Hi,
Ranus wrote:
My thoughts on the two issues Bryan brings:
On fairness
Both are saying something true. The point is whether the experiments
carried out capture any situation that seeders are scarce.
yes. You can see in the paper that there are torrents with 0 or only 1 seed.
But generally I don't think free-riders can be eliminated even with a
byte-for-byte fairness heuristic. For example, free-riders contributes
nothing, but if their downloading bandwidths are also small (compare
with other leechers), the fair share offered by seeders can well
saturate this capacity. Also joining peers without any information of
the free riders and optimistic unchoking inevitably feed the free
riders. [1] tries to replace optimistic unchoking by estimate the
bandwidth between peers, how well does this work? I don't know if any
better approaches can solve this problem.
Ashwin Bharambe et al. [1] did a very nice study on BitTorrent. We agree
on several points, even if we used
a different methodology (simulations versus experimentations). However,
the results on fairness (that justify the proposed policies)
was performed with the old choke algorithm in seed state. With the new
choke algorithm in seed state, I do not believe the problems to hold.
The good thing is, bitTorrent introduce seeders to provide extra
resources. And if we want to prove/disprove the current algorithm is
not efficient esp. when uploading capacity is rare, two things should
be demonstrated:
1. whether peers can pick up optimal remote downloaders in the
neighborhood quickly.
2. If they can do 1, how much performance loss they undertake by the
current optimistic unchoking algorithm; if they cannot, how far from
optimal do we expect the downloader selection.
You are right, but this is an extremely complex problem to model.
On Network Coding
I am not really familiar with network coding, the only example in P2P
I read is "Network Coding for Large Scale Content Distribution" [2].
But network coding often requires a global view or good heuristics on
what to generate. In [2], to generate highly needed (innovative in
this article) needs communication among the neighbors. Anybody knows
some real-world, large-scale P2P system that provides realistic data?
Network coding does not require a global view of the network, you simply
always generate new blocks of data as a linear combination of all the
blocks you have.
You choose the coefficient randomly and send them with the block. I
simplify a lot, of course, but this is the idea.
Thanks a lot for the comments,
Arnaud.
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