<s>
The	O
Session	B-Application
Initiation	I-Application
Protocol	I-Application
(	O
SIP	O
)	O
is	O
the	O
signaling	O
protocol	O
selected	O
by	O
the	O
3rd	O
Generation	O
Partnership	O
Project	O
(	O
3GPP	O
)	O
to	O
create	O
and	O
control	O
multimedia	O
sessions	O
with	O
two	O
or	O
more	O
participants	O
in	O
the	O
IP	B-Protocol
Multimedia	I-Protocol
Subsystem	I-Protocol
(	O
IMS	B-Application
)	O
,	O
and	O
therefore	O
is	O
a	O
key	B-Data_Structure
element	O
in	O
the	O
IMS	B-Application
framework	O
.	O
</s>
<s>
SIP	O
was	O
developed	O
by	O
the	O
Internet	O
Engineering	O
Task	O
Force	O
(	O
IETF	O
)	O
as	O
a	O
standard	O
for	O
controlling	O
multimedia	O
communication	O
sessions	O
in	O
Internet	B-Protocol
Protocol	I-Protocol
(	O
IP	O
)	O
networks	O
,	O
working	O
in	O
the	O
application	B-Application
layer	I-Application
of	O
the	O
Internet	B-Protocol
Protocol	I-Protocol
Suite	I-Protocol
.	O
</s>
<s>
The	O
3GPP	O
,	O
which	O
is	O
a	O
collaboration	O
between	O
groups	O
of	O
telecommunications	O
associations	O
aimed	O
at	O
developing	O
and	O
maintaining	O
the	O
IMS	B-Application
,	O
stated	O
a	O
series	O
of	O
requirements	O
for	O
SIP	O
to	O
be	O
successfully	O
used	O
in	O
the	O
IMS	B-Application
.	O
</s>
<s>
In	O
any	O
case	O
,	O
the	O
IETF	O
evolves	O
SIP	O
in	O
a	O
generic	O
basis	O
,	O
so	O
that	O
the	O
use	O
of	O
its	O
extensions	O
is	O
not	O
restricted	O
to	O
the	O
IMS	B-Application
framework	O
.	O
</s>
<s>
The	O
3GPP	O
has	O
stated	O
several	O
general	O
requirements	O
for	O
operation	O
of	O
the	O
IMS	B-Application
.	O
</s>
<s>
These	O
include	O
an	O
efficient	O
use	O
of	O
the	O
radio	O
interface	O
by	O
minimizing	O
the	O
exchange	O
of	O
signaling	O
messages	O
between	O
the	O
mobile	O
terminal	O
and	O
the	O
network	O
,	O
a	O
minimum	O
session	O
setup	O
time	O
by	O
performing	O
tasks	O
prior	O
to	O
session	O
establishment	O
instead	O
of	O
during	O
session	O
establishment	O
,	O
a	O
minimum	O
support	O
required	O
in	O
the	O
terminal	O
,	O
the	O
support	O
for	O
roaming	O
and	O
non-roaming	O
scenarios	O
with	O
terminal	O
mobility	O
management	O
(	O
supported	O
by	O
the	O
access	O
network	O
,	O
not	O
SIP	O
)	O
,	O
and	O
support	O
for	O
IPv6	B-Protocol
addressing	O
.	O
</s>
<s>
Other	O
requirements	O
involve	O
protocol	O
extensions	O
,	O
such	O
as	O
SIP	O
header	B-General_Concept
fields	I-General_Concept
to	O
exchange	O
user	O
or	O
server	O
information	O
,	O
and	O
SIP	O
methods	O
to	O
support	O
new	O
network	O
functionality	O
:	O
requirement	O
for	O
registration	O
,	O
re-registration	O
,	O
de-registration	O
,	O
event	O
notifications	O
,	O
instant	B-Application
messaging	I-Application
or	O
call	O
control	O
primitives	O
with	O
additional	O
capabilities	O
such	O
as	O
call	O
transference	O
.	O
</s>
<s>
Security	O
between	O
users	O
and	O
the	O
network	O
and	O
among	O
network	O
nodes	O
is	O
a	O
major	O
issue	O
to	O
be	O
addressed	O
by	O
using	O
mutual	O
authentication	O
mechanisms	O
such	O
as	O
private	B-Application
and	I-Application
public	I-Application
keys	I-Application
and	O
digests	B-Algorithm
,	O
as	O
well	O
as	O
media	O
authorization	O
extensions	O
.	O
</s>
<s>
Protection	O
of	O
SIP	O
signaling	O
with	O
integrity	O
and	O
confidentiality	O
support	O
based	O
on	O
initial	O
authentication	O
and	O
symmetric	B-Algorithm
cryptographic	I-Algorithm
keys	I-Algorithm
;	O
error	O
recovery	O
and	O
verification	O
are	O
also	O
needed	O
.	O
</s>
<s>
Source-routing	B-Protocol
mechanisms	I-Protocol
.	O
</s>
<s>
The	O
routing	B-Protocol
of	O
SIP	O
messages	O
has	O
its	O
own	O
requirements	O
in	O
the	O
IMS	B-Application
as	O
all	O
terminal	O
originated	O
session	O
setup	O
attempts	O
must	O
transit	O
both	O
the	O
P-CSCF	B-Protocol
and	O
the	O
S-CSCF	B-Protocol
so	O
that	O
these	O
call	O
session	O
control	O
functions	O
(	O
CSCFs	O
)	O
servers	O
can	O
properly	O
provide	O
their	O
services	O
.	O
</s>
<s>
Interoperation	O
between	O
IMS	B-Application
and	O
the	O
public	O
switched	O
telephone	O
network	O
(	O
PSTN	O
)	O
.	O
</s>
<s>
Finally	O
,	O
it	O
is	O
also	O
necessary	O
that	O
other	O
protocols	O
and	O
network	O
services	O
such	O
as	O
DHCP	B-Protocol
or	O
DNS	B-Application
are	O
adapted	O
to	O
work	O
with	O
SIP	O
,	O
for	O
instance	O
for	O
outbound	O
proxy	O
(	O
P-CSCF	B-Protocol
)	O
location	O
and	O
SIP	O
Uniform	B-Protocol
Resource	I-Protocol
Identifier	I-Protocol
(	O
URI	B-Protocol
)	O
to	O
IP	B-Protocol
address	I-Protocol
resolution	O
,	O
respectively	O
.	O
</s>
<s>
There	O
is	O
a	O
mechanism	O
in	O
SIP	O
for	O
extension	O
negotiation	O
between	O
user	O
agents	O
(	O
UA	O
)	O
or	O
servers	O
,	O
consisting	O
of	O
three	O
header	B-General_Concept
fields	I-General_Concept
:	O
supported	O
,	O
require	O
and	O
unsupported	O
,	O
which	O
UAs	O
or	O
servers	O
(	O
i.e.	O
</s>
<s>
user	O
terminals	O
or	O
