[bittorrent] what is bitTorrent piece download mechanism?

योगी / Devendra devendra.vyavahare at gmail.com
Fri Mar 7 06:20:49 EST 2008


We are trying to build a toy simulator for bitTorrent for analyzing its
behavior as a part of our course work.
For reading about "how it works?" I referred some of the research papers.
But I am not much clear with piece download mechanism used by the peer.

What understood is
1. Peer contacts tracker to get its peer list.
2. Peer selects "which piece to download?" based on rarest first policy.
(except for first 4 packets.)
now it has to make download requests for blocks within this chosen piece.

I have following questions:
1. Whether this entire piece is requested for download only from one remote
peer ;
or does it send multiple requests to different  remote peers (in case if
that piece is available with more than one)?
--> (I think it should request multiple remote peers to have better
performance but not sure about what actually happens?)
2. Is there any standard algorithm to decide "which block should be
requested from which remote peer?"
3. Suppose  peer requests remote peer for a particular block; and then
remote peer chokes that request.
Then does this peer request this block to some other peer? Is there any
time-out limit for that?
4. In general (excluding the case of end-game) does peer requests for one
block to multiple remote peers ?
5. Can we initiate requests for some other piece before the download of the
current piece is finished?
 (i.e. during the time when all requests for currently chosen piece are
made; but those requests are still being processed.)

Thanks and Regards
~ Devendra Vyavahare
M.tech Student
Dept. of CSE,
Indian institute of technology, bombay, India.
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