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Multi-Protocol Label Switching

-MPLS-

S.N.Mishra

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TE-Mid 1990s

IGP route calculation is topology based Some links become congested while other

remained under utilised

11

11

11 11

11

22

Numbers are metricsNumbers are metrics

R1R1 R2R2

R4R4

R3R3

R5R5 R6R6

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TE-Mid to Late 1990s

Routed edge/ATM core L3 decision at edge router

L2 decision at each core switch

PVCs are mapped edge to edge to evenlydistribute traffic across all the links

Routed PVCRouted PVC

PhysicalPhysicalTopologyTopology

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Routing

Internet delivers the data packets on a BestEffort basis using the TCP/IP protocol suite

The routing job is accomplished by a device calledRouter

The router sends the packets from one network toanother by processing layer-3 network header

Different routing protocols discover the routes atlayer-3 to create Routing Information Base-RIB orRouting Table

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Routing-Data Traffic

S0 S1

N

D

P

3

2

1

Routing Information Base

Receive Logic Transmit Logic

Input port Output port

Forwarding Information Base

101001001 11

DH4H3H2 T2

DH4H3 DH4H3 ICI

DH4H3

DH4H3H2 T2

101001001 11

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Routing-Control Traffic

S0 S1

N

D

P

3

2

1

Receive Logic Receive Logic

Input port Output port

Forwarding Information Base

OSPF

Routing Information Base

101001001 11101001001 11

COH3 COH3

COH3H2 T2 COH3H2 T2

CO CO

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Fast Path vs Slow Path

Control Traffic

Control traffic is the protocols traffic at layer-3

Control packets are processed slowly because

they need to be examined by the software Data Traffic

Follows the fast path and is processed by thenetwork devices in an efficient and timely

manner Generally realised in hardware

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Fast Path vs Slow Path

Control Path

Forwarding Path

Software

Hardware

Router-A

Control Path

Forwarding Path

Software

Hardware

Router-B

C

ontrol

Slow Path

Data

Fast Path

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Router Planes

S0 S1

N

D

P

3

2

1

Routing Information Base

Receive Logic Transmit Logic

Input port Output port

Forwarding Information Base

Control Plane

Forwarding Plane

Routing

Switching

Forwarding

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Routing-Switching-Forwarding

Routing

Process of setting up routes to understand thenext hop a packet should take towards itsdestination

Switching

The knowledge of directing the forwardingprocess to choose the correct output port

Forwarding The process of receiving a packet on an input

port and sending it out an output port

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Evolution of MPLS

IETF in the year 1997 met to achieve thefollowing objectives

Enhance the performance and scalability of IP

routing Facilitate explicit routing and traffic engineering

Separate control from forwarding mechanism

Develop a single forwarding algorithm to

support wide range of routing and switchingfunctionality

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Evolution of MPLS

Solution to support multi-layer switching:

IP Switching (Ipsilon/Nokia)

Tag Switching (Cisco)

IP Navigator (Cascade/Ascend/Lucent)

ARIS (IBM)

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MPLS

S0 S1

N

D

P

3

2

1

Routing Information Base

Receive Logic Transmit Logic

Input port Output port

Forwarding Information Base

Layer 2.5MPLS

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MPLS

MPLS is a binding of the control plane at the

bottom of the network layer with the data

forwarding plane at the top of data link layer

MPLS is a hybrid of a traditional networks layer-3routing protocols and layer-2 switching

technologies

MPLS is not a new network layer protocol because

it does not have its own routing capabilities andaddressing schemes

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MPLS

MPLS is not a new data link layer protocol

because it is designed to work over many of the

data link technologies that provides requisite layer-

2 addressing and functionality

MPLS is also known by the name of Layer 2.5

Technology

MPLS allows current layer-2 transport

technologies like ATM, FR and Ethernet toseamlessly interoperate and co-exits with layer-3

protocols

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MPLS Model

Upper Layers

IPv4 IPv6 IPX Apple Talk DCEnet CLNP Others

MPLS

PPP ATM FR Ethernet FDDI Others

Physical Layer

3

2

1

4-7

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MPLS Data Encapsulation

Data

DataH4

Application

TPT Layer

NW Layer

Data Link

MPLS

H3 DataH4

LVL H3 DataH4

H2 T2LVL H3 DataH4

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Traditional Systemof Mail Sorting