call	O
session	O
control	O
function	O
(	O
CSCF	O
)	O
in	O
IMS	B-Application
)	O
may	O
use	O
to	O
specify	O
the	O
extensions	O
they	O
understand	O
.	O
</s>
<s>
In	O
that	O
case	O
,	O
user	O
agents	O
or	O
servers	O
use	O
the	O
Allow	O
header	B-General_Concept
to	O
state	O
which	O
methods	O
they	O
support	O
.	O
</s>
<s>
These	O
two	O
extensions	O
allow	O
users	O
to	O
specify	O
their	O
preferences	O
about	O
the	O
service	O
the	O
IMS	B-Application
provides	O
.	O
</s>
<s>
whether	O
it	O
is	O
fixed	O
or	O
mobile	O
,	O
a	O
voicemail	O
or	O
a	O
human	O
,	O
personal	O
or	O
for	O
business	O
,	O
which	O
services	O
it	O
is	O
capable	O
to	O
provide	O
,	O
or	O
which	O
methods	O
it	O
supports	O
)	O
and	O
how	O
to	O
search	O
for	O
it	O
,	O
with	O
three	O
header	B-General_Concept
fields	I-General_Concept
:	O
Accept-Contact	O
to	O
describe	O
the	O
desired	O
destination	O
user	O
agents	O
,	O
Reject-Contact	O
to	O
state	O
the	O
user	O
agents	O
to	O
avoid	O
,	O
and	O
Request-Disposition	O
to	O
specify	O
how	O
the	O
request	O
should	O
be	O
handled	O
by	O
servers	O
in	O
the	O
network	O
(	O
i.e.	O
</s>
<s>
By	O
using	O
the	O
user	O
agent	O
capabilities	O
extension	O
,	O
user	O
agents	O
(	O
terminals	O
)	O
can	O
describe	O
themselves	O
when	O
they	O
register	O
so	O
that	O
others	O
can	O
search	O
for	O
them	O
according	O
to	O
their	O
caller	O
preferences	O
extension	O
headers	B-General_Concept
.	O
</s>
<s>
For	O
this	O
purpose	O
,	O
they	O
list	O
their	O
capabilities	O
in	O
the	O
Contact	O
header	B-General_Concept
field	O
of	O
the	O
REGISTER	O
message	O
.	O
</s>
<s>
Event	O
notification	O
is	O
necessary	O
in	O
the	O
IMS	B-Application
framework	O
to	O
inform	O
about	O
the	O
presence	O
of	O
a	O
user	O
(	O
i.e.	O
</s>
<s>
"	O
online	O
"	O
or	O
"	O
offline	O
"	O
)	O
to	O
others	O
that	O
may	O
be	O
waiting	O
to	O
contact	O
them	O
,	O
or	O
to	O
notify	O
a	O
user	O
and	O
its	O
P-CSCF	B-Protocol
of	O
its	O
own	O
registration	O
state	O
,	O
so	O
that	O
they	O
know	O
if	O
they	O
are	O
reachable	O
and	O
what	O
public	O
identities	O
they	O
have	O
registered	O
.	O
</s>
<s>
To	O
this	O
end	O
,	O
the	O
specific	O
event	O
notification	O
extension	O
defines	O
a	O
framework	O
for	O
event	O
notification	O
in	O
SIP	O
,	O
with	O
two	O
new	O
methods	O
:	O
SUBSCRIBE	O
and	O
NOTIFY	O
,	O
new	O
header	B-General_Concept
fields	I-General_Concept
and	O
response	B-Protocol
codes	I-Protocol
and	O
two	O
roles	O
:	O
the	O
subscriber	O
and	O
the	O
notifier	O
.	O
</s>
<s>
The	O
entity	O
interested	O
in	O
the	O
state	O
information	O
of	O
a	O
resource	O
(	O
the	O
subscriber	O
)	O
sends	O
a	O
SUBSCRIBE	O
message	O
with	O
the	O
Uniform	B-Protocol
Resource	I-Protocol
Identifier	I-Protocol
(	O
URI	B-Protocol
)	O
of	O
the	O
resource	O
in	O
the	O
request	O
initial	O
line	O
,	O
and	O
the	O
type	O
of	O
event	O
in	O
the	O
Event	O
header	B-General_Concept
.	O
</s>
<s>
Then	O
the	O
entity	O
in	O
charge	O
of	O
keeping	O
track	O
of	O
the	O
state	O
of	O
the	O
resource	O
(	O
the	O
notifier	O
)	O
,	O
receives	O
the	O
SUBSCRIBE	O
request	O
and	O
sends	O
back	O
a	O
NOTIFY	O
message	O
with	O
a	O
subscription-state	O
header	B-General_Concept
as	O
well	O
as	O
the	O
information	O
about	O
the	O
status	O
of	O
the	O
resource	O
in	O
the	O
message	O
body	O
.	O
</s>
<s>
An	O
event	O
package	O
describes	O
a	O
new	O
value	O
for	O
the	O
SUBSCRIBE	O
Event	O
header	B-General_Concept
,	O
as	O
well	O
as	O
a	O
MIME	B-Protocol
type	O
to	O
carry	O
the	O
event	O
state	O
information	O
in	O
the	O
NOTIFY	O
message	O
.	O
</s>
<s>
There	O
is	O
also	O
an	O
allow-events	O
header	B-General_Concept
to	O
indicate	O
event	O
notification	O
capabilities	O
,	O
and	O
the	O
202	O
accepted	O
and	O
489	O
bad	O
event	O
response	B-Protocol
codes	I-Protocol
to	O
indicate	O
if	O
a	O
subscription	O
request	O
has	O
been	O
preliminary	O
accepted	O
or	O
has	O
been	O
turned	O
down	O
because	O
the	O
notifier	O
does	O
not	O
understand	O
the	O
kind	O
of	O
event	O
requested	O
.	O
</s>
<s>
The	O
state	O
publication	O
framework	O
defines	O
a	O
new	O
method	O
:	O
PUBLISH	O
,	O
which	O
is	O
used	O
to	O
request	O
the	O
publication	O
of	O
the	O
state	O
of	O
the	O
resource	O
specified	O
in	O
the	O
request-URI	O
,	O
with	O
reference	O
to	O
the	O
event	O
stated	O
in	O
the	O
Event	O
header	B-General_Concept
,	O
and	O
with	O
the	O
information	O
carried	O
in	O
the	O
message	O
body	O
.	O
</s>
<s>
The	O
functionality	O
of	O
sending	O
instant	B-Application
messages	I-Application
to	O
provide	O
a	O
service	O
similar	O
to	O
text	O
messaging	O
is	O
defined	O
in	O
the	O
instant	B-Application
messaging	I-Application
extension	O
.	O
</s>
<s>
This	O
functionality	O
is	O
supported	O
by	O
the	O
new	O
MESSAGE	O
method	O
,	O
that	O
can	O
be	O
used	O
to	O
send	O
an	O
instant	B-Application
message	I-Application
to	O
the	O
