[BitTorrent] Re: Trackerless BitTorrent

Olaf van der Spek OvdSpek at LIACS.NL
Tue Dec 28 18:46:29 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

That depends on design details. It's not needed if it's implemented in 
addition to the tracker.

> keep the swarm healthy you'll likely need to transfer several times that
> much data.

Peer traffic is cheaper than tracker traffic and even 150 k wouldn't be such 
a big issue, considering torrents are frequently in the gb range.

>> > 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

Or you connect to a degree 2 peer to get a list of degree 3 peers. Connect 
to degree 3 to get degree 4, etc.

> 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

My get_peers/peers idea was in addition to the tracker.

> 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?

Not if you disconnect and then reconnect a week later to the peers you used 
to be connected too. 



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