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How torrent works?

How torrent works?


bittorrent Decentralization Draft

Decoding the BitTorrent Protocol

Table of Contents:

  1. Introduction
  2. The Treasure Map: Decoding the .torrent File
  3. Connecting to the Tracker: Finding Fellow Adventurers
  4. The Handshake: Establishing a Connection
  5. The Bitfield: A Glimpse into the Peer’s Inventory
  6. From Bitfield to Bytes: Navigating the Data Maze
  7. The Data Symphony: A Chorus of Messages
  8. The Grand Finale: Assembling the Masterpiece
  9. Conclusion: The Power of Decentralization

1. Introduction

The BitTorrent protocol is a marvel of decentralized engineering, enabling efficient file sharing without the need for central servers. This blog post will guide you through the intricate steps involved in downloading a file using this revolutionary technology.

2. The Treasure Map: Decoding the .torrent File

Our journey begins with a .torrent file, a compact guide containing essential information about the file we seek. This file acts as a treasure map, leading us to the digital bounty:

.torrent file (The Treasure Map)
-------------------------------
 Tracker URL:   xXx.tracker.land:6969 (Address of a server that helps peers find each other)
 Info Hash:     d9b6148819fb6760ff1ac2b823b5a239c9153831 (Unique identifier for the file)
 File Name:     EpicCatVideo.mp4 (Name of the file)
 File Size:     10485760 bytes (Total size of the file)
 Piece Length:  262144 bytes (Size of each individual chunk of the file)
 Piece Hashes:  [hash1, hash2, ..., hash40] (Cryptographic fingerprints of each piece)

3. Connecting to the Tracker: Finding Fellow Adventurers

Armed with the Tracker URL, our BitTorrent client contacts the tracker server:

Client -> Tracker:  "Hello! I'm looking for peers sharing the file with Info Hash d9b61488..."

The tracker responds with a list of IP addresses of other peers currently sharing the same file.

4. The Handshake: Establishing a Connection

Before exchanging data, we must establish a connection with a peer. This is done through a handshake:

Client -> Peer: "Greetings! Are you a fellow seeker of EpicCatVideo.mp4? My handshake: [pstrlen=19, pstr="BitTorrent protocol", reserved, d9b61488..., MyPeerID]"

Peer -> Client: "Indeed I am! My handshake: [pstrlen=19, pstr="BitTorrent protocol", reserved, d9b61488..., Peer'sPeerID]"

The matching Info Hash confirms they are both interested in the same file.

5. The Bitfield: A Glimpse into the Peer’s Inventory

Once connected, we need to know which pieces the peer possesses. This information is conveyed through the bitfield:

Client -> Peer: "Show me your inventory! Which pieces of EpicCatVideo.mp4 do you have?"

Peer -> Client: "Here is my bitfield: 110010110101110..." (1 represents a possessed piece, 0 represents a missing one)

6. From Bitfield to Bytes: Navigating the Data Maze

Armed with the bitfield, our client knows which pieces to request. However, it must first navigate the “choke/unchoke” protocol:

client -> peer: "Interested!"
client -> peer: "Unchoke me please!"

peer -> client: "Unchoke"

client -> peer:  "Piece 5, from byte 0, 262144 bytes please!"

7. The Data Symphony: A Chorus of Messages

As pieces are requested, various messages flow between peers:

  • Keep-alive: Ensures the connection stays active.
  • Choke/Unchoke: Controls the data flow.
  • Have: Announces that a peer has acquired a new piece.
  • Piece: Contains the actual data of a requested piece.

8. The Grand Finale: Assembling the Masterpiece

Piece by piece, the client gathers the fragments of EpicCatVideo.mp4. Each piece is verified against its cryptographic hash. Finally, the complete video is assembled.

9. Conclusion: The Power of Decentralization

The BitTorrent protocol demonstrates the power of decentralized systems. By connecting peers directly, it allows for efficient file sharing without relying on central servers.