resource	O
stated	O
in	O
the	O
request-URI	O
,	O
with	O
the	O
content	O
carried	O
in	O
the	O
message	O
body	O
.	O
</s>
<s>
This	O
content	O
is	O
defined	O
as	O
a	O
MIME	B-Protocol
type	O
,	O
being	O
text/plain	O
the	O
most	O
common	O
one	O
.	O
</s>
<s>
In	O
order	O
to	O
have	O
an	O
instant	B-Application
messaging	I-Application
session	O
with	O
related	O
messages	O
,	O
the	O
Message	B-Protocol
Session	I-Protocol
Relay	I-Protocol
Protocol	I-Protocol
(	O
MSRP	B-Protocol
)	O
should	O
be	O
used	O
.	O
</s>
<s>
The	O
REFER	O
method	O
extension	O
defines	O
a	O
mechanism	O
to	O
request	O
a	O
user	O
agent	O
to	O
contact	O
a	O
resource	O
which	O
is	O
identified	O
by	O
a	O
URI	B-Protocol
in	O
the	O
Refer-To	O
header	B-General_Concept
field	O
of	O
the	O
request	O
message	O
.	O
</s>
<s>
A	O
typical	O
use	O
of	O
this	O
mechanism	O
is	O
call	O
transfer	O
:	O
during	O
a	O
call	O
,	O
the	O
participant	O
who	O
sends	O
the	O
REFER	O
message	O
tells	O
the	O
recipient	O
to	O
contact	O
to	O
the	O
user	O
agent	O
identified	O
by	O
the	O
URI	B-Protocol
in	O
the	O
corresponding	O
header	B-General_Concept
field	O
.	O
</s>
<s>
However	O
,	O
this	O
mechanism	O
is	O
not	O
restricted	O
to	O
call	O
transfer	O
,	O
since	O
the	O
Refer-To	O
header	B-General_Concept
field	O
can	O
be	O
any	O
kind	O
of	O
URI	B-Protocol
,	O
for	O
instance	O
,	O
an	O
HTTP	B-Protocol
URI	B-Protocol
,	O
to	O
require	O
the	O
recipient	O
to	O
visit	O
a	O
web	O
page	O
.	O
</s>
<s>
2XX	O
response	B-Protocol
codes	I-Protocol
)	O
are	O
transmitted	O
reliably	O
,	O
this	O
is	O
,	O
they	O
are	O
retransmitted	O
by	O
the	O
sender	O
until	O
the	O
acknowledge	B-Protocol
message	O
arrives	O
(	O
i.e.	O
</s>
<s>
the	O
corresponding	O
response	B-Protocol
code	I-Protocol
to	O
a	O
request	O
,	O
or	O
the	O
ACK	B-Protocol
request	O
corresponding	O
to	O
a	O
2XX	O
response	B-Protocol
code	I-Protocol
)	O
.	O
</s>
<s>
This	O
mechanism	O
is	O
necessary	O
since	O
SIP	O
can	O
run	O
not	O
only	O
over	O
reliable	O
transport	O
protocols	O
(	O
TCP	B-Protocol
)	O
that	O
assure	O
that	O
the	O
message	O
is	O
delivered	O
,	O
but	O
also	O
over	O
unreliable	O
ones	O
(	O
UDP	B-Protocol
)	O
that	O
offer	O
no	O
delivery	O
guarantees	O
,	O
and	O
it	O
is	O
even	O
possible	O
that	O
both	O
kinds	O
of	O
protocols	O
are	O
present	O
in	O
different	O
parts	O
of	O
the	O
transport	O
network	O
.	O
</s>
<s>
However	O
,	O
in	O
such	O
an	O
scenario	O
as	O
the	O
IMS	B-Application
framework	O
,	O
it	O
is	O
necessary	O
to	O
extend	O
this	O
reliability	B-Protocol
to	O
provisional	O
responses	O
to	O
INVITE	O
requests	O
(	O
for	O
session	O
establishment	O
,	O
this	O
is	O
,	O
to	O
start	O
a	O
call	O
)	O
.	O
</s>
<s>
The	O
reliability	B-Protocol
of	O
provisional	O
responses	O
extension	O
provides	O
a	O
mechanism	O
to	O
confirm	O
that	O
provisional	O
responses	O
such	O
as	O
the	O
180	O
Ringing	O
response	B-Protocol
code	I-Protocol
,	O
that	O
lets	O
the	O
caller	O
know	O
that	O
the	O
callee	O
is	O
being	O
alerted	O
,	O
are	O
successfully	O
received	O
.	O
</s>
<s>
This	O
message	O
includes	O
a	O
RACK	O
header	B-General_Concept
field	O
which	O
is	O
a	O
sequence	B-Protocol
number	I-Protocol
that	O
matches	O
the	O
RSeq	O
header	B-General_Concept
field	O
of	O
the	O
provisional	O
response	O
that	O
is	O
being	O
acknowledged	O
,	O
and	O
also	O
contains	O
the	O
CSeq	O
number	O
that	O
identifies	O
the	O
corresponding	O
INVITE	O
request	O
.	O
</s>
<s>
In	O
the	O
IMS	B-Application
framework	O
,	O
it	O
is	O
required	O
that	O
once	O
the	O
callee	O
is	O
alerted	O
,	O
the	O
chances	O
of	O
a	O
session	O
failure	O
are	O
minimum	O
.	O
</s>
<s>
An	O
important	O
source	O
of	O
failure	O
is	O
the	O
inability	O
to	O
reserve	O
network	O
resources	O
to	O
support	O
the	O
session	O
,	O
so	O
these	O
resources	O
should	O
be	O
allocated	B-Algorithm
before	O
the	O
phone	O
rings	O
.	O
</s>
<s>
However	O
,	O
in	O
the	O
IMS	B-Application
,	O
to	O
reserve	O
resources	O
the	O
network	O
needs	O
to	O
know	O
the	O
callee	O
's	O
IP	B-Protocol
address	I-Protocol
,	O
port	O
and	O
session	O
parameters	O
and	O
therefore	O
it	O
is	O
necessary	O
that	O
the	O
initial	O
offer/answer	O
exchange	O
to	O
establish	O
a	O
session	O
has	O
started	O
(	O
INVITE	O
request	O
)	O
.	O
</s>
<s>
codecs	B-General_Concept
and	O
QoS	O
requirements	O
)	O
in	O
the	O
offer	O
,	O
and	O
the	O
callee	O
responds	O
to	O
the	O
offer	O
without	O
establishing	O
the	O
session	O
or	O
alerting	O
the	O
user	O
.	O
</s>
<s>
The	O
preconditions	O
SIP	O
extension	O
affects	O
both	O
SIP	O
,	O
with	O
a	O
new	O
option	O
tag	O
(	O
precondition	O
)	O
and	O
defined	O
offer/answer	O
exchanges	O
,	O
and	O
Session	B-Language
Description	I-Language
Protocol	I-Language
(	O
SDP	O
)	O
,	O
which	O
is	O
a	O
format	O
used	O
to	O
describe	O
streaming	O
media	O
initialization	O
parameters	O
