<s>
A	O
BitTorrent	B-Protocol
tracker	I-Protocol
is	O
a	O
special	O
type	O
of	O
server	B-Application
that	O
assists	O
in	O
the	O
communication	O
between	O
peers	O
using	O
the	O
BitTorrent	B-Protocol
protocol	I-Protocol
.	O
</s>
<s>
In	O
peer-to-peer	O
file	O
sharing	O
,	O
a	O
software	B-Protocol
client	I-Protocol
on	O
an	O
end-user	O
PC	O
requests	O
a	O
file	O
,	O
and	O
portions	O
of	O
the	O
requested	O
file	O
residing	O
on	O
peer	O
machines	O
are	O
sent	O
to	O
the	O
client	B-Protocol
,	O
and	O
then	O
reassembled	O
into	O
a	O
full	O
copy	O
of	O
the	O
requested	O
file	O
.	O
</s>
<s>
The	O
"	O
tracker	B-Protocol
"	O
server	B-Application
keeps	O
track	O
of	O
where	O
file	O
copies	O
reside	O
on	O
peer	O
machines	O
,	O
which	O
ones	O
are	O
available	O
at	O
time	O
of	O
the	O
client	B-Protocol
request	O
,	O
and	O
helps	O
coordinate	O
efficient	O
transmission	B-Language
and	O
reassembly	O
of	O
the	O
copied	O
file	O
.	O
</s>
<s>
Clients	O
that	O
have	O
already	O
begun	O
downloading	O
a	O
file	O
communicate	O
with	O
the	O
tracker	B-Protocol
periodically	O
to	O
negotiate	O
faster	O
file	O
transfer	O
with	O
new	O
peers	O
,	O
and	O
provide	O
network	O
performance	O
statistics	O
;	O
however	O
,	O
after	O
the	O
initial	O
peer-to-peer	O
file	O
download	O
is	O
started	O
,	O
peer-to-peer	O
communication	O
can	O
continue	O
without	O
the	O
connection	O
to	O
a	O
tracker	B-Protocol
.	O
</s>
<s>
Modern	O
BitTorrent	B-Protocol
clients	O
may	O
implement	O
a	O
distributed	B-Operating_System
hash	I-Operating_System
table	I-Operating_System
and	O
the	O
peer	O
exchange	O
protocol	O
to	O
discover	O
peers	O
without	O
trackers	B-Protocol
;	O
however	O
,	O
trackers	B-Protocol
are	O
still	O
often	O
included	O
with	O
torrents	B-Protocol
to	O
improve	O
the	O
speed	O
of	O
peer	O
discovery	O
.	O
</s>
<s>
Public	O
or	O
open	O
trackers	B-Protocol
can	O
be	O
used	O
by	O
anyone	O
by	O
adding	O
the	O
tracker	B-Protocol
address	O
to	O
an	O
existing	O
torrent	B-Protocol
,	O
or	O
they	O
can	O
be	O
used	O
by	O
any	O
newly	O
created	O
torrent	B-Protocol
,	O
like	O
OpenBitTorrent	O
.	O
</s>
<s>
The	B-Application
Pirate	I-Application
Bay	I-Application
operated	O
one	O
of	O
the	O
most	O
popular	O
public	B-Protocol
trackers	I-Protocol
until	O
disabling	O
it	O
in	O
2009	O
due	O
to	O
legal	O
trouble	O
,	O
and	O
thereafter	O
offered	O
only	O
magnet	B-Protocol
links	I-Protocol
.	O
</s>
<s>
A	O
private	O
tracker	B-Protocol
is	O
a	O
BitTorrent	B-Protocol
tracker	I-Protocol
that	O
restricts	O
use	O
by	O
requiring	O
users	O
to	O
register	O
with	O
the	O
site	O
.	O
</s>
<s>
The	O
method	O
for	O
controlling	O
registration	O
