<s>
A	O
session	B-Protocol
border	I-Protocol
controller	I-Protocol
(	O
SBC	O
)	O
is	O
a	O
network	O
element	O
deployed	O
to	O
protect	O
SIP	B-Application
based	O
voice	B-Application
over	I-Application
Internet	I-Application
Protocol	I-Application
(	O
VoIP	B-Application
)	O
networks	O
.	O
</s>
<s>
It	O
is	O
the	O
job	O
of	O
a	O
session	B-Protocol
border	I-Protocol
controller	I-Protocol
to	O
exert	O
influence	O
over	O
the	O
data	O
flows	O
of	O
sessions	O
.	O
</s>
<s>
As	O
a	O
simple	O
example	O
,	O
at	O
the	O
edge	O
of	O
a	O
corporate	O
network	O
,	O
a	O
firewall	B-Application
demarcates	O
the	O
local	O
network	O
(	O
inside	O
the	O
corporation	O
)	O
from	O
the	O
rest	O
of	O
the	O
Internet	O
(	O
outside	O
the	O
corporation	O
)	O
.	O
</s>
<s>
It	O
is	O
the	O
job	O
of	O
a	O
session	B-Protocol
border	I-Protocol
controller	I-Protocol
to	O
assist	O
policy	O
administrators	O
in	O
managing	O
the	O
flow	O
of	O
session	O
data	O
across	O
these	O
borders	O
.	O
</s>
<s>
The	O
term	O
"	O
controller	O
"	O
refers	O
to	O
the	O
influence	O
that	O
session	B-Protocol
border	I-Protocol
controllers	I-Protocol
have	O
on	O
the	O
data	O
streams	O
that	O
comprise	O
sessions	O
,	O
as	O
they	O
traverse	O
borders	O
between	O
one	O
part	O
of	O
a	O
network	O
and	O
another	O
.	O
</s>
<s>
Additionally	O
,	O
session	B-Protocol
border	I-Protocol
controllers	I-Protocol
often	O
provide	O
measurement	O
,	O
access	O
control	O
,	O
and	O
data	O
conversion	O
facilities	O
for	O
the	O
calls	O
they	O
control	O
.	O
</s>
<s>
SBCs	O
commonly	O
maintain	O
full	O
session	O
state	B-Application
and	O
offer	O
the	O
following	O
functions	O
:	O
</s>
<s>
With	O
the	O
advent	O
of	O
WebRTC	B-Protocol
some	O
SBCs	O
have	O
also	O
assumed	O
the	O
role	O
of	O
SIP	B-Application
to	O
WebRTC	B-Protocol
Gateway	I-Protocol
and	O
translate	O
SIP	B-Application
.	O
</s>
<s>
While	O
no	O
one	O
signalling	O
protocol	O
is	O
mandated	O
by	O
the	O
WebRTC	B-Protocol
specifications	O
,	O
SIP	B-Application
over	O
WebSockets	B-Protocol
(	O
RFC	O
7118	O
)	O
is	O
often	O
used	O
partially	O
due	O
to	O
the	O
applicability	O
of	O
SIP	B-Application
to	O
most	O
of	O
the	O
envisaged	O
communication	O
scenarios	O
as	O
well	O
as	O
the	O
availability	O
of	O
open	O
source	O
software	O
such	O
as	O
JsSIP	B-Language
.	O
</s>
<s>
In	O
such	O
a	O
case	O
the	O
SBC	O
acts	O
as	O
a	O
gateway	O
between	O
the	O
WebRTC	B-Protocol
applications	O
and	O
SIP	B-Application
end	O
points	O
.	O
</s>
<s>
SBCs	O
are	O
inserted	O
into	O
the	O
signaling	O
and/or	O
media	O
paths	O
between	O
calling	O
and	O
called	O
parties	O
in	O
a	O
VoIP	B-Application
call	I-Application
,	O
predominantly	O
those	O
using	O
the	O
Session	B-Application
Initiation	I-Application