,	O
carried	O
in	O
the	O
body	O
of	O
SIP	O
messages	O
.	O
</s>
<s>
In	O
the	O
IMS	B-Application
,	O
the	O
initial	O
session	O
parameter	O
negotiation	O
can	O
be	O
done	O
by	O
using	O
the	O
provisional	O
responses	O
and	O
session	O
description	O
updating	O
extensions	O
,	O
along	O
with	O
SDP	O
in	O
the	O
body	O
of	O
the	O
messages	O
.	O
</s>
<s>
The	O
first	O
offer	O
,	O
described	O
by	O
means	O
of	O
SDP	O
,	O
can	O
be	O
carried	O
by	O
the	O
INVITE	O
request	O
and	O
will	O
deal	O
with	O
the	O
caller	O
's	O
supported	O
codecs	B-General_Concept
.	O
</s>
<s>
This	O
request	O
will	O
be	O
answered	O
by	O
the	O
provisional	O
reliable	O
response	B-Protocol
code	I-Protocol
183	B-Protocol
Session	I-Protocol
Progress	I-Protocol
,	O
that	O
will	O
carry	O
the	O
SDP	O
list	O
of	O
supported	O
codecs	B-General_Concept
by	O
both	O
the	O
caller	O
and	O
the	O
callee	O
.	O
</s>
<s>
The	O
corresponding	O
PRACK	O
to	O
this	O
provisional	O
answer	O
will	O
be	O
used	O
to	O
select	O
a	O
codec	B-General_Concept
and	O
initiate	O
the	O
QoS	O
negotiation	O
.	O
</s>
<s>
The	O
QoS	O
negotiation	O
is	O
supported	O
by	O
the	O
PRACK	O
request	O
,	O
that	O
starts	O
resource	O
reservation	O
in	O
the	O
calling	O
party	O
network	O
,	O
and	O
it	O
is	O
answered	O
by	O
a	O
2XX	O
response	B-Protocol
code	I-Protocol
.	O
</s>
<s>
Once	O
this	O
response	O
has	O
been	O
sent	O
,	O
the	O
called	O
party	O
has	O
selected	O
the	O
codec	B-General_Concept
too	O
,	O
and	O
starts	O
resource	O
reservation	O
on	O
its	O
side	O
.	O
</s>
<s>
Subsequent	O
UPDATE	O
requests	O
are	O
sent	O
to	O
inform	O
about	O
the	O
reservation	O
progress	O
,	O
and	O
they	O
are	O
answered	O
by	O
2XX	O
response	B-Protocol
codes	I-Protocol
.	O
</s>
<s>
In	O
the	O
IMS	B-Application
framework	O
it	O
is	O
fundamental	O
to	O
handle	O
user	O
identities	O
for	O
authentication	O
,	O
authorization	O
and	O
accounting	O
purposes	O
.	O
</s>
<s>
The	O
IMS	B-Application
is	O
meant	O
to	O
provide	O
multimedia	O
services	O
over	O
IP	B-Protocol
networks	I-Protocol
,	O
but	O
also	O
needs	O
a	O
mechanism	O
to	O
charge	O
users	O
for	O
it	O
.	O
</s>
<s>
All	O
this	O
functionality	O
is	O
supported	O
by	O
new	O
special	O
header	B-General_Concept
fields	I-General_Concept
.	O
</s>
<s>
The	O
Private	O
Header	B-General_Concept
Extensions	O
to	O
SIP	O
,	O
also	O
known	O
as	O
P-Headers	O
,	O
are	O
special	O
header	B-General_Concept
fields	I-General_Concept
whose	O
applicability	O
is	O
limited	O
to	O
private	B-Protocol
networks	I-Protocol
with	O
a	O
certain	O
topology	O
and	O
characteristics	O
of	O
lower	O
layers	B-Architecture
 '	O
protocols	O
.	O
</s>
<s>
These	O
header	B-General_Concept
fields	I-General_Concept
are	O
used	O
for	O
a	O
variety	O
of	O
purposes	O
including	O
charging	O
and	O
information	O
about	O
the	O
networks	O
a	O
call	O
traverses	O
:	O
</s>
<s>
P-Charging-Vector	O
:	O
A	O
collection	O
of	O
charging	O
information	O
,	O
such	O
as	O
the	O
IMS	B-Application
Charging	O
Identity	O
(	O
ICID	O
)	O
value	O
,	O
the	O
address	O
of	O
the	O
SIP	O
proxy	O
that	O
creates	O
the	O
ICID	O
value	O
,	O
and	O
the	O
Inter	O
Operator	O
Identifier	O
(	O
IOI	O
)	O
.	O
</s>
<s>
P-Charging-Function-Address	O
:	O
The	O
addresses	B-Protocol
of	O
the	O
charging	O
functions	O
(	O
functional	O
entities	O
that	O
receive	O
the	O
charging	O
records	O
or	O
events	O
)	O
in	O
the	O
user	O
's	O
home	O
network	O
.	O
</s>
<s>
P-Called-Party-ID	O
:	O
The	O
URI	B-Protocol
originally	O
indicated	O
in	O
the	O
request-URI	O
of	O
a	O
request	O
generated	O
by	O
the	O
calling	O
user	O
agent	O
.	O
</s>
<s>
When	O
the	O
request	O
reaches	O
the	O
registrar	O
(	O
S-CSCF	B-Protocol
)	O
of	O
the	O
called	O
user	O
,	O
the	O
registrar	O
re-writes	O
the	O
request-URI	O
on	O
the	O
first	O
line	O
of	O
the	O
request	O
with	O
the	O
registered	O
contact	O
address	O
(	O
i.e.	O
</s>
<s>
IP	B-Protocol
address	I-Protocol
)	O
of	O
the	O
called	O
user	O
,	O
and	O
stores	O
the	O
replaced	O
request-URI	O
in	O
this	O
header	B-General_Concept
field	O
.	O
</s>
<s>
In	O
the	O
IMS	B-Application
,	O
a	O
user	O
may	O
be	O
identified	O
by	O
several	O
SIP	O
URIs	B-Protocol
(	O
address-of-record	O
)	O
,	O
for	O
instance	O
,	O
a	O
SIP	O
URI	B-Protocol
for	O
work	O
and	O
another	O
SIP	O
URI	B-Protocol
for	O
personal	O
use	O
,	O
and	O
when	O
the	O
registrar	O
replaces	O
the	O
request-URI	O
with	O
the	O
effective	O
contact	O
address	O
,	O
the	O
original	O
request-URI	O
must	O
be	O
stored	O
so	O
that	O
the	O
called	O
party	O
knows	O
to	O
which	O
address-of-record	O
was	O
the	O
invitation	O
sent	O
.	O
</s>
<s>
P-Associated-URI	O
:	O
Additional	O
URIs	B-Protocol
that	O
are	O
associated	O
with	O
a	O
user	O
that	O
is	O
registering	O
.	O
</s>
<s>
It	O
is	O
included	O
in	O
the	O
200	O
OK	O
response	O
to	O
a	O
REGISTER	O
request	O
to	O
inform	O
