BitTorrent

[akashvarad]

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Introduction
BitTorrent is a peer to peer communication protocol for file sharing.
The data to be shared is divided into many equal sized portions called pieces.
Each piece is further sub-divided into equal-sized sub-pieces called blocks.
All clients interested in sharing the same data are grouped into a swarm.
It does not require a user to download a file completely from a single server.
Instead a file can be downloaded from many such users who are indeed downloading the same file.
A user who has the complete file, called the seed will initiate the download by transferring pieces of file to the users.
Once a user has some considerable number of such pieces of a file then even he can start sharing them with other users who are yet to receive those pieces.
Pieces are not downloaded in sequential order.
This concept enables a client not to depend on a server completely .
It also reduces the overall load on the server.
Terminology
Torrent : refers to the small metadata file you receive from the web server.
Peer : A peer is another computer on the internet that you connect to and transfer data.
Leech: peer that s downloading the file
Seed : A computer that has a complete copy of a certain torrent.
Swarm : The group of machines that are collectively connected for a particular file.
Tracker : A server on the Internet that acts to coordinate the action of BitTorrent clients. The clients are in constant touch with this server to know about the peers in the swarm.
Share ratio : This is ratio of amount of a file downloaded to that of uploaded.
Choked : It is a state of an uploader where he does not want to send anything on his link. In such cases, the connection is said to be choked.
Working of BitTorrent
BitTorrent protocol shares data through what are known as torrents.
For a torrent to be alive or active it must have several key components to function.
These components include a tracker server, a .torrent file, a web server where the .torrent file is stored and a complete copy of the file being exchanged.
The first step in the BitTorrent exchange occurs when a peer downloads a .torrent file from a server.
The role of .torrent files is to provide the metadata that allows the protocol to function.
The .torrent files contains :
The URL of the tracker
Pieces <hash1, hash 2, , hash n>
Piece length
Name of the file
Length of the file
When a .torrent file is opened by the peer s client software, the peer then connects to the tracker server responsible for coordinating activity for that specific torrent.
BitTorrent: publishing a file
Architecture of BitTorrent
The BitTorrent protocol can be split into the following five main components:
Metainfo File - a file which contains all details necessary for the protocol to operate.
Tracker - A server which helps manage the BitTorrent protocol.
Peers - Users exchanging data via the BitTorrent protocol.
Data - The files being transferred across the protocol
Architecture of BitTorrent
Client - The program which sits on a peers computer and implements the protocol
Peers use TCP to communicate and send data
A tracker uses HTTP/HTTPS service and typically works on port 6969.
BitTorrent clients communicate with the tracker using HTTP GET requests, which is a standard CGI method.
BitTorrent uses ports 6881-6889 to send messages and data between peers.
Tracker
Piece Selection
There are three stages of piece selection, which change depending on which stage of completion a peer is at.
Random First Piece
When downloading first begins, as the peer has nothing to upload, a piece is selected at random to get the download started. Once this happens, the 'rarest first' strategy begins.
Rarest First
When a peer selects which piece to download next, the rarest piece will be chosen from the current swarm.
Piece Selection
End-game mode:
When requests sent for all sub-pieces, (re)send requests to all peers.
To speed up completion of download
Cancel request for downloaded sub-pieces
Tit-for-tat as incentive to upload
Want to encourage all peers to contribute
Peer A 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
Simple example
Conclusion
BitTorrent pioneered mesh-based file distribution that effectively utilizes all the uplinks of participating nodes.
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

[mafst]

BitTorrent
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1. BitTorrent is created by Bram Cohen in 2001.
2. It is peer to peer (P2P) file sharing communication protocol.It is a peer to peer in nature ,users connected to each other directly to send and receive the portions of the file.
It used to transfer very huge files.
The key philosophy of Bit Torrent is that user should upload at the same time they downloading.
5. BitTorrent provides protocol to all torrent clients.
6. This is most popular and efficient file transferring system.
7. More efficient than client server system.
Client server working
Client send the request to server
Server send files
Transfer is handled by FTP and HTTP.
Client Responsibility
Handle the user interface.
Translate the user's request into the desired protocol.
Send the request to the server.
Wait for the server's response.
Translate the response into "human-readable" format.
Present the results to the user.
Server Responsibilty
Wait for a client's request.
Process that request.
Return the results back to the client.
Client-Server Disadvantages
When the number of client request to server increases,server become overloaded.
Crashes whole system when Server crashes.
Peer to peer Architecture
File to be transfer is divided into equal size of pieces.
These pieces are downloaded from different peers parallely.
components
BitTorrent Client Software
.torrent file or METAinfo file
Leechers
Peers
Seeds
Tracker
Piece (of a file)
Data
Torrent index server
Torrent File / Metafile
It has extension as .torrent
It is extremely small ( up to 100KB )
It is downloaded from torrent index server
contains the information that points to the actual file and the people who are sharing it.
Contents of .torrent file
Info
size of piece
total no. piece
SHA number
announce
creation date
comment
created by
seeds
These are the clients who has entire file
They uploads files to other peers
This is the peer which does not have entire file
They uploads part of file at same time when they downloads other part.
peers
It is the client which is responsible for file transfer
Downloading speed of any file using torrent depends on number of peers.
Peers = leech + seed
Piece(of a file)
The file which to be download is divided similar size of pieces.
These pieces are downloaded parallel
Each pieces is download from different peers
Tracker
Tracker is the server
It contains all database about peers of a torrent
It provides all list of peers which are associated with a particular torrent
These trackers are mentioned in the Metafile
Client S/W sends request to send list of peers
connects to all peers
Torrent index website
Source of .torrent files
Torrent search engine
advantages
BitTorrent is an open-source program offers a spyware- and nuisance-free installation.
Allows users to share large amounts of data in a short span of time.
Discourages freeloading by rewarding fastest uploaders.
The more popular a file is the more people want a copy of it the faster it can be downloaded, because there are more places to get pieces of it.

