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What is BitTorrent?
BitTorrent is a peer-to-peer file distribution system (built for intensive daily use by a very large user community)
Good P2P design is : Decentralized computing Every peer acts as a client and a server
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The key philosophy of BitTorrent
Problem: more customers require more bandwidth
The BitTorrent Solution: customers help distribute content, they should upload at the same time they are downloading
In this manner, network bandwidth is utilized as efficiently as possible.
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The purpose of BitTorrent
BitTorrent developed for transfering big files (700 Mb and up) at minimal time such as Linux images, video files etc, and the download rates is the best as the file request popularity increase!!!
Therefore BitTorrent specializes in downloading fast new files such as newly released TV shows, films ,DVD rips, application.
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The unique in BitTorrent
Redistributes cost of upload to downloaders (thus making 1. hosting a file with a potentially unlimited number of downloaders affordable 2.downlowds large files)
Suitable for file swarming (a lot of people are trying to
download the same copy simultaneously) Has best connections to the missing pieces
(has more the 7 methods to get the files download efficiently )
Uses a Tit For Tat technique for selection of peers (encourage downloaders to upload)
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What the user should do?
Find a .torrent file of interest (any Web site can host this)
Download it BitTorrent does the rest…
A peer contacts the tracker to find other peers Peers contact one another and share information
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But What is Really Going On?
BitTorrent breaks up large files into pieces, then allows users to swarm - exchanging small portions of files with one another in a tit-for-tat-like manner (technique)- until everyone has a complete set.
A peer first downloads a random piece of the file and then attempts to find rare pieces
As soon as the client finishes importing a block, it hashes it to make sure that the block matches what the torrent file said it should be
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Terms - Seeds and Leech
Seed: a peer which after finishing download a file, staying online for a while and sharing the file for free, i.e., without bartering
Leech: a peer which trying to download the file without uploading it to anyone.
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The feature of seed
If all seeds get taken offline, the file may no longer be available for download, even if the torrent file is possessed.
However, even if there are no seeds, as long as there is at least one distributed copy of the file everyone can eventually get the complete file.
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Measurement features:
Download performance Integrity Flashcrowds Availability (Technical analyses of BitTorrent, Johan Pouwelse, 2004 - A detailed study
of the BitTorrent network)
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Download performance
The more you share, the faster your downloads.
The more people trading a file, the more options for obtaining its pieces.
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Availability
The availability of content is unpredictable. When the popularity drops and the last peer/seed with certain content goes offline, the content dies
Popularity <=> downloads <=> availability
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Integrity
a .torrent file saves a hash of each block in the file (which allows users to make sure they are downloading the real thing).
The BitTorrent system seems to be very effective in removing fake and corrupted files.
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Flashcrowds
BitTorrent focus on the system's reaction to the sudden popularity of a single file.
BitTorrent components are capable of efficiently handling very large flashcrowds. It be designated for it.
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BitTorrent developer claims
For very popular files, BitTorrent can support about a thousand times as many downloads as HTTP
BitTorrent achieves a higher level of robustness and resource utilization than any currently known cooperative technique
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Studies found:
only very few P2P system have stood the test of intensive daily use
The BitTorrent file-sharing system is one of these systems and most of popular networks
In fact, BitTorrent traffic made up 53 per cent of all P2P traffic in June 2004. (Johan Pouwelse, 18th December, 2004 )
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Advantages/Strengths
Better bandwidth utilization Limit free riding – tit-for-tat Limit leech attack – coupling upload &
download Spurious files not propagated Ability to resume a download Open Source implementations
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Drawbacks Scalability
Millions of peers – Tracker behavior uses 1/1000 of bandwidth
The tracker limits scalability and creates a single point of failure
Robustness System progress dependent on altruistic
nature of seeds and peers BitTorrent the availability of content is unpredictable
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Compare to Napster
In Napster every peer is also a client and server
BitTorrent is like napster but uses file swarming
Unlike the old Napster, popularity doesn't bog down the process -- it gives it a shot of adrenaline
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Compare to Emule
Emule has a credits system whereby a client stores the amount of data it has uploaded and downloaded from every client; and gives clients that have best uploaded to it a higher priority in the queue.
The BitTorrent is "leech resistance" by design.
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Compare to eDonkey2000 network
The original eDonkey2000 client provided little "leech resistance“; Most new eDonkey clients have some sort of system to encourage uploaders
The method used by BitTorrent to distribute files parallels to a large extent the one used by the eDonkey2000 network, but nodes in eDonkey's file sharing network usually share and download a much larger number of files, making the bandwidth available to each transfer much smaller.
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Scenario1
Web page with link to .torrent
A
B
C
Peer
[Leech]
Downloader
“US”
Peer
[Seed]
Peer
[Leech]
TrackerWeb Server
.to
rren
t
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Scenario2
Web page with link to .torrent
A
B
C
Peer
[Leech]
Downloader
“US”
Peer
[Seed]
Peer
[Leech]
Tracker
Get-
announce
Web Server
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Scenario3
Web page with link to .torrent
A
B
C
Peer
[Leech]
Downloader
“US”
Peer
[Seed]
Peer
[Leech]
Tracker
Response-peer list
Web Server
25
Scenario4
Web page with link to .torrent
A
B
C
Peer
[Leech]
Downloader
“US”
Peer
[Seed]
Peer
[Leech]
Tracker
Shake-hand
Web Server
Shake-hand
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Scenario5
Web page with link to .torrent
A
B
C
Peer
[Leech]
Downloader
“US”
Peer
[Seed]
Peer
[Leech]
Tracker
pieces
pieces
Web Server
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Scenario6
Web page with link to .torrent
A
B
C
Peer
[Leech]
Downloader
“US”
Peer
[Seed]
Peer
[Leech]
Tracker
piecespieces
pieces
Web Server
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Scenario7-all scenario
Web page with link to .torrent
A
B
C
Peer
[Leech]
Downloader
“US”
Peer
[Seed]
Peer
[Leech]
Tracker
Get-announce
Response-peer list
piecespieces
pieces
Web Server