<s>
The	O
Session	B-Application
Initiation	I-Application
Protocol	I-Application
(	O
SIP	O
)	O
is	O
a	O
signaling	O
protocol	O
used	O
for	O
initiating	O
,	O
maintaining	O
,	O
and	O
terminating	O
communication	B-Protocol
sessions	I-Protocol
that	O
include	O
voice	O
,	O
video	O
and	O
messaging	O
applications	O
.	O
</s>
<s>
SIP	O
is	O
used	O
in	O
Internet	B-Application
telephony	I-Application
,	O
in	O
private	O
IP	B-Application
telephone	I-Application
systems	O
,	O
as	O
well	O
as	O
mobile	O
phone	O
calling	O
over	O
LTE	O
(	O
VoLTE	O
)	O
.	O
</s>
<s>
SIP	O
is	O
a	O
text-based	O
protocol	O
,	O
incorporating	O
many	O
elements	O
of	O
the	O
Hypertext	B-Protocol
Transfer	I-Protocol
Protocol	I-Protocol
(	O
HTTP	B-Protocol
)	O
and	O
the	O
Simple	B-Protocol
Mail	I-Protocol
Transfer	I-Protocol
Protocol	I-Protocol
(	O
SMTP	B-Protocol
)	O
.	O
</s>
<s>
Most	O
commonly	O
,	O
media	O
type	O
and	O
parameter	O
negotiation	O
and	O
media	O
setup	O
are	O
performed	O
with	O
the	O
Session	B-Language
Description	I-Language
Protocol	I-Language
(	O
SDP	O
)	O
,	O
which	O
is	O
carried	O
as	O
payload	O
in	O
SIP	O
messages	O
.	O
</s>
<s>
SIP	O
is	O
designed	O
to	O
be	O
independent	O
of	O
the	O
underlying	O
transport	B-Architecture
layer	I-Architecture
protocol	O
and	O
can	O
be	O
used	O
with	O
the	O
User	B-Protocol
Datagram	I-Protocol
Protocol	I-Protocol
(	O
UDP	O
)	O
,	O
the	O
Transmission	B-Protocol
Control	I-Protocol
Protocol	I-Protocol
(	O
TCP	O
)	O
,	O
and	O
the	O
Stream	B-Protocol
Control	I-Protocol
Transmission	I-Protocol
Protocol	I-Protocol
(	O
SCTP	B-Protocol
)	O
.	O
</s>
<s>
For	O
secure	B-General_Concept
transmissions	I-General_Concept
of	O
SIP	O
messages	O
over	O
insecure	O
network	O
links	O
,	O
the	O
protocol	O
may	O
be	O
encrypted	O
with	O
Transport	B-Protocol
Layer	I-Protocol
Security	I-Protocol
(	O
TLS	O
)	O
.	O
</s>
<s>
For	O
the	O
transmission	O
of	O
media	O
streams	O
(	O
voice	O
,	O
video	O
)	O
the	O
SDP	O
payload	O
carried	O
in	O
SIP	O
messages	O
typically	O
employs	O
the	O
Real-time	B-Protocol
Transport	I-Protocol
Protocol	I-Protocol
(	O
RTP	O
)	O
or	O
the	O
Secure	B-Protocol
Real-time	I-Protocol
Transport	I-Protocol
Protocol	I-Protocol
(	O
SRTP	O
)	O
.	O
</s>
<s>
SIP	O
was	O
originally	O
designed	O
by	O
Mark	O
Handley	O
,	O
Henning	O
Schulzrinne	O
,	O
Eve	O
Schooler	O
and	O
Jonathan	O
Rosenberg	O
in	O
1996	O
to	O
facilitate	O
establishing	O
multicast	O
multimedia	O
sessions	O
on	O
the	O
Mbone	B-Protocol
.	O
</s>
<s>
In	O
November	O
2000	O
,	O
SIP	O
was	O
accepted	O
as	O
a	O
3GPP	O
signaling	O
protocol	O
and	O
permanent	O
element	O
of	O
the	O
IP	B-Protocol
Multimedia	I-Protocol
Subsystem	I-Protocol
(	O
IMS	B-Application
)	O
