A P2P file distribution system ——BitTorrent

Pegasus Team

CMPE 208

Outline

• 1 What is it

• 2 Why do we need BitTorrent

• 3 Architecture And Operation

• 4 Why does it work so well

• 5 Strengths & Shortcomings

• 6 Conclusion

Overview

• Peer 2 Peer File transfer Protocol

• Works over public Internet.

• Critical components – Web Server (MetaInfo Server)– MetaData file (.torrent)– Central Tracker– End User Peers

Critical Elements

• 1 A web server– To provide the ‘metainfo’ file by HTTP– For example:

• http://my.torrentInfo.net• http://smashhitsongs.com/

Web Server

Snakes on the Plane torrent

US open final torrent

Critical Elements

• 2 The .torrent file– Static ‘metainfo’ file to contain necessary infor

mation :• Name• Size• Checksum• IP address of the Tracker

Matrix.torrent

Critical Elements

• 3 A BitTorrent tracker– Non-content-sharing node– Track peers– For example:

• http://my.moviestracker.com:8080/announce• http://games.records.org:6969/announce

Critical Elements

• 4 An end user (peer)– Guys who want to use BitTorrent must install

corresponding software or plug-in for web browsers.

– Downloader (leecher) : Peer has only a part ( or none ) of the file.

– Seeder: Peer has the complete file, and chooses to stay in the system to allow other peers to download

Outline

• 1 What is it

• 2 Problem And BitTorrent Solution

• 3 Architecture And Operation

• 4 Why does it work so well

• 5 Strengths & Shortcomings

• 6 Conclusion

Problems

• Traditional Client/Server Sharing– Performance deteriorates rapidly as the

number of clients increases

• Free-riding in P2P network – Free riders only download without

contributing to the network.

BitTorrent

• Scalability of BitTorrent is very good. Even more peers means better performance.

• Strong incentives to prevent free-riding.– Priority downloading– High availability of downloads

Outline

• 1 What is it

• 2 Why do we need BitTorrent

• 3 Architecture And Operation

• 4 Why does it work so well

• 5 Strengths & Shortcomings

• 6 Conclusion

Connectivity

`

Web ServerJohn Doe

Tracker

Matrix.torrent

User

UserUser

User

Downloader:Joe

Seeder:Mike

Downloader:Michelle

Piece

• A file is cut into pieces of fixed size, typically 256Kb

• Each downloader reports to all of its peers what pieces it has.

• To verify data, Hash codes are used for all the pieces, included in .torrent files.

A trivial example

User

User

Seeder:John

DownloaderMike

{1,2,3,4,5,6,7,8,9,10}

{}{1,2,3}User

DownloaderJoe

{}{1,2,3}

{1,2,3,4}

{1,2,3,5}

{1,2,3,4,5}

Messages

• Peer – Peer messages– TCP Sockets

• Peer – Tracker messages – HTTP Request/Response

• B-encoding– The "B" encoding is identical to the "BASE64"

encoding defined by RFC 1521.

Tracker Function

• Peer cache– IP, port, peer id

• State information– Completed– Downloading

• Returns random list

Peer Functions

• File space allocated• Connect to peers• Bitfield

– have(1) and not have(0)• Have <piece>

– Advertise pieces• Interest /Not_interested

– Expressing interest in the pieces published by the peer

• Requesting for a chunk– Request <index, offset, length>

Outline

• 1 What is it

• 2 Why do we need BitTorrent

• 3 Architecture And Operation

• 4 Why does it work so well

• 5 Strengths & Shortcomings

• 6 Conclusion

Peer Selection

• Built-in incentive mechanism (where all the magic happens):– Choking Algorithm– Optimistic Unchoking– Anti-snubbing

Choking Algorithm

• Choking is a temporal refusal to upload

• Choking evaluation is performed every 10 seconds.

• Each peer unchokes a fixed number of peers (default = 4)

• The decision on which peers to un/choke is based solely on download rate, which is evaluated on a rolling, 20-second average

Reasons for Choking

• TCP congestion control.

• To ensure the peers to get a consistent download rate.

User

User

Joe

mike

ChokedChoked

Optimistic Unchoking

• a BitTorrent peer has a single ‘optimistic unchoke’ which is uploaded regardless of the current download rate from it. This peer rotates avery 30s

• Reason:– To discover currently unused connections are

better than the ones being used

Anti-snubbing

• When a peer received no data in 60s, we assume it is choked by all other peers, and refuse to upload to it except for the optimistic unchoking.

• Reason– It may cause several concurrent optimistic un

chokes.

Example

mikeUser

User

User

Downloader:Albert

Downloader:Rob

User

Downloader:Ron

User

Downloader:Kelly

40kb/s

30kb/s10kb/s

100kb/s

20kb/s

70kb/s

15kb/s

10kb/s

70kb/s

110kb/s

70kb/s

5kb/s

DownloaderJoe

User

Strengths

• Better bandwidth utilization– Never before speeds.

• Up to 7 MB/s from the Internet.

• Limit free riding – tit-for-tat

• Limit leech attack – coupling upload & download

• Spurious files not propagated

• Ability to resume a download

Shortcomings

• Small files – latency, overhead• Random list of peers - naive• Scalability

– Millions of peers – Tracker behavior (uses 1/1000 of bandwidth)

– Single point of failure

• Robustness– System progress dependent on altruistic nature of

seeds (and peers)– Malicious attacks and leeches.

Conclusion

• BitTorrent is a well thought-out protocol that embraces aspects of cooperation and self-optimizing mechanisms.

• BitTorrent propose solutions for current optimization and scalability problems

References

• [1] Bram Cohen, Incentives build robustness in Bittorrent, May 2003• [2] Stefan Saroiu, P. Krishna Gummadi, Steven D. Gribble, A

Measurement Study of Peer-to-Peer File sharing Systems, Jan 2002

• [3] Bram Cohen, Bitconjurer.org, BitTorrent Protocol Specification • [4] J R. Douceur, The Sybil Attack• [5] A W Tucker, Prisoner’s Dilemma, Serendip.com• [6] V Vishnumurthy, S Chandrakumar, EG Sirer, KARMA : A

Secure Echonomic Framework for Peer-to-Peer Resource Sharing.• [7] E. Adar and B.A. Huberman, Free Riding on Gnutella.• [8] www.kazaa.com