[bittorrent] partial download

arvid at cs.umu.se arvid at cs.umu.se
Sun Nov 15 16:21:10 EST 2009

Quoting Adrian Ulrich <torrent at blinkenlights.ch>:
> > a only downloading pieces from file a, the same for peers interested in
> file
> > b. once it got the file a or file b, it will leave.(selfish)
> No, this is not true and does not happen in 'real life':
> Let's say we got 3 peers: A, B and AB where only AB is a seeder:
> All peers might be connected to eachother:
>          --> AB <--
>          |         |
>          \/       \/
>          A <-----> B
> A does not know that B will never-ever try to download data from it nor does
> it
> know that B will never be able to send any data to A (because B does not
> download
> the chunks needed by A)
> So what will happen to the connection A<->B ? It won't get killed but
> instead
> it will just time-out because no 'interesting' traffic is happening between
> this 2 peers.

Some clients, like libtorrent (as in http://libtorrent.org), uTorrent, and I
believe Vuze as well, implement an upload-only extension where the fact that a
torrent is  finished is advertised. That way the peers can disconnect
immediately if both are upload-only.

Arvid Norberg

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