[BitTorrent] Free rider problem

Elliott Mitchell ehem at m5p.com
Mon Dec 27 03:37:28 EST 2004


>From: guanying_wang <guanying_wang at yahoo.com.cn>
> I have figured out a solution, but I have never implemented it. 
> First, I assume the peers bind to unique IP addresses.   So I can just
> remember every IP and their total upload to me, and base on these
> info, decide which peers should I choke.  Another restriction is only
> one connection per ip, because I have to remember IP-total pairs, two
> connections from the same ip will be confusing.

Not quite right. Multiple peers /could/ be NATed behind a single IP
address. If these are bandwidth limited individually behind the NAT, you
could get better bandwidth talking to both. At the same time if you throw
out the port, then you could end up unchoking the wrong one.

At the same time you should be controlling optimistic unchokes to on a
per-address basis so that you avoid the above. This then gets interesting
that an attacker with multiple IP addresses (either all on one machine or
a cluster of systems) could do a group attack against you. Also with a
sufficiently large swarm you could avoid reconnecting to the same peer
twice.


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