architecture	O
for	O
IP-based	O
streaming	O
multimedia	O
services	O
in	O
cellular	O
networks	O
.	O
</s>
<s>
It	O
has	O
been	O
extended	O
for	O
video	B-Application
conferencing	I-Application
,	O
streaming	O
media	O
distribution	O
,	O
instant	B-Application
messaging	I-Application
,	O
presence	B-Protocol
information	I-Protocol
,	O
file	B-Protocol
transfer	I-Protocol
,	O
Internet	O
fax	O
and	O
online	B-Application
games	I-Application
.	O
</s>
<s>
SIP	O
has	O
been	O
standardized	O
primarily	O
by	O
the	O
Internet	O
Engineering	O
Task	O
Force	O
(	O
IETF	O
)	O
,	O
while	O
other	O
protocols	O
,	O
such	O
as	O
H.323	B-Protocol
,	O
have	O
traditionally	O
been	O
associated	O
with	O
the	O
International	O
Telecommunication	O
Union	O
(	O
ITU	O
)	O
.	O
</s>
<s>
SIP	O
is	O
only	O
involved	O
in	O
the	O
signaling	O
operations	O
of	O
a	O
media	O
communication	B-Protocol
session	I-Protocol
and	O
is	O
primarily	O
used	O
to	O
set	O
up	O
and	O
terminate	O
voice	O
or	O
video	B-Application
calls	I-Application
.	O
</s>
<s>
SIP	O
can	O
be	O
used	O
to	O
establish	O
two-party	O
(	O
unicast	B-Protocol
)	O
or	O
multiparty	O
(	O
multicast	O
)	O
sessions	O
.	O
</s>
<s>
The	O
modification	O
can	O
involve	O
changing	O
addresses	O
or	O
ports	B-Protocol
,	O
inviting	O
more	O
participants	O
,	O
and	O
adding	O
or	O
deleting	O
media	O
streams	O
.	O
</s>
<s>
SIP	O
has	O
also	O
found	O
applications	O
in	O
messaging	O
applications	O
,	O
such	O
as	O
instant	B-Application
messaging	I-Application
,	O
and	O
event	O
subscription	O
and	O
notification	O
.	O
</s>
<s>
For	O
call	O
setup	O
,	O
the	O
body	O
of	O
a	O
SIP	O
message	O
contains	O
a	O
Session	B-Language
Description	I-Language
Protocol	I-Language
(	O
SDP	O
)	O
data	O
unit	O
,	O
which	O
specifies	O
the	O
media	O
format	O
,	O
codec	O
and	O
media	O
communication	O
protocol	O
.	O
</s>
<s>
Voice	O
and	O
video	O
media	O
streams	O
are	O
typically	O
carried	O
between	O
the	O
terminals	O
using	O
the	O
Real-time	B-Protocol
Transport	I-Protocol
Protocol	I-Protocol
(	O
RTP	O
)	O
or	O
Secure	B-Protocol
Real-time	I-Protocol
Transport	I-Protocol
Protocol	I-Protocol
(	O
SRTP	O
)	O
.	O
</s>
<s>
Every	O
resource	O
of	O
a	O
SIP	O
network	O
,	O
such	O
as	O
user	B-Protocol
agents	I-Protocol
,	O
call	O
routers	O
,	O
and	O
voicemail	O
boxes	O
,	O
are	O
identified	O
by	O
a	O
Uniform	B-Protocol
Resource	I-Protocol
Identifier	I-Protocol
(	O
URI	B-Protocol
)	O
.	O
</s>
<s>
The	O
syntax	O
of	O
the	O
URI	B-Protocol
follows	O
the	O
general	O
standard	O
syntax	O
also	O
used	O
in	O
Web	O
services	O
and	O
e-mail	O
.	O
</s>
<s>
The	O
URI	B-Protocol
scheme	O
used	O
for	O
SIP	O
is	O
sip	O
and	O
a	O
typical	O
SIP	O
URI	B-Protocol
has	O
the	O
form	O
