<s>
Secure	B-Protocol
Socket	I-Protocol
Tunneling	I-Protocol
Protocol	I-Protocol
(	O
SSTP	O
)	O
is	O
a	O
form	O
of	O
virtual	B-Application
private	I-Application
network	I-Application
(	O
VPN	B-Application
)	O
tunnel	O
that	O
provides	O
a	O
mechanism	O
to	O
transport	O
PPP	B-Protocol
traffic	O
through	O
an	O
SSL/TLS	B-Protocol
channel	O
.	O
</s>
<s>
SSL/TLS	B-Protocol
provides	O
transport-level	O
security	O
with	O
key	O
negotiation	O
,	O
encryption	O
and	O
traffic	O
integrity	O
checking	O
.	O
</s>
<s>
The	O
use	O
of	O
SSL/TLS	B-Protocol
over	O
TCP	B-Protocol
port	O
443	O
(	O
by	O
default	O
,	O
port	O
can	O
be	O
changed	O
)	O
allows	O
SSTP	O
to	O
pass	O
through	O
virtually	O
all	O
firewalls	B-Application
and	O
proxy	B-Application
servers	I-Application
except	O
for	O
authenticated	O
web	O
proxies	O
.	O
</s>
<s>
SSTP	O
servers	O
must	O
be	O
authenticated	O
during	O
the	O
SSL/TLS	B-Protocol
phase	O
.	O
</s>
<s>
SSTP	O
clients	O
can	O
optionally	O
be	O
authenticated	O
during	O
the	O
SSL/TLS	B-Protocol
phase	O
and	O
must	O
be	O
authenticated	O
in	O
the	O
PPP	B-Protocol
phase	O
.	O
</s>
<s>
The	O
use	O
of	O
PPP	B-Protocol
allows	O
support	O
for	O
common	O
authentication	O
methods	O
,	O
such	O
as	O
EAP-TLS	O
and	O
MS-CHAP	B-Algorithm
.	O
</s>
<s>
SSTP	O
is	O
available	O
for	O
Linux	B-Application
,	O
BSD	B-Operating_System
,	O
and	O
Windows	O
.	O
</s>
<s>
It	O
is	O
fully	O
integrated	O
with	O
the	O
RRAS	O
architecture	O
in	O
these	O
operating	O
systems	O
,	O
allowing	O
its	O
use	O
with	O
Winlogon	O
or	O
smart-card	O
authentication	O
,	O
remote-access	O
policies	O
and	O
the	O
Windows	O
VPN	B-Application
client	I-Application
.	O
</s>
<s>
The	O
protocol	O
is	O
also	O
used	O
by	O
Windows	B-Application
Azure	I-Application
for	O
Point-to-Site	O
Virtual	O
Network	O
.	O
</s>
<s>
SSTP	O
is	O
intended	O
only	O
for	O
remote	O
client	O
access	O
,	O
it	O
generally	O
does	O
not	O
support	O
site-to-site	O
VPN	B-Application
tunnels	O
.	O
</s>
<s>
SSTP	O
suffers	O
from	O
the	O
same	O
performance	O
limitations	O
as	O
any	O
other	O
IP-over-TCP	O
tunnel	O
.	O
</s>
<s>
In	O
general	O
,	O
performance	O
will	O
be	O
acceptable	O
only	O
as	O
long	O
as	O
there	O
is	O
sufficient	O
excess	O
bandwidth	O
on	O
the	O
un-tunneled	O
network	O
link	O
to	O
guarantee	O
that	O
the	O
tunneled	O
TCP	B-Protocol
timers	O
do	O
not	O
expire	O
.	O
</s>
<s>
This	O
is	O
known	O
as	O
the	O
"	O
TCP	B-Protocol
meltdown	O
problem	O
"	O
.	O
</s>
<s>
At	O
the	O
moment	O
,	O
this	O
can	O
only	O
be	O
PPP	B-Protocol
.	O
</s>
