The current state of anonymous file-sharing - Marc's Blog

(e.g. 80.121.24.76).
4.2.3 Downloading/Uploading a file
To download this file, a client now has to gain access to the meta-information
embedded into the torrent file. Trackers often have a web-interface that provides
an HTTP-based download for the registered torrent files. After downloading the
torrent file, the client now announces its presence on the file’s network (which is
identified by the hash of the torrent file) and starts "scraping" (asking) the tracker
for the IP addresses of other nodes that also are on the files logical network. The
tracker then sends a list of IP+port combinations to the client containing the other
peers on the files network. The client starts to connect to the given IP addresses
and asks the peers for chunks that itself is missing and sends a list of chunks that
it has already acquired. To signal which pieces the client still misses and which
ones are available for others to download, the Bittorrent protocol uses a special
"bitfield message" with a bitmap-datastructure like this:
000000000000000000 nothing downloaded yet (it’s optional to send this)
100000000000100010 three blocks completed
111111111111111111 successfully downloaded the whole file
This way, every node on the network is able to upload/share what they have
acquired so far and download the blocks they still miss at the same time. To dis-
tribute the load put on the tracker by every client asking about other peers, some
additions like PEX 3 and the DHT 4 were integrated in the widespread clients.
3 PEX: Peer Exchange
4 DHT: Distributed Hashtable
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