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How does Bittorrent work?

While downloading torrent, I notice that BitTorrent simultaneously uploads and downloads. Download is obvious, but why upload?

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possible duplicate superuser.com/questions/245390/how-does-bittorrent-work – Shakehar Apr 11 '11 at 8:22
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closed as exact duplicate by ChrisF, techie007, studiohack♦ Apr 11 '11 at 18:36

This question covers exactly the same ground as earlier questions on this topic; its answers may be merged with another identical question. See the FAQ for guidance on how to improve it.

4 Answers

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Why upload? Your "upload" is somebody else's "download". If nobody uploads, then nobody can download.

Why simultaneously? Because, if it waited until you had the whole file before making it available to others, then anyone who turns off their client when they're not actively downloading would never upload, and the network would suffer for it.

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up vote 41 down vote

well, why not? torrents have no central server - the data is not produced by magic bunny rabbits or the toothfairy - bitorrent is a peer to peer networking system - basically your computer goes "say old chap, would you happen to have chunk 1234 ?" and another computer goes "sure, old chap, i have that" and sends it. The only way this works is that people share chunks they have with those who don't.

The only reason Bitorrent works at all* is that uploading

*not entirely true. webseeds are an exception.

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+1 Anyone who uses "Say Old Chap, would you happen to have" in a technical description deserves an upvote. – Joe Taylor Apr 11 '11 at 10:09
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Sorry, but this is Incorrect.... My torrent bandwidth comes from magic bunny rabbits AND the toothfairy. – William Hilsum Apr 11 '11 at 10:21
@wil first linux, then torrents. those pesky mythical creatures have a lot to answer for – Journeyman Geek Apr 11 '11 at 10:26
I don't have magic bunny rabbits or a tooth fairy :( I have to rely on the leprechauns, and they are somewhat unreliable. – Majenko Apr 11 '11 at 10:57
@Matt at least its not gnomes. they're creepy. – Journeyman Geek Apr 11 '11 at 11:11
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up vote 5 down vote

Bittorent uses a decentralized Peer to Peer protocol, it works because you (a peer) upload the already downloaded part of the file to other peers which also are uploading their already downloaded parts to the peers who need it to complete the file.

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up vote 5 down vote

Because BitTorrent is not about downloading files. It is about sharing files, which implies both downloading and uploading.

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