Tracker (BitTorrent)
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Tracker (BitTorrent)
A tracker is a specialized server that helps in peer-to-peer file sharing using the BitTorrent protocol. Its main job is to keep a record of which computers (peers) are currently active in a swarm – the group of users sharing a specific torrent.
When a BitTorrent client wants to download or upload a file, it first contacts the tracker listed in the torrent file. The tracker doesn't store the file content. Instead, it provides the client with a list of other peers in the swarm to connect with and exchange file pieces.
Originally, the tracker was a central and essential part of the BitTorrent protocol. However, with the rise of Distributed Hash Table (DHT) and Magnet links, many BitTorrent operations can now happen without needing a central tracker.
Role in the BitTorrent Protocol
A tracker's primary role is to help clients find each other. When a client starts sharing or downloading, it sends an "announce" request to the tracker. This request includes details like:
- The torrent's unique identifier (info hash).
- The client's ID, IP address, and port.
- Downloaded and uploaded amounts.
- An event (e.g., starting, stopping, completing).
In return, the tracker sends back a list of other active peers in the swarm. The client then connects directly to these peers to exchange data. Trackers can also provide statistics about a torrent, such as the number of seeders and leechers.
Limitations and Evolution
Central trackers had several drawbacks:
- Single Point of Failure: If the tracker went offline, new peers couldn't join, potentially stopping the swarm.
- Privacy Concerns: Trackers knew which IP addresses were involved in specific torrents.
- Scalability Issues: Popular torrents could overload a single tracker.
To overcome these, the BitTorrent protocol evolved to include decentralized peer discovery methods, mainly DHT. Modern BitTorrent clients can often find peers using DHT alone or with trackers, making torrents more resilient. Magnet links also reduce the need for central torrent files.
Legality
Because trackers facilitate file sharing, they have often been involved in legal disputes concerning Copyright infringement. While trackers don't host copyrighted content, courts have sometimes ruled that facilitating infringement can be illegal, leading to the shutdown of many tracker websites.In Pulsed Media servers, each user is solely responsible for files they download or distribute/share, harbor, upload, change, modify.