[manishdriems]

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INTRODUCTION

BitTorrent is a peer-to-peer file sharing protocol used for distributing large amounts of data. BitTorrent is one of the most common protocols for transferring large files, and it has been estimated that it accounts for approximately 27-55% of all Internet traffic (depending on geographical location) as of February 2009.
BitTorrent protocol allows users to distribute large amounts of data without the heavy demands on their computers that would be needed for standard Internet hosting. A standard host's servers can easily be brought to a halt if high levels of simultaneous data flow are reached. The protocol works as an alternative data distribution method that makes even small computers (e.g. mobile phones) with low bandwidth capable of participating in large data transfers.
First, a user playing the role of file-provider makes a file available to the network. This first user's file is called a seed and its availability on the network allows other users, called peers, to connect and begin to download the seed file. As new peers connect to the network and request the same file, their computer receives a different piece of the data from the seed. Once multiple peers have multiple pieces of the seed, BitTorrent allows each to become a source for that portion of the file. The effect of this is to take on a small part of the task and relieve the initial user, distributing the file download task among the seed and many peers. With BitTorrent, no one computer needs to supply data in quantities which could jeopardize the task by overwhelming all resources, yet the same final result each peer eventually receiving the entire file is still reached.
After the file is successfully and completely downloaded by a given peer, the peer is able to shift roles and become an additional seed, helping the remaining peers to receive the entire file. This eventual shift from peers to seeders determines the overall 'health' of the file (as determined by the number of times a file is available in its complete form).
This distributed nature of BitTorrent leads to a flood like spreading of a file throughout peers. As more peers join the swarm, the likelihood of a successful download increases. Relative to standard Internet hosting, this provides a significant reduction in the original distributor's hardware and bandwidth resource costs. It also provides redundancy against system problems, reduces dependence on the original distributor and provides a source for the file which is generally temporary and therefore harder to trace than when provided by the enduring availability of a host in standard file distribution techniques.
Programmer Bram Cohen designed the protocol in April 2001 and released a first implementation on 2 July 2001. It is now maintained by Cohen's company BitTorrent, Inc. There are numerous BitTorrent clients available for a variety of computing platforms.

[basheer]

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This article is presented by:
SURYANARAYANA RAJU .G
COMPUTER SCIENCE & ENGINEERING
From
JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY
HYDERABAD

ABSTRACT

Existing studies on Bit Torrent systems are single-torrent based, while more than 85% of all peers participate in multiple torrents according to our trace analysis. In addition, these studies are not sufficiently insightful and accurate even for single-torrent models, due to some unrealistic assumptions. Our analysis of representative Bit Torrent traffic provides several new findings
regarding the limitations of Bit Torrent systems
(1) Due to the exponentially decreasing peer arrival rate in reality, service availability in such systems becomes poor quickly, after which it is difficult for the file to be located and downloaded.
(2) Client performance in the Bit Torrent-like systems is unstable, and fluctuates widely with the peer population.
(3) Existing systems could provide unfair services to peers, where peers with high downloading speed tend to download more and upload less. In this paper, we study these limitations on torrent evolution in realistic environments. Motivated by the analysis and modeling results, we further build a graph based multi-torrent model to study inter-torrent collaboration. Our model quantitatively provides strong motivation for inter-torrent collaboration instead of directly stimulating seeds to stay longer. We also discuss a system design to show the feasibility of multi-torrent collaboration.

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[ckhari88]

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BitTorrent

BitTorrent consumes significant amount of internet traffic today
In 2004, BitTorrent accounted for 30% of all internet traffic (Total P2P was 60%), according to CacheLogic
Slightly lower share in 2005 (possibly because of legal action), but still significant
BT always used for legal software (linux iso) distribution too.
Millions want to download the same popular huge files (for free)
Media (the real example!)
Client-server model fails
Single server fails
Can t afford to deploy enough servers

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