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1MPLS Basics

MPLS BASICS

MODULE ID: TMPLBAS001

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2MPLS Basics

Data networking and need for MPLS

Basic concepts and working of MPLS

Label - Allocation and Distribution

MPLS Forwarding

Topics Covered

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3MPLS Basics

Traditional Data Networks

Voice CircuitTDM ATM Frame RelayIP

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Data Network Concepts

A data network is a set of nodes connected by links.

Nodes could be Routers, Switches, Multiplexers connected by links from 64 kbps till 10 gigabit ethernet.

Fundamental property of data networks is multiplexing.

Two main types of multiplexing are - Time Division Multiplexing ( TDM )Statistical Multiplexing ( StatMux)

Others are FDM , WDM etc.

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Data Network Concepts

TDM is practice of allocating certain amount of time on a physical circuit for a particular connection. This translates to bandwidth allocation as the circuits are fixed rate.

Examples for TDM being E1 (2Mbps circuit) , SDH

- Bandwidth permanently allocated for a connection whether connection used ( carrying traffic) or not.

+ No traffic congestion issues, bandwidth guaranteed

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Data Network Concepts

Statmux - is the practice of sharing available bandwidth between all users.

Examples are : IP , Frame Relay, ATM & now MPLS

It works by way of dividing traffic (data) into discrete units which are handled separately. IP units are Packets, ATM units are Cells, Frame Relay units are Frames.

+ Better utilisation of bandwidth, allows oversubscription

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Data Network Concepts

- introduces resource contention.

Therefore statmux technologies have to deal with Buffering of data unitsQueuing of data unitsDropping of data units

Running one statmux technology over other eg. IP over ATM . Mechanisms available in one technology to deal with contention does not translate properly into another.

Requirement of translating Layer 3 contention controls mechanisms to Layer 2.

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Data Network Concepts

Circuit Switching : Dedicated Transmission Path Continuous transmission of data Path established for entire session Fixed bandwidth transmission No overhead after call setup Call Setup delay, negligible

transmission delay

Packet Switching : No dedicated path Packets may be stored Route selection may change Dynamic bandwidth use Overhead in each packet Significant setup and transmission

delay

Virtual Circuit SwitchingNo dedicated path but path established for entire sessionDynamic use of bandwidthPackets stored till delivered

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Traditional IP Forwarding

Routing Protocols distribute layer 3 routing information. IP header is parsed at each hop, resulting in low efficiency. Forwarding is based on destination address. It is hard to deploy QoS as the path is not fixed.

Parse IP header mapped to next hop

Parse IP header mapped to next hop

Parse IP header mapped to next hop

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Most traffic goes between using only the primary link Destination-based routing does not provide any mechanism

for load balancing across unequal paths. Policy-based routing can be used to forward packets based

on other parameters, but this is not a scalable solution Requirements based routing difficult

R1

R2

R3 R4

R5

R6

R7

512 kbps512 kbps

2 Mbps2 Mbps

2 Mbps2 Mbps

Satellite LinkSatellite Link

Traditional IP Forwarding

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Connectionless: packet route Path 1 = S1, S2, S6, S8 Path 2 = S1, S4, S7, S8 The data reach their destination

out of order along different paths

Connection-oriented: cell switching

VC = S1, S4, S7, S8 The data reach their destination in

order along the same connection Fixed time delay, easy to control Connection types: PVC SVC

Virtual Connection

S2 S6

S4 S7

S3 S5

S1 S81

1

1

2 2

2

S2 S6

S4 S7

VC

S1 S8S3 S5

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Connection-oriented Routing depends on link layer, based on VPI/VCI or label Ensures QoS and real-time service

Virtual Channel Connection (VCC)

VCswitching

VCswitching

NNI NNI

VPI = 2VCI = 44

VPI = 1VCI = 1

VPI = 26VCI = 44

VPI = 20VCI = 30

UNIUNI

ATM Switching Process

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MPLS Origin

Connectionless control plane

Connectionless forwarding plane

IP

Connection-oriented control plane

Connection-oriented forwarding plane

ATM

Connectionless control plane

Connection-oriented forwarding plane

MPLS

To integrate IP with ATM

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MPLS Origin

To bring advantages of connection oriented

protocols to packet switched networks.

Faster switching - Replace IP header with short

and fixed-length labels as forwarding basis.

To substitute ATM & Frame Relay & provide

Integrated services with QoS without the overhead

of call segmentation.

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+ XR = X

Router ATM switch MPLS Router

MPLS Multi-Protocol Label Switching

Layer 3 routing Scalable and FlexibleLayer 2 switching Reliable and Traffic Engineering capability