sip:username	O
@domainname	O
or	O
sip:username	O
@hostport	O
,	O
where	O
domainname	O
requires	O
DNS	B-Protocol
SRV	I-Protocol
records	O
to	O
locate	O
the	O
servers	O
for	O
SIP	O
domain	O
while	O
hostport	O
can	O
be	O
an	O
IP	B-Protocol
address	I-Protocol
or	O
a	O
fully	B-Protocol
qualified	I-Protocol
domain	I-Protocol
name	I-Protocol
of	O
the	O
host	O
and	O
port	O
.	O
</s>
<s>
If	O
secure	B-General_Concept
transmission	I-General_Concept
is	O
required	O
,	O
the	O
scheme	O
sips	O
is	O
used	O
.	O
</s>
<s>
SIP	O
employs	O
design	O
elements	O
similar	O
to	O
the	O
HTTP	B-Protocol
request	O
and	O
response	O
transaction	O
model	O
.	O
</s>
<s>
SIP	O
reuses	O
most	O
of	O
the	O
header	O
fields	O
,	O
encoding	O
rules	O
and	O
status	O
codes	O
of	O
HTTP	B-Protocol
,	O
providing	O
a	O
readable	O
text-based	O
format	O
.	O
</s>
<s>
SIP	O
can	O
be	O
carried	O
by	O
several	O
transport	B-Architecture
layer	I-Architecture
protocols	O
including	O
Transmission	B-Protocol
Control	I-Protocol
Protocol	I-Protocol
(	O
TCP	O
)	O
,	O
User	B-Protocol
Datagram	I-Protocol
Protocol	I-Protocol
(	O
UDP	O
)	O
,	O
and	O
Stream	B-Protocol
Control	I-Protocol
Transmission	I-Protocol
Protocol	I-Protocol
(	O
SCTP	B-Protocol
)	O
.	O
</s>
<s>
SIP	O
clients	O
typically	O
use	O
TCP	O
or	O
UDP	O
on	O
port	B-Protocol
numbers	I-Protocol
5060	B-Application
or	O
5061	O
for	O
SIP	O
traffic	O
to	O
servers	O
and	O
other	O
endpoints	O
.	O
</s>
<s>
Port	B-Application
5060	I-Application
is	O
commonly	O
used	O
for	O
non-encrypted	O
signaling	O
traffic	O
whereas	O
port	O
5061	O
is	O
typically	O
used	O
for	O
traffic	O
encrypted	O
with	O
Transport	B-Protocol
Layer	I-Protocol
Security	I-Protocol
(	O
TLS	O
)	O
.	O
</s>
<s>
SIP	O
is	O
a	O
client-server	B-Operating_System
protocol	I-Operating_System
of	O
equipotent	O
peers	O
.	O
</s>
<s>
The	O
network	O
elements	O
that	O
use	O
the	O
Session	B-Application
Initiation	I-Application
Protocol	I-Application
for	O
communication	O
are	O
called	O
SIP	O
user	B-Protocol
agents	I-Protocol
.	O
</s>
<s>
Each	O
user	B-Protocol
agent	I-Protocol
(	O
UA	O
)	O
performs	O
the	O
function	O
of	O
a	O
user	B-Protocol
agent	I-Protocol
client	O
(	O
UAC	O
)	O
when	O
it	O
is	O
requesting	O
a	O
service	O
function	O
,	O
and	O
that	O
of	O
a	O
user	B-Application
agent	I-Application
server	I-Application
(	O
UAS	O
)	O
when	O
responding	O
to	O
a	O
request	O
.	O
</s>
<s>
Each	O
of	O
these	O
service	O
elements	O
also	O
communicates	O
within	O
the	O
client-server	B-Operating_System
model	I-Operating_System
implemented	O
in	O
user	B-Protocol
agent	I-Protocol
clients	O
and	O
servers	O
.	O
</s>
<s>
A	O
user	B-Protocol
agent	I-Protocol
is	O
a	O
logical	O
network	O
endpoint	O
