BitTorrent download strategy: The end game
Downloads in BitTorrent take place according to a number of strategies, which map to stages. Initiating a torrent download has one strategy, normal operation has another strategy, and finally pulling down the last remaining pieces has yet another strategy.
The End Game is the name for the final download strategy - there is a tendency for the last few pieces of a torrent to download quite slowly. To avoid this, many BitTorrent implementations issue requests for the same remaining blocks to all its peers. When a block comes in from one peer, you send CANCEL messages to all the other peers requested from, in order to save bandwidth. Its cheaper to send a CANCEL message than to receive the full block and just discard it.
However, there is no formal definition of when to enter End Game Mode. I found two popular definitions:
1. All blocks have been requested
2. Number of blocks in transit is greater than number of blocks left, and no more than 20
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