Multi Protocol Label Switching

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29 Οκτ 2013 (πριν από 3 χρόνια και 7 μήνες)

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Multi Protocol Label Switching





Presented by


Ayan Kumar Roy

Roll No. 05IT6020

M.Tech (IT),SIT,

IIT,KGP





Under Guidance of


Dr. S.K.Ghosh








Table of Contents


1.

Abstract…………………………………………………………………………………………………3

2.

MPLS Characteristics……………………………………………
……………………3

3.

Label Edge routers and Label Switch routers……4


4.

Forward Equivalence Class……………………………………………………4

5.

Labels and Label binding………………………………………………………5

6.

Label Creation…………………………………………………………………………………6

7.

Label Distribution………………………………………………………………………7

8.

Label Swap
ping Forwarding Algorithm…………………………7

9.

Label Switched Path (LSP)……………………………………………………9

10.

MPLS Operations………………………………………………………………………………10

11.

MPLS Advantages………………………………………………………………………………12

12.

MPLS Disadvantages………………………………………………………………………13

13.

Reference…………………………………………………
……………………………………………13




































Abstract


In early 1997, the Internet Engineering Task Force (IETF) established the Multi
-
protocol Label Switching (MPLS) working group to produce a unified and
interoperable multilayer switching stand
ard as each vendor (Cisco Systems, Lucent,
and so on) developed a proprietary multilayer switching solution, maintaining the IP
control component and label
-
swapping components in different ways. The majority
of these multilayer switching solutions required

an ATM transport because they
could not operate over mixed media infrastructures, such as Frame Relay, PPP,
SONET, and LANs.


MPLS Characteristics

MPLS performs the following functions:



It specifies mechanisms to manage traffic flows of various granularit
ies, such
as flows among different hardware, machines, or even flows among different
applications.



It remains independent of the Layer 2 and Layer 3 protocols.



It provides a means to map IP addresses to simple, fixed
-
length labels used by
different packet
-
forwarding and packet
-
switching technologies.



It interfaces to existing routing protocols such as Resource Reservation
Protocol (RSVP) and Open Shortest Path First (OSPF).



It supports the IP, ATM, and frame
-
relay Layer 2 protocols.


Multi Protocol Label S
witching is a versatile solution to address the problems faced
by present day networks


speed, scalability, Quality of Service (QoS) management,
traffic engineering.


Label
-
Edge Routers (LERs) and Label
\
_Switching
Routers (LSRs)


The devices that particip
ate in the MPLS Protocol mechanisms can be classified into
label
-
edge routers (LERs) and label
-
switching routers (LSRs).


An
LSR
is a high
-
speed router device in the core of an MPLS network that
participates in the establishment of LSPs using the appropri
ate label signaling
protocol and high
-
speed switching of the data traffic based on the

established paths.


An
LER
is a device that operates at the edge of the access network and MPLS
network. LERs support multiple ports connected to dissimilar networks (su
ch as
frame relay, ATM, and Ethernet) and forward this traffic on to the MPLS network
after establishing LSPs. LERs use the label
-
signaling protocol at the ingress and
distribute the traffic back to the access networks at the egress. LERs play an
important

role in the assignment and removal of labels, as traffic enters or exits an
MPLS network.




Forward Equivalence Classes (FECs)


A forward equivalence class (FEC) is a representation of a group of packets that
share the same requirements for their transpo
rt. All packets in such a group are
provided the same treatment en route to the destination. As opposed to conventional
IP forwarding, in MPLS, the assignment of a particular packet to a particular FEC is
done just once, as the packet enters the network. F
ECs are based on service
requirements for a given set of packets or simply for an address prefix. Each LSR
builds a table to specify how a packet must be forwarded. This table, called a label
information base (LIB), comprises FEC
-
to
-
label bindings.









