<s>
Multiprotocol	B-Protocol
Label	I-Protocol
Switching	I-Protocol
(	O
MPLS	B-Protocol
)	O
is	O
a	O
routing	O
technique	O
in	O
telecommunications	O
networks	O
that	O
directs	O
data	O
from	O
one	O
node	B-Protocol
to	O
the	O
next	O
based	O
on	O
labels	O
rather	O
than	O
network	O
addresses	O
.	O
</s>
<s>
Whereas	O
network	O
addresses	O
identify	O
endpoints	B-Protocol
the	O
labels	O
identify	O
established	O
paths	O
between	O
endpoints	B-Protocol
.	O
</s>
<s>
MPLS	B-Protocol
can	O
encapsulate	O
packets	B-Protocol
of	O
various	O
network	O
protocols	O
,	O
hence	O
the	O
multiprotocol	O
component	O
of	O
the	O
name	O
.	O
</s>
<s>
MPLS	B-Protocol
supports	O
a	O
range	O
of	O
access	O
technologies	O
,	O
including	O
T1/E1	B-Protocol
,	O
ATM	O
,	O
Frame	B-Protocol
Relay	I-Protocol
,	O
and	O
DSL	O
.	O
</s>
<s>
In	O
an	O
MPLS	B-Protocol
network	O
,	O
labels	O
are	O
assigned	O
to	O
data	B-Protocol
packets	I-Protocol
.	O
</s>
<s>
Packet-forwarding	O
decisions	O
are	O
made	O
solely	O
on	O
the	O
contents	O
of	O
this	O
label	O
,	O
without	O
the	O
need	O
to	O
examine	O
the	O
packet	B-Protocol
itself	O
.	O
</s>
<s>
The	O
primary	O
benefit	O
is	O
to	O
eliminate	O
dependence	O
on	O
a	O
particular	O
OSI	O
model	O
data	B-Protocol
link	I-Protocol
layer	I-Protocol
(	O
layer	B-Protocol
2	I-Protocol
)	O
technology	O
,	O
and	O
eliminate	O
the	O
need	O
for	O
multiple	O
layer-2	B-Protocol
networks	O
to	O
satisfy	O
different	O
types	O
of	O
traffic	O
.	O
</s>
<s>
Multiprotocol	B-Protocol
label	I-Protocol
switching	I-Protocol
belongs	O
to	O
the	O
family	O
of	O
packet-switched	B-Protocol
networks	I-Protocol
.	O
</s>
<s>
MPLS	B-Protocol
operates	O
at	O
a	O
layer	O
that	O
is	O
generally	O
considered	O
to	O
lie	O
between	O
traditional	O
definitions	O
of	O
OSI	B-Protocol
Layer	I-Protocol
2	I-Protocol
(	O
data	B-Protocol
link	I-Protocol
layer	I-Protocol
)	O
and	O
Layer	B-Application
3	I-Application
(	O
network	B-Application
layer	I-Application
)	O
,	O
and	O
thus	O
is	O
often	O
referred	O
to	O
as	B-Architecture
a	O
layer	O
2.5	O
protocol	O
.	O
</s>
<s>
It	O
was	O
designed	O
to	O
provide	O
a	O
unified	O
data-carrying	O
service	O
for	O
both	O
circuit-based	O
clients	O
and	O
packet-switching	B-Protocol
clients	O
which	O
provide	O
a	O
datagram	B-Protocol
service	I-Protocol
model	O
.	O
</s>
<s>
It	O
can	O
be	O
used	O
to	O
carry	O
many	O
different	O
kinds	O
of	O
traffic	O
,	O
including	O
IP	B-Protocol
packets	B-Protocol
,	O
as	B-Architecture
well	O
as	B-Architecture
native	O
Asynchronous	O
Transfer	O
Mode	O
(	O
ATM	O
)	O
,	O
Frame	B-Protocol
Relay	I-Protocol
,	O
Synchronous	B-Protocol
Optical	I-Protocol
Networking	I-Protocol
(	O
SONET	B-Protocol
)	O
or	O
Ethernet	O
.	O
</s>
<s>
A	O
number	O
of	O
different	O
technologies	O
were	O
previously	O
deployed	O
with	O
essentially	O
identical	O
goals	O
,	O
such	O
as	B-Architecture
Frame	B-Protocol
Relay	I-Protocol
and	O
ATM	O
.	O
</s>
<s>
Frame	B-Protocol
Relay	I-Protocol
and	O
ATM	O
use	O
labels	O
to	O
move	O
frames	B-Protocol
or	O
cells	O
through	O
a	O
network	O
.	O
</s>
<s>
The	O
header	O
of	O
the	O
Frame	B-Protocol
Relay	I-Protocol
frame	O
and	O
the	O
ATM	O
cell	O
refers	O
to	O
the	O
virtual	B-Protocol
circuit	I-Protocol
that	O
the	O
frame	O
or	O
cell	O
resides	O
on	O
.	O
</s>
<s>
The	O
similarity	O
between	O
Frame	B-Protocol
Relay	I-Protocol
,	O
ATM	O
,	O
and	O
MPLS	B-Protocol
is	O
that	O
at	O
each	O
hop	O
throughout	O
the	O
network	O
,	O
the	O
label	O
value	O
in	O
the	O
header	O
is	O
changed	O
.	O
</s>
<s>
This	O
is	O
different	O
from	O
the	O
forwarding	B-Protocol
of	I-Protocol
IP	I-Protocol
packets	I-Protocol
.	O
</s>
<s>
MPLS	B-Protocol
technologies	O
have	O
evolved	O
with	O
the	O
strengths	O
and	O
weaknesses	O
of	O
ATM	O
in	O
mind	O
.	O
</s>
<s>
MPLS	B-Protocol
is	O
designed	O
to	O
have	O
lower	O
overhead	O
than	O
ATM	O
while	O
providing	O
connection-oriented	B-Protocol
services	I-Protocol
for	O
variable-length	O
frames	B-Protocol
,	O
and	O
has	O
replaced	O
much	O
use	O
of	O
ATM	O
in	O
the	O
market	O
.	O
</s>
<s>
MPLS	B-Protocol
dispenses	O
with	O
the	O
cell-switching	O
and	O
signaling-protocol	O
baggage	O
of	O
ATM	O
.	O
</s>
<s>
MPLS	B-Protocol
recognizes	O
that	O
small	O
ATM	O
cells	O
are	O
not	O
needed	O
in	O
the	O
core	O
of	O
modern	O
