Seeding (BitTorrent)

In the BitTorrent peer-to-peer file sharing protocol, seeding is the act of uploading pieces of a file or set of files **after** having already successfully downloaded the complete file(s) associated with a specific torrent. A peer that is currently performing this action is called a seeder.

The primary purpose of seeding is to contribute back to the swarm and ensure the continued availability of the file(s) for other peers who are still in the process of downloading (the leechers). Unlike a leecher, a seeder possesses all the pieces of the torrent's content.

Seeders are a crucial component for the health and longevity of a swarm. Without seeders, once the original distributor (the initial seeder) stops sharing, the file would eventually become unavailable as peers leave the swarm after finishing their downloads.

Role in the Swarm

Seeders are vital to the swarm because they are the source for all the file's pieces. While peers who are still downloading (leechers) also upload pieces they have acquired, only a seeder has every single piece.

The more seeders there are in a swarm, and the more upload bandwidth they collectively provide, the faster other peers (especially new ones) can download the file. A swarm with a good number of seeders is considered "healthy" because it ensures that all pieces of the file are readily available and that new peers can join and complete their downloads efficiently.

If the number of seeders becomes too low compared to the number of leechers, the overall download speed for the swarm will decrease, and there's a risk that some pieces may become unavailable if the only peers who have them leave.

Seeder vs. Leecher

The distinction between a seeder and a leecher is based on whether the peer has a complete copy of the torrent's file(s):

Leecher: A peer that is actively downloading but does **not** yet have 100% of the file(s). A peer begins as a leecher.
Seeder: A peer that has 100% of the file(s) and is actively uploading those files to other peers in the swarm. A peer becomes a seeder after completing its download.

How Seeding Works

Once a BitTorrent client finishes downloading the last pieces of a torrent and verifies the integrity of the entire file(s) using the hashes provided in the torrent file, the client transitions from being a leecher to a seeder.

The client then continues to participate in the swarm, listening for requests from other peers who need pieces that the seeder possesses. The seeder's upload bandwidth is then dedicated to serving pieces to the remaining leechers in the swarm. Most clients are configured to automatically begin seeding upon completion of a download.

Importance of Seeding

The act of seeding is often considered a fundamental part of the BitTorrent ethos and is crucial for the protocol's effectiveness:

Maintaining File Availability: Ensures that files remain downloadable even long after the original source has stopped sharing.
Improving Download Speeds: More seeders mean more sources for pieces, leading to faster downloads for peers in the swarm.
Supporting the Decentralized Model: Reinforces the peer-to-peer nature by having users contribute resources (upload bandwidth and file availability) back to the network, rather than relying solely on centralized servers.

Community etiquette in BitTorrent often encourages users to continue seeding for a certain amount of time or until they reach a specific Seed ratio.

Seed Ratio and Seed Time

Two common metrics related to seeding are:

Seed Ratio: The ratio of the total amount of data uploaded by a client for a specific torrent compared to the total amount of data downloaded for that same torrent. A ratio of 1.0 means the client has uploaded as much data as it downloaded (e.g., uploaded the full file size). A ratio greater than 1.0 means the client has uploaded more than it downloaded. Community guidelines often recommend aiming for a ratio of at least 1.0 or higher.
Seed Time: The total amount of time a client continues to seed a torrent after completing the download. Recommendations vary, but commonly include seeding for a minimum duration (e.g., 24 hours) or until the Seed ratio target is met.

Maintaining a good Seed ratio and Seed time is seen as contributing positively to the BitTorrent community and helps ensure the health of swarms.

References

BitTorrent.org - The BitTorrent Protocol Specification - The technical specification that defines the roles of peers, including seeders.
HowStuffWorks - How BitTorrent Works - Explains the concepts of peers, seeders, and leechers for a general audience.
Digital Trends - What is Torrenting? - Includes basic definitions of torrenting terms like seeding.

Seeding (BitTorrent)

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