1. BitTorrent

2. Introduction to BitTorrent
BT: BitTorrent
BT is not itself a network
it allows small Internet networks to be created to share files
Does not perform all the functions of a typical p2p system, like searching
Its virtual network is called data-oriented overlay
Written by Bram Cohen in 2001
Written in Python and it uses GTK for its GUI
It is an efficient content distribution system using file swarming
Each file split into smaller pieces
equal-sized blocks (typically KB)
Nodes request desired pieces from neighbors
Encourages contribution by all nodes
The throughput increases with the number of downloaders via the efficient use of network bandwidth

3. About GTK
GTK a cross-platform toolkit for creating graphical user interfaces
GTK stands for GIMP ToolKit
GIMP stands for GNU Image Manipulation Program
It is an image retouching and editing tools
It is open-source software
It is developed by a self-organized group of volunteers under the banner of the GNOME Project
It is licensed under the terms of the GNU LGPL
GNU Lesser General Public License
It allows developers and companies to use and integrate LGPL software into their own (even proprietary) software without being required to release the source code of their own software-parts
GNU is a recursive acronym meaning "GNU's not Unix"

4. Terminology Leecher Initial seeder Leecher Seeder
Seeder = a peer that provides the complete file
Initial seeder = a peer that provides the file that is torrented
Leecher
Initial seeder
One who is downloading
Leecher
Seeder

5. Terminology Peer: a client to the network dedicated to a torrent
Seeding: serving a file for download
Leeching: downloading without serving a complete file for download
Leech: peer that’s downloading the file
Fairer term might have been “downloader”
Subpiece: Further subdivision of a piece
The “unit for requests” is a subpiece
But a peer uploads only after assembling complete piece

6. Leecher can become seeder
As a leecher downloads pieces of the file, replicas of the pieces are created
More downloads mean more replicas available
As soon as a leecher has a complete piece, it can potentially share it with other downloaders
Eventually each leecher becomes a seeder by
obtaining all the pieces,
assembles the file, and
verifies the checksum

7. Swarm
Swarm
Set of peers all downloading the same file
Organized as a random mesh
BT is differentiated from the traditional file sharing as swarming
File is divided into many small pieces for distribution
Each node knows list of pieces downloaded by neighbors
Node requests pieces it does not own from neighbors
Clients request different pieces from the seeder or from other clients
Clients become seeders for those pieces downloaded
When all pieces are downloaded, clients can reconstruct the whole file
There exists no single BT network, but thousands of temporary networks consisting of clients downloading the same file

8. Tracker
The tracker is a central server keeping a list of all peers participating in the swarm
A peer joins a swarm by asking the tracker for a peer list and connects to those peers
The tracker gives the requester peers random selection of peers
Get Request consists of:
File ID
Peer ID
Peer IP
Peer Port
Tracker responses with:
List of peers, containing ID, IP and Port of each peer
Peers may rerequest on nonscheduled times, if they need more peers

9. How a node enters a swarm for file “popeye.mp4”
The file distributor publishes details of the .torrent file on (well-known) web server
File popeye.mp4.torrent hosted at the webserver
The .torrent has address of tracker for file
The tracker running on a webserver as well, keeps track of all peers downloading file
Tracker supplies peers with addresses of other peers that share the wanted files

10. How a node enters a swarm for file “popeye.mp4”
URL of tracker
.torrent file refers to the tracker
which steers the download process
This method makes it very clear who is responsible for the legitimacy of the content:
the operator of the tracker server 1
Peer
popeye.mp4.torrent

11. How a node enters a swarm for file “popeye.mp4”
File popeye.mp4.torrent hosted at a (well-known) webserver
The .torrent has address of tracker for file
The tracker, which runs on a webserver as well, keeps track of all peers downloading file 2
Peer
Addresses of peers
Tracker
To download the file, peers access the tracker and join the torrent
torrent is a group of peers connected to the same tracker
The torrent is downloaded and the peer registers with the tracker, which provides a list of available peers and seeds

12. How a node enters a swarm for file “popeye.mp4”
Piece length – Usually 256 KB
SHA-1 hashes of each piece in file
For reliability
Peer 3
Tracker
Swarm

13. BT Client Software
The BT client enables a host of features including multiple parallel downloads
The BT client also intermediates peering between itself, source file servers (trackers) and other clients
Thereby yielding great distribution efficiencies
The BT client also enables users to create and share torrent files
When a peer has finished downloading a file, it may become a seed by staying online for a while and sharing the file for free
i.e., without bartering

14. Requirements from the Web Server and the Tracker
The requirements from the Web hosting end are not too much
To transmit a torrent you only need a standard HTTP Web server and a free program called a tracker
The Web server should be configured to use MIME type application/x-bittorrent for any file with the ".torrent" extension
The tracker's job is:
to keep track of which clients can serve which files to other clients
The tracker
can be installed
either on individual Web servers
or operated centrally by the Web host
Its traffic load is relatively light, and
offering a tracker to your hosting customers can make using BT to distribute content a much simpler process for your customers

15. Overall Architecture Tracker Web Server .torrent C A [Seed] Peer
Web page
with link
to .torrent A B C
Peer
[Leech]
Downloader
“US”
[Seed]
Tracker
Web Server
.torrent

16. Overall Architecture Tracker Web Server Get-announce C A [Seed] Peer
Web page
with link
to .torrent A B C
Peer
[Leech]
Downloader
“US”
[Seed]
Tracker
Get-announce
Web Server

