[vox-tech] bittorrent - no seeds but distributed copies increase

[Rob Rogers]

On Mon, Jul 26, 2004 at 02:46:48PM -0700, Peter Jay Salzman wrote:
> On Mon 26 Jul 04,  2:37 PM, Samuel N. Merritt <spam at andcheese.org> said:
> > On Mon, Jul 26, 2004 at 02:08:22PM -0700, Peter Jay Salzman wrote:
> > > Question:
> > > 
> > > How does the "distributed copies" get larger when there are no seeds?
> > 
> > I think that "distributed copies" measures how many complete copies of
> > the file you could get if you took all the pieces that everyone has and
> > assembled them. 
> > 
> > For example, consider a five-part file.
> > Alice has: 1 2 3 4
> > Bob has:       3 4 5 
> > Carol has: 1 2   4 
> > 
> > You could make one complete copy of the file from all this, so there'd
> > be 1 distributed copy. If Carol got piece 5 from Bob, then you could
> > assemble two complete copies. 
> > 
> > That's the integer part of distributed copies; I'm not sure where the
> > fractional part comes from. Maybe it's the size of the largest
> > distributed incomplete subfile divided by size of file, but that's just
> > a shot in the dark. 

IIRC, that's exactly how it works. So in the above example you would
have 1.8 distributed copies. (Alice's 4 peices and Bob's #5 is one.
Carol's 1 and 2, and Bob's 3 and 4 make another, and we ignore the last
Carol's last peice because we alread counted Bob's)

> ok.  this was my understanding.
> 
> > > Does the tracker ever inject packets into the torrent when needed (like
> > > when seeds == 0 and distributed copies < 1.0)?
> > 
> > No. The tracker doesn't have a local copy of the file. If there are no
> > seeds and < 1 distributed copy, everyone's download will stall before
> > finishing 
> 
> this was also my understanding.  but my question still stands: how does
> the distributed copies increase if there are no seeds?
> 
> i'm looking at a bittorrent right now.  it's remained constant at:
> 
>    seeds: 0 seen now, plus 0.983 distributed copies
>    peers: 19 seen now, 98.4% done at 0.2 kB/s
> 
> that ".983 distributed copies" has been creeping upwards.  last i looked
> at it, about 15 minutes ago, it was at .97.   i've noticed this happen
> before too.
> 
> how exactly does that number increase when there are no seeds?
> 
> pete

In some newer BT clients there's an option for a mode called superseed.
If you activate superseed mode (usually only used by the original
seeder) you won't appear as a seed to the tracker, even though you have
a full copy of the torrent. It also activates a more intelligent seeding
mode. Clients will know which peices are available from other clients,
and will usually try to request the most rare peice the find to try to
increase the number of distributed copies faster. In superseed mode, a
seed will tell the tracker it has no peices. When a client connects to
the superseed, it will tell the client it has just received a peice (and
I believe tell the client that is the only peice it has) forcing the
client to only request that peice. Once that peice has been downloaded
by the client, the superseed will not offer another peice to that client
until it sees another client with that peice (meaning that the first
client had sent it on)

The main advantage of this mode is that the original seed usually ends
up sending less data out before the second seed appears (meaning one
client has a full copy). IIRC, it's something like 1.1 copies on average
in superseed mode, compared to 1.25 - 1.5 copies in normal seeding mode.