Popular BitTorrent Clients in Review - Brief Overview
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BitTorrent by its very nature is a peer-to-peer protocol designed for transferring files. It works on the methodology of multiple users uploading and downloading small segments as parts of a file or files. The client is a software application that enables the user to take advantage of the BitTorrent protocol. It communicates through it.
The central server that manages the connections is called the tracker. Basically, it logs the seeds and peers that are in the swarm. It is really important to point out that the tracker does not have copies of the files; therefore, it cannot be involved directly in the data transfer. The tracker exchanges information with the clients. That is its job.
In BT terminology, a peer stands for an instance of a BitTorrent client that is connected to the Internet and is able to accept connections from other clients. By definition, a peer does not have the complete file yet, just parts of it. Nevertheless, if the client has the complete file and offers to upload it, then we call that a seeder.
Other terms that you should be familiar with are leech, lurker, share ratio, swarm, and snubbed. The swarm is the combined group of peers and seeders that share a specific file. Therefore, all of the clients that are involved in the data transfer are building up a swarm.
The leech is a person that downloads more than uploads. This is when the share ratio of a leech is called poor or negative (it's below 1). Share ratio is calculated as follows: total uploaded/total downloaded. That's why below one means that the leech downloaded more than uploaded and thereby has a negative effect on the swarm.
A lurker is a seeder that does not add new content. By no means should a lurker be confused with a leech because the lurker seeds all of the already downloaded files. Furthermore, the term snubbed is used to describe a client that does not upload any data for more than 60 seconds, for whatever reason. This is a flag that works based on the feedback given by downloading clients.
Last, but definitely not least, we need to introduce the torrent file. It's a file that contains the metadata (name, size, checksum) of the files that the particular file makes available to download. The metadata also contains the address of the tracker that organizes the connections between peers and seeders. In general, this file sports a .torrent extension.
Now that we've covered all of the required BT terminology, let's talk a bit about how the system works and why it's so effective. Sharing content directly via websites is a very challenging approach, even though umpteen mbps bandwidth services are dominating. Keeping up with the high number of downloads is a headache for network and system administrators. The sharing option is definitely not a cheap one.
The underlying reason that supports the development of the BitTorrent protocol is that it popularizes the technique so that users are both uploaders and downloaders. The file is split into numerous segments and the redistribution approach is usually the "rarest first" or random. The more users are seeding the content, the higher the transfer speed gets and healthier the swarm becomes. The server acts just like a manager (tracker).
It really makes sense that organizing exchanges between users and hosting metadata files, which are very small in size, is very efficient and less resource-dependent than hosting the file as a whole and handling all of the work. As a result, file-sharing gets decentralized and the bandwidth of the central server is freed up.
Moreover, the second outstanding advantage of the BT protocol is that it offers much greater redundancy because it eliminates the not-so-negligible unfortunate possibility that the server will go offline for whatever reason. The transfer speed can also reach extreme peaks because of the dynamic bandwidth allocation on all of the clients.
Recently the BitTorrent protocol was enhanced with DHTs, which are Distributed Hash Tables. Thanks to its distributed systems nature, it allows users to connect and continue transferring data even with the absence of a dedicated tracker. It was a huge step toward decentralization, scalability, and fault tolerance. Lately, DHT has gained prestige.
Companies such as Blizzard (World of Warcraft) and Valve (Steam) adopted this protocol as their main distribution method in order to deliver updates, patches, and so forth. It isn't uncommon at all to find various Linux distributions downloadable via BT protocols.
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