<s>
Port	B-Protocol
Control	I-Protocol
Protocol	I-Protocol
(	O
PCP	O
)	O
is	O
a	O
computer	B-Architecture
networking	I-Architecture
protocol	O
that	O
allows	O
hosts	B-Device
on	O
IPv4	B-Protocol
or	O
IPv6	B-Protocol
networks	O
to	O
control	O
how	O
the	O
incoming	O
IPv4	B-Protocol
or	O
IPv6	B-Protocol
packets	B-Protocol
are	O
translated	O
and	O
forwarded	O
by	O
an	O
upstream	O
router	B-Protocol
that	O
performs	O
network	B-Protocol
address	I-Protocol
translation	I-Protocol
(	O
NAT	O
)	O
or	O
packet	B-Application
filtering	I-Application
.	O
</s>
<s>
By	O
allowing	O
hosts	B-Device
to	O
create	O
explicit	O
port	B-Protocol
forwarding	I-Protocol
rules	O
,	O
handling	O
of	O
the	O
network	O
traffic	O
can	O
be	O
easily	O
configured	O
to	O
make	O
hosts	B-Device
placed	O
behind	O
NATs	O
or	O
firewalls	B-Application
reachable	O
from	O
the	O
rest	O
of	O
the	O
Internet	O
(	O
so	O
they	O
can	O
also	O
act	O
as	O
network	B-Application
servers	I-Application
)	O
,	O
which	O
is	O
a	O
requirement	O
for	O
many	O
applications	O
.	O
</s>
<s>
Additionally	O
,	O
explicit	O
port	B-Protocol
forwarding	I-Protocol
rules	O
available	O
through	O
PCP	O
allow	O
hosts	B-Device
to	O
reduce	O
the	O
amount	O
of	O
generated	O
traffic	O
by	O
eliminating	O
workarounds	B-Application
in	O
form	O
of	O
outgoing	O
NAT	O
keepalive	B-Protocol
messages	O
,	O
which	O
are	O
required	O
for	O
maintaining	O
connections	O
to	O
servers	O
and	O
for	O
various	O
NAT	B-Protocol
traversal	I-Protocol
techniques	O
such	O
as	O
TCP	B-Protocol
hole	O
punching	O
.	O
</s>
<s>
At	O
the	O
same	O
time	O
,	O
less	O
generated	O
traffic	O
reduces	O
the	O
power	O
consumption	O
,	O
directly	O
improving	O
the	O
battery	O
runtime	O
for	O
mobile	B-Application
devices	I-Application
.	O
</s>
<s>
PCP	O
was	O
standardized	O
in	O
2013	O
as	O
a	O
successor	O
to	O
the	O
NAT	B-Protocol
Port	I-Protocol
Mapping	I-Protocol
Protocol	I-Protocol
(	O
NAT-PMP	B-Protocol
)	O
,	O
with	O
which	O
it	O
shares	O
similar	O
protocol	O
concepts	O
and	O
packet	B-Protocol
formats	O
.	O
</s>
<s>
In	O
environments	O
where	O
a	O
Universal	B-Application
Plug	I-Application
and	I-Application
Play	I-Application
Internet	B-Protocol
Gateway	I-Protocol
Device	I-Protocol
(	O
UPnP	B-Application
IGD	O
)	O
is	O
used	O
in	O
the	O
local	B-General_Concept
network	I-General_Concept
,	O
an	O
interworking	O
function	O
between	O
the	O
UPnP	B-Application
IGD	O
and	O
PCP	O
is	O
required	O
to	O
be	O
embedded	O
in	O
the	O
IGD	O
.	O
</s>
<s>
The	O
UPnP	B-Application
IGD-PCP	O
Interworking	O
Function	O
is	O
specified	O
in	O
RFC6970	O
.	O
</s>
<s>
DHCP	B-Protocol
(	O
IPv4	B-Protocol
and	O
IPv6	B-Protocol
)	O
options	O
to	O
configure	O
hosts	B-Device
with	O
Port	B-Protocol
Control	I-Protocol
Protocol	I-Protocol
(	O
PCP	O
)	O
server	O
IP	B-Protocol
addresses	I-Protocol
are	O
specified	O
in	O
RFC7291	O
.	O
</s>
<s>
In	O
environments	O
where	O
NAT64	B-Protocol
