Rare piece seems common to downloader

The availability data collected by BT from it's peer is good in most 
cases. I'd say there's an important exception though.

When a peer is can download a rare piece and chooses whether to or not, 
it's perception of availability is skewed regarding that piece, making 
it choosing the rare piece less likely (it _is_ available after all), 
which obviously is bad.

Has this been brought up before?

One would get a better view of availability by from a random peer (such 
as one of your neighbours).

See also: http://wiki.theory.org/Availability

Choosing a neighbour and keeping track of his view of availability. He 
forwards his HAVEs, meaning 2x HAVE-overhead (at most) in network. 
(Forwarding HAVE would be a special command.) Keep track of all 
neighbours? Gives a better view, but more overhead. One neighbour is 
probably better.

Then again, thinking some more I realize that this approach is quite 
exploitable. An optimization highlighting this is to simply let the 
neighbour decide what to download next, since this is how the 
availability data is used anyway.

But how much of a problem is lying here? I think this actually would 
work in the majority of cases, since peers' interest often coincide. 
(I.e. it's usually in the interest of the neighbour as well that you 
download the rare piece, so there's nothing gained in lying.) Yet, I 
wouldn't call it a satisfactory solution. Especially in cases where 
interests don't coincide, such as when the neighbour is only downloading 
some files of a multi-file torrent.

Has this issue been probed more deeply? Do we have to simply accept this 
inefficiency (rare piece seeming common to downloader) as something that 
can't be helped?

(A sidenote is that when optimizing HAVE messages, such as delaying, 
batching (e.g. into bitfields) it's worth considering how they are used, 
i.e. piece selection, meaning that rare piece-HAVEs are more important 
and should be prioritized.)

/Vaste


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