networks	O
,	O
since	O
modern	O
optical	O
networks	O
are	O
fast	O
enough	O
that	O
even	O
full-length	O
1500	O
byte	O
packets	B-Protocol
do	O
not	O
incur	O
significant	O
real-time	O
queuing	O
delays	O
.	O
</s>
<s>
At	O
the	O
same	O
time	O
,	O
MPLS	B-Protocol
attempts	O
to	O
preserve	O
the	O
traffic	O
engineering	O
(	O
TE	O
)	O
and	O
out-of-band	B-General_Concept
control	I-General_Concept
that	O
made	O
Frame	B-Protocol
Relay	I-Protocol
and	O
ATM	O
attractive	O
for	O
deploying	O
large-scale	O
networks	O
.	O
</s>
<s>
Their	O
IP	B-Protocol
Switching	O
technology	O
,	O
which	O
was	O
defined	O
only	O
to	O
work	O
over	O
ATM	O
,	O
did	O
not	O
achieve	O
market	O
dominance	O
.	O
</s>
<s>
Cisco	O
Systems	O
introduced	O
a	O
related	O
proposal	O
,	O
not	O
restricted	O
to	O
ATM	O
transmission	O
,	O
called	O
Tag	B-Protocol
Switching	I-Protocol
with	O
its	O
Tag	O
Distribution	O
Protocol	O
(	O
TDP	O
)	O
.	O
</s>
<s>
It	O
was	O
a	O
Cisco	O
proprietary	O
proposal	O
,	O
and	O
was	O
renamed	O
Label	B-Protocol
Switching	I-Protocol
.	O
</s>
<s>
One	O
original	O
motivation	O
was	O
to	O
allow	O
the	O
creation	O
of	O
simple	O
high-speed	O
switches	O
since	O
for	O
a	O
significant	O
length	O
of	O
time	O
it	O
was	O
impossible	O
to	O
forward	O
IP	B-Protocol
packets	B-Protocol
entirely	O
in	O
hardware	O
.	O
</s>
<s>
Advances	O
in	O
VLSI	O
have	O
made	O
hardware	O
forwarding	B-Protocol
of	I-Protocol
IP	I-Protocol
packets	I-Protocol
possible	O
and	O
common	O
.	O
</s>
<s>
The	O
current	O
advantages	O
of	O
MPLS	B-Protocol
primarily	O
revolve	O
around	O
the	O
ability	O
to	O
support	O
multiple	O
service	O
models	O
and	O
perform	O
traffic	O
management	O
.	O
</s>
<s>
MPLS	B-Protocol
also	O
offers	O
a	O
robust	O
recovery	O
framework	O
that	O
goes	O
beyond	O
the	O
simple	O
protection	O
rings	O
of	O
synchronous	B-Protocol
optical	I-Protocol
networking	I-Protocol
(	O
SONET/SDH	B-Protocol
)	O
.	O
</s>
<s>
MPLS	B-Protocol
works	O
by	O
prefixing	O
packets	B-Protocol
with	O
an	O
MPLS	B-Protocol
header	O
,	O
containing	O
one	O
or	O
more	O
labels	O
.	O
</s>
<s>
This	O
is	O
called	O
a	O
label	O
stack	B-Application
.	O
</s>
<s>
Each	O
entry	O
in	O
the	O
label	O
stack	B-Application
contains	O
four	O
fields	O
:	O
</s>
<s>
A	O
label	O
with	O
the	O
value	O
of	O
1	O
represents	O
the	O
router	B-Protocol
alert	I-Protocol
label	I-Protocol
.	O
</s>
<s>
A	O
1-bit	O
bottom	O
of	O
stack	B-Application
flag	O
.	O
</s>
<s>
If	O
this	O
is	O
set	O
,	O
it	O
signifies	O
that	O
the	O
current	O
label	O
is	O
the	O
last	O
in	O
the	O
stack	B-Application
.	O
</s>
<s>
An	O
8-bit	O
TTL	O
(	O
time	B-Protocol
to	I-Protocol
live	I-Protocol
)	O
field	O
.	O
</s>
<s>
These	O
MPLS-labeled	O
packets	B-Protocol
are	O
switched	O
based	O
on	O
the	O
label	O
instead	O
of	O
a	O
lookup	B-Data_Structure
in	O
the	O
IP	B-Protocol
routing	I-Protocol
table	O
.	O
</s>
<s>
When	O
MPLS	B-Protocol
was	O
conceived	O
,	O
label	B-Protocol
switching	I-Protocol
was	O
faster	O
than	O
a	O
routing	B-Protocol
table	I-Protocol
lookup	B-Data_Structure
because	O
switching	O
could	O
take	O
place	O
directly	O
within	O
the	O
switched	B-Architecture
fabric	I-Architecture
and	O
avoided	O
CPU	O
and	O
software	O
involvement	O
.	O
</s>
<s>
In	O
the	O
case	O
of	O
Ethernet	O
frames	B-Protocol
this	O
is	O
done	O
through	O
the	O
use	O
of	O
EtherType	B-Protocol
values	O
0x8847	O
and	O
0x8848	O
,	O
for	O
unicast	B-Protocol
and	O
multicast	O
connections	O
respectively	O
.	O
</s>
<s>
An	O
MPLS	B-Protocol
router	O
that	O
performs	O
routing	O
based	O
only	O
on	O
the	O
label	O
is	O
called	O
a	O
label	B-Protocol
switch	I-Protocol
router	I-Protocol
(	O
LSR	O
)	O
or	O
transit	O
router	O
.	O
</s>
<s>
This	O
is	O
a	O
type	O
of	O
router	O
located	O
in	O
the	O
middle	O
of	O
an	O
MPLS	B-Protocol
network	O
.	O
</s>
<s>
It	O
is	O
responsible	O
for	O
switching	O
the	O
labels	O
used	O
to	O
route	O
packets	B-Protocol
.	O
</s>
<s>
When	O
an	O
LSR	O
receives	O
a	O
packet	B-Protocol
,	O
it	O
uses	O
the	O
label	O
included	O
in	O
the	O
packet	B-Protocol
header	O
as	B-Architecture
an	O
index	O
to	O
determine	O
the	O
next	O
hop	O
on	O
the	O
label-switched	O
path	O
(	O
LSP	O
)	O
and	O
a	O
corresponding	O
label	O
for	O
the	O
packet	B-Protocol
from	O
a	O
lookup	B-Data_Structure
table	I-Data_Structure
