<s>
The	O
SIP	B-Protocol
URI	I-Protocol
scheme	I-Protocol
is	O
a	O
Uniform	B-Protocol
Resource	I-Protocol
Identifier	I-Protocol
(	O
URI	B-Protocol
)	O
scheme	O
for	O
the	O
Session	B-Application
Initiation	I-Application
Protocol	I-Application
(	O
SIP	B-Application
)	O
multimedia	O
communications	O
protocol	O
.	O
</s>
<s>
A	O
SIP	B-Protocol
address	I-Protocol
is	O
a	O
URI	B-Protocol
that	O
addresses	O
a	O
specific	O
telephone	B-Application
extension	I-Application
on	O
a	O
voice	B-Application
over	I-Application
IP	I-Application
system	O
.	O
</s>
<s>
Such	O
a	O
number	O
could	O
be	O
a	O
private	B-Device
branch	I-Device
exchange	I-Device
or	O
an	O
E.164	O
telephone	O
number	O
dialled	O
through	O
a	O
specific	O
gateway	O
.	O
</s>
<s>
A	O
SIP	B-Protocol
address	I-Protocol
is	O
written	O
in	O
user@domain.tld	O
format	O
in	O
a	O
similar	O
fashion	O
to	O
an	O
email	O
address	O
.	O
</s>
<s>
instructs	O
a	O
SIP	B-Application
client	O
to	O
use	O
the	O
NAPTR	B-Protocol
and	O
SRV	B-Protocol
schemes	O
to	O
look	O
up	O
the	O
SIP	B-Application
server	O
associated	O
with	O
the	O
DNS	B-Application
name	O
voip-provider.example.net	O
and	O
connect	O
to	O
that	O
server	O
.	O
</s>
<s>
If	O
those	O
records	O
are	O
not	O
found	O
,	O
but	O
the	O
name	O
is	O
associated	O
with	O
an	O
IP	O
address	O
,	O
the	O
client	O
will	O
directly	O
contact	O
the	O
SIP	B-Application
server	O
at	O
that	O
IP	O
address	O
on	O
port	B-Application
5060	I-Application
,	O
by	O
default	O
using	O
the	O
UDP	B-Protocol
transport	O
protocol	O
.	O
</s>
<s>
The	O
gateway	O
may	O
require	O
the	O
user	O
REGISTER	O
using	O
SIP	B-Application
before	O
placing	O
this	O
call	O
.	O
</s>
<s>
If	O
a	O
destination	O
port	O
is	O
provided	O
as	O
part	O
of	O
the	O
SIP	B-Application
URI	B-Protocol
,	O
the	O
NAPTR/SRV	O
lookups	O
are	O
not	O
used	O
;	O
rather	O
,	O
the	O
client	O
directly	O
connects	O
to	O
the	O
specified	O
host	O
and	O
port	O
.	O
</s>
<s>
As	O
a	O
SIP	B-Protocol
address	I-Protocol
is	O
text	O
,	O
much	O
like	O
an	O
e-mail	O
address	O
,	O
it	O
may	O
contain	O
non-numeric	O
characters	O
.	O
</s>
<s>
As	O
the	O
client	O
may	O
be	O
a	O
SIP	B-Device
phone	I-Device
or	O
other	O
device	O
with	O
just	O
a	O
numeric	O
,	O
telephone-like	O
keypad	O
,	O
various	O
schemes	O
exist	O
to	O
associate	O
an	O
entirely	O
numeric	O
identifier	O
to	O
a	O
publicly	O
reachable	O
SIP	B-Protocol
address	I-Protocol
.	O
</s>
<s>
These	O
include	O
the	O
iNum	B-Protocol
Initiative	I-Protocol
(	O
which	O
issues	O
E.164-formatted	O
numbers	O
,	O
where	O
the	O
corresponding	O
SIP	B-Protocol
address	I-Protocol
is	O
the	O
number	O
'	O
@sip	O
.inum.net	O
'	O
)	O
,	O
SIP	B-Application
Broker-style	O
services	O
(	O
which	O
associate	O
a	O
numeric	O
*	O
prefix	O
to	O
the	O
SIP	B-Application
domain	O
name	O
)	O
and	O
the	O
e164.org	O
and	O
e164.arpa	O
domain	O
name	O
servers	O
(	O
which	O
convert	O
numbers	O
to	O
addresses	O
one-by-one	O
as	O
DNS	B-Application
reverse-lookups	O
)	O
.	O
</s>
<s>
SIP	B-Application
addresses	O
may	O
be	O
used	O
directly	O
in	O
configuration	O
files	O
(	O
for	O
instance	O
,	O
in	O
Asterisk	B-Language
(	O
PBX	O
)	O
installations	O
)	O
or	O
specified	O
through	O
the	O
web	O
interface	O
of	O
a	O
voice-over-IP	B-Application
gateway	O
provider	O
