<s>
In	O
the	O
context	O
of	O
computer	B-Architecture
networking	I-Architecture
,	O
a	O
tunnel	B-Protocol
broker	I-Protocol
is	O
a	O
service	O
which	O
provides	O
a	O
network	B-Protocol
tunnel	I-Protocol
.	O
</s>
<s>
There	O
are	O
a	O
variety	O
of	O
tunnel	B-Protocol
brokers	I-Protocol
,	O
including	O
IPv4	B-Protocol
tunnel	B-Protocol
brokers	I-Protocol
,	O
though	O
most	O
commonly	O
the	O
term	O
is	O
used	O
to	O
refer	O
to	O
an	O
IPv6	B-Protocol
tunnel	I-Protocol
broker	I-Protocol
as	O
defined	O
in	O
.	O
</s>
<s>
IPv6	B-Protocol
tunnel	I-Protocol
brokers	I-Protocol
typically	O
provide	O
IPv6	B-Protocol
to	O
sites	O
or	O
end	O
users	O
over	O
IPv4	B-Protocol
.	O
</s>
<s>
In	O
general	O
,	O
IPv6	B-Protocol
tunnel	I-Protocol
brokers	I-Protocol
offer	O
so	O
called	O
'	O
protocol	B-Protocol
41	O
 '	O
or	O
proto-41	O
tunnels	O
.	O
</s>
<s>
These	O
are	O
tunnels	O
where	O
IPv6	B-Protocol
is	O
tunneled	O
directly	O
inside	O
IPv4	B-Protocol
packets	B-Protocol
by	O
having	O
the	O
protocol	B-Protocol
field	O
set	O
to	O
'	O
41	O
 '	O
(	O
IPv6	B-Protocol
)	O
in	O
the	O
IPv4	B-Protocol
packet	B-Protocol
.	O
</s>
<s>
In	O
the	O
case	O
of	O
IPv4	B-Protocol
tunnel	B-Protocol
brokers	I-Protocol
IPv4	B-Protocol
tunnels	O
are	O
provided	O
to	O
users	O
by	O
encapsulating	O
IPv4	B-Protocol
inside	O
IPv6	B-Protocol
as	O
defined	O
in	O
.	O
</s>
<s>
Configuration	O
of	O
IPv6	B-Protocol
tunnels	O
is	O
usually	O
done	O
using	O
the	O
Tunnel	B-Protocol
Setup	I-Protocol
Protocol	I-Protocol
(	O
TSP	B-Protocol
)	O
,	O
or	O
using	O
Tunnel	O
Information	O
Control	O
protocol	B-Protocol
(	O
TIC	O
)	O
.	O
</s>
<s>
A	O
client	O
capable	O
of	O
this	O
is	O
AICCU	B-Language
(	O
Automatic	O
IPv6	B-Protocol
Connectivity	O
Client	O
Utility	O
)	O
.	O
</s>
<s>
In	O
addition	O
to	O
IPv6	B-Protocol
tunnels	O
TSP	B-Protocol
can	O
also	O
be	O
used	O
to	O
set	O
up	O
IPv4	B-Protocol
tunnels	O
.	O
</s>
<s>
Proto-41	O
tunnels	O
(	O
direct	O
IPv6	B-Protocol
in	O
IPv4	B-Protocol
)	O
may	O
not	O
operate	O
well	O
situated	O
behind	O
NATs	O
.	O
</s>
<s>
One	O
way	O
around	O
this	O
is	O
to	O
configure	O
the	O
actual	O
endpoint	O
of	O
the	O
tunnel	O
to	O
be	O
the	O
DMZ	B-General_Concept
on	O
the	O
NAT-utilizing	O
equipment	O
.	O
</s>
<s>
Another	O
method	O
is	O
to	O
either	O
use	O
AYIYA	B-Protocol
or	O
TSP	B-Protocol
,	O
both	O
of	O
which	O
send	O
IPv6	B-Protocol
inside	O
UDP	B-Protocol
packets	I-Protocol
,	O
which	O
is	O
able	O
to	O
cross	O
most	O
NAT	B-Protocol
setups	O
and	O
even	O
firewalls	B-Application
