Torrents are THE most popular way of downloading large volume of data ranging from small files to terrabytes including movies, games and more. Downloading a file with torrent is very cost effective and advantageous especially if the torrent file enjoys high popularity and many people are currently downloading the torrent file. The more people download a torrent file the higher the individual download speed gets.

This can be explained with the torrent principle. The torrent download speed increases to a large extent because more people also upload at the same time, and therefore the more speed becomes available for everyone. When downloading a movie through a torrent the user shares the already downloaded part of the torrent file with other torrent users, and when a new torrent user starts downloading, the application contacts a ‘tracker server’ which coordinates the upload. From the tracker the torrent applications find out where to download from, which parts of the torrent file are to be uploaded and the torrent application also informs the tracker about what part of the torrent file a user has been able to download until now.

Problems with torrents The torrent tracker server may not be functional. In this case you can’t begin downloading, but it may not necessarily be a problem to finish the download. A common torrent problem is that there may be no one downloading and sharing but yourself. Therefore a torrent user has nowhere to download from. This happens with old and unpopular torrents. Also you may run into a situation when there is nobody who has the complete file and one simply can’t get the whole torrent file from the parts other torrent users have. There is a large amount of free torrent client applications which are able to download torrents.

The word torrent usually refers to a small file you receive from a web server. The file extension for this is *.torrent. So for example, myMusicTune.torrent. This particular file is called a metadata file. This means that the file contains information about the data you want to download, not the data itself. Each torrent receives a unique Info Hash identifier.

In the torrent world, a peer is a computer that you are downloading from (another users computer). Once you download and run a torrent in a torrent application, peers are computers that you download the file from.

A leech is usually a peer who has a negative effect on the swarm by having a very poor share ratio - in other words, downloading much more than they upload. Most leeches are users on asynchronous internet connections who through either ignorance or choice do not leave their BitTorrent client open to seed the file after their download has completed. However, some leeches intentionally hurt the swarm to avoid uploading by using modified clients or excessively limiting their upload speed. The term leech is also incorrectly used to refer to what should properly be called a peer, a member of the swarm who has not yet downloaded the complete file.

A seed is a computer that has a copy of the full file the that torrent is representing. Seeds provide the bandwidth so that others can download the represented file.

When there are zero seeds for a given torrent (and not enough peers to have a distributed copy), then eventually all the peers will get stuck with an incomplete file, since no one in the swarm has the missing pieces. When this happens, someone with a complete file (a seed) must connect to the swarm so that those missing pieces can be transferred. This is called reseeding.

The group of machines that are collectively connected for a particular file. For example, if you start a BitTorrent client and it tells you that you’re connected to 10 peers and 3 seeds, then the swarm consists of you and those 13 other people.

A server on the Internet that acts to coordinate the action of BitTorrent clients. When you open a torrent, your machine contacts the tracker and asks for a list of peers to contact. Periodically throughout the transfer, your machine will check in with the tracker, telling it how much you’ve downloaded and uploaded, how much you have left before finishing, and the state you’re in (starting, finished download, stopping.) If a tracker is down and you try to open a torrent, you will be unable to connect. If a tracker goes down during a torrent (i.e., you have already connected at some point and are already talking to peers), you will be able to continue transferring with those peers, but no new peers will be able to contact you. Often tracker errors are temporary, so the best thing to do is just wait and leave the client open to continue trying.

If you are using the experimental client with the stats-patch, you will see a share rating displayed on the GUI panel. This is simply the ratio of your amount uploaded divided by your amount downloaded. The amounts used are for the current session only, not over the history of the file. If you achieve a share ratio of 1.0, that would mean you’ve uploaded as much as you’ve downloaded. The higher the number, the more you have contributed. If you see a share ratio of "oo", this means infinity, which will happen if you open a BT client with a complete file (i.e., you seed the file.) In this case you download nothing since you have the full file, and so anything you send will cause the ratio to reach infinity. Note: The share rating is just a number that is displayed for your convenience. It does not directly affect any aspect of the client at all. In general, out of courtesy to others you should strive to keep this ratio as high as possible, of course.

distributed copies

In some versions of the torrent client, you will see the text "Connected to n seeds; also seeing n.nnn distributed copies." A seed is a machine with the complete file. However, the torrent swarm can collectively have a complete copy (or copies) of the file, and that is what this is telling you. Referring again to the "people at a table" analogy, consider the case where the book has 10 pages, and person A has pp.1-5 and B has pp.6-10. Collectively, A and B have a complete copy of the book, even though no one person has the whole thing. In other words, even if there are no torrent seeds, as long as there is at least one distributed copy of the file everyone can eventually get a complete file. Meditate on this, the Zen of BitTorrent.

choked

This is a term used in the description of the BitTorrent protocol. It refers to the state of an uploader, i.e. the thread that sends data to another peer. When a torrent connection is choked, it means that the transmitter doesn’t currently want to send anything on that link. A BT client signals that it’s choked to other torrent clients for a number of reasons, but the most common is that by default a client will only maintain -max_uploads active simultaneous uploads, the rest will be marked choked. (The default value is 4 and this is the same setting that experimental client GUI lets you adjust.) A connection can also be choked for other reasons, for example a peer downloading from a torrent seed will mark his connection as choked since the seed is not interested in receiving anything. Note that since each connection is bidirectional and symmetrical, there are two choked flags for each connection, one for each Tx endpoint.

interested

Another term used in the torrent protocol specification. This is the corollary to the choked flag, in that interested refers to the state of a downloader with respect to a connection. A downloader is marked as interested if the other end of the link has any pieces that the client wants, otherwise the connection is marked as not interested.

snubbed

If the client has not received anything after a certain period (default: 60 seconds), it marks a connection as snubbed, in that the peer on the other end has chosen not to send in a while. See the definition of choked for reasons why an uploader might mark a connection as choked. The real function of keeping track of this variable is to improve torrent download speeds. Occasionally the client will find itself in a state where even though it is connected to many torrent peers, it is choked by all of them. The client uses the snubbed flag in an attempt to prevent this situation. It notes that a torrent peer with whom it would like to trade pieces with has not sent anything in a while, and rather than leaving it up to the optimistic choking to eventuall select that peer, it instead reserves one of its upload slots for sending to that peer.

optimistic unchoking

Periodically, the torrent client shakes up the list of uploaders and tries sending on different connections that were previously choked, and choking the connections it was just using. You can observe this action every 10 or 20 seconds or so, by watching the "Advanced" panel of one of the experimental torrent clients.