1. All the sortingstations should havelist of all the cities,towns and villages

2. Searching fordestination from huge

list of cities, townsand villages was a

time consuming job

3. Language problemalso poses a problem

and tends to delay the

mail sorting

Mr.XYZQr.No-1,RTTC ,

Bhubaneswar-751007

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3. Beyond main cities lettersare sorted & forwarded basedon the actual village or town

address

2. At main citiesLetters are sorted

& forwardedquickly based on

the PIN Code

Postal IndexNumber System of

Mail Sorting

4. Language alsodoes not pose anyproblem in writing

the address

Mr.XYZ

Qr.No-1,RTTC ,

Bhubaneswar751007

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Normal IP Routing

1. Routers maintainRouting Tables

2. Packets are routed on thebasis of Destination IP

Address and Big RoutingTables

IP Header is Carryingthe Routing Information

Source IP Address andDestination IP Address

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2. Outside MPLS Domainpackets are routed on the basisof information in the IP Header

1. In MPLS Domain packets areswitched on the basis of labelinformation in the MPLS Header

MPLS BasedRouting

MPLS Router

Normal Router

MPLS Router

MPLS Router

MPLS Domain

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MPLS Labels

MPLS uses Label Switched Path (LSP) for layer-2

switching that have been set up with layer-3

routing and signaling protocols

MPLS signaling and label distributing protocol willdistribute the proper labels within the MPLS

domain to create LSP

LSPs are roughly equivalent to virtual circuit

Label Switched Path is unidirectional If the traffic flow on the same route in opposite direction

is required, two label paths are to be set up

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LSP

Label-Switched Path

Simplex L2 tunnel across a network

Concatenation of one or more label switched hops

Analogous to an ATM or Frame Relay PVC

New DelhiNew Delhi

MumbaiMumbai

LSPLSP

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LSR

MPLS enabled router is called label SwitchingRouter

Forwards MPLS packets using label-switching Capable of forwarding native IP packets

Executes one or more IP routing protocols

Participates in MPLS control protocols

New DelhiNew Delhi

MumbaiMumbai

LSPLSP

LSRLSR

LSRLSR

LSRLSRLSRLSR

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MPLS Terminology

SanSanFranciscoFrancisco

NewNewYorkYork

LSPLSP

Ingress LSR (head-end LSR) Examines inbound IP packets and assigns them to an FEC

Generates MPLS header and assigns initial label

Transit LSR Forwards MPLS packets using label swapping

Egress LSR (tail-end LSR) Removes the MPLS header

IngressIngressLSRLSR TransitTransit

LSRLSR TransitTransitLSRLSR

EgressEgressLSRLSR

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FEC

A forwarding equivalence class (FEC) is a stream

of IP packets that are forwarded over the same

path, treated in the same manner and mapped to

the same label

LDP associates a set of destinations with each

data link LSP

This set of destinations is called the FEC

These destinations all share a common data LSPpath egress and a common unicast routing path

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FEC

Packets could be assigned to a LSP based

on

A combination of destination address and

application type A combination of destination address and

source address

A specific quality of service requirement

A VPN identifier

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MPLS Label Assignment/Binding

Label values and FEC-bindings are

negotiated before any MPLS traffic ever

flows on data path for a particular label

Labels are downstream-assigned

FEC-Label bindings are distributed

downstream-to- upstream direction

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MPLS Label Assignment/Binding

LSR-A LSR-B

Assign label

Label assigned

DataData

Control

Data

Control

Upstream Node Downstream Node

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Label Request for FEC

10.0.0.0

MPLS Label Assignment/Binding

UpstreamUpstream

LDP peerLDP peer

DownstreamDownstream

LDP peerLDP peerLSRLSR

325413

Net: 10.0.0.0Net: 10.0.0.0

Label: 52Label: 52

(3, 29)

Net: 10.0.0.0Net: 10.0.0.0

Label: 29Label: 29

MPLS TableMPLS Table

In Out

(2, 52)

MPLS TableMPLS TableIn Out

(1, 17)

MPLS TableMPLS Table

In Out

(5, 52)

ReceiveReceiveoutgoingoutgoing

labellabel(4, 17)

Net: 10.0.0.0Net: 10.0.0.0

Label: 17Label: 17

AdvertiseAdvertiseincomingincoming

labellabel

(3, 35)

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MPLS Labels

MPLS label assignment can be done in two

ways

Control Driven

Labels are assigned and distributed before any labelswapping of data occurs

Mostly used

Traffic Driven

Labels are assigned and distributed as packetsbelonging to a specified flow or class enter the

network

Rarely used

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Label Distribution

MPLS architecture does not mandate a

single method of signalling for label

distribution. Various schemes for label

exchange is as follows : LDP- maps unicast destinations 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)

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Label Processing

Push

Add a new label to the top of the packet

The TTL, stack and CoS fields are derived from the IP

packet header

Can be performed on an existing MPLS packet-Label

Stacking

Pop

Remove the label

TTL is copied from the label to the IP header

IP packet is forwarded as a native IP packet

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Label Processing

Swap Replace the label at the top of the label stack with a

new label

The TTL, stack and CoS fields are copied from the

previous label Multiple Push

Adding multiple labels (up to 3)

Swap and Push

Replace the existing top of the label stack with a newlabel followed by pushing another new label on top

