Intro:
BitTorrent is a p2p system that distributes downloads among peer downloaders. When you are downloading, you are simultaneously uploading the same file to other users.
The actual process of file transferring is decentralized. The Fundamental idea of BitTorrent is that users upload at the same time they are downloading. By doing so network bandwidth is used in an efficient manner.
BitTorrent is designed to work better as the demand to download a specific file increases. Which is in direct contrast to WinMX and other p2p programs queue concept. The more that try to download a bittorrent at the same time the better.
Unlike most other p2p models there is no search function for BitTorrent. To Download a file using bittorrent, you must find .torrent files from a variety of sources. These sources include websites, irc and even emailing torrent files from one person to another.
There are central servers (tracker) which coordinates the action of all peers engaged in downloading. The tracker does not have any knowledge of content of files bittorrent users download, it only manages connections. Allowing a large quantity of downloaders with limited tracker bandwidth usage.
BitTorrent is best understood by analogy. Visualize a group of people (BitTorrent users) sitting around a table (Tracker). Each person at the table can both talk and listen to any other person at the table. These people are each trying to get a complete copy of the book.
Person 1 has pages 1-9, 35-46 and 88-100.
Person 2 has 10,28-35,44-60 and 72
Person 3 has 5-20 32-39, 80-92
Person 4 has 9-27 and 58-73.
Between them, they have every page except 74-87. So each user could continue to complete (Download) the book from each other, each one getting a page from the other, in exchange for a page they do not have. Until a time that another person shows up who has the missing 74-87, or even a person who has the entire book. In which the other users would each gather pages that were missing from the whole to distribute among themselves until every copy was complete.
A user who has the entire "book" would be called a seed. A person with the entire file (seed) is required for the process to begin, but it is possible to have a distributed copy without having any single user possessing a complete copy as other users continue to share their partial long after there are no seeds in a swarm.
It is always good to have a complete copy, but it is not vital.
Terms:
torrent
A small metadata file which contains information about the data you want to download, not the data itself. It is downloaded from a web site (BitTorrent file extension is .torrent) by clicking on a download link. It can also be saved to your computer. This is useful if you want to be able to re-open the torrent later on without having to find the link again.
peer
A peer is another computer on the internet that is sharing the file you wish to download. Typically a peer does not have the complete file, if it did it would be called a seed. Peers are also called leeches, to distinguish them from those who have completed their download and continue to leave their BitTorrent Client running and act as a seed.
seed
A computer that has a complete copy of the specific torrent you are downloading . Once your client finishes downloading, it will remain open until you click the Finish button. This is known as seeding. You can also start a BitTorrent Client with a complete file, and once BitTorrent has checked the file it will connect and seed the file to others. It is good to continue seeding a file after you have finished downloading, to help others finish. Also, when a new torrent is posted to a tracker, someone must seed it in order for it to be available to others. The tracker doesn't know anything of the actual contents of a file, so it's important to follow through and seed a file if you upload the torrent to a tracker.
reseed
When there are zero seeds for a given torrent (and not enough peers to have a distributed copy), 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. Usually a request for a reseed comes with an implicit promise that the requester will leave his or her client open for some time period after finishing (to add longevity to the torrent).
distributed copies
In some versions of the client, you will see "Connected to x seeds; also seeing x.xxx distributed copies." A seed is a computer with the complete file. However, the swarm can collectively have a complete copy (or copies) of the file, even without seeds a complete distributed copy can be available if the sum total of the users have a complete copy among themselves. BitTorrent doesn't distribute in sequential order, so this is possible.
swarm
The group of users that are collectively connected for a particular file. Example, if you start a BitTorrent Client and it tells you that you're connected to 5 peers and 1 seeds, then the swarm consists of you and those 6 other people.
tracker
Server on the Internet that coordinates the action of BitTorrent Clients. Upon opening a torrent, you contact the tracker and receive a list of peers to connect to. Throughout the transfer, your computer will query the tracker, telling it how much you've downloaded and uploaded and how much before finishing. 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 (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. Tracker errors are often temporary, leave the client open and continue trying.
share rating
A 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 this means infinity, which will happen if you open a BitTorrent 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. While share ratings are just displayed for your convenience, courtesy to others should cause you to keep this ratio as high as possible.
choked
When a connection is choked, it means that the transmitter isn't currently sending anything else on the link. A BitTorrent Client signals that it's choked to other clients for a number of reasons, the most common is that by default a client will only maintain --max_uploads active simultaneous uploads, the rest will be marked choked. A connection can also be choked for other reasons, for example a peer downloading from a seed will mark his connection as choked since the seed has no need to receive.
optimistic unchoking
Periodically, the 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 clients.
interested
Term used in the protocol specification. 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. 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 download speeds. Occasionally the client will find itself in a state where even though it is connected to many 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 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 eventually select that peer, it instead reserves one of its upload slots for sending to that peer.
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