Thread: BitTorrent FAQ
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Old 2007-02-28, 12:04   #1
pp
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Exclamation BitTorrent FAQ

There are literally hundreds of BitTorrent FAQs already on the Internet and I will not repeat everything that's already been said in them. I'll try to answer the most basic questions here, enough for you to download the files in this forum.
1. What is BitTorrent?

BitTorrent is a filesharing protocol, just like http, ftp, eMule, Limewire, Gnutella or DC. You may wonder why we need yet another way to share files but BitTorrent adds two important innovations:

First of all it allows you to download one piece of a file from one source and at the same time download another piece of that file from another source. In theory there's actually no limit how many sources you can download from at the same time so BitTorrent has the ability to share files quickly. However, there are many factors controlling the speed so don't always expect to fully utilize your download.

Second, every piece of a file you download is error checked. When you have finished a download you can be certain that the file is error free.

You can also stop and resume your download at any time. Nothing will disappear if you turn off your PC over night.

2. Do I need special software to use BitTorrent?

Yes. As with all filesharing you need some sort of software, it's nothing strange about that. For BitTorrent you need a BitTorrent-client and there are many to choose from. The following clients are the ones that are most widely used:

µTorrent - http://www.utorrent.com/

This is only for Windows. It's a very small client but it has all modern features you can wish for.

Azureus - http://azureus.sourceforge.net/

Azureus is Java-based and can therefore be used on any operating system - Windows, Linux and OSX. Because of Java it's a bit trickier to install and it's also a very advanced program with tons of features and you probably won't need any of them if you're new to BitTorrent. But there is a "simple mode" so don't be afraid to try it.

3. When I click on a torrent-link I only end up with a file with a .torrent extension. What is that and what do I do with it?

A vital part of the BitTorrent protocol is the tracker. The tracker is the server that knows what computers actually share the file you want and keep this information up-to-date. The .torrent-file contains the name of the tracker. The tracker itself doesn't have the file you want but when your client contacts the tracker it gets a list of computers to connect to that DOES have the file.

The .torrent-file also contains checksums of all pieces of the file you want to download. So this is the file that certifies that what you download is error free.

If the .torrent extension is correctly registered then your BitTorrent client should start automatically when you click on a torrent-link. If it doesn't you either have to correct the registration or you can always open the .torrent-file manually in your client.

4. The speed isn't that great really...

As I mentioned earlier there are several factors that controls the speed. If there is only one computer sharing the file you can't get more speed than that single computer gives you. But, and this is one of the smart things with BitTorrent, when you download a file you also share the same file, or at least those parts that you have downloaded so far. If there are several people downloading this file at the same time they can also exchange data between them, adding to the overall speed.

Because the file doesn't exist on any central server it's only possible to download it as long as anyone is sharing it. When you have finished downloading the file you still share it automatically until you close your BitTorrent client. Out of respect to all other people who want the file you should at least try to share it until you have uploaded as much data as you have downloaded but dial-up users are excused of course. You don't have to share the file non-stop. You can turn off your computer and restart your BitTorrent client the next day. If you let the file remain in its queue you can continue sharing the file whenever you feel like it.

One more thing to mention about speed. A common ADSL connection usually has a huge download/upload ratio. TCP/IP wasn't designed for this and if you use your upload fully (which is common with BitTorrent) your download will practically die. Unless you're technically gifted and know how to shape your traffic you should limit your upload to about 80% of its maximum speed. But don't limit it more because the sources you download from will notice if your upload is low and will refuse to send data to you if this is the case. After all, BitTorrent is all about sharing. Also note that if you have a broadband router and several computers behind it, you should limit the upload in the router if it's possible. Because if you only limit it on your own computer, another computer in your home may fill the remaining upload and you haven't gained anything.

5. I can't download anything...

Well, this can be for several reasons. I'll mention the most important ones here.

First of all there might be noone sharing the file at the moment. Just let your client run for a while and see if anyone joins the torrent.

If you have a software firewall (and for your sake I hope you do) make sure that your BitTorrent client is allowed to communicate freely through it both in and out. It won't work otherwise.

Also, if you have a broadband router you have to configure it to forward the data port your BitTorrent client use. You have to consult your router manual on how to do this. The port number to use is normally selected during the installation of your BitTorrent client but if you don't remember it you can always check it in the preferences. You can get help with port forwarding here: http://www.portforward.com/english/r...outerindex.htm

Note: It is vital that you do the port forward and the firewall unblocking because if you don't you can't download at all from other people who have also skipped this. If everyone neglects it, it means that nothing can be downloaded by anyone and if only a few people are doing it correctly, they will be overloaded and the speed will drop for everyone who wants to download the file.

Some ISPs actively try to block all P2P traffic, be it BitTorrent or any other type. To circumvent this, first of all choose a port number for BitTorrent that isn't one of the standard ports 6881-6889. You should choose a random port between 49152 and 65535. Ports outside this interval may be blocked also by some peers! If your client supports it, make sure to activate encryption. Some peers only allow encrypted communication and you won't be able to download from them otherwise. Both µTorrent and Azureus support encryption and I believe µTorrent even has it activated by default. Unfortunately encrypting your data isn't foolproof but it will still work with many ISPs. However, US Internet provider Comcast e.g., is using technology now that can identify this traffic too.

6. My router hangs/reboots/gets unresponsive

Broadband routers today are crap to put it mildly. Hardware is probably ok but the software (firmware) controlling them is not. It costs huge amount of money to have programmers create and test this software and to be able to sell these products cheaply, the manufacturer have to save money somewhere. For normal day-to-day Internet use they'll work just fine but p2p-traffic puts strain on them and many just can't handle that.

There are several things that can be the problem:

UPnP (Universal Plug and Play) is a feature you find in many modern broadband routers. Among other things UPnP allows a program on your computer to automatically open the ports it needs and forward them correctly. This is bad for at least two reasons. First of all the UPnP implementation in many routers is buggy and leaves you with stalled transfers. You seem to be connected and there seems to be people sharing what you want to download but no data is transferred. Secondly, UPnP doesn't care if it's a legitimate program on your computer who wants access or if it's a virus. The security implications are enormous, leaving your home network wide open to anything the virus wants to do with it. TURN UPNP OFF IN YOUR ROUTER! You really should learn how to forward the ports you need, to keep your computer safe.

The vast amount of connections that p2p software potentially can open up can exhaust the memory of your router. Common symptoms are reboots and lock-ups. Disabling DHT/DDB in your BitTorrent client might help. Limiting maximum number of global connections probably won't hurt either.

Also, always check if there is an updated firmware for your router and update if possible.

Azureus' wiki has a list of troublesome routers and possible ways of circumventing their limitations:
http://www.azureuswiki.com/index.php/Bad_routers
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