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Alexey Zagalsky
INTRODUCTION TO
BITTORRENT
Based on data from Wikipedia
slides from “Introduction to BitTorent” by Arvid Norberg
slides from”BitTorrent Background” by Hilel
What is BitTorrent
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


BitTorrent is a peer-to-peer file sharing protocol
used for distributing large amounts of data
It has been estimated that it accounted for roughly
27% to 55% of all Internet traffic (depending on
geographical location) as of February 2009.
Programmer Bram Cohen designed the protocol in
April 2001
Reasons for Adoption
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
Better performance through “pull-based” transfer
 Slow


nodes don’t bog down other nodes
Allows uploading from hosts that have downloaded
parts of a file
Practical Reasons (perhaps more important!)
 Working
implementation with simple well-defined
interfaces for plugging-in new content
 Many recent competitors got sued / shut down
 Napster,
Kazaa
How It Works
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
The file to be distributed is split up in pieces and a
SHA-1 hash is calculated for each piece
How It Works
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
A metadata file (.torrent) is distributed to all peers
 Usually

via Web, Email, etc…
The metadata contains:
 SHA-1
hashes of all pieces
 Tracker reference (URL)
 Piece Length: usually 256KB
 is
it better smaller or bigger pieces ?
How It Works
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

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The user makes the file itself available through a
BitTorrent node acting as a seed
The Tracker is a central server keeping a list of all
peers participating in the swarm
A swarm is the set of peers that are participating in
distributing the same file
A peer joins a swarm by asking the tracker for a
peer list and connects to those peers
A Peer Joins The Swarm
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A Peer Joins The Swarm
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Seeding A File
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Terminology
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

A downloader is any peer that does not have the
entire file and is downloading the file
A leecher is:
A
peer who has a negative effect on the swarm by
having a very poor share ratio
 A downloader

A seeder is a peer that has an entire copy of the
torrent and offers it for upload
Goals
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
Efficiency
 Ability
to download from many peers yields fast
downloads
 Minimize piece overlap among peers
 Download
random pieces
 Rarest First algorithm

Reliability
 Tolerant
to dropping peers
 Ability to verify data integrity (SHA-1 hashes)
Rarest First
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



The piece picking algorithm used in BitTorrent is called
Rarest First
To maximize the distributed copies, maximize the
availability of the rarest pieces
Picks a random piece from the set of rarest pieces
No peer has global knowledge of piece availability,
it is approximated by the availability among
neighbors
Incentive to Share
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Policies to determine to whom to send data:
 Tit-for-Tat
Upload to whoever uploads the most to you
 "Survival of the fittest“
 Theoretically increases performance by encouraging fast
peers to upload to you and giving them even more pieces to
upload to others
 May result in suboptimal situations


Optimistic Unchoking
In hope of discovering better partners
 To ensure that newcomers get a chance to join the swarm

Tit-for-tat as Incentive to Upload



Want to encourage all peers to contribute
Peer A is said to choke peer B if it (A) decides not to
upload to B
Each peer (say A) unchokes at most 4 interested peers
at any time

The three with the largest upload rates to A


Where the tit-for-tat comes in
Another randomly chosen (Optimistic Unchoke)

To periodically look for better choices
Limitations
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
Content unavailability
 Although
swarming scales well to tolerate flash crowds
for popular content, it is less useful for unpopular
content

The leech problem
A
user may often choose to leave the swarm as soon as
they have a complete copy of the file they are
downloading

Pieces not downloaded in sequential order (think
VOD)
Trackerless Torrents
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
Common problems with Trackers:
 Single

point of failure
Solutions:
 Multiple
 DHT
Trackers (splits swarms)
Distributed Hash Table
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

Works as a hash table with SHA-1 hashes as keys
The key is the info-hash, the hash of the metadata
 It

uniquely identifies a torrent
The data is a peer list of the peers in the swarm