The BitTorrent protocol A peer-to-peer file sharing protocol.

Problem Addressed How do we transfer a piece of data quickly to a group of people interested in it. –Web server –Distributors –Redhat patches Soln: BitTorrent based file swarming

Philosophy Author: Bram Cohen Based on Tit-for-tat Incentive - Uploading while downloading Pieces of files

Overall Architecture Web page with link to.torrent A B C Peer [Leech] Downloader “US” Peer [Seed] Peer [Leech] Tracker Web Server.torrent

Overall Architecture Web page with link to.torrent A B C Peer [Leech] Downloader “US” Peer [Seed] Peer [Leech] Tracker Get-announce Web Server

Overall Architecture Web page with link to.torrent A B C Peer [Leech] Downloader “US” Peer [Seed] Peer [Leech] Tracker Response-peer list Web Server

Overall Architecture Web page with link to.torrent A B C Peer [Leech] Downloader “US” Peer [Seed] Peer [Leech] Tracker Shake-hand Web Server Shake-hand

Overall Architecture Web page with link to.torrent A B C Peer [Leech] Downloader “US” Peer [Seed] Peer [Leech] Tracker pieces Web Server

Overall Architecture Web page with link to.torrent A B C Peer [Leech] Downloader “US” Peer [Seed] Peer [Leech] Tracker pieces Web Server

Overall Architecture Web page with link to.torrent A B C Peer [Leech] Downloader “US” Peer [Seed] Peer [Leech] Tracker Get-announce Response-peer list pieces Web Server

Messages Peer – Peer messages –TCP Sockets Peer – Tracker messages –HTTP Request/Response B-encoding

.torrent url of the tracker Pieces Piece length Name Length Files –Path –length

Tracker Peer cache –IP, port, peer id State information –Completed –Downloading Returns random list

Peer Operation File space allocated Connect to peers Bitfield –have(1) and not have(0) Have –Advertise pieces Interest /Not_interested –Expressing interest in the pieces published by the peer Requesting for a chunk –Request

Peer Operation Choking algorithm –Choke/Unchoke –Preferred peers –Optimistic unchoke –Snubbing behavior Prevented by Anti-snubbing. Upload to interested peers who are not choking.

Peer Operation Verify on receiving complete piece Endgame Behavior –Cancel

Experimental Client

Strengths Better bandwidth utilization –Never before speeds. Up to 7 MB/s from the Internet. Limit free riding – tit-for-tat Limit leech attack – coupling upload & download Spurious files not propagated Ability to resume a download

Drawbacks Small files – latency, overhead Random list of peers - naive Scalability –Millions of peers – Tracker behavior (uses 1/1000 of bandwidth) –Single point of failure Robustness –System progress dependent on altruistic nature of seeds (and peers) –Malicious attacks and leeches.

MNL’s Interest Robustness –Devise possible attacks and find loopholes –Secure against attacks Scalability –Sharing across files –Millions of users Possible extensions –Peer optimization Upload just enough to maximize your download. –Intelligent tracker Peer selection policy –Dead sites Share html content

Optimism - hack Connect to as many peers as possible Benefit –Optimistic unchoking –Probability of free download increases

Peer upload optimization Upload enough to stay within the preferred list of as many peers as possible Get the best download deal with minimum required upload download upload

Smarter tracker Classify peers Pick at random from each of these subsets No compromise on random choice Will lead to a more progressive peer network. Peers FastSlowCompleted

Interesting links Official site: BitTorrent FAQ: Torrent sites – – – Remember –leave your download windows open –Big brother is watching!