(	O
usually	O
as	O
a	O
call	O
forwarding	O
destination	O
or	O
an	O
address	O
book	O
entry	O
)	O
.	O
</s>
<s>
Systems	O
which	O
allow	O
speed	O
dial	O
from	O
a	O
user	O
's	O
address	O
book	O
using	O
a	O
vertical	O
service	O
code	O
may	O
allow	O
a	O
short	O
numeric	O
code	O
(	O
like	O
*	O
75xx	O
)	O
to	O
be	O
translated	O
to	O
a	O
pre-stored	O
alphanumeric	O
SIP	B-Protocol
address	I-Protocol
.	O
</s>
<s>
In	O
theory	O
,	O
the	O
owner	O
of	O
a	O
SIP-capable	O
telephone	O
handset	O
could	O
publish	O
a	O
SIP	B-Protocol
address	I-Protocol
from	O
which	O
they	O
could	O
be	O
freely	O
and	O
directly	O
reached	O
worldwide	O
,	O
in	O
much	O
the	O
same	O
way	O
that	O
SMTP	B-Protocol
e-mail	O
recipients	O
may	O
be	O
contacted	O
from	O
anywhere	O
at	O
almost	O
no	O
cost	O
to	O
the	O
message	O
sender	O
.	O
</s>
<s>
Anyone	O
with	O
a	O
broadband	O
connection	O
could	O
install	O
a	O
softphone	B-Protocol
(	O
such	O
as	O
Ekiga	B-Language
)	O
and	O
call	O
any	O
of	O
these	O
SIP	B-Application
addresses	O
for	O
free	O
.	O
</s>
<s>
In	O
practice	O
,	O
various	O
forms	O
of	O
network	O
abuse	O
are	O
discouraging	O
creation	O
and	O
publication	O
of	O
openly	O
reachable	O
SIP	B-Application
addresses	O
:	O
</s>
<s>
The	O
spam	O
which	O
has	O
rendered	O
SMTP	B-Protocol
the	O
"	O
spam	O
mail	O
transport	O
protocol	O
"	O
could	O
potentially	O
make	O
published	O
sip	B-Application
:	O
numbers	O
unusable	O
as	O
the	O
numbers	O
are	O
flooded	O
with	O
VoIP	B-Protocol
spam	I-Protocol
,	O
usually	O
automatic	O
announcement	O
devices	O
delivering	O
pre-recorded	O
advertisements	O
.	O
</s>
<s>
Unlike	O
mailto	O
:	O
,	O
sip	B-Application
:	O
establishes	O
a	O
voice	O
call	O
which	O
interrupts	O
the	O
human	O
recipient	O
in	O
real	O
time	O
with	O
a	O
ringing	O
telephone	O
.	O
</s>
<s>
SIP	B-Application
is	O
vulnerable	O
to	O
Caller	O
ID	O
spoofing	O
as	O
the	O
displayed	O
name	O
and	O
number	O
,	O
much	O
like	O
the	O
return	O
address	O
on	O
e-mail	O
,	O
is	O
supplied	O
by	O
the	O
sender	O
and	O
not	O
authenticated	O
.	O
</s>
<s>
Servers	O
supporting	O
inbound	O
sip	B-Application
:	O
connections	O
are	O
also	O
targeted	O
with	O
unsolicited	O
attempts	O
to	O
reach	O
outside	O
numbers	O
,	O
usually	O
premium-rate	O
destinations	O
such	O
as	O
caller-pays-airtime	O
mobile	O
exchanges	O
in	O
foreign	O
countries	O
.	O
</s>
<s>
Security	O
tools	O
such	O
as	O
firewalls	B-Application
or	O
fail2ban	B-Protocol
must	O
therefore	O
be	O
deployed	O
to	O
prevent	O
unauthorised	O
outside	O
call	O
attempts	O
;	O
many	O
VoIP	B-Application
providers	O
also	O
disable	O
overseas	O
calls	O
to	O
all	O
but	O
countries	O
specifically	O
requested	O
as	O
enabled	O
by	O
the	O
subscriber	O
.	O
</s>
<s>
The	O
SIPS	B-Protocol
URI	I-Protocol
scheme	I-Protocol
adheres	O
to	O
the	O
syntax	O
of	O
the	O
SIP	B-Application
URI	B-Protocol
,	O
differing	O
only	O
in	O
that	O
the	O
scheme	O
is	O
sips	O
rather	O
than	O
sip	B-Application
.	O
</s>
<s>
The	O
default	O
Internet	O
port	O
address	O
for	O
SIPS	O
is	O
5061	O
unless	O
explicitly	O
specified	O
in	O
the	O
URI	B-Protocol
.	O
</s>
<s>
It	O
mandates	O
that	O
each	O
hop	O
over	O
which	O
the	O
request	O
is	O
forwarded	O
up	O
to	O
the	O
target	O
domain	O
must	O
be	O
secured	O
with	O
TLS	B-Protocol
.	O
</s>
