[p2p-hackers] Incentive to trade with newcomers, catch 22

[Hal Finney]

Vaste writes:
> I want to discuss a problem with torrent and similar tradebased swarming 
> networks. It is how a new peer is suppose to begin participating in the 
> trade. No rational greedy peer would be interested in trading with it 
> since it (naturally) has no pieces to offer in exchange. Yet, the swarm 
> should (likely) benefit from it joining, providing additional bandwidth. 
> Catch 22.

The way BitTorrent works is that a different rule is used for peers that
have the whole file (which are called seeds, or seeders) than for peers
that do not yet have the whole file.

Ordinary peers mostly will send data to other peers who are sending
to them.  This is the "tit for tat" rule and it works well.  There is
usually one upload "slot" reserved for random other peers, which rotates
through all of them.  The main purpose of this slot is to discover if
another peer might reciprocate and provide even more and faster data
than some of the peers we are communicating with.  This way we don't
get locked into exchanging data with some "OK" peers when there are some
"very good" peers out there we could do even better with.

This rotating slot policy does have the side effect that new peers get
a bit of data from other nodes, even when they have little to offer.

The rule for seeds is different.  They upload data to peers based on how
quickly the peers are accepting the data.  (Compared to regular peers,
which decide based on how fast their partner peers are providing data
back to them.)  Peers that are downloading from the seeders the fastest
get priority.

This is one of the main ways that new peers can do well.  If they can
accept data quickly, they can get it very rapidly from seeder nodes,
even faster than other nodes that may be sharing with many peers.
And by biasing the data from seeder nodes towards peers that accept it
the fastest, BT spreads the data around as fast as possible, so that
there can be more peers with data and the whole network works well.

This set of policies works very well in practice and is a good part of
the reason for the success of BT.  It's pretty hard to tweak it and get
something that works better.

Hal