Used when a LDP signaled LSP transits an RSVP-TSsignaled core

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Label Pushing

S0 S1

N

D

P

3

2

1

Routing Information Base

Receive Logic Transmit Logic

Input port Output port

Forwarding Information Base

MPLS

Ingress Node

DH4H3H2 T2

DH4H3

101001001 11S

DH4H3H2 T2S

DH4H3S

101001001 11

DH4H3

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Label Swapping

S0 S1

N

D

P

3

2

1

Routing Information Base

Receive Logic Transmit Logic

Input port Output port

Forwarding Information Base

MPLS

Transit Node

101001001 11S

DH4H3S

101001001 11S

DH4H3H2 T2S

DH4H3S

DH4H3H2 T2S

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Label Popping

S0 S1

N

D

P

3

2

1

Routing Information Base

Receive Logic Transmit Logic

Input port Output port

Forwarding Information Base

MPLS

Egress Node

DH4H3H2 T2S

101001001 11

DH4H3H2 T2

101001001 11S

DH4H3

DH4H3S

DH4H3

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MPLS Forwarding Model

Ingress LSR determines FEC and assigns a label

Forwards Mumbai traffic on the Green LSP

ForwardsC

hennai traffic on theB

lue LSP Traffic is label swapped at each transit LSR

Egress LSR

Removes MPLS header

Forwards packet based on destination address

EgressEgressLSRLSRIngressIngress

LSRLSRMumbaiMumbai

ChennaiChennai

SourceSource

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LSR Operation

Each LSR maintains a connection table

Port 1

Port 3

Port 2

Port 4

Connection TableConnection Table

In(port, label)

Out(port, label)

(1, 22)

(1, 24)

(1, 25)

(2, 23)

(2, 17)

(3, 17)

(4, 19)

(3, 12)

LabelOperation

Swap

Swap

Swap

Swap

25IP

19IP

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1 2Ingress Routing TableIngress Routing TableDestination Next Hop

134.5/16

200.3.2/24

(2, 84)

(3, 99)

MPLS TableMPLS Table

In Out

(2, 84) (6, 0)

MPLS TableMPLS TableIn Out

(1, 99) (2, 56)

MPLS TableMPLS TableIn Out

(3, 56) (5, 0)

3 5

2

3

2 6134.5.1.5134.5.1.5

200.3.2.7200.3.2.7200.3.2.1200.3.2.1

134.5.6.1134.5.6.1

Destination

Egress Routing TableEgress Routing TableNext Hop

134.5/16

200.3.2/24

134.5.6.1

200.3.2.1

MPLS Forwarding Example

200.3.2.7

200.3.2.7

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MPLS v IP Routing

SourceSourceIP Routing DomainIP Routing Domain

Examine IP headerExamine IP header

Assign to FECAssign to FECForwardForward

Examine IP headerExamine IP header

Assign to FECAssign to FECForwardForward

Examine IP headerExamine IP header

Assign to FECAssign to FECForwardForward

Examine IP headerExamine IP header

Assign to FECAssign to FECForwardForward

EgressEgressLSRLSR

IngressIngressLSRLSR

SourceSourceMPLS DomainMPLS Domain

Label swapLabel swapForwardForward

Label swapLabel swapForwardForward

Examine IP headerExamine IP headerAssign to FECAssign to FEC

ForwardForward

Examine IP headerExamine IP headerAssign to FECAssign to FEC

ForwardForward

DestinationDestination

DestinationDestination

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MPLS Header

Label-(20 Bits)

Used for all of the subsequent label operations Push

Pop

Swap

Multiple Push

Swap and Push

A label only has a local significance between two

communicating LSRs

LABEL EXP S TTL

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MPLS Header

2-IPv6 Explicit NULL Label

Label stack must be popped

Subsequent forwarding of the packet will be based on the

IPv6network header

Used only as the bottom most label

3-Implicit NULL Label

Label is virtual in the sense that this value can be distributed but

never appears in the MPLS header encapsulation

Causes the LSR to pop the stack instead of normal swap

operation, where a new label value will be introduced 4:15-Reserved

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Label Spaces

Each label space consists of the assignable labelsfrom 0-1048574 (0-15 Reserved)

Two basic notions of label spaces Per-Platform Label Space

There is one set of labels for the entire LSR All interfaces share this common label pool

Per-Interface Label Space Each interface has its own label pool

Used particularly with ATM-LSRs

Decision to choose the label platform to beimplemented on a particular LSR is a function ofhow the interfaces are used

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Label Spaces

Platform Labels

(0-1048574)

Label Manager Process

LSR

Int-1 Int-n

Per-Platform Label Space

Int-1 Labels

(0-1048574)

Label Manager Process

LSR

Int-1 Int-n

Per-Interface Label Space

Int-n Labels

(0-1048574)

