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BitTorrent - Why is My Download So Slow?
February 23, 2004
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Bittorrent is often referred to as a fast protocol. While that is true under some circumstances, it would be more accurate to say that it has the *potential* to be a fast protocol. Several things can limit the speed, including the size of the swarm, your connection to the Internet, and even how fast you upload!

The first thing to look for is how large the swarm is. A larger swarm means more bandwidth, which can mean faster downloads. One way to check is to see what the tracker details are for that file. However, some websites often have very inaccurate details. A better way, some have found, is to use a program called Torrentspy. It checks the tracker in real-time, and gives you up-to-date information. If the swarm is small, then you probably will not get a very fast download speed. If the swarm is large, then check your upload capacity.

It might seem strange to hear that your upload speed can directly affect your download speed, but it can, and quite dramatically. The reason behind it is based on the way the Internet works. When you download a file (or in this case, chunks of a file), your computer sends a tiny message back to the source computer, called an ACKnowledgement packet. These ACK packets tell the source computer that chunk #1 has arrived OK, please send #2. When #2 is down, it sends another, and so on. The source computer will not send #3 until it gets the ACK from #2. It uses a small amount of your upload bandwidth to do this. This works fine when downloading files off the Internet, but with P2P, you tend to upload as well.

This is especially true with BitTorrent – because you upload as well. If done efficiently, downloads can be very fast. The problem lies with the fact that you upload *so much* that the ACK's do not get sent out fast enough, which slows your download. The simplest way to solve this is to limit your upload bandwidth. 80% of the maximum is usually recommended, but I find I can set it to 90% with no adverse effects. This gives your ACKs a bit of breathing space to get out with.

If that does not work, one other thing you can try is checking your connection to the Net. Firewalls and routers can both cause problems with download speed, as they can block the ingress and egress of chunks and ACK packets. Allowing your client through your firewall on ports 6881-6999, and setting up Port Forwarding in your router (if you have one) are essential. Some of the newer clients will let you operate several downloads on a single port, including Azureus.

Happy downloading.

This story is filed in these Slyck News categories
BitTorrent :: Independent/Other Development
File-Sharing/P2P Related :: Software

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