that	O
sends	O
or	O
receives	O
SIP	O
messages	O
and	O
manages	O
SIP	O
sessions	O
.	O
</s>
<s>
User	B-Protocol
agents	I-Protocol
have	O
client	O
and	O
server	O
components	O
.	O
</s>
<s>
The	O
user	B-Protocol
agent	I-Protocol
client	O
(	O
UAC	O
)	O
sends	O
SIP	O
requests	O
.	O
</s>
<s>
The	O
user	B-Application
agent	I-Application
server	I-Application
(	O
UAS	O
)	O
receives	O
requests	O
and	O
returns	O
a	O
SIP	O
response	O
.	O
</s>
<s>
Unlike	O
other	O
network	O
protocols	O
that	O
fix	O
the	O
roles	O
of	O
client	O
and	O
server	O
,	O
e.g.	O
,	O
in	O
HTTP	B-Protocol
,	O
in	O
which	O
a	O
web	O
browser	O
only	O
acts	O
as	O
a	O
client	O
,	O
and	O
never	O
as	O
a	O
server	O
,	O
SIP	O
requires	O
both	O
peers	O
to	O
implement	O
both	O
roles	O
.	O
</s>
<s>
A	O
SIP	B-Device
phone	I-Device
is	O
an	O
IP	B-Device
phone	I-Device
that	O
implements	O
client	O
and	O
server	O
functions	O
of	O
a	O
SIP	O
user	B-Protocol
agent	I-Protocol
and	O
provides	O
the	O
traditional	O
call	O
functions	O
of	O
a	O
telephone	O
,	O
such	O
as	O
dial	O
,	O
answer	O
,	O
reject	O
,	O
call	O
hold	O
,	O
and	O
call	O
transfer	O
.	O
</s>
<s>
SIP	B-Device
phones	I-Device
may	O
be	O
implemented	O
as	O
a	O
hardware	O
device	O
or	O
as	O
a	O
softphone	B-Protocol
.	O
</s>
<s>
As	O
vendors	O
increasingly	O
implement	O
SIP	O
as	O
a	O
standard	O
telephony	O
platform	O
,	O
the	O
distinction	O
between	O
hardware-based	O
and	O
software-based	O
SIP	B-Device
phones	I-Device
is	O
blurred	O
and	O
SIP	O
elements	O
are	O
implemented	O
in	O
the	O
basic	O
firmware	O
functions	O
of	O
many	O
IP-capable	O
communications	O
devices	O
such	O
as	O
smartphones	B-Application
.	O
</s>
<s>
In	O
SIP	O
,	O
as	O
in	O
HTTP	B-Protocol
,	O
the	O
user	B-Protocol
agent	I-Protocol
may	O
identify	O
itself	O
using	O
a	O
message	O
header	O
field	O
(	O
User-Agent	B-Protocol
)	O
,	O
containing	O
a	O
text	O
description	O
of	O
the	O
software	O
,	O
hardware	O
,	O
or	O
the	O
product	O
name	O
.	O
</s>
<s>
The	O
user	B-Protocol
agent	I-Protocol
field	O
is	O
sent	O
in	O
request	O
messages	O
,	O
which	O
means	O
that	O
the	O
receiving	O
SIP	O
server	O
can	O
evaluate	O
this	O
information	O
to	O
perform	O
device-specific	O
configuration	O
or	O
feature	O
activation	O
.	O
</s>
<s>
SIP	O
proxy	O
servers	O
that	O
route	O
messages	O
to	O
more	O
than	O
one	O
destination	O
are	O
called	O
forking	B-Application
proxies	I-Application
.	O
</s>
<s>
A	O
redirect	O
server	O
is	O
a	O
user	B-Application
agent	I-Application
server	I-Application
that	O
generates	O
3xx	O
(	O
redirection	O
)	O
responses	O
to	O
requests	O
it	O
receives	O
,	O
directing	O
the	O
client	O
to	O
contact	O
an	O
alternate	O
set	O
of	O
URIs	B-Protocol
.	O