.	O
</s>
<s>
The	O
old	O
label	O
is	O
then	O
removed	O
from	O
the	O
header	O
and	O
replaced	O
with	O
the	O
new	O
label	O
before	O
the	O
packet	B-Protocol
is	O
routed	O
forward	O
.	O
</s>
<s>
A	O
label	B-Protocol
edge	I-Protocol
router	I-Protocol
(	O
LER	O
,	O
also	O
known	O
as	B-Architecture
edge	O
LSR	O
)	O
is	O
a	O
router	O
that	O
operates	O
at	O
the	O
edge	O
of	O
an	O
MPLS	B-Protocol
network	O
and	O
acts	O
as	B-Architecture
the	O
entry	O
and	O
exit	O
points	O
for	O
the	O
network	O
.	O
</s>
<s>
LERs	O
push	O
an	O
MPLS	B-Protocol
label	O
onto	O
an	O
incoming	O
packet	B-Protocol
and	O
pop	O
it	O
off	O
an	O
outgoing	O
packet	B-Protocol
.	O
</s>
<s>
Alternatively	O
,	O
under	O
penultimate	B-General_Concept
hop	I-General_Concept
popping	I-General_Concept
this	O
function	O
may	O
instead	O
be	O
performed	O
by	O
the	O
LSR	O
directly	O
connected	O
to	O
the	O
LER	O
.	O
</s>
<s>
When	O
forwarding	O
an	O
IP	B-Protocol
datagram	B-Protocol
into	O
the	O
MPLS	B-Protocol
domain	O
,	O
an	O
LER	O
uses	O
routing	O
information	O
to	O
determine	O
the	O
appropriate	O
label	O
to	O
be	O
affixed	O
,	O
labels	O
the	O
packet	B-Protocol
accordingly	O
,	O
and	O
then	O
forwards	O
the	O
labeled	O
packet	B-Protocol
into	O
the	O
MPLS	B-Protocol
domain	O
.	O
</s>
<s>
Likewise	O
,	O
upon	O
receiving	O
a	O
labeled	O
packet	B-Protocol
that	O
is	O
destined	O
to	O
exit	O
the	O
MPLS	B-Protocol
domain	O
,	O
the	O
LER	O
strips	O
off	O
the	O
label	O
and	O
forwards	O
the	O
resulting	O
IP	B-Protocol
packet	B-Protocol
using	O
normal	O
IP	B-Protocol
forwarding	I-Protocol
rules	O
.	O
</s>
<s>
In	O
the	O
specific	O
context	O
of	O
an	O
MPLS-based	O
virtual	B-Application
private	I-Application
network	I-Application
(	O
VPN	B-Application
)	O
,	O
LERs	O
that	O
function	O
as	B-Architecture
ingress	B-General_Concept
or	O
egress	B-General_Concept
routers	I-General_Concept
to	O
the	O
VPN	B-Application
are	O
often	O
called	O
provider	B-Protocol
edge	I-Protocol
(	O
PE	O
)	O
routers	O
.	O
</s>
<s>
Devices	O
that	O
function	O
only	O
as	B-Architecture
transit	O
routers	O
are	O
similarly	O
called	O
provider	B-General_Concept
(	O
P	O
)	O
routers	O
.	O
</s>
<s>
The	O
job	O
of	O
a	O
P	B-General_Concept
router	I-General_Concept
is	O
significantly	O
easier	O
than	O
that	O
of	O
a	O
PE	B-Protocol
router	I-Protocol
.	O
</s>
<s>
Labels	O
may	O
be	O
distributed	O
between	O
LERs	O
and	O
LSRs	O
using	O
the	O
Label	B-Protocol
Distribution	I-Protocol
Protocol	I-Protocol
(	O
LDP	B-Protocol
)	O
or	O
Resource	B-Protocol
Reservation	I-Protocol
Protocol	I-Protocol
(	O
RSVP	B-Protocol
)	O
.	O
</s>
<s>
LSRs	O
in	O
an	O
MPLS	B-Protocol
network	O
regularly	O
exchange	O
label	O
and	O
reachability	O
information	O
with	O
each	O
other	O
using	O
standardized	O
procedures	O
in	O
order	O
to	O
build	O
a	O
complete	O
picture	O
of	O
the	O
network	O
so	O
that	O
they	O
can	O
then	O
use	O
that	O
information	O
to	O
forward	O
the	O
packets	B-Protocol
.	O
</s>
<s>
Label-switched	O
paths	O
(	O
LSPs	O
)	O
are	O
established	O
by	O
the	O
network	O
operator	O
for	O
a	O
variety	O
of	O
purposes	O
,	O
such	O
as	B-Architecture
to	O
create	O
network-based	O
IP	B-Protocol
virtual	B-Application
private	I-Application
networks	I-Application
or	O
to	O
route	O
traffic	O
along	O
specified	O
paths	O
through	O
the	O
network	O
.	O
</s>
<s>
In	O
many	O
respects	O
,	O
LSPs	O
are	O
not	O
different	O
from	O
permanent	O
virtual	B-Protocol
circuits	I-Protocol
(	O
PVCs	O
)	O
in	O
ATM	O
or	O
Frame	B-Protocol
Relay	I-Protocol
networks	I-Protocol
,	O
except	O
that	O
they	O
are	O
not	O
dependent	O
on	O
a	O
particular	O
layer-2	B-Protocol
technology	O
.	O
</s>
<s>
When	O
an	O
unlabeled	O
packet	B-Protocol
enters	O
the	O
ingress	B-General_Concept
router	I-General_Concept
and	O
needs	O
to	O
be	O
passed	O
on	O
to	O
an	O
MPLS	B-Protocol
tunnel	B-Protocol
,	O
the	O
router	O
first	O
determines	O
the	O
forwarding	B-General_Concept
equivalence	I-General_Concept
class	I-General_Concept
(	O
FEC	B-General_Concept
)	O
for	O
the	O
packet	B-Protocol
and	O
then	O
inserts	O
one	O
or	O
more	O
labels	O
in	O
the	O
packet	B-Protocol
's	O
newly	O
created	O
MPLS	B-Protocol
header	O
.	O
</s>
<s>
The	O
packet	B-Protocol
is	O
then	O
passed	O
on	O
to	O
the	O
next	O
hop	O
router	O
for	O