is	O
deployed	O
,	O
PCP	O
allows	O
to	O
learn	O
the	O
IPv6	B-Protocol
prefix(es )	O
used	O
by	O
a	O
PCP-controlled	O
NAT64	B-Protocol
device	O
to	O
build	O
IPv4-converted	O
IPv6	B-Protocol
addresses	O
by	O
the	O
NAT64	B-Protocol
(	O
RFC7225	O
)	O
.	O
</s>
<s>
Many	O
applications	O
and	O
network	B-Device
equipment	I-Device
deployments	O
require	O
their	O
network	O
locations	O
to	O
be	O
reachable	O
from	O
outside	O
their	O
local	B-General_Concept
networks	I-General_Concept
,	O
following	O
the	O
originally	O
envisioned	O
model	O
of	O
IP	B-Protocol
end-to-end	B-Application
connectivity	I-Application
across	O
the	O
Internet	O
,	O
so	O
they	O
can	O
operate	O
as	O
network	B-Application
servers	I-Application
and	O
accept	O
connections	O
from	O
remote	O
clients	B-Protocol
.	O
</s>
<s>
An	O
example	O
of	O
such	O
equipment	O
is	O
an	O
IP	B-Algorithm
camera	I-Algorithm
,	O
which	O
includes	O
a	O
network	B-Application
server	I-Application
that	O
provides	O
remote	O
surveillance	O
over	O
IP	B-Protocol
networks	O
.	O
</s>
<s>
Usually	O
,	O
network	B-Device
equipment	I-Device
deployments	O
place	O
the	O
devices	O
behind	O
routers	B-Protocol
or	O
firewalls	B-Application
that	O
perform	O
NAT	O
(	O
to	O
enable	O
sharing	O
of	O
an	O
IPv4	B-Protocol
address	I-Protocol
,	O
for	O
example	O
)	O
or	O
packet	B-Application
filtering	I-Application
(	O
for	O
improved	O
network	O
security	O
and	O
protection	O
)	O
,	O
ending	O
up	O
with	O
breaking	O
the	O
end-to-end	B-Application
connectivity	I-Application
and	O
rendering	O
the	O
equipment	O
and	O
applications	O
inaccessible	O
from	O
the	O
rest	O
of	O
the	O
Internet	O
.	O
</s>
<s>
Making	O
the	O
deployed	O
equipment	O
accessible	O
,	O
by	O
extending	O
its	O
server	O
role	O
beyond	O
the	O
local	B-General_Concept
network	I-General_Concept
,	O
requires	O
either	O
manual	O
configuration	O
of	O
port	B-Protocol
forwarding	I-Protocol
at	O
the	O
network	B-Application
gateway	I-Application
(	O
which	O
is	O
usually	O
a	O
CPE	O
)	O
,	O
or	O
application-level	O
workarounds	B-Application
that	O
initiate	O
connections	O
from	O
the	O
deployed	O
equipment	O
to	O
additional	O
intermediate	O
servers	O
used	O
for	O
"	O
merging	O
"	O
those	O
"	O
firewall	B-Application
punching	O
"	O
connections	O
and	O
connections	O
from	O
the	O
actual	O
clients	B-Protocol
.	O
</s>
<s>
For	O
example	O
,	O
an	O
online	B-Application
computer	I-Application
game	I-Application
(	O
which	O
acts	O
as	O
a	O
client	O
)	O
requires	O
communication	O
with	O
a	O
game	O
server	O
for	O
exchanging	O
gameplay	O
data	O
.	O
</s>
<s>
In	O
order	O
to	O
make	O
it	O
possible	O
for	O
a	O
game	O
server	O
to	O
provide	O
data	O
to	O
its	O
clients	B-Protocol
,	O
those	O
clients	B-Protocol
must	O
be	O
made	O
accessible	O
to	O
the	O
server	O
.	O
</s>
<s>
Usually	O
,	O
clients	B-Protocol
initiate	O
connections	O
to	O
the	O
game	O
server	O
to	O
open	O
communication	O