</s>
<s>
It	O
accepts	O
REGISTER	O
requests	O
,	O
recording	O
the	O
address	O
and	O
other	O
parameters	O
from	O
the	O
user	B-Protocol
agent	I-Protocol
.	O
</s>
<s>
The	O
location	O
service	O
links	O
one	O
or	O
more	O
IP	B-Protocol
addresses	I-Protocol
to	O
the	O
SIP	O
URI	B-Protocol
of	O
the	O
registering	O
agent	O
.	O
</s>
<s>
Multiple	O
user	B-Protocol
agents	I-Protocol
may	O
register	O
for	O
the	O
same	O
URI	B-Protocol
,	O
with	O
the	O
result	O
that	O
all	O
registered	O
user	B-Protocol
agents	I-Protocol
receive	O
the	O
calls	O
to	O
the	O
URI	B-Protocol
.	O
</s>
<s>
Session	B-Protocol
border	I-Protocol
controllers	I-Protocol
(	O
SBCs	O
)	O
serve	O
as	O
middleboxes	O
between	O
user	B-Protocol
agents	I-Protocol
and	O
SIP	O
servers	O
for	O
various	O
types	O
of	O
functions	O
,	O
including	O
network	O
topology	O
hiding	O
and	O
assistance	O
in	O
NAT	B-Protocol
traversal	I-Protocol
.	O
</s>
<s>
Gateways	B-Application
can	O
be	O
used	O
to	O
interconnect	O
a	O
SIP	O
network	O
to	O
other	O
networks	O
,	O
such	O
as	O
the	O
PSTN	O
,	O
which	O
use	O
different	O
protocols	O
or	O
technologies	O
.	O
</s>
<s>
SIP	O
is	O
a	O
text-based	O
protocol	O
with	O
syntax	O
similar	O
to	O
that	O
of	O
HTTP	B-Protocol
.	I-Protocol
</s>
<s>
The	O
first	O
line	O
of	O
a	O
request	O
has	O
a	O
method	O
,	O
defining	O
the	O
nature	O
of	O
the	O
request	O
,	O
and	O
a	O
Request-URI	O
,	O
indicating	O
where	O
the	O
request	O
should	O
be	O
sent	O
.	O
</s>
<s>
They	O
are	O
sent	O
by	O
a	O
user	B-Protocol
agent	I-Protocol
client	O
to	O
the	O
server	O
and	O
are	O
answered	O
with	O
one	O
or	O
more	O
SIP	B-Protocol
responses	I-Protocol
,	O
which	O
return	O
a	O
result	O
code	O
of	O
the	O
transaction	O
,	O
and	O
generally	O
indicate	O
the	O
success	O
,	O
failure	O
,	O
or	O
other	O
state	O
of	O
the	O
transaction	O
.	O
</s>
<s>
+	O
SIP	O
requests	O
Request	O
name	O
Description	O
Notes	O
RFC	O
references	O
REGISTER	O
Register	O
the	O
URI	B-Protocol
listed	O
in	O
the	O
To-header	O
field	O
with	O
a	O
location	O
server	O
and	O
associates	O
it	O
with	O
the	O
network	O
address	O
given	O
in	O
a	O
Contact	O
header	O
field	O
.	O
</s>
<s>
The	O
request	O
is	O
sent	O
by	O
a	O
user	B-Protocol
agent	I-Protocol
client	O
to	O
a	O
user	B-Application
agent	I-Application
server	I-Application
.	O
</s>
<s>
Used	O
in	O
instant	B-Application
messaging	I-Application
applications	O
.	O
</s>
<s>
It	O
is	O
often	O
used	O
for	O
NAT	O
keepalive	B-Protocol
purposes	O
.	O
</s>
<s>
Responses	O
are	O
sent	O
by	O
the	O
user	B-Application
agent	I-Application
server	I-Application
indicating	O
the	O
result	O
of	O
a	O
received	O
request	O
.	O
</s>
<s>
The	O
Session	B-Application
Initiation	I-Application
