1. Introduction to BitTorrent
Alexey Zagalsky
Introduction to BitTorrent
Based on data from Wikipedia
slides from “Introduction to BitTorent” by Arvid Norberg
slides from”BitTorrent Background” by Hilel

2. What is BitTorrent
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

3. Reasons for Adoption 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

4. How It Works
The file to be distributed is split up in pieces and a SHA-1 hash is calculated for each piece

5. How It Works A metadata file (.torrent) is distributed to all peers
Usually via Web, , etc…
The metadata contains:
SHA-1 hashes of all pieces
Tracker reference (URL)
Piece Length: usually 256KB
is it better smaller or bigger pieces ?

6. How It Works
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

7. A Peer Joins The Swarm

8. A Peer Joins The Swarm

9. Seeding A File

10. Terminology
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

11. Goals Efficiency Reliability
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)

12. Rarest First
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

13. Incentive to Share 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

14. 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

15. Limitations Content unavailability The leech problem
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)

16. Trackerless Torrents Common problems with Trackers: Solutions:
Single point of failure
Solutions:
Multiple Trackers (splits swarms)
DHT

17. Distributed Hash Table
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