this	O
tunnel	B-Protocol
.	O
</s>
<s>
From	O
an	O
OSI	O
model	O
perspective	O
,	O
the	O
MPLS	B-Protocol
Header	O
is	O
added	O
between	O
the	O
network	B-Application
layer	I-Application
header	O
and	O
link	B-Protocol
layer	I-Protocol
header	O
.	O
</s>
<s>
When	O
a	O
labeled	O
packet	B-Protocol
is	O
received	O
by	O
an	O
MPLS	B-Protocol
router	O
,	O
the	O
topmost	O
label	O
is	O
examined	O
.	O
</s>
<s>
Based	O
on	O
the	O
contents	O
of	O
the	O
label	O
a	O
swap	O
,	O
push	O
or	O
pop	O
operation	O
is	O
performed	O
on	O
the	O
packet	B-Protocol
's	O
label	O
stack	B-Application
.	O
</s>
<s>
Routers	O
can	O
have	O
prebuilt	O
lookup	B-Data_Structure
tables	I-Data_Structure
that	O
tell	O
them	O
which	O
kind	O
of	O
operation	O
to	O
do	O
based	O
on	O
the	O
topmost	O
label	O
of	O
the	O
incoming	O
packet	B-Protocol
so	O
they	O
can	O
process	O
the	O
packet	B-Protocol
very	O
quickly	O
.	O
</s>
<s>
In	O
a	O
swap	O
operation	O
the	O
label	O
is	O
swapped	O
with	O
a	O
new	O
label	O
,	O
and	O
the	O
packet	B-Protocol
is	O
forwarded	O
along	O
the	O
path	O
associated	O
with	O
the	O
new	O
label	O
.	O
</s>
<s>
In	O
a	O
push	O
operation	O
a	O
new	O
label	O
is	O
pushed	O
on	O
top	O
of	O
the	O
existing	O
label	O
,	O
effectively	O
encapsulating	O
the	O
packet	B-Protocol
in	O
another	O
layer	O
of	O
MPLS	B-Protocol
.	O
</s>
<s>
This	O
allows	O
hierarchical	B-Protocol
routing	I-Protocol
of	O
MPLS	B-Protocol
packets	B-Protocol
.	O
</s>
<s>
Notably	O
,	O
this	O
is	O
used	O
by	O
MPLS	B-Application
VPNs	I-Application
.	O
</s>
<s>
In	O
a	O
pop	O
operation	O
the	O
label	O
is	O
removed	O
from	O
the	O
packet	B-Protocol
,	O
which	O
may	O
reveal	O
an	O
inner	O
label	O
below	O
.	O
</s>
<s>
If	O
the	O
popped	O
label	O
was	O
the	O
last	O
on	O
the	O
label	O
stack	B-Application
,	O
the	O
packet	B-Protocol
leaves	O
the	O
MPLS	B-Protocol
tunnel	B-Protocol
.	O
</s>
<s>
This	O
can	O
be	O
done	O
by	O
the	O
egress	B-General_Concept
router	I-General_Concept
,	O
or	O
at	O
the	O
penultimate	O
hop	O
.	O
</s>
<s>
During	O
these	O
operations	O
,	O
the	O
contents	O
of	O
the	O
packet	B-Protocol
below	O
the	O
MPLS	B-Protocol
Label	O
stack	B-Application
are	O
not	O
examined	O
.	O
</s>
<s>
Indeed	O
,	O
transit	O
routers	O
typically	O
need	O
only	O
to	O
examine	O
the	O
topmost	O
label	O
on	O
the	O
stack	B-Application
.	O
</s>
<s>
The	O
forwarding	O
of	O
the	O
packet	B-Protocol
is	O
done	O
based	O
on	O
the	O
contents	O
of	O
the	O
labels	O
,	O
which	O
allows	O
protocol-independent	O
packet	B-Protocol
forwarding	O
that	O
does	O
not	O
need	O
to	O
look	O
at	O
a	O
protocol-dependent	O
routing	B-Protocol
table	I-Protocol
and	O
avoids	O
the	O
expensive	O
IP	B-Protocol
longest	B-Protocol
prefix	I-Protocol
match	I-Protocol
at	O
each	O
hop	O
.	O
</s>
<s>
At	O
the	O
egress	B-General_Concept
router	I-General_Concept
,	O
when	O
the	O
last	O
label	O
has	O
been	O
popped	O
,	O
only	O
the	O
payload	O
remains	O
.	O
</s>
<s>
This	O
can	O
be	O
an	O
IP	B-Protocol
packet	B-Protocol
or	O
any	O
type	O
of	O
packet	B-Protocol
.	O
</s>
<s>
The	O
egress	B-General_Concept
router	I-General_Concept
must	O
,	O
therefore	O
,	O
have	O
routing	O
information	O
for	O
the	O
packet	B-Protocol
's	O
payload	O
since	O
it	O
must	O
forward	O
it	O
without	O
the	O
help	O
of	O
label	O
lookup	B-Data_Structure
tables	I-Data_Structure
.	O
</s>
<s>
An	O
MPLS	B-Protocol
transit	O
router	O
has	O
no	O
such	O
requirement	O
.	O
</s>
<s>
Usually	O
,	O
the	O
last	O
label	O
is	O
popped	O
off	O
at	O
the	O
penultimate	O
hop	O
(	O
the	O
hop	O
before	O
the	O
egress	B-General_Concept
router	I-General_Concept
)	O
.	O
</s>
<s>
This	O
is	O
called	O
penultimate	B-General_Concept
hop	I-General_Concept
popping	I-General_Concept
(	O
PHP	O
)	O
.	O
</s>
<s>
This	O
is	O
useful	O
in	O
cases	O
where	O
the	O
egress	B-General_Concept
router	I-General_Concept
has	O
many	O
packets	B-Protocol
leaving	O
MPLS	B-Protocol
tunnels	O
and	O
thus	O
spends	O
significant	O
CPU	O
resources	O
on	O
these	O
transitions	O
.	O
</s>
<s>
By	O
using	O
PHP	O
,	O
transit	O
routers	O
connected	O
directly	O
to	O
this	O
egress	B-General_Concept
router	I-General_Concept
effectively	O
offload	O
it	O
,	O
by	O
popping	O
the	O
last	O
label	O
themselves	O
.	O
</s>
<s>
In	O
the	O
label	B-Protocol
distribution	I-Protocol