Labels and Label Bindings


A label, in its simplest form, identifies the path that a packet should traverse. A label
is carried or encapsulated in a Layer 2 header along with the packet. The receiving
router examines the packet for its label content to de
termine the next hop. After a
packet has been labeled, the rest of the journey of the packet through the backbone is
based on label switching. The label values are of local significance only, meaning
that they pertain only to hops between LSRs. After a pac
ket has been classified as a
new or existing FEC, a label is assigned to the packet. The label values are derived
from the underlying data link layer. These data link layer identifiers, such as Frame
Relay DLCIs or ATM VPI/VCIs, can be used directly as lab
els. The packets are then
forwarded based on their label value.

Labels are bound to an FEC as a result of some event or policy that indicates a need
for such binding. These events can be either data
-
driven bindings or control
-
driven
bindings. The latter is

preferable because of its advanced scaling properties that can
be used in MPLS.

Label assignment decisions can be based on forwarding criteria such as the
following:


Destination unicast routing


Traffic engineering (TE)


Multicast


Virtual private network (VPN)


Quality of Service (QoS)


Generic Label Format



Labels attached to Different Types of Frames




Label Creation




Topology
-
based method


uses normal processing of routing protocols (such
as OSPF and BGP)



Request
-
based method


uses processing of request
-
based control traffic (such
as RSVP)



Traffic
-
based method


uses the reception of a packet to trigg
er the
assignment and distribution of a label



The topology
-

and request
-
based methods are examples of control
-
driven
label bindings, while the traffic
-
based method is an example of data
-
driven binding



Label Distribution


MPLS architecture dose not mand
ate a single method of signaling for label
distribution. Existing routing protocols, such as the border gateway protocol (BGP),
have been enhanced to piggyback the label information within the contents of the
protocol. The RSVP has been extended to support

piggybacked exchange of labels.
The IETF has also defined a new protocol known as the label distribution protocol
(LDP) for explicit signaling and management of the label space.

A summary of the various schemes for label exchange is as follows:




LDP


ma
ps unicast IP destination into labels



RSVP,CR
-
LDP


used for traffic engineering and resource reservation



Protocol
-
independent multicast (PIM)


used for multicast states label
mapping



BGP


external labels (VPN)



Label
-
Swapping Forwarding Algorithm


MPLS
's forwarding component is based on a label
-
swapping forwarding
algorithm

the same algorithm used to forward data in ATM and Frame Relay
switches. The label
-
swapping forwarding algorithm requires packet classification at
the ingress edge of the network to
assign an initial label to each packet described in
the following list.
Figure 27
-
2
illustrates the path of an unlabeled packet with a
destination of 192.4.2.1.








Figure 27
-
2. Packet Traversing a Label
-
Switched Path




1.

The ingress label switch rec
eives an unlabeled packet with a destination

address of 192.4.2.1.

2.

The label switch performs a longest
-
match routing table lookup and maps the

packet to an FEC

192.4/16.

3.

The ingress label switch then assigns a label (with a value of 5) to the packe
t

and forwards it to the next hop in the label
-
switched path (LSP). This next hop

is specified as an outgoing interface on the LSR.

4.

This process repeats until the destination interface is reached and the packet is

delivered to the intended destinatio
n network, via the specified outgoing

interface. When the last LSR is reached, the delivered packet has been

stripped, or "popped," of its last label.


In the network core, label switches ignore the packet's network layer header and
forward the packet ba
sed on decisions made using the label
-
swapping algorithm.
When a labeled packet arrives at a switch, the forwarding component uses the input
port number and label as an index to perform an exact match search of its forwarding
table. After the forwarding co
mponent finds a match, it retrieves the outgoing label,
the outgoing interface, and the next
-
hop address from the forwarding table. The
forwarding component then swaps (or replaces) the incoming label with the
outgoing label and directs the packet to the o
utbound interface for transmission to
the next hop in the LSP.

When the labeled packet arrives at the egress label switch, the forwarding
component searches its forwarding table.

If the next hop is not a label switch, the egress switch discards the label a
nd
forwards the packet using conventional longest
-
match IP forwarding.