channels	O
.	O
</s>
<s>
However	O
,	O
such	O
open	O
connections	O
can	O
become	O
idle	O
and	O
can	O
subsequently	O
be	O
closed	O
by	O
network	B-Application
gateways	I-Application
,	O
leading	O
to	O
the	O
necessity	O
of	O
maintaining	O
them	O
by	O
using	O
a	O
form	O
of	O
keepalive	B-Protocol
messages	O
.	O
</s>
<s>
Keepalive	B-Protocol
messages	O
are	O
small	O
messages	O
that	O
are	O
sent	O
between	O
client	O
and	O
server	O
that	O
create	O
traffic	O
over	O
a	O
communication	O
channel	O
and	O
therefore	O
prevent	O
gateway	B-Application
servers	O
from	O
closing	O
it	O
.	O
</s>
<s>
This	O
increases	O
network	O
chatter	O
,	O
wastes	O
network	O
bandwidth	O
and	O
CPU	B-General_Concept
cycles	I-General_Concept
,	O
and	O
decreases	O
the	O
autonomy	O
of	O
battery-powered	O
devices	O
.	O
</s>
<s>
Additionally	O
,	O
some	O
network	O
applications	O
(	O
for	O
example	O
,	O
FTP	B-Protocol
)	O
require	O
dynamic	O
opening	O
of	O
multiple	O
connections	O
,	O
which	O
involves	O
application-level	O
gateways	B-Application
(	O
ALGs	O
)	O
and	O
additionally	O
increases	O
complexity	O
.	O
</s>
<s>
PCP	O
allows	O
equipment	O
and	O
applications	O
to	O
create	O
explicit	O
mappings	O
between	O
an	O
external	O
IP	B-Protocol
address	I-Protocol
,	O
protocol	O
and	O
port	B-Protocol
,	O
and	O
an	O
internal	O
IP	B-Protocol
address	I-Protocol
,	O
protocol	O
and	O
port	B-Protocol
.	O
</s>
<s>
With	O
such	O
explicit	O
mappings	O
in	O
place	O
,	O
inbound	O
communication	O
can	O
reach	O
the	O
hosts	B-Device
behind	O
a	O
NAT	O
or	O
firewall	B-Application
,	O
which	O
either	O
expands	O
their	O
server	O
roles	O
beyond	O
boundaries	O
of	O
local	B-General_Concept
networks	I-General_Concept
,	O
or	O
makes	O
use	O
of	O
various	O
services	O
simplified	O
and	O
less	O
resource-consuming	O
.	O
</s>
<s>
Created	O
mappings	O
are	O
permanent	O
to	O
the	O
extent	O
of	O
having	O
a	O
known	O
lifetime	O
that	O
can	O
be	O
extended	O
,	O
which	O
is	O
similar	O
to	O
the	O
way	O
Dynamic	B-Protocol
Host	I-Protocol
Configuration	I-Protocol
Protocol	I-Protocol
(	O
DHCP	B-Protocol
)	O
implements	O
its	O
leases	O
.	O
</s>
<s>
At	O
the	O
same	O
time	O
,	O
PCP	O
allows	O
applications	O
to	O
create	O
additional	O
mappings	O
dynamically	O
as	O
required	O
,	O
which	O
reduces	O
or	O
eliminates	O
the	O
need	O
for	O
having	O
ALG-enabled	O
NAT	B-Protocol
devices	I-Protocol
and	O
firewalls	B-Application
.	O
</s>
<s>
Created	O
explicit	O
mappings	O
have	O
a	O
known	O
lifetime	O
,	O
commonly	O
several	O
hours	O
,	O
with	O
no	O
need	O
for	O
application-level	O
keepalive	B-Protocol
messages	O
to	O
be	O
exchanged	O
between	O
hosts	B-Device
and	O
servers	O
for	O
the	O
purpose	O
of	O
preserving	O
the	O
mapping	O
.	O
</s>
<s>
As	O
a	O
result	O
,	O
network	O
usage	O
and	O
power	O
consumption	O
are	O
reduced	O
,	O
and	O
application-level	O
keepalive	B-Protocol
logic	O
no	O
longer	O
needs	O
to	O
be	O
implemented	O