Protocol	I-Application
for	O
Instant	B-Application
Messaging	I-Application
and	O
Presence	O
Leveraging	O
Extensions	O
(	O
SIMPLE	O
)	O
is	O
the	O
SIP-based	O
suite	O
of	O
standards	O
for	O
instant	B-Application
messaging	I-Application
and	O
presence	B-Protocol
information	I-Protocol
.	O
</s>
<s>
Message	B-Protocol
Session	I-Protocol
Relay	I-Protocol
Protocol	I-Protocol
(	O
MSRP	B-Protocol
)	O
allows	O
instant	B-Application
message	I-Application
sessions	O
and	O
file	B-Protocol
transfer	I-Protocol
.	O
</s>
<s>
The	O
TTCN-3	B-Protocol
test	O
specification	O
language	O
,	O
developed	O
by	O
a	O
task	O
force	O
at	O
ETSI	O
(	O
STF	O
196	O
)	O
,	O
is	O
used	O
for	O
specifying	O
conformance	O
tests	O
for	O
SIP	O
implementations	O
.	O
</s>
<s>
The	O
software	O
measures	O
performance	O
indicators	O
like	O
answer	O
delay	O
,	O
answer/seizure	O
ratio	O
,	O
RTP	O
jitter	O
and	O
packet	B-Protocol
loss	I-Protocol
,	O
round-trip	O
delay	O
time	O
.	O
</s>
<s>
SIP	B-Application
connection	I-Application
is	O
a	O
marketing	O
term	O
for	O
voice	B-Application
over	I-Application
Internet	I-Application
Protocol	I-Application
(	O
VoIP	B-Application
)	O
services	O
offered	O
by	O
many	O
Internet	B-Protocol
telephony	I-Protocol
service	I-Protocol
providers	I-Protocol
(	O
ITSPs	B-Protocol
)	O
.	O
</s>
<s>
The	O
service	O
provides	O
routing	O
of	O
telephone	O
calls	O
from	O
a	O
client	O
's	O
private	B-Device
branch	I-Device
exchange	I-Device
(	O
PBX	O
)	O
telephone	O
system	O
to	O
the	O
PSTN	O
.	O
</s>
<s>
Such	O
services	O
may	O
simplify	O
corporate	O
information	O
system	O
infrastructure	O
by	O
sharing	O
Internet	O
access	O
for	O
voice	O
and	O
data	O
,	O
and	O
removing	O
the	O
cost	O
for	O
Basic	B-Protocol
Rate	I-Protocol
Interface	I-Protocol
(	O
BRI	O
)	O
or	O
Primary	B-Protocol
Rate	I-Protocol
Interface	I-Protocol
(	O
PRI	O
)	O
telephone	O
circuits	O
.	O
</s>
<s>
SIP	B-Protocol
trunking	I-Protocol
is	O
a	O
similar	O
marketing	O
term	O
preferred	O
for	O
when	O
the	O
service	O
is	O
used	O
to	O
simplify	O
a	O
telecom	O
infrastructure	O
by	O
sharing	O
the	O
carrier	O
access	O
circuit	O
for	O
voice	O
,	O
data	O
,	O
and	O
Internet	O
traffic	O
while	O
removing	O
the	O
need	O
for	O
PRI	O
circuits	O
.	O
</s>
<s>
SIP	O
is	O
used	O
in	O
audio	O
over	O
IP	O
for	O
broadcasting	B-Operating_System
applications	O
where	O
it	O
provides	O
an	O
interoperable	O
means	O
for	O
audio	O
interfaces	O
from	O
different	O
manufacturers	O
to	O
make	O
connections	O
with	O
one	O
another	O
.	O
</s>
<s>
The	O
U.S.	O
National	O
Institute	O
of	O
Standards	O
and	O
Technology	O
(	O
NIST	O
)	O
,	O
Advanced	O
Networking	O
Technologies	O
Division	O
provides	O
a	O
public-domain	O