17. Overall Architecture Tracker Web Server Response-peer list C A [Seed]
Web page
with link
to .torrent A B C
Peer
[Leech]
Downloader
“US”
[Seed]
Tracker
Response-peer list
Web Server

18. Overall Architecture Tracker Web Server Shake-hand C A [Seed] Peer
Web page
with link
to .torrent A B C
Peer
[Leech]
Downloader
“US”
[Seed]
Tracker
Shake-hand
Web Server

19. Overall Architecture Tracker Web Server C A pieces [Seed] Peer [Leech]
Web page
with link
to .torrent A B C
Peer
[Leech]
Downloader
“US”
[Seed]
Tracker
pieces
Web Server

20. Overall Architecture Tracker Web Server C A pieces [Seed] Peer [Leech]
Web page
with link
to .torrent A B C
Peer
[Leech]
Downloader
“US”
[Seed]
Tracker
pieces
Web Server

21. Overall Architecture Tracker Web Server Get-announce
Web page
with link
to .torrent A B C
Peer
[Leech]
Downloader
“US”
[Seed]
Tracker
Get-announce
Response-peer list
pieces
Web Server

22. Simple example {1,2,3,4,5,6,7,8,9,10} Seeder: A {} {1,2,3} {1,2,3,5}
{1,2,3,4}
{1,2,3,4,5}
Downloader C
Downloader B

23. Pipelining
When transferring data over TCP, always have several requests pending at once (typically 5), to avoid a delay between pieces being sent
Every time a piece or a sub-piece arrives, a new request is sent out

24. Piece Selection
The order in which pieces are selected by different peers is critical for good performance
If an inefficient policy is used, then peers may end up in a situation where each has all identical set of easily available pieces, and none of the missing ones.
If the original seed is prematurely taken down, then the file cannot be completely downloaded

25. Piece Selection Small overlap is good Large overlap is bad
-- wastes bandwidth

26. Piece selection Strict Priority Rarest First Random First Piece
General rule
Random First Piece
Special case, at the beginning
Endgame Mode
Special case

27. Random First Piece Initially, a peer has nothing to trade
Important to get a complete piece ASAP
So as to assemble first complete piece quickly
Then participate in uploads
Select a random piece of the file and download it
When first complete piece assembled, switch to rarest-first

28. Rarest Piece First
Determine the pieces that are most rare among your peers, and download those first
This ensures that the most commonly available pieces are left till the end to download
Increases diversity in the pieces downloaded
avoids case where a node and each of its peers have exactly the same pieces; increases throughput
Increases likelihood all pieces still available even if original seed leaves before any one node has downloaded entire file

29. Endgame Mode
Near the end, missing pieces are requested from every peer containing them
This ensures that a download is not prevented from completion due to a single peer with a slow transfer rate
This can potentially delay the finishing of a download
When all the sub-pieces a peer lacks are requested
this request is flooded to all peers
This helps to get the last chunk of the file as quickly as possible
To speed up completion of download
Once a sub-piece arrives, it sends a cancel-message indicating that the peer has obtained it
Some bandwidth is of course wasted by this flooding
but not much because of the short period of the endgame mode

30. Difficulties with BitTorrent
Works best with files that are widely copied on the network
In practice, files appear and disappear
There is no permanent archive, or incentive for users to keep old files
Since blocks are not downloaded sequentially, a partial file is not useful

31. Reason of the success
The success of Bittorent is unlikely to be due purely to the use of the tit-for-tat inspired protocol
The real driving force behind the high cooperation might be the by-product of the lack of meta-data search within BitTorrent
This results in the creation of a number of disconnected “tribes” at both the swarm and the tracker level
The users are active in the tribal dynamics by selecting those tribes that best satisfy their needs
hence tribes filled with free-riders will tend not to operate

32. An example
The BitTorrent client BitTornado displays information about the peers and seeders engaged in sharing and distributing a torrent

33. Additional features BitTorrent uses 6881 as the default port
if that port is unreachable BitTorrent tries to connect to a number of successive ports up to 6889
If the client cannot connect to port 6889, it gives up
BitTorrent supports resuming, it resumes where it left off after checking the partial download
How do the user know the download is not corrupted?
BitTorrent does cryptographic hashing (SHA1) of all data
When seeing "Download succeeded!" the user can be sure that BitTorrent has already verified the integrity of the data
The integrity and authenticity of a BitTorrent download is as good as the original request to the tracker
Checking the MD5/CRC32/other hash of a file downloaded via BitTorrent is redundant

34. An application of the BitTorrent Technology
Cranberry Publishing uses Bittorrent as one of the means of distribution for its free Home Computer Magazine
As the magazine is free to its readers, it is available simultaneously as a free download from the magazine website and as a torrent file for Bittorrent users
However, the potential load on their server is enormous, so they wanted a way to ensure that it could be delivered
Bittorrent offered the means to make sure the users can get the files they want distributed faster
The best thing about it is that the more people that download it, the faster it gets for everyone, not slower
Torrent users can just grab the torrent and download

35. Why is (studying) BitTorrent important?
(From CacheLogic, 2004)

36. Legal Issues
You should know about some inherent dangers to using BitTorrent to download movies and TV shows
In the USA, organizations like the Recording Industry Association of America (RIAA) and the Motion Picture Association of America (MPAA) actively prosecute people and companies that are engaged in making copyrighted content available to others illegally
Internationally, the laws are even more complex
You are not anonymous when you use BitTorrent, because the process itself involves sharing of identifying information about your computer
This lack of anonymity puts you at risk if you use BitTorrent or other file-sharing technologies to download or share music, movies, TV shows, and other content