When it comes to file sharing, one technology reigns supreme: BitTorrent. It has transformed the way we share files, making it faster and more efficient.

But what is BitTorrent? For most of us, it is just an application, or a tool to download free software, games or files. But actually BitTorrent is a protocol.

How does BitTorrent work? In this series we will try to figure out, how BitTorrent works under the hood. The data structures and the algorithms it uses to do what it does. But before let us see how it begins.

The magic starts with Peer-to-Peer (P2P) technology.

The Essence of Peer-to-Peer (P2P) Technology:

BitTorrent is built on P2P technology, which allows users to share files directly with each other. It’s like a network where everyone is both a consumer and a provider. No central servers are needed. This decentralized approach makes BitTorrent powerful.

Pure P2P and Hybrid P2P:

P2P technology comes in two variations: pure P2P and hybrid P2P.

a. Pure P2P:
In a pure P2P network, participants are equal and contribute to the network’s resources. No central authority or dedicated servers are required. Each user acts as a peer, downloading and uploading files. This decentralized model ensures robustness and scalability.

b. Hybrid P2P:
Hybrid P2P networks combine the strengths of P2P and client-server architectures. They use central servers or “trackers” to help peers find each other initially. The actual file transfers occur directly between peers. This hybrid model balances scalability, decentralization, and coordination.

Understanding P2P technology is essential to harnessing the true potential of BitTorrent. In the next sections, we’ll explore how BitTorrent works, the roles of peers and seeds, the mechanics of file downloading and uploading, and the advantages it offers for file sharing.

Now that we know the infrastructure on top of which it works, let us deep dive into what BitTorrent.


BitTorrent is a protocol that simplifies file sharing, especially large files, while minimizing the bandwidth requirements for the publisher. It achieves this by using the upload capacity of the peers who are currently downloading the file. The effect is, even when the number of downloaders increases significantly, the impact on the publisher’s hosting load remains relatively low.

The figure above provides a visual representation of the fundamental flow of BitTorrent. On the left side of the illustration, a traditional client-server approach to downloading is depicted. In this scenario, peers download the file simultaneously from the server. If we assume that the server’s upload capacity is equivalent to the download capacity of a peer, the total download time will be twice as long compared to a situation where only one peer is downloading from the server. On the right side of the figure, a similar approach to BitTorrent is showcased. By fragmenting the file and distributing different parts to each peer, and allowing peers to download the missing parts from one another, both the download time and the server load are significantly reduced. It’s important to note that the BitTorrent protocol is much more sophisticated than this simplified example, but it effectively conveys the underlying concept.

Ok, so now that we know what BitTorrent actually is, let us try to understand it’s internal working.

BitTorrent Architecture

BitTorrent operates on a well-defined architecture, with key entities working together to facilitate efficient file sharing. Understanding this architecture is crucial to grasp how BitTorrent operates and why it excels in distributing files. Let’s explore the main components of the BitTorrent architecture:

  1. The Torrent File: A Roadmap for Sharing

At the heart of BitTorrent’s architecture is the torrent file, which serves as a static manifest containing important information about the shared content, such as its name, size, and a list of trackers. It acts as a roadmap guiding the file sharing process. The torrent file is created by the original downloader, known as the seed, who initiates the sharing by making it available for download.

  1. The Seed: Initiating the Distribution

The seed is the initial source of the shared file. They create the torrent file and kickstart the distribution process by making it accessible for download. The seed holds the complete file and forms the backbone of the BitTorrent network. As other users start downloading the file, they become part of the swarm.

  1. The Tracker: Coordinating Connections

The tracker plays a crucial role in the BitTorrent architecture by coordinating connections between peers. It assists both seeds and leeches in discovering and connecting with one another. When a peer wants to download a file, it communicates with the tracker to obtain a list of other peers in the swarm. This information enables the peer to establish direct connections and start exchanging file data.

  1. The Leech: Downloading and Uploading Simultaneously

The end users, often referred to as leeches, are the peers who download files using BitTorrent. They connect to the tracker to obtain a list of other peers and establish direct connections to download file pieces. Interestingly, as leeches receive file pieces, they simultaneously upload those pieces to other peers, contributing to the overall file distribution and increasing its availability within the swarm.

BitTorrent’s architecture optimizes the sharing process by leveraging the collaborative efforts of all entities involved. As more users join the swarm, the file’s download speed and overall availability improve. Peers dynamically adapt and contribute based on their download and upload capacities, ensuring efficient and balanced file distribution.

