豆知识 2011-05-15&05-21 比特风暴

 Our computer files aren’t like they used to be. These days a short HD video can be over a gigabyte in size. That's huge. The problem is that the bigger they are, the harder they are to share. Thankfully, we have options that make sharing big files over the Internet fast and easy. This is BitTorrent explained by Common Craft 2. 

Blake was so excited. He’s the present of his local Sasquatch fan club, and recently heard that a new video was circulating 3. He couldn't rest until he had that video on his computer. He tried to download it from the original website, but it was painfully slow. Because so many fans were trying to download it at the same time, the website crashed. He was discouraged. 
Then a fellow fan said the video could also be downloaded via BitTorrent. She said that unlike typical downloads, file using BitTorrent actually download faster when more people are involved, and it works 4 with any kind of file. 
To see for himself, Blake downloaded BitTorrent for free from BitTorrent.com. He opened the program and was a little confused. There were no videos and he wasn't sure where to start. But soon he realized that’s because BitTorrent is a little like a web browser 5. Just like a browser needs websites to be useful, BitTorrent needs special files called torrents 6 to work. A quick web search pointed 7 him to websites with torrents galore.  Soon enough, he found the Sasquatch video torrent 1, downloaded the torrent file to his computer, and with a simple click, BitTorrent immediately sprang to action and the file started downloading. 
He could see it was working and wondered how. Later his friend made it clear by talking about how downloads typically work without BitTorrent. When someone downloads a file, it comes to their computer in a stream from a single source. When multiple 8 people want the file, that single source can get overworked and can even shout down. BitTorrent solves this problem by making each downloader a source. This way, they get pieces of the file, but also provide pieces to each other. Together the downloaders become a network of multiple sources all working to provide pieces to one another. This makes downloads fast and realizable. 
A couple of things behind the scenes make it happen. First, when he clicked “Download”, Blake received the torrent file. This file has all the information about the video file, like what pieces are needed to complete it, but something is missing 9. Blake needs a way to connect to other users 10 who have the pieces he needs for his file. BitTorrent uses a computer called a tracker that helps Blake's computer find other computers called peers 11. The tracker keeps track of computers that are downloading or already have the whole file and introduces Blake's computer to them. With the connections in place, he receives what he needs, and also distributes pieces to others who are also downloading. 
Blake got it. BitTorren works because it turns downloaders into sources. And more downloaders means more sources and faster downloads.  
In the end, Brike's video downloaded in no time at all. But he was a little disappointed. What he thought was a Sasquatch was only a wookie. But at least he now knows that BitTorrent is a faster, easier way to download files from the Internet.