Summary

BitTorrent

• file divided into 256Kb chunks
• peers in torrent send/receive file chunks

Distribution

• Peer joining torrent:
  • has no chunks, but will accumulate them over time from other peers
  • registers with tracker to get list of peers, connects to subset of peers (neighbors)
• While downloading, peer uploads chunks to other peers.
• Churn: peers may come and go

Requesting File

• At any time, a peer may request a chunk from a neighbor.
• Periodically, peer asks each peer for list of chunks that they have.
• Peer requests missing chunks from peers, rarest first

Sending File: tit-for-tat

• Peer sends chunks to those 4 peers currently sending the peer chunks at highest rate (choke others) reevaluate every 10 seconds
• every 30 secs: randomly select another peer, starts sending chunks (optimistically unchoke)