used	O
among	O
many	O
private	O
trackers	B-Protocol
is	O
an	O
invitation	O
system	O
,	O
in	O
which	O
active	O
and	O
contributing	O
members	O
are	O
given	O
the	O
ability	O
to	O
grant	O
a	O
new	O
user	O
permission	O
to	O
register	O
at	O
the	O
site	O
,	O
or	O
a	O
new	O
user	O
goes	O
through	O
an	O
interview	O
process	O
.	O
</s>
<s>
For	O
example	O
,	O
the	O
Creative	O
Commons	O
license	O
family	O
for	O
free	B-Application
cultural	I-Application
works	I-Application
in	O
text	O
,	O
audio	O
,	O
video	O
or	O
image	O
format	O
;	O
or	O
software	O
licenses	O
for	O
Free	B-Application
Software	I-Application
/	O
Open-source	B-Application
software	I-Application
like	O
the	O
BSD	B-Operating_System
License	I-Operating_System
and	O
others	O
.	O
</s>
<s>
Wikipedia	O
itself	O
can	O
be	O
distributed	O
via	O
BitTorrent	B-Protocol
for	O
the	O
same	O
reason	O
.	O
</s>
<s>
There	O
are	O
also	O
experiments	O
that	O
legally	O
sell	O
content	O
that	O
is	O
distributed	O
over	O
BitTorrent	B-Protocol
using	O
a	O
"	O
secure	O
"	O
tracker	B-Protocol
system	O
.	O
</s>
<s>
Trackers	B-Protocol
are	O
the	O
primary	O
reason	O
for	O
a	O
damaged	O
BitTorrent	B-Protocol
"	O
swarm	O
"	O
.	O
</s>
<s>
The	O
reliability	O
of	O
trackers	B-Protocol
has	O
been	O
improved	O
through	O
two	O
main	O
innovations	O
in	O
the	O
BitTorrent	B-Protocol
protocol	I-Protocol
.	O
</s>
<s>
Multi-tracker	O
torrents	B-Protocol
contain	O
multiple	O
trackers	B-Protocol
in	O
a	O
single	O
torrent	B-Protocol
file	O
.	O
</s>
<s>
This	O
provides	O
redundancy	O
in	O
the	O
case	O
that	O
one	O
tracker	B-Protocol
fails	O
,	O
the	O
other	O
trackers	B-Protocol
can	O
continue	O
to	O
maintain	O
the	O
swarm	O
for	O
the	O
torrent	B-Protocol
.	O
</s>
<s>
One	O
disadvantage	O
to	O
this	O
is	O
that	O
it	O
becomes	O
possible	O
to	O
have	O
multiple	O
unconnected	O
swarms	O
for	O
a	O
single	O
torrent	B-Protocol
where	O
some	O
users	O
can	O
connect	O
to	O
one	O
specific	O
tracker	B-Protocol
while	O
being	O
unable	O
to	O
connect	O
to	O
another	O
.	O
</s>
<s>
This	O
can	O
create	O
a	O
disjoint	O
set	O
which	O
can	O
impede	O
the	O
efficiency	O
of	O
a	O
torrent	B-Protocol
to	O
transfer	O
the	O
files	O
it	O
describes	O
.	O
</s>
<s>
Additional	O
extensions	O
such	O
as	O
Peer	O
exchange	O
and	O
DHT	B-Protocol
mitigate	O
this	O
effect	O
by	O
rapidly	O
merging	O
otherwise	O
disjoint	O
graphs	O
of	O
peers	O
.	O
</s>
<s>
Vuze	B-Application
(	O
formerly	O
Azureus	B-Application
)	O
was	O
the	O
first	O
BitTorrent	B-Protocol
client	B-Protocol
to	O
implement	O
such	O
a	O
system	O
through	O
the	O
distributed	B-Operating_System
hash	I-Operating_System
