Why BitTorrent Works
BitTorrent
is often referred to as the best filesharing protocol for sharing large
files. But why? What makes BitTorrent so unique?
Well,
lets take a sneak peek under the hood of your BitTorrent client, and
list a couple of features that make BitTorrent as fast, stable, and
robust as it is.
How it all Starts
BitTorrent starts with chopping a file into small pieces, the person who starts sharing the file (initial seed) sends those small pieces to available peers in the swarm.
The BitTorrent protocol makes sure that the seed sends a unique piece
to everyone, so they can immediately exchange these pieces with each
other.
Thus, the great advantage is that everyone starts sharing pieces of the file right away.
Fair trading
Once
you have a little piece of the file, your BitTorrent client tries to
find other people who might be interested in the pieces you have. The
BitTorrent protocol works as a tit-for-tat game, you only upload pieces to people who offer something to you. The more you upload to others, the more you receive.
This fair trading principle prevents free-riders from stealing your precious bandwidth.
Once you're trading pieces with someone, the BitTorrent client keeps a close eye on this peer,
as soon as the other side stops sending you data it will stop sending
as well (choke). In the meantime, your client keeps searching for other
peers It randomly sends data to peers who are interested (optimistic
unchoke) to see if that peer is a potential trading partner.
Selecting pieces
So what pieces does your client sent first? Well, BitTorrent works with a "rarest piece first"
scheme. Your BitTorrent client generally looks for the rarest piece
that's available among the peers in the swarm. This makes sure that the
most common pieces are still available at the end and may prevent
people from getting stuck at 99%.
Seeders & New Peers
Seeders don't stop sending out pieces of course, but the rules
change a bit. As a downloader the client determines the best peer by
the upload/download ratio. A seeder however is only interested in peers
with a high upload speed, so these fast uploaders can send the pieces
to other peers, and speed up the overall speed of the swarm (seeds +
peers).
New peers (without pieces) are more likely to receive a piece of the
file at random. Besides this, new peers are not bound to the rarest
piece first restriction. This makes sure that a new peer receives a
"piece" as quick as possible, so he can share it with others.
This is a brief, and simplified outline of some features that aid to the success of the BitTorrent protocol.
for more information
http://torrent-heaven.blogspot.com/
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