pieces as quickly as possible

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Piece Selection (II)

• Rarest First
• Download first the pieces that the fewest of
  their own peers have
• Ensures that peers have the pieces that most of
  their peers want

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Piece Selection (III)

• Random First Piece
• New peer should get its first complete piece as
  quickly as possible
• Rare pieces can be downloaded from fewer peers
  than other pieces
• New peer will select first pieces to download at
  random until it has obtained a complete piece

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Piece Selection (IV)

• Endgame Mode
• At end of download
• Peer will send to all other peers requests for
  sub-pieces it doesnt have yet from all other
• Will send cancels for all sub-pieces which arrive
• Objective is to speed up end of download

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Overview
Torrent file
Tracker
Two nodes and theirpeers
Downloaders
A seed
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BT Choking Algorithm

• Penalizes peers that do not reciprocate
• Tit-for-tat policy
• Every ten seconds, each peer selects four less
  cooperating peers it will choke
• Will refuse to upload data to these peers for
  ten seconds
• Long enough for TCP to reach full capacity with
  the new transfers

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Optimistic Unchoking

• At all times, each BT peer has
  singleoptimistic unchoke,
• Unchoked regardless of its current upload
• Rotated every third rechoke period (30 seconds)
• Correspond very strongly to always cooperating
  on the first move in prisoners dilemma.

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Upload only mode

• A peer that has the whole content of the file it
  wanted to download starts privileging peers that
  use best its upload bandwidth

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Anti-snubbing

• A peer might be sometimes choked by all peers
  from which it was downloading
• To mitigate this problem, when over a minute
  goes by without getting a single piece from a
  particular peer, BitTorrent assumes it is
  snubbed by that peer and doesnt upload to it
  except as an optimistic unchoke

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Actual deployments

• BT routinely serves
• Files hundreds of megabytes in size
• To hundreds of simultaneous downloaders
• Can have over a thousand concurrent downloaders.
• Sole scaling bottleneck appears to be the
  bandwidth overhead of the tracker
• One thousandth of total traffic