at	O
client	O
and	O
server	O
sides	O
.	O
</s>
<s>
The	O
PCP	O
mapping	O
response	O
provides	O
the	O
application	O
with	O
associated	O
externally	O
visible	O
parameters	O
(	O
IP	B-Protocol
address	I-Protocol
,	O
protocol	O
and	O
port	B-Protocol
)	O
that	O
can	O
then	O
be	O
announced	O
to	O
other	O
clients	B-Protocol
in	O
application-specific	O
ways	O
so	O
incoming	O
connections	O
can	O
be	O
established	O
.	O
</s>
<s>
Additionally	O
,	O
PCP	O
can	O
inform	O
applications	O
when	O
the	O
external	O
IP	B-Protocol
address	I-Protocol
is	O
changed	O
while	O
a	O
mapping	O
is	O
already	O
established	O
.	O
</s>
<s>
Various	O
types	O
of	O
NAT	O
can	O
be	O
handled	O
by	O
PCP	O
,	O
providing	O
support	O
for	O
NAT64	B-Protocol
,	O
NAT66	B-Protocol
,	O
and	O
NAT44	B-Protocol
;	O
inclusion	O
of	O
PCP	O
into	O
IPv4	B-Protocol
and	O
IPv6	B-Protocol
firewall	B-Application
devices	O
is	O
also	O
supported	O
.	O
</s>
<s>
PCP	O
is	O
designed	O
to	O
be	O
used	O
on	O
both	O
large-scale	O
aggregation	O
points	O
(	O
for	O
example	O
,	O
as	O
part	O
of	O
carrier-grade	B-Protocol
NATs	I-Protocol
)	O
,	O
and	O
inside	O
less	O
expensive	O
consumer-grade	O
devices	O
.	O
</s>
<s>
Both	O
long-term	O
(	O
for	O
an	O
IP	B-Algorithm
camera	I-Algorithm
or	O
a	O
temperature	O
sensor	O
acting	O
as	O
a	O
server	O
,	O
for	O
example	O
)	O
and	O
short-term	O
mappings	O
(	O
while	O
playing	O
an	O
online	B-Application
computer	I-Application
game	I-Application
,	O
for	O
example	O
)	O
are	O
supported	O
.	O
</s>
<s>
PCP	O
supports	O
transport	B-Architecture
layer	I-Architecture
protocols	O
that	O
use	O
16-bit	O
port	B-Protocol
numbers	I-Protocol
(	O
for	O
example	O
,	O
TCP	B-Protocol
,	O
UDP	B-Protocol
,	O
Stream	B-Protocol
Control	I-Protocol
Transmission	I-Protocol
Protocol	I-Protocol
(	O
SCTP	B-Protocol
)	O
or	O
Datagram	B-Protocol
Congestion	I-Protocol
Control	I-Protocol
Protocol	I-Protocol
(	O
DCCP	B-Protocol
)	O
.	O
</s>
<s>
Protocols	O
that	O
do	O
not	O
use	O
port	B-Protocol
numbers	I-Protocol
(	O
for	O
example	O
,	O
Resource	B-Protocol
Reservation	I-Protocol
Protocol	I-Protocol
(	O
RSVP	B-Protocol
)	O
,	O
Encapsulating	O
Security	O
Payload	O
(	O
ESP	O
)	O
,	O
ICMP	B-Protocol
or	O
ICMPv6	B-Protocol
)	O
are	O
supported	O
for	O
IPv4	B-Protocol
firewall	B-Application
,	O
IPv6	B-Protocol
firewall	B-Application
and	O
NPTv6	B-Protocol
(	O
IPv6	B-Protocol
prefix	I-Protocol
translation	I-Protocol
)	O
functions	O
,	O
but	O
cannot	O
be	O
supported	O
by	O
more	O
than	O
one	O
client	O
per	O
external	O
IP	B-Protocol
address	I-Protocol
in	O
the	O
case	O
of	O
NAT	O
.	O
</s>
<s>
The	O
PCP	O
specification	O
does	O
not	O
define	O
a	O
mechanism	O
for	O
dealing	O
with	O
multi-homed	B-Protocol
networks	I-Protocol
(	O
which	O
have	O
multiple	O
network	B-Application
gateways	I-Application
or	O
default	B-Protocol
routes	I-Protocol
)	O
.	O
</s>
<s>
However	O
,	O
if	O
the	O
different	O
networks	O
