[BitTorrent] receive piece twice

John Prevost j.prevost at gmail.com
Wed Jul 21 17:30:26 EDT 2004


On Wed, 21 Jul 2004 07:05:13 -0000, sandboy8 <sandboy8 at yahoo.com> wrote:
> at some case, when i send request for file piece, it seems the peer
> send the same piece for twice. i don't know how it happen, if the
> protocol is that? i think it will cost some ext bandwidth.

When your client is nearing completion (which is defined rather
nebulously), it may enter "end-game mode", in which it requests all
the remaining pieces from all peers that are talking to it at once. 
This is meant to speed up the process of finishing the last few pieces
of a file--and not everyone is convinced that it's a good idea. 
Regardless, once your client receives a piece, it cancels the requests
for that piece to the other peers.  But, if another peer has already
started sending, you'll get a duplicate copy.

Another case (and I'm less sure of this one) where you might get
duplicates is if you make a request to a peer and then later snub that
peer because it doesn't seem to be sending data.  Your client will
probably then request the pieces that had been requested from the
snubbed peer from different peers.  In the meanwhile, it could be that
the peer you snubbed is just glacially slow, and finally gets around
to sending your data.  I'm less sure of this because I'm not sure how
snubbing and cancelling requests are handled together in most clients.

John.


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