a	O
user	O
which	O
other	O
URIs	B-Protocol
the	O
service	O
provider	O
has	O
associated	O
with	O
an	O
address-of-record	O
(	O
AOR	O
)	O
URI	B-Protocol
.	O
</s>
<s>
More	O
private	O
headers	B-General_Concept
have	O
been	O
defined	O
for	O
user	O
database	O
accessing	O
:	O
</s>
<s>
Although	O
the	O
HSS	O
is	O
a	O
unique	O
master	O
database	O
,	O
it	O
can	O
be	O
distributed	O
into	O
different	O
nodes	O
for	O
reliability	B-Protocol
and	O
scalability	B-Architecture
reasons	O
.	O
</s>
<s>
When	O
a	O
user	O
request	O
reaches	O
the	O
I-CSCF	B-Protocol
at	O
the	O
edge	O
of	O
the	O
administrative	O
domain	O
,	O
this	O
entity	O
queries	O
the	O
SLF	O
for	O
the	O
corresponding	O
HSS	O
and	O
then	O
,	O
to	O
prevent	O
the	O
S-CSCF	B-Protocol
from	O
having	O
to	O
query	O
the	O
SLF	O
again	O
,	O
sends	O
the	O
HSS	O
address	O
to	O
the	O
S-CSCF	B-Protocol
in	O
the	O
P-User-Database	O
header	B-General_Concept
.	O
</s>
<s>
Then	O
the	O
S-CSCF	B-Protocol
will	O
be	O
able	O
to	O
directly	O
query	O
the	O
HSS	O
to	O
get	O
information	O
about	O
the	O
user	O
(	O
e.g.	O
</s>
<s>
P-Profile-Key	O
:	O
The	O
key	B-Data_Structure
to	O
be	O
used	O
to	O
query	O
the	O
user	O
database	O
(	O
HSS	O
)	O
for	O
a	O
profile	O
corresponding	O
to	O
the	O
destination	O
SIP	O
URI	B-Protocol
of	O
a	O
particular	O
SIP	O
request	O
.	O
</s>
<s>
It	O
is	O
transmitted	O
among	O
proxies	O
to	O
perform	O
faster	O
database	O
queries	O
:	O
the	O
first	O
proxy	O
finds	O
the	O
key	B-Data_Structure
and	O
the	O
others	O
query	O
the	O
database	O
by	O
directly	O
using	O
the	O
key	B-Data_Structure
.	O
</s>
<s>
This	O
is	O
useful	O
when	O
Wildcarded	O
Service	O
Identities	O
are	O
used	O
,	O
this	O
is	O
,	O
Public	O
Service	O
Identities	O
that	O
match	O
a	O
regular	B-Language
expression	I-Language
,	O
because	O
the	O
first	O
query	O
has	O
to	O
resolve	O
the	O
regular	B-Language
expression	I-Language
to	O
find	O
the	O
key	B-Data_Structure
.	O
</s>
<s>
To	O
indicate	O
so	O
,	O
they	O
must	O
insert	O
the	O
privacy	O
token	O
id	O
into	O
the	O
Privacy	O
header	B-General_Concept
field	O
.	O
</s>
<s>
The	O
main	O
functionality	O
is	O
supported	O
by	O
the	O
P-Asserted-Identity	O
extension	O
header	B-General_Concept
.	O
</s>
<s>
verifies	O
that	O
the	O
user	O
is	O
who	O
he	O
or	O
she	O
says	O
that	O
he	O
or	O
she	O
is	O
)	O
,	O
it	O
then	O
inserts	O
this	O
header	B-General_Concept
with	O
the	O
identity	O
that	O
has	O
been	O
authenticated	O
,	O
and	O
then	O
forwards	O
the	O
request	O
as	O
usual	O
.	O
</s>
<s>
the	O
network	O
of	O
trusted	O
entities	O
with	O
previously	O
agreed	O
security	O
policies	O
)	O
can	O
safely	O
rely	O
on	O
the	O
identity	O
information	O
carried	O
in	O
the	O
P-Asserted-Identity	O
header	B-General_Concept
without	O
the	O
necessity	O
of	O
re-authenticating	O
the	O
user	O
.	O
</s>
<s>
The	O
P-Preferred-Identity	O
extension	O
header	B-General_Concept
is	O
also	O
defined	O
,	O
so	O
that	O
a	O
user	O
with	O
several	O
public	O
identities	O
is	O
able	O
to	O
tell	O
the	O
proxy	O
which	O
public	O
identity	O
should	O
be	O
included	O
in	O
the	O
P-Asserted-Identity	O
header	B-General_Concept
when	O
the	O
user	O
is	O
authenticated	O
.	O
</s>
<s>
Finally	O
,	O
when	O
privacy	O
is	O
requested	O
,	O
proxies	O
must	O
withhold	O
asserted	O
identity	O
information	O
outside	O
the	O
trusted	O
domain	O
by	O
removing	O
P-Asserted-Identity	O
headers	B-General_Concept
before	O
forwarding	O
user	O
requests	O
to	O
untrusted	O
identities	O
(	O
outside	O
the	O
Trust	O
Domain	O
)	O
.	O
</s>
<s>
There	O
exist	O
analogous	O
extension	O
headers	B-General_Concept
for	O
handling	O
the	O
identification	O
of	O
services	O
of	O
users	O
,	O
instead	O
of	O
the	O
users	O
themselves	O
.	O
</s>
<s>
In	O
this	O
case	O
,	O
Uniform	B-Protocol
Resource	I-Protocol
Names	I-Protocol
are	O
used	O
to	O
identify	O
a	O
service	O
(	O
e.g.	O
</s>
<s>
Access	O
security	O
in	O
the	O
IMS	B-Application
consists	O
of	O
first	O
authenticating	O
and	O
authorizing	O
the	O
user	O
,	O
which	O
is	O
done	O
by	O
the	O
S-CSCF	B-Protocol
,	O
and	O
then	O
establishing	O
secure	O
connections	O
between	O
the	O
P-CSCF	B-Protocol
and	O
the	O
user	O
.	O
</s>
<s>
HTTP	B-Protocol
digest	B-Protocol
access	I-Protocol
authentication	I-Protocol
,	O
which	O
is	O
part	O
of	O
the	O
basic	O
SIP	O
specification	O
and	O
leads	O
to	O
a	O
Transport	B-Protocol
Layer	I-Protocol
Security	I-Protocol
connection	O
between	O
the	O
user	O
and	O
the	O
proxy	O
.	O
</s>
<s>
HTTP	B-Protocol
digest	B-Protocol
access	I-Protocol
authentication	I-Protocol
using	O
AKA	B-Protocol
,	O
a	O
more	O
secure	O
version	O
of	O
the	O
previous	O
mechanism	O
for	O
cellular	O
networks	O
that	O
uses	O
the	O
information	O
from	O
the	O
user	O
's	O
smart	O
card	O
and	O
commonly	O
creates	O
two	O
IPsec	B-Protocol
security	O
associations	O