each	O
have	O
their	O
own	O
external	O
IP	O
address(es )	O
,	O
a	O
given	O
PCP	O
mapping	O
can	O
only	O
use	O
one	O
or	O
the	O
other	O
because	O
the	O
protocol	O
requires	O
one	O
specific	O
external	O
IP	B-Protocol
address	I-Protocol
to	O
be	O
provided	O
to	O
the	O
client	O
.	O
</s>
<s>
If	O
that	O
network	O
should	O
then	O
become	O
unavailable	O
the	O
PCP	O
mapping	O
would	O
have	O
to	O
be	O
updated	O
to	O
use	O
an	O
external	O
IP	B-Protocol
address	I-Protocol
from	O
the	O
other	O
network	O
.	O
</s>
<s>
PCP	O
was	O
standardized	O
in	O
2013	O
as	O
a	O
successor	O
to	O
the	O
NAT	B-Protocol
Port	I-Protocol
Mapping	I-Protocol
Protocol	I-Protocol
(	O
NAT-PMP	B-Protocol
)	O
,	O
sharing	O
similar	O
protocol	O
concepts	O
and	O
packet	B-Protocol
formats	O
with	O
it	O
.	O
</s>
<s>
As	O
one	O
of	O
the	O
design	O
differences	O
,	O
NAT-PMP	B-Protocol
is	O
pretty	O
much	O
limited	O
to	O
the	O
deployment	O
on	O
consumer-grade	O
devices	O
,	O
while	O
PCP	O
is	O
designed	O
to	O
also	O
support	O
carrier-grade	O
equipment	O
.	O
</s>
<s>
Since	O
2005	O
,	O
NAT-PMP	B-Protocol
has	O
been	O
implemented	O
in	O
various	O
Apple	O
products	O
.	O
</s>
<s>
PCP	O
relates	O
to	O
the	O
Internet	B-Protocol
Gateway	I-Protocol
Device	I-Protocol
Protocol	I-Protocol
(	O
IGDP	O
)	O
,	O
which	O
was	O
standardized	O
in	O
2001	O
as	O
part	O
of	O
the	O
Universal	B-Application
Plug	I-Application
and	I-Application
Play	I-Application
(	O
UPnP	B-Application
)	O
specification	O
.	O
</s>
<s>
The	O
NAT-PMP	B-Protocol
specification	O
contains	O
a	O
list	O
of	O
the	O
problems	O
with	O
IGDP	O
that	O
prompted	O
the	O
creation	O
of	O
NAT-PMP	B-Protocol
,	O
and	O
subsequently	O
,	O
its	O
successor	O
PCP	O
.	O
</s>
<s>
Excluding	O
the	O
attackers	O
capable	O
of	O
altering	O
network	B-Protocol
packets	I-Protocol
exchanged	O
while	O
an	O
explicit	O
PCP	O
mapping	O
is	O
created	O
(	O
packets	B-Protocol
that	O
contain	O
negotiation	O
required	O
for	O
establishing	O
an	O
explicit	O
mapping	O
,	O
which	O
is	O
exchanged	O
between	O
hosts	B-Device
and	O
PCP-enabled	O
NAT	B-Protocol
devices	I-Protocol
or	O
firewalls	B-Application
)	O
,	O
PCP	O
is	O
considered	O
to	O
be	O
secure	O
as	O
long	O
as	O
created	O
explicit	O
mappings	O
do	O
not	O
exceed	O
the	O
domain	O
of	O
implicit	O
mappings	O
.	O
</s>
<s>
In	O
other	O
words	O
,	O
implicit	O
mappings	O
are	O
created	O
as	O
a	O
result	O
of	O
the	O
way	O
NAT	B-Protocol
devices	I-Protocol
and	O
firewalls	B-Application
are	O
handling	O
regular	O
outbound	O
client	O
connections	O
,	O
meaning	O
that	O
PCP	O
is	O
safe	O
as	O
long	O
as	O
no	O
new	O
mapping	O
possibilities	O
are	O
introduced	O
through	O
the	O
explicit	O
mapping	O
mechanism	O
.	O
</s>
<s>
From	O
the	O
security	O
standpoint	O
,	O
an	O
important	O
PCP	O
feature	O
is	O
the	O
mapping	O
request	B-Protocol
option	O
.	O
</s>
<s>
When	O
used	O
,	O
this	O
option	O
signifies	O
that	O
the	O
