The Modern Way

The BitTorrent protocol was designed in such a way that it eliminates almost all the negative points of the method previously described.

Unlike the previous method, BitTorrent uses a central server just to keep track of the users downloading the files, therefore it can scale to a very large number of users downloading the files (even thousands) and that actual files can even be stored on other server.

The BitTorrent central server (also called "tracker" ) simply sends a small file called "torrent file" (after the file extension, ".torrent") to each user, a file that contains just the list of files the torrent contains, the list of users that are already downloading the files and a small packet of data containing informations about the files (size, error correction checksums). Users report back to the tracker once in while, telling what parts of the files they managed to download already and to obtain information about the users that have started downloading the files since the last time a connection was established with the tracker.

As you see in the picture below, the central server sends information about the files to the users (in the picture below just two users receive information from the tracker). Users learn about each other this way and, as soon as someone completes a small part of a file (called "chunk"), it offers the chunk to the other users.

The central server's only purpuse is to keep the users informed about each other

As time passes, more and more users have at least one chunk of the torrent completed and will start exchanging the chunks between them, using the error correction information in the torrent file downloaded from the central server to check the chunks for errors during transfer.

If you check the picture you will see that the central server only sends data to 2 computers in the group of users ("swarm"). Only once, there is real need to connect to the tracker, to obtain the torrent file, which is great because, if there are network problems or the tracker is no longer available, you can obtain all the files from the people in your swarm. You don't have to connect to the tracker again, although it is best to do it (and most BitTorrent programs do it for you) because it refreshes the list of users that are downloading the files and you have more users to connect to and increase your download speed.

In conclusion, using the BitTorrent protocol, people help each other download the files in the torrent by exchanging chunks of data between them as soon as those chunks are completed. The central server just keeps track of the swarm and helps users complete the torrent in a shorter amount of time by telling users what chunks are worst spread in the swarm and on what parts they should focus on.

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