between	O
the	O
P-CSCF	B-Protocol
and	O
the	O
terminal	O
.	O
</s>
<s>
The	O
security	O
mechanisms	O
agreement	O
extension	O
for	O
SIP	O
was	O
then	O
introduced	O
to	O
provide	O
a	O
secure	O
mechanism	O
for	O
negotiating	O
the	O
security	O
algorithms	O
and	O
parameters	O
to	O
be	O
used	O
by	O
the	O
P-CSCF	B-Protocol
and	O
the	O
terminal	O
.	O
</s>
<s>
This	O
extension	O
uses	O
three	O
new	O
header	B-General_Concept
fields	I-General_Concept
to	O
support	O
the	O
negotiation	O
process	O
:	O
</s>
<s>
First	O
,	O
the	O
terminal	O
adds	O
a	O
security	O
–	O
client	O
header	B-General_Concept
field	O
containing	O
the	O
mechanisms	O
,	O
authentication	O
and	O
encryption	O
algorithms	O
it	O
supports	O
to	O
the	O
REGISTER	O
request	O
.	O
</s>
<s>
Then	O
,	O
the	O
P-CSCF	B-Protocol
adds	O
a	O
security-server	O
header	B-General_Concept
field	O
to	O
the	O
response	O
that	O
contains	O
the	O
same	O
information	O
as	O
the	O
client	O
's	O
but	O
with	O
reference	O
to	O
the	O
P-CSCF	B-Protocol
.	O
</s>
<s>
Finally	O
,	O
the	O
user	O
agent	O
sends	O
a	O
new	O
REGISTER	O
request	O
over	O
the	O
just	O
created	O
secure	O
connection	O
with	O
the	O
negotiated	O
parameters	O
,	O
including	O
a	O
security-verify	O
header	B-General_Concept
field	O
that	O
carries	O
the	O
same	O
contents	O
as	O
the	O
previously	O
received	O
security-server	O
header	B-General_Concept
field	O
.	O
</s>
<s>
This	O
procedure	O
protects	O
the	O
negotiation	O
mechanism	O
from	O
Man-in-the-middle	O
attacks	O
:	O
if	O
an	O
attacker	O
removed	O
the	O
strongest	O
security	O
mechanisms	O
from	O
the	O
Security-Server	O
header	B-General_Concept
field	O
in	O
order	O
to	O
force	O
the	O
terminal	O
to	O
choose	O
weaker	O
security	O
algorithms	O
,	O
then	O
the	O
Security-Verify	O
and	O
Security-Server	O
header	B-General_Concept
fields	I-General_Concept
would	O
not	O
match	O
.	O
</s>
<s>
The	O
contents	O
of	O
the	O
Security-Verify	O
header	B-General_Concept
field	O
cannot	O
be	O
altered	O
as	O
they	O
are	O
sent	O
through	O
the	O
new	O
established	O
secure	O
association	O
,	O
as	O
long	O
as	O
this	O
association	O
is	O
no	O
breakable	O
by	O
the	O
attacker	O
in	O
real	O
time	O
(	O
i.e.	O
</s>
<s>
before	O
the	O
P-CSCF	B-Protocol
discovers	O
the	O
Man-in-the-middle	O
attack	O
in	O
progress	O
.	O
</s>
<s>
The	O
necessity	O
in	O
the	O
IMS	B-Application
of	O
reserving	O
resources	O
to	O
provide	O
quality	O
of	O
service	O
(	O
QoS	O
)	O
leads	O
to	O
another	O
security	O
issue	O
:	O
admission	O
control	O
and	O
protection	O
against	O
denial-of-service	O
attacks	O
.	O
</s>
<s>
This	O
token	O
will	O
be	O
obtained	O
from	O
its	O
P-CSCF	B-Protocol
,	O
which	O
may	O
be	O
in	O
charge	O
of	O
QoS	O
policy	O
control	O
or	O
have	O
an	O
interface	O
with	O
the	O
policy	O
control	O
entity	O
in	O
the	O
network	O
(	O
i.e.	O
</s>
<s>
The	O
private	O
extensions	O
for	O
media	O
authorization	O
link	O
session	O
signaling	O
to	O
the	O
QoS	O
mechanisms	O
applied	O
to	O
media	O
in	O
the	O
network	O
,	O
by	O
defining	O
the	O
mechanisms	O
for	O
obtaining	O
authorization	O
tokens	O
and	O
the	O
P-Media-Authorization	O
header	B-General_Concept
field	O
to	O
carry	O
these	O
tokens	O
from	O
the	O
P-CSCF	B-Protocol
to	O
the	O
user	O
agent	O
.	O
</s>
<s>
It	O
was	O
particularly	O
designed	O
for	O
specialized	O
SIP	O
networks	O
like	O
the	O
IMS	B-Application
,	O
and	O
not	O
for	O
the	O
general	O
Internet	O
.	O
</s>
<s>
Source	B-Protocol
routing	I-Protocol
is	O
the	O
mechanism	O
that	O
allows	O
the	O
sender	O
of	O
a	O
message	O
to	O
specify	O
partially	O
or	O
completely	O
the	O
route	O
the	O
message	O
traverses	O
.	O
</s>
<s>
In	O
SIP	O
,	O
the	O
route	O
header	B-General_Concept
field	O
,	O
filled	O
by	O
the	O
sender	O
,	O
supports	O
this	O
functionality	O
by	O
listing	O
a	O
set	O
of	O
proxies	O
the	O
message	O
will	O
visit	O
.	O
</s>
<s>
In	O
the	O
IMS	B-Application
context	O
,	O
there	O
are	O
certain	O
network	O
entities	O
(	O
i.e.	O
</s>
<s>
certain	O
CSCFs	O
)	O
that	O
must	O
be	O
traversed	O
by	O
requests	O
from	O
or	O
to	O
a	O
user	O
,	O
so	O
they	O
are	O
to	O
be	O
listed	O
in	O
the	O
Route	O
header	B-General_Concept
field	O
.	O
</s>
<s>
To	O
allow	O
the	O
sender	O
to	O
discover	O
such	O
entities	O
and	O
populate	O
the	O
route	O
header	B-General_Concept
field	O
,	O
there	O
are	O
mainly	O
two	O
extension	O
header	B-General_Concept
fields	I-General_Concept
:	O
path	O
and	O
service-route	O
.	O
</s>
<s>
The	O
extension	O
header	B-General_Concept
field	O
for	O
registering	O
non-adjacent	O
contacts	O
provides	O
a	O
Path	O
header	B-General_Concept
field	O
which	O
accumulates	O
and	O
transmits	O
the	O
SIP	O
URIs	B-Protocol
of	O
the	O
proxies	O
that	O
are	O
situated	O
between	O
a	O
user	O
agent	O
and	O
its	O
registrar	O
as	O
the	O
REGISTER	O
message	O
traverses	O
then	O
.	O
</s>
<s>