Protocol	I-Application
(	O
SIP	B-Application
)	O
,	O
H.323	B-Protocol
,	O
and	O
MGCP	B-Protocol
call-signaling	O
protocols	O
.	O
</s>
<s>
In	O
technical	O
terms	O
,	O
when	O
used	O
with	O
the	O
SIP	B-Application
protocol	O
,	O
this	O
defines	O
a	O
back-to-back	B-Protocol
user	I-Protocol
agent	I-Protocol
(	O
B2BUA	B-Protocol
)	O
.	O
</s>
<s>
In	O
other	O
cases	O
,	O
the	O
SBC	O
simply	O
modifies	O
the	O
stream	O
of	O
call	O
control	O
(	O
signaling	O
)	O
data	O
involved	O
in	O
each	O
call	O
,	O
perhaps	O
limiting	O
the	O
kinds	O
of	O
calls	O
that	O
can	O
be	O
conducted	O
,	O
changing	O
the	O
codec	B-General_Concept
choices	O
,	O
and	O
so	O
on	O
.	O
</s>
<s>
Ultimately	O
,	O
SBCs	O
allow	O
the	O
network	O
operators	O
to	O
manage	O
the	O
calls	O
that	O
are	O
made	O
on	O
their	O
networks	O
,	O
fix	O
or	O
change	O
protocols	O
and	O
protocol	O
syntax	O
to	O
achieve	O
interoperability	O
,	O
and	O
also	O
overcome	O
some	O
of	O
the	O
problems	O
that	O
firewalls	B-Application
and	O
network	B-Protocol
address	I-Protocol
translators	I-Protocol
(	O
NATs	O
)	O
present	O
for	O
VoIP	B-Application
calls	I-Application
.	O
</s>
<s>
Proxies	O
also	O
do	O
not	O
alter	O
any	O
information	O
in	O
the	O
SIP	B-Application
message	O
bodies	O
.	O
</s>
<s>
Note	O
that	O
during	O
the	O
session	O
initiation	O
phase	O
the	O
user	O
agents	O
exchange	O
SIP	B-Application
messages	O
with	O
the	O
SDP	O
bodies	O
that	O
include	O
addresses	O
at	O
which	O
the	O
agents	O
expect	O
the	O
media	O
traffic	O
.	O
</s>
<s>
For	O
example	O
,	O
most	O
SBCs	O
are	O
implemented	O
as	O
back-to-back	B-Protocol
user	I-Protocol
agent	I-Protocol
.	O
</s>
<s>
A	O
B2BUA	B-Protocol
is	O
a	O
proxy-like	O
server	O
that	O
splits	O
a	O
SIP	B-Application
transaction	O
in	O
two	O
call	O
legs	O
:	O
on	O
the	O
side	O
facing	O
the	O
user	O
agent	O
client	O
(	O
UAC	O
)	O
,	O
it	O
acts	O
as	O
server	O
,	O
on	O
the	O
side	O
facing	O
user	B-Application
agent	I-Application
server	I-Application
(	O
UAS	O
)	O
it	O
acts	O
as	O
a	O
client	O
.	O
</s>
<s>
While	O
a	O
proxy	O
usually	O
keeps	O
only	O
state	B-Application
information	O
related	O
to	O
active	O
transactions	O
,	O
B2BUAs	B-Protocol
keep	O
state	B-Application
information	O
about	O
active	O
dialogs	O
,	O
e.g.	O
,	O
calls	O
.	O
</s>
<s>
That	O
is	O
,	O
once	O
a	O
proxy	O
receives	O
a	O
SIP	B-Application
request	O
it	O
will	O
save	O
some	O
state	B-Application
information	O
.	O
</s>
<s>
Once	O
the	O
transaction	O
is	O
over	O
,	O
e.g.	O
,	O
after	O
receiving	O
a	O
response	O
,	O
the	O
state	B-Application
information	O
will	O
soon	O
after	O
be	O
deleted	O
.	O
</s>
<s>
A	O
B2BUA	B-Protocol
will	O
maintain	O
state	B-Application
information	O
for	O
active	O
calls	O
and	O
only	O
delete	O
