[BitTorrent] Bit Torrent Spec

Vitenka vitenka at ntlworld.com
Fri Mar 4 15:30:09 EST 2005

> 1. Does a client choke all peers in which it is not interested, i.e.,
> the only way to get a piece is by EXCHANGE (both client and selected
> peer have pieces of interest to each other)?

Generally no.  Peers will usually unchoke any and all peers.  Though 
there's nothing in the actual spec requiring this - as long as you can 
talk the protocol you can be a valid bittorrent client.

> 2. If the above is true, the only way to get a piece upon joining the
> system would be from a seed, since none of the other peers would be
> interested in you, is this what happens?

Not always.  Though generally seeds favor new clients, and existing 
lclients tend to favour those giving them good rates - so they don't 
usually get to the new peer for a while.

> The above would be correct from a game theoretic (selfish) point of view.

Not quite.  Remember that peers keep a history of what rates they got 
from other peers.  Since seeds tend to do rarest first sending (not all, 
but many), you can end up with the only available copy of a rare piece 
being held by the 'new' peer (perhaps even as its only piece.)  Then 
you'd want a good record of having sent data to that peer, in order to 
get ahold of it.

> Sorry if the question seems very basic, but I cannot find a definite
> answer to this in the "Incentive Build Robustness in Bit Torrent" or
> the protocol spec.

In general, though - you are right.  A peer joining an existing swarm 
gets a lower data rate until they have acquired a few pieces.  If that 
peer is sensible, it will acquire pieces which are in demand from other 
clients (and many seeds will help with this) and then it will be able to 
actively join the normal peer swarm.

In closing:  It doesn't HAVE to be that way, but it oftren is.

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