In	O
the	O
IMS	B-Application
every	O
user	O
agent	O
is	O
served	O
by	O
its	O
P-CSCF	B-Protocol
,	O
which	O
is	O
discovered	O
by	O
using	O
the	O
Dynamic	B-Protocol
Host	I-Protocol
Configuration	I-Protocol
Protocol	I-Protocol
or	O
an	O
equivalent	O
mechanism	O
when	O
the	O
user	O
enters	O
the	O
IMS	B-Application
network	O
,	O
and	O
all	O
requests	O
and	O
responses	O
from	O
or	O
to	O
the	O
user	O
agent	O
must	O
traverse	O
this	O
proxy	O
.	O
</s>
<s>
When	O
the	O
user	O
registers	O
to	O
the	O
home	O
registrar	O
(	O
S-CSCF	B-Protocol
)	O
,	O
the	O
P-CSCF	B-Protocol
adds	O
its	O
own	O
SIP	O
URI	B-Protocol
in	O
a	O
Path	O
header	B-General_Concept
field	O
in	O
the	O
REGISTER	O
message	O
,	O
so	O
that	O
the	O
S-CSCF	B-Protocol
receives	O
and	O
stores	O
this	O
information	O
associated	O
with	O
the	O
contact	O
information	O
of	O
the	O
user	O
.	O
</s>
<s>
This	O
way	O
,	O
the	O
S-CSCF	B-Protocol
will	O
forward	O
every	O
request	O
addressed	O
to	O
that	O
user	O
through	O
the	O
corresponding	O
P-CSCF	B-Protocol
by	O
listing	O
its	O
URI	B-Protocol
in	O
the	O
route	O
header	B-General_Concept
field	O
.	O
</s>
<s>
The	O
extension	O
for	O
service	O
route	O
discovery	O
during	O
registration	O
consists	O
of	O
a	O
Service-Route	O
header	B-General_Concept
field	O
that	O
is	O
used	O
by	O
the	O
registrar	O
in	O
a	O
2XX	O
response	O
to	O
a	O
REGISTER	O
request	O
to	O
inform	O
the	O
registering	O
user	O
of	O
the	O
entity	O
that	O
must	O
forward	O
every	O
request	O
originated	O
by	O
him	O
or	O
her	O
.	O
</s>
<s>
In	O
the	O
IMS	B-Application
,	O
the	O
registrar	O
is	O
the	O
home	O
network	O
's	O
S-CSCF	B-Protocol
and	O
it	O
is	O
also	O
required	O
that	O
all	O
requests	O
are	O
handled	O
by	O
this	O
entity	O
,	O
so	O
it	O
will	O
include	O
its	O
own	O
SIP	O
URI	B-Protocol
in	O
the	O
service-route	O
header	B-General_Concept
field	O
.	O
</s>
<s>
The	O
user	O
will	O
then	O
include	O
this	O
SIP	O
URI	B-Protocol
in	O
the	O
Route	O
header	B-General_Concept
field	O
of	O
all	O
his	O
or	O
her	O
requests	O
,	O
so	O
that	O
they	O
are	O
forwarded	O
through	O
the	O
home	O
S-CSCF	B-Protocol
.	O
</s>
<s>
In	O
the	O
IMS	B-Application
it	O
is	O
possible	O
for	O
a	O
user	O
to	O
have	O
multiple	O
terminals	O
(	O
e.g.	O
</s>
<s>
video	B-Application
telephony	I-Application
,	O
instant	B-Application
messaging	I-Application
,	O
voice	O
mail	O
)	O
that	O
are	O
identified	O
with	O
the	O
same	O
public	O
identity	O
(	O
i.e.	O
</s>
<s>
SIP	O
URI	B-Protocol
)	O
.	O
</s>
<s>
That	O
is	O
what	O
a	O
Globally	B-Protocol
Routable	I-Protocol
User	I-Protocol
Agent	I-Protocol
URI	I-Protocol
(	O
GRU	O
)	O
is	O
:	O
a	O
URI	B-Protocol
that	O
identifies	O
a	O
specific	O
user	O
agent	O
instance	O
(	O
i.e.	O
</s>
<s>
These	O
URIs	B-Protocol
are	O
constructed	O
by	O
adding	O
the	O
gr	O
parameter	O
to	O
a	O
SIP	O
URI	B-Protocol
,	O
either	O
to	O
the	O
public	O
SIP	O
URI	B-Protocol
with	O
a	O
value	O
that	O
identifies	O
the	O
user	O
agent	O
instance	O
,	O
or	O
to	O
a	O
specially	O
created	O
URI	B-Protocol
that	O
does	O
not	O
reveal	O
the	O
relationship	O
between	O
the	O
GRUU	O
and	O
the	O
user	O
's	O
identity	O
,	O
for	O
privacy	O
purposes	O
.	O
</s>
<s>
They	O
are	O
commonly	O
obtained	O
during	O
the	O
registration	O
process	O
:	O
the	O
registering	O
user	O
agent	O
sends	O
a	O
Uniform	B-Protocol
Resource	I-Protocol
Name	I-Protocol
(	O
URN	B-Protocol
)	O
that	O
uniquely	O
identifies	O
that	O
SIP	O
instance	O
,	O
and	O
the	O
registrar	O
(	O
i.e.	O
</s>
<s>
S-CSCF	B-Protocol
)	O
builds	O
the	O
GRUU	O
,	O
associates	O
it	O
to	O
the	O
registered	O
identity	O
and	O
SIP	O
instance	O
and	O
sends	O
it	O
back	O
to	O
the	O
user	O
agent	O
in	O
the	O
response	O
.	O
</s>
<s>
When	O
the	O
S-CSCF	B-Protocol
receives	O
a	O
request	O
for	O
that	O
GRUU	O
,	O
it	O
will	O
be	O
able	O
to	O
route	O
the	O
request	O
to	O
the	O
registered	O
SIP	O
instance	O
.	O
</s>
<s>
The	O
efficient	O
use	O
of	O
network	O
resources	O
,	O
which	O
may	O
include	O
a	O
radio	O
interface	O
or	O
other	O
low-bandwidth	O
access	O
,	O
is	O
essential	O
in	O
the	O
IMS	B-Application
in	O
order	O
to	O
provide	O
the	O
user	O
with	O
an	O
acceptable	O
experience	O
in	O
terms	O
of	O
latency	O
.	O
</s>
<s>
To	O
achieve	O
this	O
goal	O
,	O
SIP	O
messages	O
can	O
be	O
compressed	B-General_Concept
using	O
the	O
mechanism	O
known	O
as	O
SigComp	O
(	O
signaling	O
compression	O
)	O
.	O
</s>
<s>
Compression	B-General_Concept
algorithms	I-General_Concept
perform	O
this	O
operation	O
by	O
substituting	O
repeated	O
words	O
in	O
the	O
message	O
by	O
its	O
position	O
in	O
a	O
dictionary	B-Algorithm
where	O
all	O
these	O
words	O
only	O
appear	O
once	O
.	O
</s>
<s>
In	O
a	O
first	O
approach	O
,	O
this	O
dictionary	B-Algorithm
may	O
be	O
built	O
for	O
each	O
message	O
by	O
the	O
compressor	O
and	O
sent	O
to	O