.	O
</s>
<s>
A	O
problem	O
that	O
still	O
might	O
occur	O
is	O
that	O
of	O
the	O
timing-out	O
of	O
the	O
state	O
in	O
the	O
NAT	B-Protocol
machine	O
.	O
</s>
<s>
As	O
a	O
NAT	B-Protocol
remembers	O
that	O
a	O
packet	B-Protocol
went	O
outside	O
to	O
the	O
Internet	O
it	O
allows	O
another	O
packet	B-Protocol
to	O
come	O
back	O
in	O
from	O
the	O
Internet	O
that	O
is	O
related	O
to	O
the	O
initial	O
proto-41	O
packet	B-Protocol
.	O
</s>
<s>
When	O
this	O
state	O
expires	O
,	O
no	O
other	O
packets	B-Protocol
from	O
the	O
Internet	O
will	O
be	O
accepted	O
.	O
</s>
<s>
This	O
therefore	O
breaks	O
the	O
connectivity	O
of	O
the	O
tunnel	O
until	O
the	O
user	O
's	O
host	O
again	O
sends	O
out	O
a	O
packet	B-Protocol
to	O
the	O
tunnel	B-Protocol
broker	I-Protocol
.	O
</s>
<s>
When	O
the	O
endpoint	O
is	O
n't	O
a	O
static	O
IP	O
address	O
,	O
the	O
user	O
,	O
or	O
a	O
program	O
,	O
has	O
to	O
instruct	O
the	O
tunnel	B-Protocol
broker	I-Protocol
to	O
update	O
the	O
endpoint	O
address	O
.	O
</s>
<s>
This	O
can	O
be	O
done	O
using	O
the	O
tunnel	B-Protocol
broker	I-Protocol
's	O
web	O
site	O
or	O
using	O
an	O
automated	O
protocol	B-Protocol
like	O
TSP	B-Protocol
or	O
,	O
as	O
used	O
by	O
AICCU	B-Language
.	O
</s>
<s>
In	O
the	O
case	O
of	O
a	O
tunnel	B-Protocol
broker	I-Protocol
using	O
TSP	B-Protocol
,	O
the	O
client	O
automatically	O
restarting	O
the	O
tunnel	O
will	O
cause	O
the	O
endpoint	O
address	O
and	O
port	O
to	O
be	O
updated	O
.	O
</s>
<s>
There	O
are	O
a	O
variety	O
of	O
tunnel	B-Protocol
brokers	I-Protocol
that	O
provide	O
their	O
own	O
custom	O
implementations	O
based	O
on	O
different	O
goals	O
.	O
</s>
<s>
Listed	O
here	O
are	O
the	O
common	O
implementations	O
as	O
used	O
by	O
the	O
listed	O
IPv6	B-Protocol
tunnel	I-Protocol
brokers	I-Protocol
.	O
</s>
<s>
gogoSERVER	O
(	O
formerly	O
Gateway6	O
)	O
is	O
used	O
by	O
the	O
Freenet6	O
service	O
,	O
which	O
is	O
the	O
second	O
IPv6	B-Protocol
tunnel	I-Protocol
broker	I-Protocol
service	O
,	O
going	O
into	O
production	O
in	O
1999	O
.	O
</s>
<s>
In	O
June	O
2009	O
,	O
Hexago	O
became	O
gogo6	O
through	O
a	O
management	O
buyout	O
and	O
the	O
Freenet6	O
service	O
became	O
part	O
of	O
gogoNET	O
,	O
a	O
social	O
network	O
for	O
IPv6	B-Protocol
professionals	O
.	O
</s>
<s>
CITC	O
Tunnel	B-Protocol
Broker	I-Protocol
,	O
run	O
by	O
the	O
Saudi	O
Arabia	O
IPv6	B-Protocol
Task	O
Force	O
,	O
uses	O
their	O
own	O
implementation	O
of	O
the	O
TSP	B-Protocol
RFC	O
named	O
'	O
ddtb	O
 '	O
.	O
</s>
