<s>
In	O
computer	B-Architecture
networking	I-Architecture
,	O
Point-to-Point	B-Protocol
Protocol	I-Protocol
(	O
PPP	O
)	O
is	O
a	O
data	B-Protocol
link	I-Protocol
layer	I-Protocol
(	O
layer	B-Protocol
2	I-Protocol
)	O
communication	O
protocol	O
between	O
two	O
routers	O
directly	O
without	O
any	O
host	O
or	O
any	O
other	O
networking	O
in	O
between	O
.	O
</s>
<s>
It	O
can	O
provide	O
loop	O
connection	O
authentication	O
,	O
transmission	O
encryption	O
,	O
and	O
data	B-General_Concept
compression	I-General_Concept
.	O
</s>
<s>
PPP	O
is	O
used	O
over	O
many	O
types	O
of	O
physical	O
networks	O
,	O
including	O
serial	O
cable	O
,	O
phone	O
line	O
,	O
trunk	O
line	O
,	O
cellular	O
telephone	O
,	O
specialized	O
radio	O
links	O
,	O
ISDN	B-Protocol
,	O
and	O
fiber	B-Application
optic	I-Application
links	I-Application
such	O
as	O
SONET	B-Protocol
.	O
</s>
<s>
Since	O
IP	B-Protocol
packets	O
cannot	O
be	O
transmitted	O
over	O
a	O
modem	B-Application
line	O
on	O
their	O
own	O
without	O
some	O
data	O
link	O
protocol	O
that	O
can	O
identify	O
where	O
the	O
transmitted	O
frame	O
starts	O
and	O
where	O
it	O
ends	O
,	O
Internet	O
service	O
providers	O
(	O
ISPs	O
)	O
have	O
used	O
PPP	O
for	O
customer	O
dial-up	O
access	O
to	O
the	O
Internet	O
.	O
</s>
<s>
Two	O
derivatives	O
of	O
PPP	O
,	O
Point-to-Point	B-Protocol
Protocol	I-Protocol
over	I-Protocol
Ethernet	I-Protocol
(	O
PPPoE	B-Protocol
)	O
and	O
Point-to-Point	B-Protocol
Protocol	I-Protocol
over	I-Protocol
ATM	I-Protocol
(	O
PPPoA	B-Protocol
)	O
,	O
are	O
used	O
most	O
commonly	O
by	O
ISPs	O
to	O
establish	O
a	O
digital	O
subscriber	O
line	O
(	O
DSL	O
)	O
Internet	O
service	O
LP	O
connection	O
with	O
customers	O
.	O
</s>
<s>
PPP	O
is	O
commonly	O
used	O
as	O
a	O
data	B-Protocol
link	I-Protocol
layer	I-Protocol
protocol	O
for	O
connection	O
over	O
synchronous	B-Application
and	O
asynchronous	B-Application
circuits	I-Application
,	O
where	O
it	O
has	O
largely	O
superseded	O
the	O
older	O
Serial	B-Protocol
Line	I-Protocol
Internet	I-Protocol
Protocol	I-Protocol
(	O
SLIP	O
)	O
and	O
telephone	O
company	O
mandated	O
standards	O
(	O
such	O
as	O
Link	B-Protocol
Access	I-Protocol
Protocol	I-Protocol
,	I-Protocol
Balanced	I-Protocol
(	O
LAPB	B-Protocol
)	O
in	O
the	O
X.25	B-Protocol
protocol	I-Protocol
suite	I-Protocol
)	O
.	O
</s>
<s>
PPP	O
was	O
designed	O
to	O
work	O
with	O
numerous	O
network	B-Application
layer	I-Application
protocols	O
,	O
including	O
Internet	B-Protocol
Protocol	I-Protocol
(	O
IP	B-Protocol
)	O
,	O
TRILL	B-Protocol
,	O
Novell	O
's	O
Internetwork	B-Protocol
Packet	I-Protocol
Exchange	I-Protocol
(	O
IPX	B-Protocol
)	O
,	O
NBF	B-Protocol
,	O
DECnet	B-Protocol
