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BitTorrent System

Efrat Oune

Bar-Ilan

2005

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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

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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

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Links

The official BitTorrent Home Page BitTorrent- Wikipedia,the free encyclopedia Technical analyses of BitTorrent Interview with the creator of BitTorrent List of BitTorrent clients List of web for Torrent Files