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BitTorrent Protocol
- 1. BitTorrent Protocol Arun Kumar(ENG18CS0047) Computer Networks Seminar
- 2. 2 Peer-to-Peer Network ● Many computerscome together and pool their resources to form a content distribution system. ● These computers don't need to be machines in data centers, they can be home computers. ● They are called peers because each can act as a client and a server. ● There is often no central point of control. ● If a P2P network has an average of N users, each with broadband connectivity at 1 Mbps, then the aggregate upload capacity is N Mbps. ● The aggregate download capacity is also the same because users has a 1-Mbps link in each direction.
- 3. 3 Peer-to-Peer Network ● Skype usesP2P. ● The two paths that P2P technology has followed are BitTorrent protocol and Distributed hash table (DHT).
- 4. 4 BitTorrent Protocol ● Developed byBram Cohen in 2001. ● Quick and easy file-sharing among a set of peers. ● There are many freely available client softwares. Eg: BitTorrent, uTorrent, Deluge ● Applications of BitTorrent ➔Distribution of games, TV shows, podcasts and music albums. ➔Sharing datasets. ➔Facebook uses BitTorrent to distribute updates to Facebook servers. ➔The British government used BitTorrent to distribute details about how the tax money of British citizens was spent.
- 5. 5 How does apeer find other peers that have the content it wants to download? ● Not all peers will have all of the content. ● Every content provider creates a torrent. ● A torrent is used by a peer to verify the integrity of the data that it downloads. ● The content is divided into equal-sized chunks (64 KB to 512 KB). ● Torrent contains 160-bit SHA1 hashes of all chunks. ● Tracker is a server that leads peers to the content. It maintains a list of peers (swarm) that are actively downloading and uploading the content. ● Peers contact tracker regularly to report that they are still active and when they leave the swarm.
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- 7. 7 How is contentreplicated by peers to provide high-speed downloads for everyone? ● Seeders: Peers that have all of the chunks. ● A peer simulataneously downloads chunks that it is missing and uploads chunks that it has. ● A peer can leave the swarm and return at any time. ● Each chunk must be available at many peers. ● If everyone were to get the chunks in the same order, many peers will depend on the seeders only. This is a bottleneck. ● Instead, peers exchange lists of chunks that they have with each other. ● They select and download rare chunks because downloading creates a copy and the rarity of the chunk reduces. ● After a short while, all chunks will be widely available.
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- 9. 9 How do peersencourage each other to upload content to others as well as download content for themselves? ● Leechers are peers that download content but do not want to upload and contribute. ● Solution: Reward peers that show good behaviour. ● Each peer randomly samples the other peers, retrieving chunks from them while it uploads chunks to them. ● The peer prefers to trade chunks with a small group that give high download performance. ● The peer will randomly try newcomer peers too. This allows newcomer peers to obtain initial chunks so that they can trade with other peers. ● This algorithm will match peers with comparable upload and download rates overtime. ● If a peer is not uploading chunks or is doing so slowly, it will be cut off / choked sooner or later.
- 10. 10 DHT ● Tracker is centralizedfor each swarm. ● Each peer keeping its own index of who has what is not worth the effort. ● DHTs impose a regular structure on the communication between nodes. So they are called structured P2P networks. ● Properties of DHT Each node keeps only a small amount of information about other nodes. Each node can look up entries in the index quickly. Each node can use the index at the same time, even as other nodes come and go. ● The first proposed DHT was Chord in 2001. ● Some BitTorrent clients use DHTs to provide a fully distributed tracker.
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