<s>
HTTP	B-Protocol
tunneling	I-Protocol
is	O
used	O
to	O
create	O
a	O
network	O
link	O
between	O
two	O
computers	O
in	O
conditions	O
of	O
restricted	O
network	O
connectivity	O
including	O
firewalls	B-Application
,	O
NATs	O
and	O
ACLs	O
,	O
among	O
other	O
restrictions	O
.	O
</s>
<s>
The	O
tunnel	O
is	O
created	O
by	O
an	O
intermediary	O
called	O
a	O
proxy	B-Application
server	I-Application
which	O
is	O
usually	O
located	O
in	O
a	O
DMZ	B-General_Concept
.	O
</s>
<s>
The	O
most	O
common	O
form	O
of	O
HTTP	B-Protocol
tunneling	I-Protocol
is	O
the	O
standardized	O
HTTP	O
CONNECT	O
method	O
.	O
</s>
<s>
In	O
this	O
mechanism	O
,	O
the	O
client	O
asks	O
an	O
HTTP	O
proxy	B-Application
server	I-Application
to	O
forward	O
the	O
TCP	B-Protocol
connection	O
to	O
the	O
desired	O
destination	O
.	O
</s>
<s>
Once	O
the	O
connection	O
has	O
been	O
established	O
by	O
the	O
server	O
,	O
the	O
proxy	B-Application
server	I-Application
continues	O
to	O
proxy	O
the	O
TCP	B-Protocol
stream	O
to	O
and	O
from	O
the	O
client	O
.	O
</s>
<s>
Only	O
the	O
initial	O
connection	O
request	O
is	O
HTTP	O
-	O
after	O
that	O
,	O
the	O
server	O
simply	O
proxies	O
the	O
established	O
TCP	B-Protocol
connection	O
.	O
</s>
<s>
This	O
mechanism	O
is	O
how	O
a	O
client	O
behind	O
an	O
HTTP	O
proxy	O
can	O
access	O
websites	O
using	O
SSL	B-Protocol
or	O
TLS	O
(	O
i.e.	O
</s>
<s>
Proxy	B-Application
servers	I-Application
may	O
also	O
limit	O
connections	O
by	O
only	O
allowing	O
connections	O
to	O
the	O
default	O
HTTPS	O
port	O
443	O
,	O
whitelisting	O
hosts	O
,	O
or	O
blocking	O
traffic	O
which	O
does	O
n't	O
appear	O
to	O
be	O
SSL	B-Protocol
.	O
</s>
<s>
The	O
client	O
connects	O
to	O
the	O
proxy	B-Application
server	I-Application
and	O
requests	O
tunneling	O
by	O
specifying	O
the	O
port	O
and	O
the	O
host	O
computer	O
to	O
which	O
it	O
would	O
like	O
to	O
connect	O
.	O
</s>
<s>
Any	O
data	O
sent	O
to	O
the	O
proxy	B-Application
server	I-Application
is	O
now	O
forwarded	O
,	O
unmodified	O
,	O
to	O
the	O
remote	O
host	O
and	O
the	O
client	O
can	O
communicate	O
using	O
any	O
protocol	O
accepted	O
by	O
the	O
remote	O
host	O
.	O
</s>
<s>
A	O
HTTP	B-Protocol
tunnel	I-Protocol
can	O
also	O
be	O
implemented	O
using	O
only	O
the	O
usual	O
HTTP	O
methods	O
as	O
POST	O
,	O
GET	O
,	O
PUT	O
and	O
DELETE	O
.	O
</s>
<s>
This	O
is	O
similar	O
to	O
the	O
approach	O
used	O
in	O
Bidirectional-streams	B-Protocol
Over	I-Protocol
Synchronous	I-Protocol
HTTP	I-Protocol
(	O
BOSH	B-Protocol
)	O
.	O
</s>
<s>
Since	O
all	O
traffic	O
is	O
encapsulated	O
inside	O
normal	O
GET	O
and	O
POST	O
requests	O
and	O
responses	O
,	O
this	O
approach	O
works	O
through	O
most	O
proxies	O
and	O
firewalls	B-Application
.	O
</s>