this	O
information	O
once	O
the	O
call	O
is	O
terminated	O
.	O
</s>
<s>
When	O
an	O
SBC	O
is	O
included	O
in	O
the	O
call	O
path	O
,	O
the	O
SBC	O
acts	O
as	O
a	O
B2BUA	B-Protocol
that	O
behaves	O
as	O
a	O
user	B-Application
agent	I-Application
server	I-Application
towards	O
the	O
caller	O
and	O
as	O
user	O
agent	O
client	O
towards	O
the	O
callee	O
.	O
</s>
<s>
Thereby	O
,	O
not	O
only	O
will	O
all	O
SIP	B-Application
messages	O
traverse	O
the	O
SBC	O
but	O
also	O
all	O
audio	O
and	O
video	O
packets	O
.	O
</s>
<s>
Note	O
that	O
the	O
list	O
of	O
header	O
manipulations	O
listed	O
here	O
is	O
only	O
a	O
subset	O
of	O
the	O
possible	O
changes	O
that	O
an	O
SBC	O
might	O
introduce	O
to	O
a	O
SIP	B-Application
message	O
.	O
</s>
<s>
SBCs	O
are	O
often	O
used	O
by	O
corporations	O
along	O
with	O
firewalls	B-Application
and	O
intrusion	O
prevention	O
systems	O
(	O
IPS	O
)	O
to	O
enable	O
VoIP	B-Application
calls	I-Application
to	O
and	O
from	O
a	O
protected	O
enterprise	O
network	O
.	O
</s>
<s>
VoIP	B-Application
service	O
providers	O
use	O
SBCs	O
to	O
allow	O
the	O
use	O
of	O
VoIP	B-Application
protocols	O
from	O
private	O
networks	O
with	O
Internet	O
connections	O
using	O
NAT	O
,	O
and	O
also	O
to	O
implement	O
strong	O
security	O
measures	O
that	O
are	O
necessary	O
to	O
maintain	O
a	O
high	O
quality	O
of	O
service	O
.	O
</s>
<s>
In	O
larger	O
enterprises	O
,	O
SBCs	O
can	O
also	O
be	O
used	O
in	O
conjunction	O
with	O
SIP	B-Protocol
trunks	I-Protocol
to	O
provide	O
call	O
control	O
and	O
make	O
routing/policy	O
decisions	O
on	O
how	O
calls	O
are	O
routed	O
through	O
the	O
LAN/WAN	O
.	O
</s>
<s>
Additionally	O
,	O
some	O
SBCs	O
can	O
allow	O
VoIP	B-Application
calls	I-Application
to	O
be	O
set	O
up	O
between	O
two	O
phones	O
using	O
different	O
VoIP	B-Application
signaling	O
protocols	O
(	O
e.g.	O
,	O
SIP	B-Application
,	O
H.323	B-Protocol
,	O
Megaco/MGCP	O
)	O
as	O
well	O
as	O
performing	O
transcoding	O
of	O
the	O
media	O
stream	O
when	O
different	O
codecs	B-General_Concept
are	O
in	O
use	O
.	O
</s>
<s>
Most	O
SBCs	O
also	O
provide	O
firewall	B-Application
features	O
for	O
VoIP	B-Application
traffic	O
(	O
denial	O
of	O
service	O
protection	O
,	O
call	O
filtering	O
,	O
bandwidth	O
management	O
)	O
.	O
</s>
<s>
From	O
an	O
IP	B-Protocol
Multimedia	I-Protocol
Subsystem	I-Protocol
(	O
IMS	O
)	O
or	O
3GPP	O
(	O
3rd	O
Generation	O
Partnership	O
Project	O
)	O
architecture	O
perspective	O
,	O
the	O
SBC	O
is	O
the	O
integration	O
of	O
the	O
P-CSCF	B-Protocol
and	O
IMS-ALG	O
at	O
the	O
signaling	O
plane	O
and	O
the	O
IMS	O
Access	O
Gateway	O
at	O
the	O
media	O
plane	O
on	O
the	O
access	O
side	O
.	O
</s>
<s>
From	O
an	O
IMS/TISPAN	O
architecture	O
perspective	O
,	O
the	O
