[BitTorrent] Re: Trackerless BitTorrent

Olaf van der Spek OvdSpek at LIACS.NL
Fri Dec 31 07:52:23 EST 2004


Elliott Mitchell wrote:
>>From: Olaf van der Spek <OvdSpek at LIACS.NL>
>>
>>>>From: Olaf van der Spek <OvdSpek at LIACS.NL>
>>>>
>>>>>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.
> 
> 
> You might think that, but this is not correct. For the purposes of this
> explanation, I'll designate directly connected clients as degree 0 peers.
> 
> When you ask a degree 0 peer you'll get a list of degree 0 and 1 peers.
> If it doesn't filter the list, you'll even get a degree -1 peer
> (yourself). Depending upon the size of the swarm it is likely you'll get
> a couple degree 0 peers, ones you're already connected to, these can be
> easily filtered out though. In a really small swarm or one where the
> swarm mesh has broken down, you might get zero degree 1 peers.
> 
> With those degree 1 peers, you can briefly connect and ask them for peers
> (if you stay connected, they're now really degree 0; but I'll ignore
> this). From the degree 1 peer's list, you're guarenteed to find at least
> one degree 0 peer, the one you used to jump to it. Assuming anything
> remains of the list (again, tiny swarm, or unhealthy mesh), there will
> be degree 1 peers (other intermediates that you haven't contacted) and
> even further out degree 2 peers. Contact a degree 2 peer, and you'll get
> a list of degree 1, 2 and 3 peers. Contact a degree N peer, and you'll
> get a list of degree N-1, N and N+1 peers.
> 
> The problem hits once you're contacted the degree 1 and greater peers,
> and gotten the list consisting of degree 1 and 2 peers. How do you tell
> what degree a peer is? If you ask *all* your degree 0 peers for their
> lists, you can build a complete list of degree 1 peers, and filter them
> out and have a list of degree 2 peers. This does mean you've got to
> spend those 300 (900 for v6) bytes on every connection asking for peers.
> So, you can build a list of degree 2 peers. How about building a list of
> degree 3 peers? Well, connect to all your degree 1 peers...
> 
> The basic issue is you're trying to detect an unhealthy mesh, and this
> quickly becomes prohibitively expensive. Yet you must do so, otherwise
> you risk getting stuck on an island completely separated from the swam.
> Wherever the border where you stop constructing complete lists of peers
> a gap could form and disconnect you from the swarm.
> 
> So the main issue becomes, how dangerous is this? The presence of a
> tracker doesn't guarentee this won't happen, it merely makes it
> extraordinarily unlikely (good old stochastic methods work). It boils
> down to the issue, do peer lists aggravate island-forming behavior?
> Looking for higher degree peers won't stop island forming behavior,
> merely make those islands larger.

Thanks for the explanation, now I understand the issue.
It's basically that because you connect to close peers, you (and others) 
are more likely to find the same close peers, and so on.

But for small swarms, 'begin mode' and 'tracker lost mode', it still 
sounds like a good idea.

If you mark (connections to) peers found via the tracker as good, peers 
via p2p discovery as bad and drop bad peers to make room for good peers, 
the island issue should not cause bad things.

>>>>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.
> 
> 
> The original poster was proposing completely getting rid of the tracker,
> so this was the model I was commenting on. As these two are diverging
> I'll respond separately.

It's related, complete trackerless operation is basically a superset of 
this issue.

>>>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. 
> 
> 
> In this circumstance though, are *any* of your old peers likely to be
> valid? (DHCP/address changes, download completions, etc)

Users of clients that support multiple torrents per instance and keep 
state during client restarts are more likely still be available.
Of course it depends on the time you've been away, but a week is 
certainly possible.


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