and	O
AppleTalk	B-Application
.	O
</s>
<s>
Like	O
SLIP	O
,	O
this	O
is	O
a	O
full	O
Internet	O
connection	O
over	O
telephone	O
lines	O
via	O
modem	B-Application
.	O
</s>
<s>
RFC	O
2516	O
describes	O
Point-to-Point	B-Protocol
Protocol	I-Protocol
over	I-Protocol
Ethernet	I-Protocol
(	O
PPPoE	B-Protocol
)	O
as	O
a	O
method	O
for	O
transmitting	O
PPP	O
over	O
Ethernet	O
that	O
is	O
sometimes	O
used	O
with	O
DSL	O
.	O
</s>
<s>
RFC	O
2364	O
describes	O
Point-to-Point	B-Protocol
Protocol	I-Protocol
over	I-Protocol
ATM	I-Protocol
(	O
PPPoA	B-Protocol
)	O
as	O
a	O
method	O
for	O
transmitting	O
PPP	O
over	O
ATM	B-Protocol
Adaptation	I-Protocol
Layer	I-Protocol
5	I-Protocol
(	O
AAL5	B-Protocol
)	O
,	O
which	O
is	O
also	O
a	O
common	O
alternative	O
to	O
PPPoE	B-Protocol
used	O
with	O
DSL	O
.	O
</s>
<s>
PPP	O
,	O
PPPoE	B-Protocol
and	O
PPPoA	B-Protocol
are	O
widely	O
used	O
in	O
WAN	O
lines	O
.	O
</s>
<s>
PPP	O
is	O
a	O
layered	B-Protocol
protocol	I-Protocol
that	O
has	O
three	O
components	O
:	O
</s>
<s>
An	O
encapsulation	O
component	O
that	O
is	O
used	O
to	O
transmit	O
datagrams	B-Protocol
over	O
the	O
specified	O
physical	B-Application
layer	I-Application
.	O
</s>
<s>
A	O
Link	B-Protocol
Control	I-Protocol
Protocol	I-Protocol
(	O
LCP	B-Protocol
)	O
to	O
establish	O
,	O
configure	O
,	O
and	O
test	O
the	O
link	O
as	O
well	O
as	O
negotiate	O
settings	O
,	O
options	O
and	O
the	O
use	O
of	O
features	O
.	O
</s>
<s>
One	O
or	O
more	O
Network	O
Control	O
Protocols	O
(	O
NCP	O
)	O
used	O
to	O
negotiate	O
optional	O
configuration	O
parameters	O
and	O
facilities	O
for	O
the	O
network	B-Application
layer	I-Application
.	O
</s>
<s>
LCP	B-Protocol
initiates	O
and	O
terminates	O
connections	O
gracefully	O
,	O
allowing	O
hosts	O
to	O
negotiate	O
connection	O
options	O
.	O
</s>
<s>
LCP	B-Protocol
provides	O
automatic	O
configuration	O
of	O
the	O
interfaces	O
at	O
each	O
end	O
(	O
such	O
as	O
setting	O
datagram	B-Protocol
size	O
,	O
escaped	O
characters	O
,	O
and	O
magic	O
numbers	O
)	O
and	O
for	O
selecting	O
optional	O
authentication	O
.	O
</s>
<s>
The	O
LCP	B-Protocol
protocol	O
runs	O
on	O
top	O
of	O
PPP	O
(	O
with	O
PPP	B-Protocol
protocol	I-Protocol
number	O
0xC021	O
)	O
and	O
therefore	O
a	O
basic	O
PPP	B-Protocol
connection	I-Protocol
has	O
to	O
be	O
established	O
before	O
LCP	B-Protocol
is	O
able	O
to	O
configure	O
it	O
.	O
</s>
<s>
RFC	O
1994	O
describes	O
Challenge-Handshake	B-Protocol
Authentication	I-Protocol
Protocol	I-Protocol
(	O
CHAP	B-Protocol
)	O
,	O
which	O
is	O
preferred	O
for	O
establishing	O
dial-up	O
connections	O
with	O
ISPs	O
.	O
</s>
<s>
Although	O
deprecated	O
,	O
Password	B-Protocol