table	I-Operating_System
(	O
DHT	B-Protocol
)	O
method	O
.	O
</s>
<s>
An	O
alternative	O
and	O
incompatible	O
DHT	B-Protocol
system	O
,	O
known	O
as	O
Mainline	B-Protocol
DHT	I-Protocol
,	O
was	O
developed	O
simultaneously	O
and	O
later	O
adopted	O
by	O
the	O
BitTorrent	B-Protocol
(	O
Mainline	O
)	O
,	O
μTorrent	B-Application
,	O
Transmission	B-Language
,	O
rTorrent	O
,	O
KTorrent	O
,	O
BitComet	B-Protocol
,	O
and	O
Deluge	O
clients	O
.	O
</s>
<s>
Current	O
versions	O
of	O
the	O
official	O
BitTorrent	B-Protocol
client	B-Protocol
,	O
μTorrent	B-Application
,	O
BitComet	B-Protocol
,	O
Transmission	B-Language
and	O
BitSpirit	O
all	O
share	O
compatibility	O
with	O
Mainline	B-Protocol
DHT	I-Protocol
.	O
</s>
<s>
Both	O
DHT	B-Protocol
implementations	O
are	O
based	O
on	O
Kademlia	B-Operating_System
.	O
</s>
<s>
As	O
of	O
version	O
3.0.5.0	O
,	O
Vuze	B-Application
also	O
supports	O
Mainline	B-Protocol
DHT	I-Protocol
in	O
addition	O
to	O
its	O
own	O
distributed	O
database	O
through	O
use	O
of	O
an	O
optional	O
application	O
plugin	O
MainlineDHT	O
Plugin	O
.	O
</s>
<s>
This	O
potentially	O
allows	O
the	O
Vuze	B-Application
client	B-Protocol
to	O
reach	O
a	O
bigger	O
swarm	O
.	O
</s>
<s>
Most	O
BitTorrent	B-Protocol
clients	O
also	O
use	O
Peer	O
exchange	O
(	O
PeX	O
)	O
to	O
gather	O
peers	O
in	O
addition	O
to	O
trackers	B-Protocol
and	O
DHT	B-Protocol
.	O
</s>
<s>
With	O
the	O
3.0.5.0	O
release	O
of	O
Vuze	B-Application
,	O
all	O
major	O
BitTorrent	B-Protocol
clients	O
now	O
have	O
compatible	O
peer	O
exchange	O
.	O
</s>
<s>
One	O
of	O
the	O
options	O
for	O
this	O
HTTP	B-Protocol
based	O
tracker	B-Protocol
protocol	O
is	O
the	O
"	O
compact	O
"	O
flag	O
.	O
</s>
<s>
This	O
flag	O
,	O
as	O
defined	O
in	O
BEP	O
23	O
,	O
specifies	O
that	O
the	O
tracker	B-Protocol
can	O
compact	O
the	O
response	O
by	O
encoding	O
IPv4	B-Protocol
addresses	O
as	O
a	O
set	O
of	O
4	O
bytes	O
(	O
32bits	O
)	O
.	O
</s>
<s>
IPv6	B-Protocol
though	O
are	O
128bits	O
long	O
,	O
and	O
as	O
such	O
,	O
the	O
"	O
compact	O
"	O
would	O
break	O
IPv6	B-Protocol
support	O
.	O
</s>
<s>
opentracker	B-Language
from	O
Dirk	O
Engling	O
powered	O
one	O
of	O
the	O
biggest	O
BitTorrent	B-Protocol
trackers	I-Protocol
,	O
The	B-Application
Pirate	I-Application
Bay	I-Application
tracker	B-Protocol
.	O
</s>
<s>
qBittorrent	B-Application
is	O
an	O
open	O
source	O
BitTorrent	B-Protocol
client	B-Protocol
with	O
a	O
built-in	O
tracker	B-Protocol
support	O
.	O
</s>
<s>
BitTorious	B-Application
is	O
an	O
open	O
source	O
,	O
commercially	O
supported	O
tracker	B-Protocol
with	O
integrated	O
web-based	O
management	O
portal	O
.	O
</s>
