How BitTorrent Works

As a process for the transfer of content over the internet, BitTorrent is clearly superior to traditional methods.

What is BitTorrent?

BitTorrent is a decentralized peer-to-peer (P2P) distribution of content which uses the upload bandwidth of each individual who is downloading the content, and those who have downloaded, to transfer the content.

The BitTorrent transfer is initiated through either a .torrent file or a magnet link. Either of these is associated with specific content and when opened in a BitTorrent client, transfer of that associated content begins.

From its inauspicious start in 2001, BitTorrent has grown to one of the major forces on the internet. Estimates of bandwidth use by BitTorrent are one third, and upwards, of all internet use. There are presently over 17 million torrents active and many petabytes of content associated with those torrents.

Unfortunately, the use of BitTorrent to illegally distribute copyrighted content has overshadowed BitTorrent’s advantages in the distribution of legal content.

Comparison of Distribution Methods

Central Server – Traditional downloads from the internet use a central server to provide the bandwidth for the transfer. All who want the download, obtain their copy from the same source.

Traditional P2P – The first incarnations of peer to peer transfers (Napster etc.) worked similarly to central server. An individual served as a server distributing content to others using the upload bandwidth of that individual’s internet connection. While the distribution method was similar, P2P was “off the main grid” and involved distribution between individuals, hence the peer to peer (P2P) designation.

BitTorrent – It was the next step in the evolution of transfers and P2P. Different from traditional central server and traditional P2P distribution, it involves the use of multiple sources for the distribution of the content. Each active user is involved in the distribution of the content through their upload bandwidth. It also provides a fail safe against the corruption of distributed content.

These diagrams roughly show these distribution methods. Since central server and traditional P2P essentially distribute the same way, the “normal” network distribution image reflects both of these methods.

Advantages of BitTorrent over Traditional Distribution Methods

Virtuous Circle vs Viscous Circle

BitTorrent – The upload bandwidth of every active user on a torrent is used to transfer the content in BitTorrent. Therefore, the more popular the content, the greater the download speed for each user.

Central Server and Traditional P2P – A central server, or an individual serving as a central server in traditional P2P, has a limited amount of bandwidth for distribution. The more users seeking the content, the less the download speed of each user.

Integrity of Content

BitTorrent – The torrent file or the magnet link that is used to initiate the distribution contains unique and specific information on the content that will be downloaded through either. The content is broken down to bits to facilitate distribution among the active users. If any of these bits becomes corrupt during transfer, and therefore does not match the information within the torrent or magnet link, then it will be rejected by the BitTorrent client and replaced with an uncorrupted bit. This ensures that it will be an exact copy of the distributed content when the download is complete.

Central Server – Distribution through a central server always involves some risk of corruption and this issue is worsened when the demand for the content is high. There is no fail-safe guard against such corruption of content.

Traditional P2P – It suffers from the same issues as a central server. Since traditional P2P is “off the grid”, the risk of distribution of malware is increased, making this the most dangerous of distribution methods.

Definitions Of Common BitTorrent Terms

Torrent – A small text file that contains information about specific (associated) content, file and folder names within the content and about trackers. These are used to begin download of the associated content when opened in a BitTorrent client. A torrent file will only download when the content, and file and folder names are identical.

Magnet Links – Similar to torrents, magnet links are used to download associated content when opened in a BitTorrent client. They differ in that they are not files, only links. A magnet link only contains information on the content, and no tracker or file and folder information.

This is an advantage for BitTorrent search sites as they do not need to store files, only bits of data relating to the magnet link. Additionally, this further decentralizes BitTorrent as a tracker is no longer needed. This is also an advantage for users as a magnet link looks for the content only. If the file or folder names are different, a magnet link may still download. This could help increase download speed, particularly if the content is rare.

Tracker – A communications hub that helps people active on a torrent connect to each other.

Seeds – Those who have 100% of the torrent’s associated content and are uploading to peers or leechers.

Peers – Those active on a torrent who have less than 100% of the content associated with the torrent. They download from seeds and other peers and upload to other peers. (Most BitTorrent clients use the term this way).

Leechers – Those who have less than 100% of the content, or those who download 100% of a torrent’s associated content, but do not upload back at least 100% of that content. (This negative connotation is why BitTorrent clients use peers for the above definition.)

Ratio – The amount uploaded divided by the amount downloaded. Usually shown per torrent and overall within the BitTorrent client.

Swarm – The group of people active on a single torrent.

DHT – Short for Distributed Hash Table. A way of connecting to other users without the tracker being involved. Especially useful when a tracker goes down or for hosting tracker-less torrents.

PEX – Short for Peer Exchange. A way of connecting to other users without the tracker, through people that you are already connected to. Especially useful if tracker is down.

How BitTorrent Works

This is a non-technical description of the BitTorrent process.

A user goes to a torrent search site to search for content. When the user finds wanted content, the user downloads the .torrent file (or magnet link) associated with the content. When the torrent (or magnet link) is opened in a BitTorrent client, transfer of the associated content begins.

When the download is begun with a torrent, the BitTorrent client communicates to a tracker through the information contained in the .torrent file. A tracker essentially serves as a communications hub for this swarm of individuals, helping them find each other. DHT and PEX also help users to find each other.

With a magnet link, the BitTorrent client uses DHT and PEX to find other users with identical content.

BitTorrent works by splitting content files into hundreds of smaller “bits” and sharing those bits, using the upload bandwidth of the active users, across a swarm of linked users. These bits are downloaded randomly, so the content is generally not usable until the download is complete. The BitTorrent client will reject any bits that do not match the information contained in the torrent file. This ensures that the completed download will be an uncorrupted copy of the content.

Conclusion

As a process for the transfer of content, BitTorrent is clearly superior to traditional methods. Any organization would be wise to use the BitTorrent protocol for the transfer of communications between separate offices. This would result in less cost in distribution and assurance of accuracy in the transferred content.

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