IP	B-Protocol
address	I-Protocol
specified	O
additionally	O
as	O
part	O
of	O
the	O
mapping	O
request	B-Protocol
should	O
be	O
used	O
as	O
the	O
internal	O
address	O
for	O
the	O
created	O
explicit	O
mapping	O
,	O
rather	O
than	O
following	O
the	O
default	O
behavior	O
of	O
using	O
source	O
IP	B-Protocol
address	I-Protocol
of	O
the	O
actual	O
mapping	O
request	B-Protocol
packet	B-Protocol
for	O
that	O
purpose	O
.	O
</s>
<s>
Such	O
mapping	O
requests	O
can	O
end	O
up	O
with	O
a	O
PCP-enabled	O
NAT	B-Protocol
device	I-Protocol
or	O
firewall	B-Application
granting	O
explicit	O
mapping	O
privileges	O
higher	O
than	O
allowed	O
by	O
implicit	O
mappings	O
due	O
to	O
unknown	O
rules	O
imposed	O
elsewhere	O
for	O
the	O
specified	O
IP	B-Protocol
address	I-Protocol
,	O
allowing	O
that	O
way	O
an	O
attacker	O
to	O
steal	O
some	O
traffic	O
,	O
or	O
to	O
conduct	O
a	O
denial-of-service	O
(	O
DoS	O
)	O
attack	O
.	O
</s>
<s>
Additionally	O
,	O
explicit	O
PCP	O
security	O
mechanisms	O
are	O
available	O
as	O
extensions	O
to	O
the	O
PCP	O
protocol	O
,	O
providing	O
authentication	O
and	O
access	O
control	O
mechanisms	O
by	O
using	O
an	O
authenticated	O
and	O
integrity-protected	O
in-band	O
signalling	O
channel	O
,	O
which	O
relies	O
on	O
Extensible	B-Protocol
Authentication	I-Protocol
Protocol	I-Protocol
(	O
EAP	O
)	O
to	O
perform	O
the	O
authentication	O
between	O
devices	O
involved	O
in	O
a	O
PCP	O
negotiation	O
session	O
.	O
</s>
<s>
Such	O
PCP-enabled	O
NAT	B-Protocol
devices	I-Protocol
or	O
firewalls	B-Application
may	O
still	O
accept	O
unauthenticated	O
mapping	O
requests	O
;	O
at	O
the	O
same	O
time	O
,	O
all	O
previously	O
described	O
explicit	O
mapping	O
constraints	O
still	O
apply	O
.	O
</s>
<s>
Internally	O
,	O
PCP	O
works	O
by	O
exchanging	O
control	O
messages	O
between	O
hosts	B-Device
and	O
PCP-enabled	O
NAT	B-Protocol
devices	I-Protocol
or	O
firewalls	B-Application
(	O
referred	O
to	O
as	O
servers	O
)	O
,	O
using	O
User	B-Protocol
Datagram	I-Protocol
Protocol	I-Protocol
(	O
UDP	B-Protocol
)	O
as	O
the	O
underlying	O
protocol	O
.	O
</s>
<s>
This	O
communication	O
consists	O
of	O
port	B-Protocol
mapping	I-Protocol
requests	O
created	O
by	O
the	O
hosts	B-Device
that	O
result	O
in	O
responses	B-Protocol
once	O
submitted	O
to	O
and	O
processed	O
by	O
the	O
servers	O
.	O
</s>
<s>
Following	O
UDP	B-Protocol
's	O
nature	O
of	O
unreliability	O
,	O
which	O
means	O
that	O
UDP	B-Protocol
datagrams	B-Protocol
can	O
be	O
lost	O
,	O
duplicated	O
or	O
reordered	O
,	O
after	O
submitting	O
a	O
request	B-Protocol
there	O
is	O
no	O
guarantee	O
for	O
a	O
response	O
of	O
any	O
kind	O
,	O
thus	O
host	B-Device
requests	I-Device
are	O
also	O
referred	O
to	O
as	O
"	O
hints	O
"	O
.	O
</s>
<s>
In	O
addition	O
to	O
direct	O
responses	B-Protocol
,	O
servers	O
also	O
generate	O
gratuitous	O
notifications	O
for	O
example	O
,	O
unicast	B-Protocol
notifications	O
to	O