protocols	I-Protocol
,	O
this	O
PHP	O
label	O
pop	O
action	O
is	O
advertised	O
as	B-Architecture
label	O
value	O
3	O
(	O
implicit	O
null	O
)	O
and	O
is	O
never	O
found	O
in	O
a	O
label	O
,	O
since	O
it	O
means	O
that	O
the	O
label	O
is	O
to	O
be	O
popped	O
.	O
</s>
<s>
This	O
optimization	O
is	O
no	O
longer	O
that	O
useful	O
(	O
like	O
for	O
initial	O
rationales	O
for	O
MPLS	B-Protocol
–	O
easier	O
operations	O
for	O
the	O
routers	O
)	O
.	O
</s>
<s>
Several	O
MPLS	B-Protocol
services	O
(	O
including	O
end-to-end	O
QoS	O
management	O
,	O
and	O
6PE	O
)	O
imply	O
to	O
keep	O
a	O
label	O
even	O
between	O
the	O
penultimate	O
and	O
the	O
last	O
MPLS	B-Protocol
router	O
,	O
with	O
a	O
label	O
disposition	O
always	O
done	O
on	O
the	O
last	O
MPLS	B-Protocol
router	O
:	O
the	O
«	O
Ultimate	O
Hop	O
Popping»	O
(	O
UHP	O
)	O
.	O
</s>
<s>
A	O
label-switched	O
path	O
(	O
LSP	O
)	O
is	O
a	O
path	O
through	O
an	O
MPLS	B-Protocol
network	O
,	O
set	O
up	O
by	O
the	O
NMS	O
or	O
by	O
a	O
signaling	O
protocol	O
such	O
as	B-Architecture
LDP	B-Protocol
,	O
RSVP-TE	B-Protocol
,	O
BGP	B-Protocol
(	O
or	O
the	O
now	O
deprecated	O
CR-LDP	B-Protocol
)	O
.	O
</s>
<s>
The	O
path	O
is	O
set	O
up	O
based	O
on	O
criteria	O
in	O
the	O
FEC	B-General_Concept
.	O
</s>
<s>
The	O
path	O
begins	O
at	O
a	O
label	B-Protocol
edge	I-Protocol
router	I-Protocol
(	O
LER	O
)	O
,	O
which	O
makes	O
a	O
decision	O
on	O
which	O
label	O
to	O
prefix	O
to	O
a	O
packet	B-Protocol
,	O
based	O
on	O
the	O
appropriate	O
FEC	B-General_Concept
.	O
</s>
<s>
It	O
then	O
forwards	O
the	O
packet	B-Protocol
along	O
to	O
the	O
next	O
router	O
in	O
the	O
path	O
,	O
which	O
swaps	O
the	O
packet	B-Protocol
's	O
outer	O
label	O
for	O
another	O
label	O
,	O
and	O
forwards	O
it	O
to	O
the	O
next	O
router	O
.	O
</s>
<s>
The	O
last	O
router	O
in	O
the	O
path	O
removes	O
the	O
label	O
from	O
the	O
packet	B-Protocol
and	O
forwards	O
the	O
packet	B-Protocol
based	I-Protocol
on	O
the	O
header	O
of	O
its	O
next	O
layer	O
,	O
for	O
example	O
IPv4	B-Protocol
.	O
</s>
<s>
Due	O
to	O
the	O
forwarding	O
of	O
packets	B-Protocol
through	O
an	O
LSP	O
being	O
opaque	O
to	O
higher	O
network	B-Application
layers	I-Application
,	O
an	O
LSP	O
is	O
also	O
sometimes	O
referred	O
to	O
as	B-Architecture
an	O
MPLS	B-Protocol
tunnel	B-Protocol
.	O
</s>
<s>
The	O
router	O
which	O
first	O
prefixes	O
the	O
MPLS	B-Protocol
header	O
to	O
a	O
packet	B-Protocol
is	O
called	O
an	O
ingress	B-General_Concept
router	I-General_Concept
.	O
</s>
<s>
The	O
last	O
router	O
in	O
an	O
LSP	O
,	O
which	O
pops	O
the	O
label	O
from	O
the	O
packet	B-Protocol
,	O
is	O
called	O
an	O
egress	B-General_Concept
router	I-General_Concept
.	O
</s>
<s>
Routers	O
in	O
between	O
,	O
which	O
need	O
only	O
swap	O
labels	O
,	O
are	O
called	O
transit	O
routers	O
or	O
label	B-Protocol
switch	I-Protocol
routers	I-Protocol
(	O
LSRs	O
)	O
.	O
</s>
<s>
Note	O
that	O
LSPs	O
are	O
unidirectional	O
;	O
they	O
enable	O
a	O
packet	B-Protocol
to	O
be	O
label	O
switched	O
through	O
the	O
MPLS	B-Protocol
network	O
from	O
one	O
endpoint	O
to	O
another	O
.	O
</s>
<s>
When	O
protection	O
is	O
considered	O
,	O
LSPs	O
could	O
be	O
categorized	O
as	B-Architecture
primary	O
(	O
working	O
)	O
,	O
secondary	O
(	O
backup	O
)	O
and	O
tertiary	O
(	O
LSP	O
of	O
last	O
resort	O
)	O
.	O
</s>
<s>
As	B-Architecture
described	O
above	O
,	O
LSPs	O
are	O
normally	O
P2P	O
(	O
point	O
to	O
point	O
)	O
.	O
</s>
<s>
A	O
new	O
concept	O
of	O
LSPs	O
,	O
which	O
are	O
known	O
as	B-Architecture
P2MP	O
(	O
point	O
to	O
multi-point	O
)	O
,	O
was	O
introduced	O
recently	O
.	O
</s>
<s>
There	O
are	O
two	O
standardized	O
protocols	O
for	O
managing	O
MPLS	B-Protocol
paths	O
:	O
the	O
Label	B-Protocol
Distribution	I-Protocol
Protocol	I-Protocol
(	O
LDP	B-Protocol
)	O
and	O
RSVP-TE	B-Protocol
,	O
an	O
extension	O
of	O
the	O
Resource	B-Protocol
Reservation	I-Protocol
Protocol	I-Protocol
(	O
RSVP	B-Protocol
)	O
for	O
traffic	O
engineering	O
.	O
</s>
<s>
Furthermore	O
,	O
there	O
exist	O
extensions	O
of	O
the	O
Border	B-Protocol
Gateway	I-Protocol
Protocol	I-Protocol
(	O
BGP	B-Protocol
)	O
that	O
can	O
be	O
used	O
to	O
manage	O
an	O
MPLS	B-Protocol
path	O
.	O
</s>
<s>
An	O
MPLS	B-Protocol
header	O
does	O
not	O
identify	O