SBC	O
is	O
the	O
integration	O
of	O
the	O
P-CSCF	B-Protocol
and	O
C-BGF	O
functions	O
on	O
the	O
access	O
side	O
,	O
and	O
the	O
IBCF	O
,	O
IWF	O
,	O
THIG	O
,	O
and	O
I-BGF	O
functions	O
on	O
the	O
peering	O
side	O
.	O
</s>
<s>
Some	O
SBCs	O
can	O
be	O
"	O
decomposed	O
"	O
,	O
meaning	O
the	O
signaling	O
functions	O
can	O
be	O
located	O
on	O
a	O
separate	O
hardware	O
platform	O
than	O
the	O
media	O
relay	O
functions	O
–	O
in	O
other	O
words	O
the	O
P-CSCF	B-Protocol
can	O
be	O
separated	O
from	O
the	O
C-BGF	O
,	O
or	O
the	O
IBCF/IWF	O
can	O
be	O
separated	O
from	O
the	O
I-BGF	O
functions	O
physically	O
.	O
</s>
<s>
During	O
its	O
infancy	O
,	O
the	O
concept	O
of	O
SBC	O
was	O
controversial	O
to	O
proponents	O
of	O
end-to-end	B-Application
systems	O
and	O
peer-to-peer	O
networking	O
because	O
:	O
</s>
<s>
However	O
,	O
many	O
times	O
there	O
are	O
obstacles	O
to	O
communication	O
such	O
as	O
firewalls	B-Application
between	O
the	O
call	O
parties	O
,	O
and	O
in	O
these	O
cases	O
SBCs	O
offer	O
an	O
efficient	O
method	O
to	O
guide	O
media	O
streams	O
towards	O
an	O
acceptable	O
path	O
between	O
caller	O
and	O
callee	O
;	O
without	O
the	O
SBC	O
the	O
call	O
media	O
would	O
be	O
blocked	O
.	O
</s>
<s>
Some	O
SBCs	O
can	O
detect	O
if	O
the	O
ends	O
of	O
the	O
call	O
are	O
in	O
the	O
same	O
subnetwork	B-Protocol
and	O
release	O
control	O
of	O
the	O
media	O
enabling	O
it	O
to	O
flow	O
directly	O
between	O
the	O
clients	O
,	O
this	O
is	O
anti-tromboning	B-Protocol
or	O
media	O
release	O
.	O
</s>
<s>
Also	O
,	O
some	O
SBCs	O
can	O
create	O
a	O
media	O
path	O
where	O
none	O
would	O
otherwise	O
be	O
allowed	O
to	O
exist	O
(	O
by	O
virtue	O
of	O
various	O
firewalls	B-Application
and	O
other	O
security	O
apparatus	O
between	O
the	O
two	O
endpoints	O
)	O
.	O
</s>
<s>
Lastly	O
,	O
for	O
specific	O
VoIP	B-Application
network	O
models	O
where	O
the	O
service	O
provider	O
owns	O
the	O
network	O
,	O
SBCs	O
can	O
actually	O
decrease	O
the	O
media	O
path	O
by	O
shortcut	O
routing	O
approaches	O
.	O
</s>
<s>
SBCs	O
can	O
restrict	O
the	O
flow	O
of	O
information	O
between	O
call	O
endpoints	O
,	O
potentially	O
reducing	O
end-to-end	B-Application
transparency	O
.	O
</s>
<s>
VoIP	B-Application
phones	O
may	O
not	O
be	O
able	O
to	O
use	O
new	O
protocol	O
features	O
unless	O
they	O
are	O
understood	O
by	O
the	O
SBC	O
.	O
</s>
<s>
Sometimes	O
end-to-end	B-Application
encryption	O
ca	O
n't	O
be	O
used	O
if	O
the	O
SBC	O
does	O
not	O
have	O
the	O
key	O
,	O
although	O
some	O
portions	O
of	O
the	O
information	O
stream	O
in	O
an	O
encrypted	O
call	O
are	O
not	O
encrypted	O
,	O
and	O
those	O
portions	O
can	O
be	O
used	O
and	O
influenced	O
by	O
the	O
SBC	O
.	O
</s>
<s>
However	O
,	O
the	O