Authentication	I-Protocol
Protocol	I-Protocol
(	O
PAP	B-Protocol
)	O
is	O
still	O
sometimes	O
used	O
.	O
</s>
<s>
Another	O
option	O
for	O
authentication	O
over	O
PPP	O
is	O
Extensible	B-Protocol
Authentication	I-Protocol
Protocol	I-Protocol
(	O
EAP	B-Protocol
)	O
described	O
in	O
RFC	O
2284	O
.	O
</s>
<s>
After	O
the	O
link	O
has	O
been	O
established	O
,	O
additional	O
network	O
(	O
layer	B-Application
3	I-Application
)	O
configuration	O
may	O
take	O
place	O
.	O
</s>
<s>
Most	O
commonly	O
,	O
the	O
Internet	B-Protocol
Protocol	I-Protocol
Control	I-Protocol
Protocol	I-Protocol
(	O
IPCP	B-Protocol
)	O
is	O
used	O
,	O
although	O
Internetwork	B-Protocol
Packet	I-Protocol
Exchange	I-Protocol
Control	I-Protocol
Protocol	I-Protocol
(	O
IPXCP	O
)	O
and	O
AppleTalk	B-Application
Control	I-Application
Protocol	I-Application
(	O
ATCP	O
)	O
were	O
once	O
popular	O
.	O
</s>
<s>
Internet	B-Protocol
Protocol	I-Protocol
Version	I-Protocol
6	I-Protocol
Control	O
Protocol	O
(	O
IPv6CP	O
)	O
will	O
see	O
extended	O
use	O
in	O
the	O
future	O
,	O
when	O
IPv6	B-Protocol
replaces	O
IPv4	B-Protocol
as	O
the	O
dominant	O
layer-3	B-Application
protocol	O
.	O
</s>
<s>
PPP	O
permits	O
multiple	O
network	B-Application
layer	I-Application
protocols	O
to	O
operate	O
on	O
the	O
same	O
communication	O
link	O
.	O
</s>
<s>
For	O
every	O
network	B-Application
layer	I-Application
protocol	O
used	O
,	O
a	O
separate	O
Network	O
Control	O
Protocol	O
(	O
NCP	O
)	O
is	O
provided	O
in	O
order	O
to	O
encapsulate	O
and	O
negotiate	O
options	O
for	O
the	O
multiple	O
network	B-Application
layer	I-Application
protocols	O
.	O
</s>
<s>
It	O
negotiates	O
network-layer	B-Application
information	O
,	O
e.g.	O
</s>
<s>
network	B-Protocol
address	I-Protocol
or	O
compression	O
options	O
,	O
after	O
the	O
connection	O
has	O
been	O
established	O
.	O
</s>
<s>
For	O
example	O
,	O
IP	B-Protocol
uses	O
IPCP	B-Protocol
,	O
and	O
Internetwork	B-Protocol
Packet	I-Protocol
Exchange	I-Protocol
(	O
IPX	B-Protocol
)	O
uses	O
the	O
Novell	O
IPX	B-Protocol
Control	O
Protocol	O
(	O
IPX/SPX	B-Protocol
)	O
.	O
</s>
<s>
NCPs	O
include	O
fields	O
containing	O
standardized	O
codes	O
to	O
indicate	O
the	O
network	B-Application
layer	I-Application
protocol	O
type	O
that	O
the	O
PPP	B-Protocol
connection	I-Protocol
encapsulates	O
.	O
</s>
<s>
When	O
the	O
node	O
sends	O
PPP	O
LCP	B-Protocol
messages	O
,	O
these	O
messages	O
may	O
include	O
a	O
magic	O
number	O
.	O
</s>
<s>
If	O
a	O
line	O
is	O
looped	O
,	O
the	O
node	O
receives	O
an	O
LCP	B-Protocol
message	O
with	O
its	O
own	O
magic	O
number	O
,	O
instead	O
of	O
getting	O
a	O
message	O
with	O
the	O
peer	O
's	O
magic	O
number	O
.	O
</s>
<s>