the	O
decompressor	O
along	O
with	O
the	O
message	O
itself	O
.	O
</s>
<s>
However	O
,	O
as	O
many	O
words	O
are	O
repeated	O
in	O
different	O
messages	O
,	O
the	O
extended	O
operations	O
for	O
SigComp	O
define	O
a	O
way	O
to	O
use	O
a	O
shared	O
dictionary	B-Algorithm
among	O
subsequent	O
messages	O
.	O
</s>
<s>
Moreover	O
,	O
in	O
order	O
to	O
speed	O
up	O
the	O
process	O
of	O
building	O
a	O
dictionary	B-Algorithm
along	O
subsequent	O
messages	O
and	O
provide	O
high	O
compression	B-General_Concept
ratios	I-General_Concept
since	O
the	O
first	O
INVITE	O
message	O
,	O
SIP	O
provides	O
a	O
static	O
SIP/SDP	O
dictionary	B-Algorithm
which	O
is	O
already	O
built	O
with	O
common	O
SIP	O
and	O
SDP	O
terms	O
.	O
</s>
<s>
There	O
is	O
a	O
mechanism	O
to	O
indicate	O
that	O
a	O
SIP	O
message	O
is	O
desired	O
to	O
be	O
compressed	B-General_Concept
.	O
</s>
<s>
This	O
mechanism	O
defines	O
the	O
comp	O
=	O
sigcomp	O
parameter	O
for	O
SIP	O
URIs	B-Protocol
,	O
which	O
signals	O
that	O
the	O
SIP	O
entity	O
identified	O
by	O
the	O
URI	B-Protocol
supports	O
SigComp	O
and	O
is	O
willing	O
to	O
receive	O
compressed	B-General_Concept
messages	O
.	O
</s>
<s>
When	O
used	O
in	O
request-URIs	O
,	O
it	O
indicates	O
that	O
the	O
request	O
is	O
to	O
be	O
compressed	B-General_Concept
,	O
while	O
in	O
Via	O
header	B-General_Concept
fields	I-General_Concept
it	O
signals	O
that	O
the	O
subsequent	O
response	O
is	O
to	O
be	O
compressed	B-General_Concept
.	O
</s>
<s>
In	O
order	O
to	O
obtain	O
even	O
shorter	O
SIP	O
messages	O
and	O
make	O
a	O
very	O
efficient	O
use	O
of	O
the	O
resources	O
,	O
the	O
content	O
indirection	O
extension	O
makes	O
it	O
possible	O
to	O
replace	O
a	O
MIME	B-Protocol
body	O
part	O
of	O
the	O
message	O
with	O
an	O
external	O
reference	O
,	O
typically	O
an	O
HTTP	B-Protocol
URI	B-Protocol
.	O
</s>
<s>
Network	B-Protocol
address	I-Protocol
translation	I-Protocol
(	O
NAT	O
)	O
makes	O
it	O
impossible	O
for	O
a	O
terminal	O
to	O
be	O
reached	O
from	O
outside	O
its	O
private	B-Protocol
network	I-Protocol
,	O
since	O
it	O
uses	O
a	O
private	B-Protocol
address	I-Protocol
that	O
is	O
mapped	O
to	O
a	O
public	O
one	O
when	O
packets	O
originated	O
by	O
the	O
terminal	O
cross	O
the	O
NAT	O
.	O
</s>
<s>
Therefore	O
,	O
NAT	B-Protocol
traversal	I-Protocol
mechanisms	O
are	O
needed	O
for	O
both	O
the	O
signaling	O
plane	O
and	O
the	O
media	O
plane	O
.	O
</s>
<s>
Internet	O
Engineering	O
Task	O
Force	O
's	O
RFC	O
6157	O
describes	O
the	O
necessary	O
mechanisms	O
to	O
guarantee	O
that	O
SIP	O
works	O
successfully	O
between	O
both	O
Internet	B-Protocol
Protocol	I-Protocol
versions	O
during	O
the	O
transition	B-Architecture
to	O
IPv6	B-Protocol
.	O
</s>
<s>
While	O
SIP	O
signaling	O
messages	O
can	O
be	O
transmitted	O
through	O
heterogeneous	O
IPv4/IPv6	O
networks	O
as	O
long	O
as	O
proxy	O
servers	O
and	O
DNS	B-Application
entries	O
are	O
properly	O
configured	O
to	O
relay	O
messages	O
across	O
both	O
networks	O
according	O
to	O
these	O
recommendations	O
,	O
user	O
agents	O
will	O
need	O
to	O
implement	O
extensions	O
so	O
that	O
they	O
can	O
directly	O
exchange	O
media	O
streams	O
.	O
</s>
<s>
These	O
extensions	O
are	O
related	O
to	O
the	O
Session	B-Language
Description	I-Language
Protocol	I-Language
offer/answer	O
initial	O
exchange	O
,	O
that	O
will	O
be	O
used	O
to	O
gather	O
the	O
IPv4	B-Protocol
and	O
IPv6	B-Protocol
addresses	B-Protocol
of	O
both	O
ends	O
so	O
that	O
they	O
can	O
establish	O
a	O
direct	O
communication	O
.	O
</s>
<s>
Apart	O
from	O
all	O
the	O
explained	O
extensions	O
to	O
SIP	O
that	O
make	O
it	O
possible	O
for	O
the	O
IMS	B-Application
to	O
work	O
successfully	O
,	O
it	O
is	O
also	O
necessary	O
that	O
the	O
IMS	B-Application
framework	O
interworks	O
and	O
exchanges	O
services	O
with	O
existing	O
network	O
infrastructures	O
,	O
mainly	O
the	O
Public	O
switched	O
telephone	O
network	O
(	O
PSTN	O
)	O
.	O
</s>
<s>
the	O
IMS	B-Application
network	O
)	O
:	O
</s>
<s>
Services	O
in	O
PSTN	O
requesting	O
Internet	B-Protocol
Services	I-Protocol
(	O
SPIRITS	O
)	O
,	O
that	O
provides	O
the	O
opposite	O
functionality	O
to	O
PINT	O
,	O
this	O
is	O
,	O
supporting	O
the	O
access	O
to	O
Internet	B-Protocol
services	I-Protocol
from	O
the	O
PSTN	O
.	O
</s>
<s>
Session	B-Application
Initiation	I-Application
Protocol	I-Application
for	O
Telephones	O
(	O
SIP-T	O
)	O
,	O
that	O
describes	O
the	O
practices	O
and	O
uses	O
of	O
these	O
gateways	O
.	O
</s>
<s>
ISDN	B-Protocol
User	I-Protocol
Part	I-Protocol
(	O
ISUP	B-Protocol
)	O
to	O
Session	B-Application
Initiation	I-Application
Protocol	I-Application
(	O
SIP	O
)	O
Mapping	O
,	O
which	O
makes	O
it	O
possible	O
to	O
translate	O
SIP	O
signaling	O
messages	O
into	O
ISUP	B-Protocol
messages	O
of	O
the	O
Signaling	O
System	O
No	O
.	O
</s>
