[BitTorrent] Re: Trackerless BitTorrent

John Hoffman theshadow at shambala.net
Fri Dec 31 14:02:42 EST 2004



>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.
>

Just realized a very simple solution to the problem, and it only 
requires one flag per connection.  Basically, make two classes of 
connection; one where peers were introduced via tracker, and one where 
peers were introduced via P2P.  If the "fast-reannounce" period comes 
due and the peer doesn't have enough tracker-introduced connections, it 
reannounces.

The question is, how many of these types of connections per peer is 
sufficient?  IMO, only a small number is required; 5 should be plenty, 
and even 3 ought to be enough.  BT establishes 40 connections (on 
average) per peer, which means the absolute minumum size of an "island" 
will be 40 peers, where if each peer has 3 tracker-initiated 
connections, it means even if only 3 connections per peer were via 
tracker, the island will have 120 "bridges" to the rest of the swarm, 
which ought to be sufficient to introduce new data to the sub-swarm.


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