The	O
previous	O
section	O
introduced	O
the	O
use	O
of	O
LCP	B-Protocol
options	O
to	O
meet	O
specific	O
WAN	O
connection	O
requirements	O
.	O
</s>
<s>
PPP	O
may	O
include	O
the	O
following	O
LCP	B-Protocol
options	O
:	O
</s>
<s>
Two	O
authentication	O
choices	O
are	O
Password	B-Protocol
Authentication	I-Protocol
Protocol	I-Protocol
(	O
PAP	B-Protocol
)	O
and	O
Challenge	B-Protocol
Handshake	I-Protocol
Authentication	I-Protocol
Protocol	I-Protocol
(	O
CHAP	B-Protocol
)	O
.	O
</s>
<s>
Compression	O
-	O
Increases	O
the	O
effective	O
throughput	O
on	O
PPP	B-Protocol
connections	I-Protocol
by	O
reducing	O
the	O
amount	O
of	O
data	O
in	O
the	O
frame	O
that	O
must	O
travel	O
across	O
the	O
link	O
,	O
using	O
a	O
agreed-on	O
algorithm	O
such	O
as	O
BSD	O
compress	B-Device
or	O
Deflate	O
.	O
</s>
<s>
If	O
both	O
peers	O
agree	O
to	O
Address	O
field	O
and	O
Control	O
field	O
compression	O
during	O
LCP	B-Protocol
,	O
then	O
those	O
fields	O
are	O
omitted	O
.	O
</s>
<s>
The	O
Protocol	O
field	O
indicates	O
the	O
type	O
of	O
payload	O
packet	O
:	O
0xC021	O
for	O
LCP	B-Protocol
,	O
0x80xy	O
for	O
various	O
NCPs	O
,	O
0x0021	O
for	O
IP	B-Protocol
,	O
0x0029	O
AppleTalk	B-Application
,	O
0x002B	O
for	O
IPX	B-Protocol
,	O
0x003D	O
for	O
Multilink	O
,	O
0x003F	O
for	O
NetBIOS	B-Protocol
,	O
0x00FD	O
for	O
MPPC	B-Device
and	O
MPPE	B-Protocol
,	O
etc	O
.	O
</s>
<s>
PPP	O
is	O
limited	O
,	O
and	O
cannot	O
contain	O
general	O
Layer	B-Application
3	I-Application
data	O
,	O
unlike	O
EtherType	B-Protocol
.	O
</s>
<s>
The	O
Information	O
field	O
contains	O
the	O
PPP	O
payload	O
;	O
it	O
has	O
a	O
variable	O
length	O
with	O
a	O
negotiated	O
maximum	O
called	O
the	O
Maximum	B-Protocol
Transmission	I-Protocol
Unit	I-Protocol
.	O
</s>
<s>
PPP	O
frames	O
are	O
encapsulated	O
in	O
a	O
lower-layer	O
protocol	O
that	O
provides	O
framing	O
and	O
may	O
provide	O
other	O
functions	O
such	O
as	O
a	O
checksum	B-Algorithm
to	O
detect	O
transmission	O
errors	O
.	O
</s>
<s>
PPP	O
on	O
serial	B-Protocol
links	I-Protocol
is	O
usually	O
encapsulated	O
in	O
a	O
framing	O
similar	O
to	O
HDLC	O
,	O
described	O
by	O
IETF	O
RFC	O
1662	O
.	O
</s>
<s>
The	O
Address	O
and	O
Control	O
fields	O
always	O
have	O
the	O
value	O
hex	O
FF	O
(	O
for	O
"	O
all	O
stations	O
"	O
)	O
and	O
hex	O
03	O
(	O
for	O
"	O
unnumbered	O
information	O
"	O
)	O
,	O
and	O
can	O
be	O
omitted	O
whenever	O
PPP	O
LCP	B-Protocol
Address-and-Control-Field-Compression	O
(	O
ACFC	O
)	O
is	O
negotiated	O
.	O
</s>
<s>
The	O
frame	B-Protocol
check	I-Protocol
sequence	I-Protocol
(	O
FCS	O
)	O
field	O
is	O
used	O
for	O
determining	O
whether	O
an	O
individual	O
frame	O
has	O
an	O
error	O
.	O
