[BitTorrent] Re: Trackerless BitTorrent

Elliott Mitchell ehem at m5p.com
Tue Dec 28 17:52:55 EST 2004


>From: Olaf van der Spek <OvdSpek at LIACS.NL>
> >>From: "Olaf van der Spek" <OvdSpek at LIACS.NL>
> >
> > First, assuming 50 peers and retrieving 50 peers from each out those.
> > Resulting in a list of 2500 address/ports, minimum bandwidth of 15,000
> > bytes (45,000 with IPv6) for the peer list. In a small swarm there will
> > also be a *lot* of redundancy here merely because there aren't that many
> > actual clients.
> 
> 15 kb total overhead doesn't sound like a *lot*.

Not huge, but a significant chunk of overhead that you *must* transfer
right at start, before anything else can be gotten. If you really want to
keep the swarm healthy you'll likely need to transfer several times that
much data.

> > The real problem is the one I was pointing to above. From your peers
> > you'll get other peers that are already close to you in network terms
> > (degree 2). This is extraordinarily unhealthy for the swarm. Any piece
> 
> And from those other peers you can get degree 3, 4, 5 and so on. Or not?
> Also, you could ask peers at random instead of asking all peers.

This would mean every client would need to keep track of a *lot* of other
clients. In order to have those higher degree peers, your neighbors must
know of the existance of the peer, and then forward that information
without ever connecting. This does give you high degree peers, but those
will only be a very slight percentage of the list, the vast majority will
be very nearby.

You still have to trust these peers. You can retaliate for peers not
sending you data, tit for tat. You cannot retaliate for peers sending you
bogus peer lists. Worse, this makes poisoning easier since they may not
detect the poisoning immediately. Choosing at random reduces the data
transfered for peer lists, but means you must place even greater trust in
those peers you do contact.

> > you get from your peers will likely already be had by most/all your peers
> > (because they're already cross-linked). This results in bad download
> > speeds because you cannot obtain any pieces your peers are interested in.
> 
> Only if you connect to those peers instead of connecting to other peers 
> (from the tracker).
> If, you connect 'in addition to', that problem doesn't happen.
> You could even drop peers (one or a few at a time) found by the p2p 
> discovery when you get new addresses from the tracker to make place for 
> 'better' peers.
> 
> > With the tracker not only will you get a list of 50 unique peers, but
> > those will also be widely spaced across the network. This *cannot* be
> > done by peers telling each other about other peers.
> 
> But that's quite useless if for whatever reason the tracker isn't reachable.

True, true. Wasn't the original proposal to completely get rid of the
tracker though? Also if the tracker is up long enough for you to get any
peers, isn't it likely you'll get a sufficient set from the tracker in
the first place?


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