Java	B-Language
implementation	O
that	O
serves	O
as	O
a	O
reference	O
implementation	O
for	O
the	O
standard	O
.	O
</s>
<s>
The	O
implementation	O
can	O
work	O
in	O
proxy	O
server	O
or	O
user	B-Protocol
agent	I-Protocol
scenarios	O
and	O
has	O
been	O
used	O
in	O
numerous	O
commercial	O
and	O
research	O
projects	O
.	O
</s>
<s>
See	O
List	B-Protocol
of	I-Protocol
SIP	I-Protocol
software	I-Protocol
.	O
</s>
<s>
SIP-I	O
,	O
Session	B-Application
Initiation	I-Application
Protocol	I-Application
with	O
encapsulated	O
ISUP	B-Protocol
,	O
is	O
a	O
protocol	O
used	O
to	O
create	O
,	O
modify	O
,	O
and	O
terminate	O
communication	B-Protocol
sessions	I-Protocol
based	O
on	O
ISUP	B-Protocol
using	O
SIP	O
and	O
IP	O
networks	O
.	O
</s>
<s>
Services	O
using	O
SIP-I	O
include	O
voice	O
,	O
video	B-Application
telephony	I-Application
,	O
fax	O
and	O
data	O
.	O
</s>
<s>
SIP-I	O
and	O
SIP-T	O
are	O
two	O
protocols	O
with	O
similar	O
features	O
,	O
notably	O
to	O
allow	O
ISUP	B-Protocol
messages	O
to	O
be	O
transported	O
over	O
SIP	O
networks	O
.	O
</s>
<s>
This	O
preserves	O
all	O
of	O
the	O
detail	O
available	O
in	O
the	O
ISUP	B-Protocol
header	O
.	O
</s>
<s>
Concerns	O
about	O
the	O
security	O
of	O
calls	O
via	O
the	O
public	O
Internet	O
have	O
been	O
addressed	O
by	O
encryption	O
of	O
the	O
SIP	O
protocol	O
for	O
secure	B-General_Concept
transmission	I-General_Concept
.	O
</s>
<s>
The	O
URI	B-Protocol
scheme	O
SIPS	O
is	O
used	O
to	O
mandate	O
that	O
SIP	O
communication	O
be	O
secured	O
with	O
Transport	B-Protocol
Layer	I-Protocol
Security	I-Protocol
(	O
TLS	O
)	O
.	O
</s>
<s>
SIPS	O
URIs	B-Protocol
take	O
the	O
form	O
.	O
</s>
<s>
While	O
a	O
direct	O
connection	O
can	O
be	O
made	O
via	O
Peer-to-peer	B-Protocol
SIP	I-Protocol
or	O
via	O
a	O
VPN	B-Application
between	O
the	O
endpoints	O
,	O
most	O
SIP	O
communication	O
involves	O
multiple	O
hops	O
,	O
with	O
the	O
first	O
hop	O
being	O
from	O
a	O
user	B-Protocol
agent	I-Protocol
to	O
the	O
user	B-Protocol
agent	I-Protocol
's	O
ITSP	B-Protocol
.	O
</s>
<s>
In	O
contrast	O
,	O
the	O
HTTPS	B-Protocol
protocol	O
provides	O
end-to-end	O
security	O
as	O
it	O
is	O
done	O
with	O
a	O
direct	O
connection	O
and	O
does	O
not	O
involve	O
the	O
notion	O
of	O
hops	O
.	O
</s>
<s>
The	O
key	O
exchange	O
for	O
SRTP	O
is	O
performed	O
with	O
SDES	B-Protocol
(	O
)	O
,	O
or	O
with	O
ZRTP	B-Protocol
(	O
)	O
.	O
</s>
<s>
When	O
SDES	B-Protocol
is	O
used	O
,	O
the	O
keys	O
will	O
be	O
transmitted	O
via	O
insecure	O
SIP	O
unless	O
SIPS	O
is	O
used	O
.	O
</s>
<s>
One	O
may	O
also	O
add	O
a	O
MIKEY	B-Protocol
(	O
)	O
exchange	O
to	O
SIP	O
to	O
determine	O
session	O
keys	O
for	O
use	O
with	O
SRTP	O
.	O
</s>
