how bit torrent works.pptx

7/28/2019 How Bit Torrent works.pptx

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How Bit Torrent Works?A PRESENTATION BY

ADITYA MISRA (10ECE035) RAJESH LIRIA (10ECE064)


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Bit Torrentan introduction

Consumed about 40% of the world bandwidth in 2012

No requirement of large capacity server or expensive bandwidth

Works on a p2p network basis

Data is much more secure over such a p2p network

The possibility of a server crash or timeout is exterminated.

In a nut shell, bit torrent is a setup to extend the number of file sourcesexponentially.


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Popular Terms

AVAILABILITY

CLIENT

HASH/HASH SUM

HIT AND RUN

LEECH

LURKER

SEED

PEER

P2P (PEER TO PEER)

SWARM

TRACKER

METADATA


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Why Bit Torrent?

Used for replication of large amounts of static data.

Scalability : the speed of data transfer or throughput increases with theincrease of peers and seeds to a swarm.

Efficientutilizes a large amount of available network bandwidth

Tolerant to dropping peers

Ability to verify data integrity (SHA-1 Hashes)

Minimise piece overlap among peers to allow each peer to exchangepieces with as many other peers as possible


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File SplittingThe file to be distributed is spl it up in pieces and an SHA-1 hash is calculated for each piece.


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Metadata File

A metadata file (commonly known as .torrent file) is distributed to all thepeers in a swarm, using a different protocol.(Generally, http or ftp)

The metadata contains

The SHA-1 hashes of all pieces (Secure Hash Algorithm)

A mapping of the pieces to files

A tracker reference

URL of tracker

Name

Length of file


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Tracker

The tracker is a central server keeping a list of all peers participating in theswarm

A swarm is the set of peers that are participating in distributing the samefiles

A peer joins a swarm by asking the tracker for a peer list and connects tothose peers.


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Graphic VisualizationThe above visualization shows a set of swarm with the pieces they have. The node connected with an arrow is a newpeer to the swarm requesting for the list of ip of its peers.


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Graphic VisualizationThe above visualization shows a set of swarm with the pieces they have. The node connected with an arrow is a ne

peer to the swarm requesting for the list of ip of its peers.


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DHT distributed hash table

Works as a hash table with sha1-hashes as keys.

The key is the info-hash, the hash of the metadata. It uniquely identifies atorrent.

The data is a peer list of the peers in the swarm.

Each node is assigned an ID

in the key space (160 bit numbers)

Nodes order themselves in a defined topography

Makes it possible to search for Ids by traversing the node topography

Bittorrent uses kademlia as DHT.


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Kademlia Bootstrap

Each node bootstraps by looking for its own ID

The search is done recursively until no closer nodes can be found

The nodes passed on the way are stored in the routing table

The routing table have more room forclose nodes than distant nodes


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Kademlia Routing Table

Each node knows much more about close nodes than distant nodes

The key space each bucket represents is growing with the power of 2 with

the distance

Querying a node for a specific ID will on average halve the distance to thtarget ID each step


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Visualization of Kademlia Bootsrap RoutingNode Bucket gives us the list of neighborhood peers. Farther you are on the Node ID bit, the closer you are to you

neighbors.


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Visualization of Kademlia Node IDEach bit level represents a node bucket. Higher on the node bucket, more the no. of neighborhood peers.


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Pieces and Sub-pieces

A piece is broken into sub-pieces typically 16KB in size.

Until a piece is assembled, only download the sub-pieces of that pieceonly

This policy lets pieces assemble quickly.


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Piece overlap

Small overlap Big overlap


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Piece Selection

The order in which pieces are selected by different peers is critical forgood 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 themissing ones.

If the original seed is prematurely taken down, then the file cannot becompletely downloaded!


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Piece Selection Algorithms

Strict Priority

First priority (commonly in a sequential order, otherwise can be reconfigured toour own choice)

Rarest First

General Rule (preset in all the torrent clientsBit Torrent, Torrent, Vuze,

Transmission)

Random First Piece

Special Case, at the beginning

Endgame Mode

Special Case for end of data transmission.


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Bit Torrent : Internal Mechanism

Built in Incentive mechanism

Choking Algorithm

Optimistic Unchoking


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Choking Algorithm

Choking is a temporary refusal to upload. It is one of BitTorrents mostpowerful idea to deal with free riders (those who only download but neverupload).

Tit-for-tat strategy is based on game-theoretic concepts. A good choking algorithm caps the number of simultaneous uploads for

good TCP performance.

Avoids choking and unchoking too quickly, known as fibrillation).


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Optimistic Unchoking

A BitTorrent peer has a single optimistic unchoke to which it uploadsregardless of the current download rate from it. This peer rotates every 30s

Reasons: To discover currently unused connections are better than the ones being used

To provide minimal service to new peers


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Upload only mode

Transition from a peer to a seed.

Once download is complete, a peer has no download rates to use for

comparison nor has any need to use them. Upload to those with the best upload rate. This ensures that pieces get

replicated faster, and new seeders are created fast.


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Optimization of Bit Torrent - Canwe further push the limits?

A centrally monitored and controlled uplink and downlink of data between two nodes may theoreprovide a larger throughput, but in real environment that has to deal with - non responsive nodes, aasymmetrical upload and download bandwidth; Bit Torrent might be the most efficient p2p sharingnetwork in the entire course of file exchange protocols.

CANACENTRALLY SCHEDULED FILE DISTRIBUTION PROTOCOL GIVEAN ANSWERTOTHE BIT TORRENT LAG ?


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Summary and Queries.

Well go through a quick revision on what we learnt. You may ask anyquestions post summary.

Thank you for your attention !

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