<s>
Peer-to-peer	B-Protocol
SIP	I-Protocol
(	O
P2P-SIP	B-Protocol
)	O
is	O
an	O
implementation	O
of	O
a	O
distributed	O
voice	B-Application
over	I-Application
Internet	I-Application
Protocol	I-Application
(	O
VoIP	B-Application
)	O
or	O
instant	B-Application
messaging	I-Application
communications	O
application	O
using	O
a	O
peer-to-peer	O
(	O
P2P	O
)	O
architecture	O
in	O
which	O
session	O
control	O
between	O
communication	O
end	O
points	O
is	O
facilitated	O
with	O
the	O
Session	B-Application
Initiation	I-Application
Protocol	I-Application
(	O
SIP	O
)	O
.	O
</s>
<s>
The	O
Session	B-Application
Initiation	I-Application
Protocol	I-Application
is	O
in	O
principle	O
a	O
client-server	B-Operating_System
protocol	I-Operating_System
,	O
however	O
it	O
has	O
been	O
described	O
in	O
analogy	O
to	O
the	O
P2P	O
relationship	O
,	O
called	O
a	O
dialog	O
,	O
which	O
is	O
defined	O
by	O
a	O
unique	O
combination	O
of	O
SIP	O
protocol	O
parameters	O
(	O
To-tag	O
,	O
From-tag	O
,	O
Call-ID	O
)	O
.	O
</s>
<s>
Both	O
endpoints	O
of	O
a	O
communication	O
session	O
implement	O
a	O
user-agent	B-Protocol
server	O
and	O
a	O
user-agent	B-Protocol
client	O
,	O
which	O
enables	O
any	O
two	O
user	B-Protocol
agents	I-Protocol
to	O
communicate	O
directly	O
with	O
one	O
another	O
without	O
the	O
mediation	O
of	O
another	O
a	O
central	O
switching	O
system	O
.	O
</s>
<s>
SIP	O
also	O
provides	O
facilities	O
of	O
registering	O
(	O
REGISTER	O
request	O
)	O
the	O
network	O
location	O
of	O
a	O
user	B-Protocol
agent	I-Protocol
with	O
other	O
SIP	O
elements	O
and	O
subscription	O
(	O
SUBSCRIBE	O
request	O
)	O
and	O
notification	O
(	O
NOTIFY	O
request	O
)	O
features	O
for	O
event	O
tracking	O
between	O
user	B-Protocol
agents	I-Protocol
.	O
</s>
<s>
The	O
search	O
key	O
in	O
such	O
a	O
mechanism	O
is	O
the	O
Uniform	B-Protocol
Resource	I-Protocol
Identifier	I-Protocol
(	O
URI	B-Protocol
)	O
of	O
a	O
user	B-Protocol
agent	I-Protocol
.	O
</s>
<s>
This	O
URI	B-Protocol
requires	O
resolution	O
to	O
a	O
particular	O
device	O
or	O
Uniform	O
Resource	O
Locator	O
(	O
URL	O
)	O
that	O
must	O
be	O
performed	O
in	O
real	O
time	O
.	O
</s>
<s>
P2P-SIP	B-Protocol
systems	O
may	O
employ	O
structured	O
peer-to-peer	O
approaches	O
,	O
as	O
well	O
as	O
unstructured	O
peer-to-peer	O
architectures	O
.	O
</s>
<s>
The	O
Internet	O
Engineering	O
Task	O
Force	O
(	O
IETF	O
)	O
is	O
conducting	O
a	O
working	O
group	O
(	O
P2PSIP	B-Protocol
)	O
that	O
develops	O
standards-track	O
specifications	O
for	O
P2P-SIP	B-Protocol
.	O
</s>
