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The BitTorrent Protocol
The BitTorrent Protocol
What is BitTorrent
What is BitTorrent
File sharing
File sharing
How it works
How it works
BT Components
BT Components
BT Components
BT Components
BitTorrent Lingo
BitTorrent Lingo
Simple example
Simple example
Basic Idea
Basic Idea
Operation
Operation
Download in progress
Download in progress
Download in progress
Download in progress
Pipelining
Pipelining
Piece Selection
Piece Selection
Piece Selection
Piece Selection
Piece selection
Piece selection
Random First Piece
Random First Piece
Rarest Piece First
Rarest Piece First
Endgame Mode
Endgame Mode
BT: internal mechanism
BT: internal mechanism
Choking
Choking
Choking
Choking
More on Choking
More on Choking
Optimistic unchoking
Optimistic unchoking
Upload-Only mode
Upload-Only mode
Questions about BT
Questions about BT
Trackerless torrents
Trackerless torrents

Презентация на тему: «The BitTorrent Protocol». Автор: Sukumar Ghosh. Файл: «The BitTorrent Protocol.ppt». Размер zip-архива: 114 КБ.

The BitTorrent Protocol

содержание презентации «The BitTorrent Protocol.ppt»
СлайдТекст
1 The BitTorrent Protocol

The BitTorrent Protocol

2 What is BitTorrent

What is BitTorrent

Efficient content distribution system using file swarming. Does not perform all the functions of a typical p2p system, like searching. The throughput increases with the number of down loaders via the efficient use of network bandwidth

3 File sharing

File sharing

To share a file or group of files, the initiator first creates a .torrent file, a small file that contains Metadata about the files to be shared, and Information about the tracker, the computer that coordinates the file distribution. Downloaders first obtain a .torrent file, and then connect to the specified tracker, which tells them from which other peers to download the pieces of the file.

4 How it works

How it works

The file to be distributed is split up into pieces and an SHA-1 hash is calculated for each piece

5 BT Components

BT Components

The peers first obtain a metadata file for each object The metadata contains: The SHA-1 hashes of all pieces A mapping of the pieces to files Piece size Length of the file A tracker reference

6 BT Components

BT Components

The tracker is a central server keeping a list of all peers participating in the swarm A swarm is the set of peers that are participating in distributing the same files A peer joins a swarm by asking the tracker for a peer list and connects to those peers.

7 BitTorrent Lingo

BitTorrent Lingo

Seeder = a peer that provides the complete file. Initial seeder = a peer that provides the initial copy.

Leecher

Leecher

Seeder

One who is downloading (not a derogatory term)

Initial seeder

8 Simple example

Simple example

Seeder: A

Downloader C

Downloader B

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

{}

{1,2,3}

{1,2,3,5}

{}

{1,2,3}

{1,2,3,4}

{1,2,3,4,5}

9 Basic Idea

Basic Idea

As a leecher downloads pieces of the file, replicas of the pieces are created. More downloads mean more replicas available As soon as a leecher has a complete piece, it can potentially share it with other downloaders. Eventually each leecher becomes a seeder by obtaining all the pieces, and assembles the file. Verifies the checksum.

10 Operation

Operation

11 Download in progress

Download in progress

12 Download in progress

Download in progress

13 Pipelining

Pipelining

When transferring data over TCP, always have several requests pending at once (typically 5), to avoid a delay between pieces being sent. Every time a piece or a sub-piece arrives, a new request is sent out.

14 Piece Selection

Piece Selection

The order in which pieces are selected by different peers is critical for good 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 the missing ones. If the original seed is prematurely taken down, then the file cannot be completely downloaded! What are “good policies?”

15 Piece Selection

Piece Selection

Small overlap is good

Large overlap is bad -- wastes bandwidth

16 Piece selection

Piece selection

Strict Priority Rarest First General rule Random First Piece Special case, at the beginning Endgame Mode Special case

17 Random First Piece

Random First Piece

Initially, a peer has nothing to trade Important to get a complete piece ASAP Select a random piece of the file and download it

18 Rarest Piece First

Rarest Piece First

Determine the pieces that are most rare among your peers, and download those first. This ensures that the most commonly available pieces are left till the end to download.

19 Endgame Mode

Endgame Mode

Near the end, missing pieces are requested from every peer containing them. This ensures that a download is not prevented from completion due to a single peer with a slow transfer rate. Some bandwidth is wasted, but in practice, this is not too much.

20 BT: internal mechanism

BT: internal mechanism

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

21 Choking

Choking

Choking is a temporary refusal to upload. It is one of BT’s most powerful idea to deal with free riders (those who only download but never upload). Tit-for-tat strategy is based on game-theoretic concepts.

22 Choking

Choking

Reasons for choking: Avoid free riders Network congestion A good choking algorithm caps the number of simultaneous uploads for good TCP performance.

23 More on Choking

More on Choking

Peers try out unused connections once in a while to find out if they might be better than the current ones (optimistic unchoking).

24 Optimistic unchoking

Optimistic unchoking

A BT peer has a single “optimistic unchoke” to which it uploads regardless of the current download rate from it. This peer rotates every 30s Reasons: To discover currently unused connections that are better than the ones being used To provide minimal service to new peers

25 Upload-Only mode

Upload-Only mode

Once download is complete, a peer can only upload. The question is, which nodes to upload to? Policy: Upload to those with the best upload rate. This ensures that pieces get replicated faster, and new seeders are created fast

26 Questions about BT

Questions about BT

What is the effect of bandwidth constraints? Is the Rarest First policy really necessary? Must nodes perform seeding after downloading is complete? How serious is the Last Piece Problem? Does the incentive mechanism affect the performance much?

27 Trackerless torrents

Trackerless torrents

BitTorrent also supports "trackerless" torrents, featuring a DHT implementation that allows the client to download torrents that have been created without using a BitTorrent tracker.

«The BitTorrent Protocol»
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