</s>
<s>
It	O
contains	O
a	O
checksum	B-Algorithm
computed	O
over	O
the	O
frame	O
to	O
provide	O
basic	O
protection	O
against	O
errors	O
in	O
transmission	O
.	O
</s>
<s>
Link	O
Establishment	O
Phase	O
This	O
phase	O
is	O
where	O
Link	B-Protocol
Control	I-Protocol
Protocol	I-Protocol
negotiation	O
is	O
attempted	O
.	O
</s>
<s>
If	O
successful	O
,	O
control	O
goes	O
either	O
to	O
the	O
authentication	O
phase	O
or	O
the	O
Network-Layer	B-Application
Protocol	O
phase	O
,	O
depending	O
on	O
whether	O
authentication	O
is	O
desired	O
.	O
</s>
<s>
If	O
successful	O
,	O
control	O
goes	O
to	O
the	O
network-layer	B-Application
protocol	O
phase	O
.	O
</s>
<s>
Network-Layer	B-Application
Protocol	O
Phase	O
This	O
phase	O
is	O
where	O
each	O
desired	O
protocols	O
 '	O
Network	O
Control	O
Protocols	O
are	O
invoked	O
.	O
</s>
<s>
For	O
example	O
,	O
IPCP	B-Protocol
is	O
used	O
in	O
establishing	O
IP	B-Protocol
service	O
over	O
the	O
line	O
.	O
</s>
<s>
This	O
can	O
happen	O
if	O
there	O
is	O
an	O
authentication	O
failure	O
,	O
if	O
there	O
are	O
so	O
many	O
checksum	B-Algorithm
errors	O
that	O
the	O
two	O
parties	O
decide	O
to	O
tear	O
down	O
the	O
link	O
automatically	O
,	O
if	O
the	O
link	O
suddenly	O
fails	O
,	O
or	O
if	O
the	O
user	O
decides	O
to	O
hang	O
up	O
a	O
connection	O
.	O
</s>
<s>
Multilink	O
PPP	O
(	O
also	O
referred	O
to	O
as	O
MLPPP	O
,	O
MP	O
,	O
MPPP	O
,	O
MLP	O
,	O
or	O
Multilink	O
)	O
provides	O
a	O
method	O
for	O
spreading	O
traffic	O
across	O
multiple	O
distinct	O
PPP	B-Protocol
connections	I-Protocol
.	O
</s>
<s>
It	O
can	O
be	O
used	O
,	O
for	O
example	O
,	O
to	O
connect	O
a	O
home	O
computer	O
to	O
an	O
Internet	O
Service	O
Provider	O
using	O
two	O
traditional	O
56k	O
modems	B-Application
,	O
or	O
to	O
connect	O
a	O
company	O
through	O
two	O
leased	O
lines	O
.	O
</s>
<s>
On	O
a	O
single	O
PPP	O
line	O
frames	O
cannot	O
arrive	O
out	O
of	O
order	O
,	O
but	O
this	O
is	O
possible	O
when	O
the	O
frames	O
are	O
divided	O
among	O
multiple	O
PPP	B-Protocol
connections	I-Protocol
.	O
</s>
<s>
Multilink	O
PPP	O
is	O
an	O
example	O
of	O
a	O
link	B-Architecture
aggregation	I-Architecture
technology	O
.	O
</s>
<s>
Cisco	B-Protocol
IOS	I-Protocol
Release	O
11.1	O
and	O
later	O
supports	O
Multilink	O
PPP	O
.	O
</s>
<s>
With	O
PPP	O
,	O
one	O
cannot	O
establish	O
several	O
simultaneous	O
distinct	O
PPP	B-Protocol
connections	I-Protocol
over	O
a	O
single	O
link	O
.	O
</s>
<s>
Multiclass	O
PPP	O
is	O
a	O
kind	O
of	O
Multilink	O
PPP	O
where	O
each	O
"	O
class	O
"	O
of	O
traffic	O
uses	O
a	O
separate	O
sequence	B-Protocol
number	I-Protocol
space	O
and	O
reassembly	O
buffer	O
.	O
</s>
<s>
PPTP	B-Protocol
(	O