inform	O
hosts	B-Device
of	O
changes	O
in	O
the	O
external	O
IP	B-Protocol
address	I-Protocol
.	O
</s>
<s>
+	O
Port	B-Protocol
Control	I-Protocol
Protocol	I-Protocol
opcodes	B-Language
Opcode	B-Language
Description	O
Creates	O
or	O
renews	O
a	O
mapping	O
for	O
inbound	O
forwarding	O
,	O
allowing	O
a	O
hosts	B-Device
to	O
act	O
as	O
a	O
server	O
and	O
receive	O
inbound	O
communication	O
.	O
</s>
<s>
Announces	O
various	O
changes	O
to	O
the	O
hosts	B-Device
,	O
including	O
server	O
restarts	O
and	O
changes	O
to	O
the	O
external	O
IP	B-Protocol
address	I-Protocol
.	O
</s>
<s>
Such	O
a	O
simplified	O
design	O
is	O
based	O
on	O
having	O
all	O
messages	O
self-describing	O
and	O
complete	O
,	O
with	O
no	O
additional	O
context	B-Operating_System
required	O
for	O
each	O
message	O
to	O
be	O
successfully	O
processed	O
.	O
</s>
<s>
Servers	O
may	O
decide	O
to	O
silently	O
ignore	O
host	B-Device
requests	I-Device
,	O
in	O
case	O
they	O
are	O
unable	O
to	O
process	O
them	O
at	O
the	O
moment	O
;	O
in	O
such	O
cases	O
,	O
hosts	B-Device
need	O
to	O
retransmit	B-Protocol
the	O
request	B-Protocol
.	O
</s>
<s>
Also	O
,	O
hosts	B-Device
may	O
safely	O
decide	O
to	O
silently	O
ignore	O
any	O
unwanted	O
mapping	O
responses	B-Protocol
.	O
</s>
<s>
For	O
the	O
purpose	O
of	O
creating	O
PCP	O
requests	O
,	O
IP	B-Protocol
address	I-Protocol
of	O
the	O
server	O
is	O
either	O
manually	O
configured	O
on	O
the	O
host	O
,	O
found	O
as	O
part	O
of	O
the	O
host	O
's	O
DHCP	B-Protocol
lease	O
,	O
or	O
set	O
to	O
the	O
host	O
's	O
configured	O
default	B-Protocol
gateway	I-Protocol
.	O
</s>
<s>
Host	B-Device
request	I-Device
messages	O
are	O
sent	O
from	O
any	O
source	O
UDP	B-Protocol
port	B-Protocol
on	O
a	O
client	O
to	O
the	O
server	O
's	O
UDP	B-Protocol
port	B-Protocol
5351	O
that	O
it	O
listens	O
to	O
;	O
unsolicited	O
multicast	O
server	O
notifications	O
(	O
such	O
as	O
server	O
restart	O
announcements	O
)	O
are	O
sent	O
from	O
the	O
server	O
's	O
UDP	B-Protocol
port	B-Protocol
5351	O
to	O
the	O
UDP	B-Protocol
port	B-Protocol
5350	O
on	O
hosts	B-Device
which	O
they	O
listen	O
to	O
.	O
</s>
<s>
Maximum	O
UDP	B-Protocol
payload	O
length	O
for	O
all	O
PCP	O
messages	O
is	O
1100	O
octets	O
.	O
</s>
<s>
Each	O
PCP	O
message	O
consists	O
of	O
a	O
request	B-Protocol
or	O
response	O
header	O
containing	O
an	O
opcode	B-Language
that	O
determines	O
the	O
associated	O
operation	O
,	O
any	O
relevant	O
opcode-specific	O
information	O
(	O
such	O
as	O
which	O
ports	O
are	O
to	O
be	O
mapped	O
)	O
,	O
and	O
zero	O
or	O
more	O
options	O
(	O
such	O
as	O
the	O
option	O
described	O
above	O
)	O
.	O
</s>
<s>
Result	O
codes	O
are	O
returned	O
as	O
part	O
of	O
server	O
responses	B-Protocol
;	O
each	O
result	O
code	O
has	O
an	O
associated	O
lifetime	O
,	O
which	O
tells	O
the	O
hosts	B-Device
when	O
certain	O
operations	O
may	O
be	O
retried	O
or	O
should	O
be	O
repeated	O
.	O
</s>