new	O
generations	O
of	O
SBCs	O
,	O
armed	O
with	O
sufficient	O
computing	O
capacity	O
,	O
are	O
able	O
to	O
offload	O
this	O
encryption	O
function	O
from	O
other	O
elements	O
in	O
the	O
network	O
by	O
terminating	O
SIP-TLS	O
,	O
IPsec	B-Protocol
,	O
and/or	O
SRTP	B-Protocol
.	O
</s>
<s>
Furthermore	O
,	O
SBCs	O
can	O
actually	O
make	O
calls	O
and	O
other	O
SIP	B-Application
scenarios	O
work	O
when	O
they	O
could	O
n't	O
have	O
before	O
,	O
by	O
performing	O
specific	O
protocol	O
"	O
normalization	O
"	O
or	O
"	O
fix-up	O
"	O
.	O
</s>
<s>
In	O
most	O
cases	O
,	O
far-end	O
or	O
hosted	O
NAT	B-Protocol
traversal	I-Protocol
can	O
be	O
done	O
without	O
SBCs	O
if	O
the	O
VoIP	B-Application
phones	O
support	O
protocols	O
like	O
STUN	B-Protocol
,	O
TURN	B-Protocol
,	O
ICE	O
,	O
or	O
Universal	B-Application
Plug	I-Application
and	I-Application
Play	I-Application
(	O
UPnP	B-Application
)	O
.	O
</s>
<s>
For	O
example	O
,	O
should	O
call	O
control	O
remain	O
with	O
Alice	O
and	O
Bob	O
(	O
two	O
callers	O
)	O
,	O
or	O
should	O
call	O
control	O
be	O
shared	O
with	O
the	O
operators	O
of	O
all	O
the	O
IP	O
networks	O
involved	O
in	O
connecting	O
Alice	O
and	O
Bob	O
's	O
VoIP	B-Application
phones	O
together	O
.	O
</s>
<s>
Those	O
who	O
wanted	O
unfettered	O
control	O
in	O
the	O
endpoints	O
only	O
,	O
were	O
also	O
greatly	O
frustrated	O
by	O
the	O
various	O
realities	O
of	O
modern	O
networks	O
,	O
such	O
as	O
firewalls	B-Application
and	O
filtering/throttling	O
.	O
</s>
<s>
An	O
SBC	O
may	O
provide	O
session	O
media	O
(	O
usually	O
RTP	B-Protocol
)	O
and	O
signaling	O
(	O
often	O
SIP	B-Application
)	O
wiretap	O
services	O
,	O
which	O
can	O
be	O
used	O
by	O
providers	O
to	O
enforce	O
requests	O
for	O
the	O
lawful	O
interception	O
of	O
network	O
sessions	O
.	O
</s>
<s>
According	O
to	O
Jonathan	O
Rosenberg	O
,	O
the	O
author	O
of	O
RFC	O
3261	O
(	O
SIP	B-Application
)	O
and	O
numerous	O
other	O
related	O
RFCs	O
,	O
Dynamicsoft	O
developed	O
the	O
first	O
working	O
SBC	O
in	O
conjunction	O
with	O
Aravox	O
,	O
but	O
the	O
product	O
never	O
truly	O
gained	O
marketshare	O
.	O
</s>
<s>
In	O
2009	O
,	O
Ingate	O
Systems’	O
Firewall	B-Application
became	O
the	O
first	O
SBC	O
to	O
earn	O
certification	O
from	O
ICSA	O
Labs	O
,	O
a	O
milestone	O
in	O
certifying	O
the	O
VoIP	B-Application
security	O
capabilities	O
of	O
an	O
SBC	O
.	O
</s>
<s>
The	O
continuing	O
growth	O
of	O
VoIP	B-Application
networks	O
pushes	O
SBCs	O
further	O
to	O
the	O
edge	O
,	O
mandating	O
adaptation	O
in	O
capacity	O
and	O
complexity	O
.	O
</s>
<s>
As	O
the	O
VoIP	B-Application
network	O
grows	O
and	O
traffic	O
volume	O
increases	O
,	O
more	O
and	O
more	O
sessions	O
are	O
passing	O
through	O
SBC	O
.	O
</s>