the	O
type	O
of	O
data	O
carried	O
inside	O
the	O
MPLS	B-Protocol
path	O
.	O
</s>
<s>
If	O
one	O
wants	O
to	O
carry	O
two	O
different	O
types	O
of	O
traffic	O
between	O
the	O
same	O
two	O
routers	O
,	O
with	O
different	O
treatment	O
by	O
the	O
core	O
routers	O
for	O
each	O
type	O
,	O
one	O
has	O
to	O
establish	O
a	O
separate	O
MPLS	B-Protocol
path	O
for	O
each	O
type	O
of	O
traffic	O
.	O
</s>
<s>
Multicast	O
was	O
,	O
for	O
the	O
most	O
part	O
,	O
an	O
after-thought	O
in	O
MPLS	B-Protocol
design	O
.	O
</s>
<s>
It	O
was	O
introduced	O
by	O
point-to-multipoint	O
RSVP-TE	B-Protocol
.	O
</s>
<s>
It	O
was	O
driven	O
by	O
service	O
provider	B-General_Concept
requirements	O
to	O
transport	O
broadband	O
video	O
over	O
MPLS	B-Protocol
.	O
</s>
<s>
Since	O
the	O
inception	O
of	O
there	O
has	O
been	O
a	O
tremendous	O
surge	O
in	O
interest	O
and	O
deployment	O
of	O
MPLS	B-Protocol
multicast	O
and	O
this	O
has	O
led	O
to	O
several	O
new	O
developments	O
both	O
in	O
the	O
IETF	O
and	O
in	O
shipping	O
products	O
.	O
</s>
<s>
The	O
hub&spoke	O
multipoint	O
LSP	O
is	O
also	O
introduced	O
by	O
IETF	O
,	O
short	O
as	B-Architecture
HSMP	B-General_Concept
LSP	I-General_Concept
.	O
</s>
<s>
HSMP	B-General_Concept
LSP	I-General_Concept
is	O
mainly	O
used	O
for	O
multicast	O
,	O
time	O
synchronization	O
,	O
and	O
other	O
purposes	O
.	O
</s>
<s>
MPLS	B-Protocol
works	O
in	O
conjunction	O
with	O
the	O
Internet	B-Protocol
Protocol	I-Protocol
(	O
IP	B-Protocol
)	O
and	O
its	O
routing	O
protocols	O
,	O
usually	O
Interior	B-Protocol
Gateway	I-Protocol
Protocols	I-Protocol
(	O
IGPs	O
)	O
.	O
</s>
<s>
MPLS	B-Protocol
LSPs	O
provide	O
dynamic	O
,	O
transparent	O
virtual	O
networks	O
with	O
support	O
for	O
traffic	O
engineering	O
,	O
the	O
ability	O
to	O
transport	O
layer-3	B-Application
(	O
IP	B-Protocol
)	O
VPNs	B-Application
with	O
overlapping	O
address	O
spaces	O
,	O
and	O
support	O
for	O
layer-2	B-Protocol
pseudowires	B-Protocol
using	O
Pseudowire	B-Protocol
Emulation	O
Edge-to-Edge	O
(	O
PWE3	O
)	O
that	O
are	O
capable	O
of	O
transporting	O
a	O
variety	O
of	O
transport	O
payloads	O
(	O
IPv4	B-Protocol
,	O
IPv6	B-Protocol
,	O
ATM	O
,	O
Frame	B-Protocol
Relay	I-Protocol
,	O
etc	O
.	O
)	O
.	O
</s>
<s>
MPLS-capable	O
devices	O
are	O
referred	O
to	O
as	B-Architecture
LSRs	O
.	O
</s>
<s>
The	O
paths	O
an	O
LSR	O
knows	O
can	O
be	O
defined	O
using	O
explicit	O
hop-by-hop	O
configuration	O
,	O
or	O
are	O
dynamically	O
routed	O
by	O
the	O
constrained	B-Protocol
shortest	I-Protocol
path	I-Protocol
first	I-Protocol
(	O
CSPF	B-Protocol
)	O
algorithm	O
,	O
or	O
are	O
configured	O
as	B-Architecture
a	O
loose	O
route	O
that	O
avoids	O
a	O
particular	O
IP	B-Protocol
address	O
or	O
that	O
is	O
partly	O
explicit	O
and	O
partly	O
dynamic	O
.	O
</s>
<s>
In	O
a	O
pure	O
IP	B-Protocol
network	O
,	O
the	O
shortest	O
path	O
to	O
a	O
destination	O
is	O
chosen	O
even	O
when	O
the	O
path	O
becomes	O
congested	O
.	O
</s>
<s>
Meanwhile	O
,	O
in	O
an	O
IP	B-Protocol
network	O
with	O
MPLS	B-Protocol
Traffic	O
Engineering	O
CSPF	B-Protocol
routing	O
,	O
constraints	O
such	O
as	B-Architecture
the	O
RSVP	B-Protocol
bandwidth	O
of	O
the	O
traversed	O
links	O
can	O
also	O
be	O
considered	O
,	O
such	O
that	O
the	O
shortest	O
path	O
with	O
available	O
bandwidth	O
will	O
be	O
chosen	O
.	O
</s>
<s>
MPLS	B-Protocol
Traffic	O
Engineering	O
relies	O
upon	O
the	O
use	O
of	O
TE	O
extensions	O
to	O
Open	B-Protocol
Shortest	I-Protocol
Path	I-Protocol
First	I-Protocol
(	O
OSPF	B-Protocol
)	O
or	O
Intermediate	B-Protocol
System	I-Protocol
To	I-Protocol
Intermediate	I-Protocol
System	I-Protocol
(	O
IS-IS	B-Protocol
)	O
and	O
RSVP	B-Protocol
.	O
</s>
<s>
In	O
addition	O
to	O
the	O
constraint	O
of	O
RSVP	B-Protocol
bandwidth	O
,	O
users	O
can	O
also	O
define	O
their	O
own	O
constraints	O
by	O
specifying	O
link	O
attributes	O
and	O
special	O
requirements	O
for	O
tunnels	O
to	O
route	O
(	O
or	O
not	O
to	O
route	O
)	O
over	O
links	O
with	O
certain	O
attributes	O
.	O
</s>
<s>
For	O
end-users	O
the	O
use	O
of	O
MPLS	B-Protocol
is	O
not	O
visible	O
directly	O
,	O
but	O
can	O
be	O
assumed	O
when	O
doing	O
a	O
traceroute	B-Protocol
:	O
only	O
nodes	O
that	O
do	O
full	O
IP	B-Protocol
routing	I-Protocol
are	O
shown	O