In addition to these main entities, BitTorrent incorporates various protocols and mechanisms like distributed hash tables (DHT) and peer exchange (PEX) to enhance efficiency and resilience within the network. These elements work together to create a robust and efficient file sharing ecosystem. In the next part of this series, we will delve into the algorithms and mechanisms that power BitTorrent, further unraveling its inner workings and demonstrating its true potential in the world of file sharing.

Certainly! Here’s an enhanced version of the section, including examples, coverage of popular and rare file use cases, SHA-1 fragmentation, and an explanation of how the bandwidth of the publisher reduces with more downloads:

How File sharing happens?

The process of file sharing in BitTorrent involves multiple steps, from the initial upload by the seeder to the arrival of leeches seeking to download the file. The tracker plays a crucial role in facilitating this journey and keeping track of the peers involved. Let’s walk through the journey of a file in BitTorrent:

  1. Uploading by the Seeder
    The file sharing journey begins with the seeder, the original uploader of the file. For example, imagine a popular video file that has gained attention online. The seeder creates the torrent file, which contains metadata about the shared content, including file name, size, and a list of trackers. The seeder uploads the torrent file to a tracker, signaling their readiness to share the file with others.
  2. Connecting with the Tracker
    When a leecher (a peer seeking to download the file) enters the scene, they connect to the tracker associated with the torrent file. The tracker plays a pivotal role in connecting peers, maintaining a log of available peers in the swarm, and coordinating the exchange of file data. For example, if the file is rare or hard to find, the tracker helps interested leechers discover and connect with the seeder.
  3. Discovering Peers and Joining the Swarm
    The tracker responds to the leecher’s request by providing a list of other peers (both seeds and leeches) who are currently sharing the file. Imagine there are multiple seeds and leeches actively participating in the swarm. The leecher establishes direct connections with the available peers, joining the swarm to start downloading the file.
  4. Requesting and Downloading File Pieces
    With the connections established, the leecher starts requesting file pieces from the available peers. BitTorrent breaks the file into small pieces, enabling concurrent downloading from multiple sources. For instance, if the video file is 1 GB in size and each piece is 1 MB, there would be 1000 pieces to download. As the leecher receives these pieces, they also become a source for other leechers, uploading the pieces they have downloaded to contribute to the swarm.
  5. Continual Sharing and Completion
    As the file transfer progresses, the leecher continues to download missing file pieces from the available peers. At the same time, they actively upload the pieces they have acquired, ensuring a healthy and collaborative sharing environment within the swarm. This process continues until the leecher has obtained all the file pieces and completed the download.

In rare file use cases, where the file is scarce and not widely available, BitTorrent’s decentralized nature helps connect interested leechers to the few seeds possessing the file, fostering a distributed sharing network.

BitTorrent employs a technique called SHA-1 fragmentation, where the file is divided into small chunks using the SHA-1 hashing algorithm. This fragmentation enhances data integrity and ensures that each piece can be independently verified and reassembled by the leecher.

Moreover, the bandwidth consumption for the publisher is reduced as more users download the file. In traditional download scenarios, as the number of downloaders increases, the publisher’s bandwidth requirements also increase. However, in BitTorrent, as more leechers join the swarm, the burden on the seeder decreases. Each leecher contributes to the distribution by uploading pieces they have acquired, effectively sharing

the load across the network and reducing the bandwidth demand on the seeder.

By leveraging the power of peer-to-peer connectivity and the coordination of the tracker, BitTorrent offers a decentralized and efficient file sharing experience. The collaborative nature of the protocol, with peers simultaneously downloading and uploading, enables faster downloads, increased availability, and reduced strain on individual connections.

This is all for this part, in the next part of this series, we will take a deep dive into the inner workings of BitTorrent and explore the algorithms that make it such an efficient and robust file sharing protocol. We will discuss the intricate details of peer interactions, piece selection strategies, as well as the mechanisms employed by the tracker to maintain a healthy and balanced swarm. By understanding these algorithms, you will gain valuable insights into how BitTorrent optimizes file distribution and enhances the overall user experience.

Stay tuned for the upcoming part, where we will unravel the algorithms behind the magic of BitTorrent!
In the meanwhile, you can read other informative posts about CI/CD , System Design, JS, etc on the same blog.

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