Point-to-Point	B-Protocol
Tunneling	I-Protocol
Protocol	I-Protocol
)	O
is	O
a	O
form	O
of	O
PPP	O
between	O
two	O
hosts	O
via	O
GRE	B-Protocol
using	O
encryption	O
(	O
MPPE	B-Protocol
)	O
and	O
compression	O
(	O
MPPC	B-Device
)	O
.	O
</s>
<s>
Many	O
protocols	O
can	O
be	O
used	O
to	O
tunnel	B-Protocol
data	O
over	O
IP	B-Protocol
networks	I-Protocol
.	O
</s>
<s>
Some	O
of	O
them	O
,	O
like	O
SSL	B-Protocol
,	O
SSH	B-Protocol
,	O
or	O
L2TP	B-Application
create	O
virtual	B-General_Concept
network	I-General_Concept
interfaces	I-General_Concept
and	O
give	O
the	O
impression	O
of	O
direct	O
physical	O
connections	O
between	O
the	O
tunnel	B-Protocol
endpoints	O
.	O
</s>
<s>
On	O
a	O
Linux	B-Application
host	O
for	O
example	O
,	O
these	O
interfaces	O
would	O
be	O
called	O
tun0	O
or	O
ppp0	O
.	O
</s>
<s>
As	O
there	O
are	O
only	O
two	O
endpoints	O
on	O
a	O
tunnel	B-Protocol
,	O
the	O
tunnel	B-Protocol
is	O
a	O
point-to-point	O
connection	O
and	O
PPP	O
is	O
a	O
natural	O
choice	O
as	O
a	O
data	B-Protocol
link	I-Protocol
layer	I-Protocol
protocol	O
between	O
the	O
virtual	B-General_Concept
network	I-General_Concept
interfaces	I-General_Concept
.	O
</s>
<s>
PPP	O
can	O
assign	O
IP	B-Protocol
addresses	O
to	O
these	O
virtual	B-General_Concept
interfaces	I-General_Concept
,	O
and	O
these	O
IP	B-Protocol
addresses	O
can	O
be	O
used	O
,	O
for	O
example	O
,	O
to	O
route	O
between	O
the	O
networks	O
on	O
both	O
sides	O
of	O
the	O
tunnel	B-Protocol
.	O
</s>
<s>
IPsec	B-Protocol
in	O
tunneling	O
mode	O
does	O
not	O
create	O
virtual	O
physical	O
interfaces	O
at	O
the	O
end	O
of	O
the	O
tunnel	B-Protocol
,	O
since	O
the	O
tunnel	B-Protocol
is	O
handled	O
directly	O
by	O
the	O
TCP/IP	B-Protocol
stack	I-Protocol
.	O
</s>
<s>
L2TP	B-Application
can	O
be	O
used	O
to	O
provide	O
these	O
interfaces	O
,	O
this	O
technique	O
is	O
called	O
L2TP/IPsec	O
.	O
</s>
<s>
In	O
this	O
case	O
too	O
,	O
PPP	O
provides	O
IP	B-Protocol
addresses	O
to	O
the	O
extremities	O
of	O
the	O
tunnel	B-Protocol
.	O
</s>
<s>
PPP	O
is	O
defined	O
in	O
RFC	O
1661	O
(	O
The	O
Point-to-Point	B-Protocol
Protocol	I-Protocol
,	O
July	O
1994	O
)	O
.	O
</s>
<s>
RFC	O
1547	O
(	O
Requirements	O
for	O
an	O
Internet	O
Standard	O
Point-to-Point	B-Protocol
Protocol	I-Protocol
,	O
December	O
1993	O
)	O
provides	O
historical	O
information	O
about	O
the	O
need	O
for	O
PPP	O
and	O
its	O
development	O
.	O
</s>
<s>
A	O
series	O
of	O
related	O
RFCs	O
have	O
been	O
written	O
to	O
define	O
how	O
a	O
variety	O
of	O
network	O
control	O
protocols-including	O
TCP/IP	B-Protocol
,	O
DECnet	B-Protocol
,	O
AppleTalk	B-Application
,	O
IPX	B-Protocol
,	O
and	O
others-work	O
with	O
PPP	O
.	O
</s>