Label
-
Switched Paths (LSPs)


A collection of MPLS
-
enabled devices represents an MPLS domain. Within an
MPLS domain, a path is set up for a given packet to travel based on an FEC.
The
LSP is set up prior to data transmission.

MPLS provides two options to set up an LSP



hop
-
by
-
hop routing




Each LSR independently selects the next hop for a given FEC. This

methodology is similar to that currently used in IP networks. The LSRs

suppor
t any available routing protocols (OSPF, ATM …).



explicit routing




Explicit routing is similar to source routing. The ingress LSR (i.e., the LSR
where the data flow to the network first starts) specifies the list of nodes through
which the packet travers
es. The path specified could be non
-
optimal, as well. Along
the path the resources may be reserved to ensure QoS to the data traffic. This eses
traffic engineering through out the network, and differentiated services can be
provided using flows based on po
licies or network management methods.


The LSP setup for an FEC is unidirectional. The return traffic must take another
LSP.


















MPLS Operation






The following steps must be taken for a data packet to travel thro
ugh an MPLS
domain.



label creation and distribution



table creation at each router



label
-
switched path creation



label insertion/table lookup



packet forwarding

The source sends its data to the destination. In an MPLS domain, not all of the
source traffi
c is necessarily transported through the same path. Depending on the
traffic characteristics, different LSPs could be created for packets with different
requirements.









MPLS Actions:





Label creation and label stribution



Before any traffic begins the

routers make the decision to bind a label to
a specific FEC and build their tables.



In LDP, downstream routers initiate the distribution of labels and the
label/FEC binding.



In addition, traffic
-
related characteristics and MPLS capabilities are
negotiat
ed using LDP.



A reliable and ordered transport protocol should be used for the
signaling protocol.



Table creation



On receipt of label bindings each LSR creates entries in the label
information base (LIB).



The contents of the table will specify the mapp
ing between a label and
an FEC.



mapping between the input port and input label table to the
output port and output label table.



The entries are updated whenever renegotiation of the label
bindings occurs.




Example of LIB Table

Input Port

Incoming Port
Label

Output Port

Outgoing Port Label

1

3

3

6

2

9

1

7






Label switched path creation



The LSPs are created in the reverse direction to the creation of entries
in the LIBs.






Label insertion/table
-
lookup



The first router (LER1) uses the LIB table to
find the next hop and a
label for the specific FEC.



Subsequent routers just use the label to find the next hop.



Once the packet reaches the egress LSR (LER4), the label is removed
and the packet is supplied to the destination.





Packet forwarding



LER1
may not have any labels for this packet as it is the first
occurrence of this request. In an IP network, it will find the longest
address match to find the next hop. Let LSR1 be the next hop for LER1.



LER1 will initiate a label request toward LSR1.



This
request will propagate through the network as indicated by the
broken green lines.



Each intermediary router will receive a label from its downstream
router starting from LER2 and going upstream till LER1. The LSP
setup is indicated by the broken blue line
s using LDP or any other
signaling protocol.



LER1 will insert the label and forward the packet to LSR1.



Each subsequent LSR, i.e., LSR2 and LSR3, will examine the label in
the received packet, replace it with the outgoing label and forward it.



When the
packet reaches LER4, it will remove the label because the
packet is departing from an MPLS domain and deliver it to the
destination.



The actual data path followed by the packet is indicated by the broken
red lines.



MPLS Advantages





Improves packet
-
forw
arding performance in the network



Supports QoS and CoS for service differentiation



Supports network scalability



Integrates IP and ATM in the network



Builds interoperable networks




MPLS Disadvantages




An additional layer is added



The router has to un
derstand MPLS



References




http://www.iec.org/online/tutorials/mpls.
pdf



Cisco Press
-
Network Consultants Handbook
-
by Mathew Castelli.pdf



http://www.iaik.tugraz.ac.at/teaching/03_advanced%20c
omputer%
20networks
/ss2004/vo3/MPLS.pdf

by Mario Ivkovic



http://ica1www.epfl.ch/cn2/0304/doc/lecture/mpls.pdf



Encyclopedia of Netwoking.pdf