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MPLS Header

EXP-(3 Bits)-Experimental Bits

Reserved for experimentation

Will be used for differentiated services (Diff-serv) and

providing class of service

S Bit-Stack Bit

Used to support hierarchical MPLS label stacking

Set to indicate the last header in the stack

Set to 0 for all other MPLS headers MPLS stack follows LIFO arrangement

L2

HDR

MPLS

HDR#3

(S=0)

MPLS

HDR#2

(S=0)

MPLS

HDR#1

(S=1)

L3

HDR Data

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MPLS Header

TTL-(8 Bits)-Time To Live

Works in a similar fashion to the TTL field in an

IP header

The valid range is 0-255

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MPLS Applications

There are 3 major MPLS applications

currently being implemented

Traffic Engineering

Virtual Private Network Internet QoS

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MPLS-Traffic Engineering

11

11

11 11

11

22

Numbers are metricsNumbers are metrics

R1R1 R2R2

R4R4

R3R3

R5R5 R6R6

IGP PathMPLS Path

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MPLS-QoS

Different types of service quality can be providedfor different types of traffic

QoS is the guarantee of providing a requested

level of service QoS requires every element from the start of an

application flow till its completion to be aware and

respond to QoS requirements

CoS is the category of service requested by eachpacket that is part of a particular application flow

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MPLS-QoS

IETF has created two efforts to directly relate QoSans CoS IntServ

Diff-Serv

IntServ and Diff-Serv can be used together withMPLS to realise an MPLS QoS application

IntServ deals with resource reservations Network resources are apportioned to the needs of the

application flows RSVP is the only currently implemented signaling

protocol used for IntServ

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MPLS-QoS

Diff-Serv is often compared to a set ofbuilding blocks that can be used to prioritisepackets in the QoS flow

Packets can be classified by designatedfields in the header and are examined ateach node for the predefined treatment

TOS field in IP Header

Diff-Serv redefines the TOS byte syntax as wellas its semantics

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TOS Byte-IP Header

76543210

Reliability0-Normal1-Maximise

Precedence000-Routine001-Priority010-Immediate011-Flash

100-Flash Override101-CRITIC/ECP110-Internetwork Control111-Network Control

Delay0-Normal1-Minimise

Throughput0-Normal1-Maximise

Cost0-Normal1-Minimise

Reserved:Always set to 0

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DS Byte-IP Header

DS byte field uses the formerTOS field location inthe IP header

Bits (0-5) are used as an index value called

DSC

P(Diff-ServC

ode Point) field 6 bits can give 26=64 independent values called

Code Points

Bits 6 & 7 are unused

76543210

DSCP-Diff-Serve Code Point Reserved

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Per Hop Behaviors (PHB)

The DSCP index maps to various data treatmentsthat handle CoS

These data treatments are called Per HopBehaviors (PHB)

A Per Hop Behavior PHB includes CoS handlingmechanism such as queue selection, schedulingand congestion control.

The currently defined PHBs include Expedited Forwarding (EF) Gold

Assured Forwarding (AF) Silver

Default Behavior (DE) - Bronze

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Per Hop Behaviors (PHB)

An EF PHB is required when loss,jitter, and

delay must be low. This PHB generally

prevents any queues in the node from

increasing in size An AF PHB assures the delivery but does

not guarantees the delay, jitter and other

QoS parameters A DE PHB is the default best effort behavior

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EXP(CoS) Bits-MPLS Header

0 0 0

0 0 1

0 1 0

0 1 1

1 0 0

1 0 1

1 1 0

1 1 1

Queue No 4

Queue No 3

Queue No 2

Queue No 1

Packet Loss

Priority (PLP) Bit

Premium Traffic (Guaranteed Bandwidth, Latency and Delivery)

Gold Traffic (Expedited Forwarding, Guaranteed Latency

And Delivery

Silver Traffic (Assured Forwarding, Guaranteed delivery)

Bronze Traffic (Best Effort)

If PLP bit is set to 1 packet discard probability is high in case of

Network congestion.

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Implementing Network Policy

Video

Conferencing

Business

Applications

Email,

Web browsing,

Pointcast

Gold

Silver

Bronze

Traffic

Classification

VPN

Application-level QoS and

bandwidth management

Multiple Service Classes

(can be customized

per customer)

IP Network

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Traffic Differentiation

CE

CE

Separate queuesfor different VPNs

Gold VPNs

preferred over

Silver VPNs.

Gold

VPN

Customer

Silver

VPN

Customer

Bronze

VPN

Customer

CCE

S

VRF

VRF Base

Tunnel

Stacked

Tunnel

C

C

L2I

nterface

VRF

Schedule

r

Classifier sets MPLS EXP bits

Could have different

traffic-engineered base

tunnels for Gold and

Silver VPNs.

C

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