as	B-Architecture
hops	O
in	O
the	O
path	O
,	O
thus	O
not	O
the	O
MPLS	B-Protocol
nodes	O
used	O
in	O
between	O
,	O
therefore	O
when	O
you	O
see	O
that	O
a	O
packet	B-Protocol
hops	O
between	O
two	O
very	O
distant	O
nodes	O
and	O
hardly	O
any	O
other	O
'	O
hop	O
 '	O
is	O
seen	O
in	O
that	O
provider	B-General_Concept
's	O
network	O
(	O
or	O
AS	B-Architecture
)	O
it	O
is	O
very	O
likely	O
that	O
network	O
uses	O
MPLS	B-Protocol
.	O
</s>
<s>
In	O
the	O
event	O
of	O
a	O
network	O
element	O
failure	O
when	O
recovery	O
mechanisms	O
are	O
employed	O
at	O
the	O
IP	B-Protocol
layer	O
,	O
restoration	O
may	O
take	O
several	O
seconds	O
which	O
may	O
be	O
unacceptable	O
for	O
real-time	O
applications	O
such	O
as	B-Architecture
VoIP	B-Application
.	O
</s>
<s>
In	O
contrast	O
,	O
MPLS	B-General_Concept
local	I-General_Concept
protection	I-General_Concept
meets	O
the	O
requirements	O
of	O
real-time	O
applications	O
with	O
recovery	O
times	O
comparable	O
to	O
those	O
of	O
shortest	B-Protocol
path	I-Protocol
bridging	I-Protocol
networks	O
or	O
SONET	B-Protocol
rings	I-Protocol
of	O
less	O
than	O
50ms	O
.	O
</s>
<s>
MPLS	B-Protocol
can	O
make	O
use	O
of	O
existing	O
ATM	O
network	O
or	O
Frame	B-Protocol
Relay	I-Protocol
infrastructure	O
,	O
as	B-Architecture
its	O
labeled	O
flows	O
can	O
be	O
mapped	O
to	O
ATM	O
or	O
Frame	B-Protocol
Relay	I-Protocol
virtual-circuit	O
identifiers	O
,	O
and	O
vice	O
versa	O
.	O
</s>
<s>
Frame	B-Protocol
Relay	I-Protocol
aimed	O
to	O
make	O
more	O
efficient	O
use	O
of	O
existing	O
physical	O
resources	O
,	O
which	O
allow	O
for	O
the	O
underprovisioning	O
of	O
data	O
services	O
by	O
telecommunications	O
companies	O
(	O
telcos	O
)	O
to	O
their	O
customers	O
,	O
as	B-Architecture
clients	O
were	O
unlikely	O
to	O
be	O
utilizing	O
a	O
data	O
service	O
100	O
percent	O
of	O
the	O
time	O
.	O
</s>
<s>
Consequently	O
,	O
oversubscription	O
of	O
capacity	O
by	O
the	O
telcos	O
(	O
excessive	O
bandwidth	O
overbooking	O
)	O
,	O
while	O
financially	O
advantageous	O
to	O
the	O
provider	B-General_Concept
,	O
can	O
directly	O
affect	O
overall	O
performance	O
.	O
</s>
<s>
Telcos	O
often	O
sold	O
Frame	B-Protocol
Relay	I-Protocol
to	O
businesses	O
looking	O
for	O
a	O
cheaper	O
alternative	O
to	O
dedicated	B-Protocol
lines	I-Protocol
;	O
its	O
use	O
in	O
different	O
geographic	O
areas	O
depended	O
greatly	O
on	O
governmental	O
and	O
telecommunication	O
companies	O
 '	O
policies	O
.	O
</s>
<s>
Many	O
customers	O
migrated	O
from	O
Frame	B-Protocol
Relay	I-Protocol
to	O
MPLS	B-Protocol
over	O
IP	B-Protocol
or	O
Ethernet	O
,	O
which	O
in	O
many	O
cases	O
reduced	O
costs	O
and	O
improved	O
manageability	O
and	O
performance	O
of	O
their	O
wide	O
area	O
networks	O
.	O
</s>
<s>
While	O
the	O
underlying	O
protocols	O
and	O
technologies	O
are	O
different	O
,	O
both	O
MPLS	B-Protocol
and	O
ATM	O
provide	O
a	O
connection-oriented	B-Protocol
service	I-Protocol
for	O
transporting	O
data	O
across	O
computer	O
networks	O
.	O
</s>
<s>
In	O
both	O
technologies	O
,	O
connections	O
are	O
signaled	O
between	O
endpoints	B-Protocol
,	O
the	O
connection	O
state	O
is	O
maintained	O
at	O
each	O
node	B-Protocol
in	O
the	O
path	O
,	O
and	O
encapsulation	O
techniques	O
are	O
used	O
to	O
carry	O
data	O
across	O
the	O
connection	O
.	O
</s>
<s>
Excluding	O
differences	O
in	O
the	O
signaling	O
protocols	O
(	O
RSVP/LDP	O
for	O
MPLS	B-Protocol
and	O
PNNI:Private	O
Network-to-Network	O
Interface	O
for	O
ATM	O
)	O
there	O
still	O
remain	O
significant	O
differences	O
in	O
the	O
behavior	O
of	O
the	O
technologies	O
.	O
</s>
<s>
MPLS	B-Protocol
is	O
able	O
to	O
work	O
with	O
variable	O
length	O
packets	B-Protocol
while	O
ATM	O
transports	O
fixed-length	O
(	O
53	O
bytes	O
)	O
cells	O
.	O
</s>
<s>
Packets	B-Protocol
must	O
be	O
segmented	O
,	O
transported	O
and	O
re-assembled	O
over	O
an	O
ATM	O
network	O
using	O
an	O
adaptation	O
layer	O
,	O
which	O
adds	O
significant	O
complexity	O
and	O
overhead	O
to	O
the	O
data	O
stream	O
.	O
</s>
<s>
MPLS	B-Protocol
,	O
on	O
the	O
other	O
hand	O
,	O
simply	O
adds	O
a	O
label	O
to	O
the	O
head	O
of	O
each	O
packet	B-Protocol
and	O
transmits	O
it	O
on	O
the	O
network	O
.	O
</s>
<s>
Differences	O
exist	O
,	O
as	B-Architecture
well	O
,	O
in	O
the	O
nature	O
