BitTorrent Guide

This BitTorrent Guide has been put together to help you get familiar with the BitTorrent file sharing network. The BitTorrent network is currently one of the most popular file sharing networks (judging from the general internet traffic usage reports) and was designed for exchanging large files.

People usually refer to a BitTorrent download as a torrent. Torrents are small files (ending with the extension .torrent) which contain the info your BitTorrent program needs to start downloading a file. Downloading and opening the torrent file in your BitTorrent program is the only way to start a download. Note: some programs also support magnet (URL) links. A torrent will always contain one item but not necessarily one file. So a torrent can contain an entire album and not just one song.

BitTorrent doesn't work with central servers or (Super)Nodes, but with trackers. Trackers are servers that allow peers to find each other so they can exchange data. The network of peers connected to a tracker is called a swarm. A tracker will also send you basic information about peers in the swarm. Every file has its own swarm so a tracker may be tracking or coordinating hundreds of torrents at the same time. A torrent may also have multiple trackers. Swarms are not connected to each other in any way, not even swarms on the same tracker. A tracker will also never have a copy of any of the torrents it's tracking.

The tracker architecture of the BitTorrent network means you cannot search "the network" for a file because there simply isn't "a network" to begin with. Some trackers allow you to search through a list of their torrents, but the best way to find a file is to use a torrent index site. These sites will list the torrents from their own tracker (if they have one) but most importantly from many other trackers. Torrent Scan allows you to search the largest torrent index sites on the net.

If there are no peers connected to the tracker for a particular torrent, the tracker will stop tracking it. As a result peers cannot find each other anymore and the network will simply disappear. Torrents in general don't last very long. Naturally, popular ones will last longer than less popular ones.

The most important question is of course how fast BitTorrent is. Obviously the key element is the internet connection that both you and the user uploading to you have. However, BitTorrent was designed to take a few other elements into account. Here's how it works:

New peers will always receive a random piece of data from other peers. Once you have a piece, BitTorrent will only upload to peers that are interested in your pieces. This ultimately leads to peers downloading only from peers they are uploading to. The more you upload, the more you will receive. BitTorrent does this by checking your share ratio. A share ratio or rating is the amount uploaded divided by the amount you downloaded. A 1:1 ratio or 1.0 rating means you uploaded as much as you downloaded.

BitTorrent will not just offer any piece to another peer; it will always check what the rarest piece in the swarm is and start with that one. When a peer uploading to you hasn't sent any data for a certain period of time BitTorrent will mark the connection as snubbed. It will do this before doing an optimistic unchoke or to prevent a situation where it is choked by all peers. A choke is when you stop sending data to a peer because that peer has stopped sending data to you. BitTorrent will then do an optimistic unchoke which is to send data to random peers that are interested in your pieces, in order to find an exchange partner. Those peers will be marked as interested.

As long as you haven't got all the pieces of the torrent you are called a leech. Once you've got a complete copy you are a seed. BitTorrent will behave a little differently when you are a seed because you aren't interested in any pieces. Instead of looking for a peer with the best share ratio, it will look for a peer with the highest upload speed. This helps spread the pieces of data more quickly across the swarm.

People may request a Reseed if peers are stuck with an incomplete download. This may happen if there are no more seeds and all leeches are missing the same pieces of the file.

Getting the best speed:

• Listen Port. Every BitTorrent program requires you to specify one open port. Without this one open port, your transfers will be very slow. Pick any port above 10000, completely open it in your firewall and if present your router. You can use the Port Scanner at ShowIP.be to check if the port is open.
• Distributed Hash Table (DHT). The DHT layer is a method of finding more sources, sources that the tracker can't find. At the moment there are only a few programs that support this protocol extension. The official BitTorrent, BitComet and µTorrent all share a DHT network. Azureus has a DHT network all to itself. The DHT network is usually on by default in all programs. Note: private BitTorrent sites do not allow DHT networks to be used.
• Protocol Encryption. Paying your ISP for a high speed connection? Well if you start using BitTorrent you may not get what you're paying for. ISP's don't like the extra traffic caused by BitTorrent so they slow it down or worse. This is called throttling. The solution is to use protocol encryption. Most BitTorrent programs don't support this; some that do are Azureus, BitComet and µTorrent. A list of ISPs that employ throttling can be found here (note: the list is not fully accurate).

Advantages:

• BitTorrent was designed to handle large files and is most efficient at it.
• Extremely high download speeds are not uncommon.
• BitTorrent stimulates sharing (without sacrificing speed)
• Very popular protocol with many users all over the globe.
• Files can be grouped in one download along with a directory structure (useful for example when downloading DVDrips).

Disadvantages:

• Files do not usually remain available as long as on other peer-to-peer networks.
• BitTorrent's main purpose is to exchange larger files (not one MP3 at a time, for example).
• There is no search function.
• It does not take into account upload-data-caps and bandwidth-per-dollar internet connections that are the standard in some countries (for example Belgium or India)

Hopefully this guide was of some use to you. You may always use our forum if you have remaining questions. Now get yourself a BitTorrent program, find a torrent and start exchanging data!

Related links

• BitTorrent Downloads - Get a BitTorrent program here
• Torrent Scan - Torrent searcher
• BitTorrent Community Forum