In this project, I will extend Project 1 to support a BitTorrent-style file sharing application. BitTorrent is a communication protocol for P2P(peer to peer) sharing, which allows users to distribute data over the Internet in a decentralized way.
The download process of BitTorrent can be roughly divided into 4 steps: 1. Parse .torrent file. 2. Request Tracker to get a list of Peers. 3. Request Peers, download Piece, and check the validity of Piece according to the pieces field. 4. Assemble Piece, get complete file.
1.Parse .torrent file: Torrent file is a bencoded dictionary, containing a bunch of information about files and folders to be distributed. In this project, I am going to assume all the torrent files are in Single File Mode, and don't consider the cases of Multiple File Mode.
2.Request Tracker for Peers: Connect to tracker by HTTP server to ask the peers' information
3.Request Peers, download Piece: I will simplify this step. Only implement handshake, bitfield, request and piece. I am going to ignore have, choke, unchoke,ect.
4.Assemble Piece, get complete file: Once each piece is correct, we can assemble the pieces to a complete file.
In addition, in the BitTorrent protocol, each host acts as both server and client, so except for the download process, a host should be able to handle the communication with other peers. For example, Handshake, Request,ect.
Another critical piece of BitTorrent is concurrent download/upload. The idea is that, while you're downloading a file, you're also helping to upload it to others who need it. And everyone will be doing the same, so it's one of the reasons why it can be so fast. I will implement this feature in my project as well.