of	O
the	O
connections	O
.	O
</s>
<s>
An	O
MPLS	B-Protocol
connection	O
(	O
LSP	O
)	O
is	O
unidirectional	O
—	O
allowing	O
data	O
to	O
flow	O
in	O
only	O
one	O
direction	O
between	O
two	O
endpoints	B-Protocol
.	O
</s>
<s>
Establishing	O
two-way	O
communications	O
between	O
endpoints	B-Protocol
requires	O
a	O
pair	O
of	O
LSPs	O
to	O
be	O
established	O
.	O
</s>
<s>
ATM	O
point-to-point	O
connections	O
(	O
virtual	B-Protocol
circuits	I-Protocol
)	O
,	O
on	O
the	O
other	O
hand	O
,	O
are	O
bidirectional	O
,	O
allowing	O
data	O
to	O
flow	O
in	O
both	O
directions	O
over	O
the	O
same	O
path	O
(	O
Both	O
SVC	O
and	O
PVC	O
ATM	O
connections	O
are	O
bidirectional	O
.	O
</s>
<s>
Both	O
ATM	O
and	O
MPLS	B-Protocol
support	O
tunneling	O
of	O
connections	O
inside	O
connections	O
.	O
</s>
<s>
MPLS	B-Protocol
uses	O
label	O
stacking	O
to	O
accomplish	O
this	O
while	O
ATM	O
uses	O
virtual	O
paths	O
.	O
</s>
<s>
MPLS	B-Protocol
can	O
stack	B-Application
multiple	O
labels	O
to	O
form	O
tunnels	O
within	O
tunnels	O
.	O
</s>
<s>
The	O
ATM	O
virtual	O
path	O
indicator	O
(	O
VPI	O
)	O
and	O
virtual	B-Protocol
circuit	I-Protocol
indicator	O
(	O
VCI	O
)	O
are	O
both	O
carried	O
together	O
in	O
the	O
cell	O
header	O
,	O
limiting	O
ATM	O
to	O
a	O
single	O
level	O
of	O
tunneling	O
.	O
</s>
<s>
The	O
biggest	O
advantage	O
that	O
MPLS	B-Protocol
has	O
over	O
ATM	O
is	O
that	O
it	O
was	O
designed	O
from	O
the	O
start	O
to	O
be	O
complementary	O
to	O
IP	B-Protocol
.	O
</s>
<s>
Modern	O
routers	O
are	O
able	O
to	O
support	O
both	O
MPLS	B-Protocol
and	O
IP	B-Protocol
natively	O
across	O
a	O
common	O
interface	O
allowing	O
network	O
operators	O
great	O
flexibility	O
in	O
network	B-Architecture
design	I-Architecture
and	O
operation	O
.	O
</s>
<s>
ATM	O
's	O
incompatibilities	O
with	O
IP	B-Protocol
require	O
complex	O
adaptation	O
,	O
making	O
it	O
comparatively	O
less	O
suitable	O
for	O
today	O
's	O
predominantly	O
IP	B-Protocol
networks	O
.	O
</s>
<s>
MPLS	B-Protocol
is	O
currently	O
(	O
as	B-Architecture
of	O
March	O
2012	O
)	O
in	O
use	O
in	O
IP-only	O
networks	O
and	O
is	O
standardized	O
by	O
the	O
IETF	O
in	O
.	O
</s>
<s>
It	O
is	O
deployed	O
to	O
connect	O
as	B-Architecture
few	O
as	B-Architecture
two	O
facilities	O
to	O
very	O
large	O
deployments	O
.	O
</s>
<s>
In	O
practice	O
,	O
MPLS	B-Protocol
is	O
mainly	O
used	O
to	O
forward	O
IP	B-Protocol
protocol	I-Protocol
data	O
units	O
(	O
PDUs	O
)	O
and	O
Virtual	B-Application
Private	I-Application
LAN	I-Application
Service	I-Application
(	O
VPLS	B-Application
)	O
Ethernet	O
traffic	O
.	O
</s>
<s>
Major	O
applications	O
of	O
MPLS	B-Protocol
are	O
telecommunications	O
traffic	O
engineering	O
,	O
and	O
MPLS	B-Application
VPN	I-Application
.	O
</s>
<s>
MPLS	B-Protocol
has	O
been	O
originally	O
proposed	O
to	O
allow	O
high-performance	O
traffic	O
forwarding	O
and	O
traffic	O
engineering	O
in	O
IP	B-Protocol
networks	O
.	O
</s>
<s>
However	O
it	O
evolved	O
in	O
Generalized	O
MPLS	B-Protocol
(	O
GMPLS	O
)	O
to	O
allow	O
the	O
creation	O
of	O
label-switched	O
paths	O
(	O
LSPs	O
)	O
also	O
in	O
non-native	O
IP	B-Protocol
networks	O
,	O
such	O
as	B-Architecture
SONET/SDH	B-Protocol
networks	I-Protocol
and	O
wavelength	B-Protocol
switched	I-Protocol
optical	I-Protocol
networks	I-Protocol
.	O
</s>
<s>
MPLS	B-Protocol
can	O
exist	O
in	O
both	O
an	O
IPv4	B-Protocol
and	O
an	O
IPv6	B-Protocol
environment	O
,	O
using	O
appropriate	O
routing	O
protocols	O
.	O
</s>
<s>
The	O
major	O
goal	O
of	O
MPLS	B-Protocol
development	O
was	O
the	O
increase	O
of	O
routing	O
speed	O
.	O
</s>
<s>
This	O
goal	O
is	O
no	O
longer	O
relevant	O
because	O
of	O
the	O
usage	O
of	O
newer	O
switching	O
methods	O
(	O
able	O
to	O
forward	O
plain	O
IPv4	B-Protocol
as	B-Architecture
fast	O
as	B-Architecture
MPLS	B-Protocol
labelled	O
packets	B-Protocol
)	O
,	O
such	O
as	B-Architecture
ASIC	O
,	O
TCAM	O
and	O
CAM-based	O
switching	O
.	O
</s>
<s>
Now	O
,	O
therefore	O
,	O
the	O
main	O
application	O
of	O
MPLS	B-Protocol
is	O
to	O
implement	O
limited	O
traffic	O
engineering	O
and	O
layer	B-Application
3	I-Application
/	O
layer	B-Protocol
2	I-Protocol
“	O
service	O
provider	B-General_Concept
type	O
”	O
VPNs	B-Application
over	O
IPv4	B-Protocol
networks	I-Protocol
.	O
</s>