Technology combining the advantages of ATM and IP

MPLS Origin

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MPLS

MPLSMulti-Protocol Label Switching

Multi-Protocol

Support multiple Layer-3 protocols, such as IP, IPv6, IPX, SNA

Label Switching

Label packets, and replace IP forwarding with label switching

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Multiple Protocol Support

OthersCLNPDCEne

tApple TalkIPXIPv6IPv4

MPLS

OthersFDDIEthernetFRATMPPP

Upper Layers

Physical Layer

3

2

1

4-7

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MPLS is the 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 network layer-3 routing

protocols and layer-2 switching technologies

MPLS is not a new network layer protocol because it does not

have its own routing capabilities and addressing schemes

MPLS is designed to work over many of the data layer

technologies that provides requisite layer-2 addressing and

functionality

MPLS is a Layer 2.5 Technology

MPLS

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MPLS Data Encapsulation

Data

DataH4

Application

TPT Layer

NW Layer

Data Link

MPLS

H3 DataH4

SHIM H3 DataH4

H2 T2SHIM H3 DataH4

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Label Position in Packet

Ethernet header/PPP header Label Layer-3 data

Ethernet /SONET/SDH packet

ATM header Label Layer-3 dataFrame modeATM packet

Cell modeATM packet VPI/VCI Layer-3 data

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Basic Working Process of MPLS

Core LSR

IP IP L1 IP L2 IP L3 IP

Traditional IP forwarding

Traditional IP forwarding Label forwarding

Edge LSR Edge LSR

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Traditional System of Mail Sorting

1. All the sorting stations should have list of all the cities, towns and villages

2. Searching for destination from huge list of cities, towns and

villages is a time consuming job

3. Language problem adds to delay the mail

sorting

Mr.R.RamachandranH.No.2132Village & PO: RajbanDistrict: Paonta Sahib

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3. Beyond main cities letters are sorted & forwarded based on the actual village or town address

2. At main cities Letters are sorted & forwarded quickly based on the PIN

Code

1. All the postal delivery post offices are allotted a six digit code

Mr.R.RamachandranH.No.2132Village & PO: RajbanDistrict: Paonta Sahib

12-13

14-16

18-19

17

11

20-2830-34

36-39

40-44

45-49

50-53

56-59

60-6467-69

70-74

80-85 78

79

75-77

Postal Index Number System of Mail Sorting

PIN Code: 170004

4. Language also does not pose any problem in writing the address

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Normal IP Routing

1. Routers maintain Routing Tables

2. Packets are routed on the basis of Destination IP

Address and Big Routing Tables

IP Header is Carrying the Routing InformationSource IP Address andDestination IP Address

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2. Outside MPLS Domain packets are routed on the basis of information in the IP Header

1. In MPLS Domain packets are switched on the basis of label information in the MPLS Header

MPLS Based Routing

MPLS Router

Normal Router

MPLS Router

MPLS Router

MPLS Domain

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MPLS Label

MPLS headerLayer 2 header IP header Data

Label SEXP TTL

200 23 24 31

32 bits

LABEL - 20 Bits - To set-up LSPEXP - Experimental - 3 Bits - Used for Prioritizing trafficS - Stack Bit - TTL - Time-to-Live

LABELS are only relevant to adjacent nodes

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Label Stack

Label stack enables nesting to provide extended service support. This is one of the biggest benefits of MPLS technology.

Inner Label and Outer Label generally used in VPN Services

MPLS header

Layer2 header IP header Data

MPLS header

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Labels

0-IPv4 Explicit NULL Label : Label stack must be poppedSubsequent forwarding of the packet will be based on the IPv4 network headerUsed only as the bottom most label1-Router Alert Label : Can be used anywhere in the stack except at the bottomLSR receiving this label will send packet to the local software to process the informationThe next innermost label determines the subsequent forwarding of the MPLS packetIf the packet is forwarded further, this label must be pushed back on to the stack before forwarding

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Labels

2-IPv6 Explicit NULL Label : Label stack must be poppedSubsequent forwarding of the packet will be based on the IPv6network headerUsed only as the bottom most label3-Implicit NULL LabelLabel is virtual in the sense that this value can be distributed but never appears in the MPLS header encapsulationCauses the LSR to pop the stack instead of normal swap operation, where a new label value will be introduced4:15-Reserved

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Labels

Label Spaces :Each label space consists of the assignable labels from 0-1048575 (0-15 Reserved)Two basic notions of using label spaces

- Per-Platform Label Space- Per-Interface Label Space

Per-Platform Label Space There is one set of labels for the entire LSR All interfaces share this common label pool

Per-Interface Label SpaceEach interface has its own label poolUsed particularly with ATM-LSRs

Decision to choose the label platform to be implemented on a particular LSR is a function of how the interfaces are used .

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City A City B

LSP

LER - Ingress

TransitLSR

TransitLSR

LER-Egress

MPLS Devices

MPLS uses Label Switched Path (LSP) for layer-2 forwarding. The path is setup using signaling protocols like Label Distribution Protocol (LDP) and routing protocols.

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MPLS Terminology

LER -Label Edge Router Examines inbound IP packets and assigns them to an FEC Generates MPLS header and assigns initial label Ingress & Egress

LSR - Label Switch Router Forwards MPLS packets using Label swapping Table lookup in Label Information Base (LIB)

LSP - Label Switch Path

- Path thro MPLS network setup by signaling protocol (LDP)

- LSPs are unidirectional , roughly equivalent to VC

- LSP setup is based on FEC criteria ,

- LSP may be different from IGP path

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MPLS Terminology

Forwarding Equivalence Class (FEC)

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. ( Same QoS ; Same Next Hop; Same Path)

LDP ( Label Distribution Protocol) associates a set of destinations with each LSP

Packets could be assigned to a LSP based on Combination of destination address and application type Combination of destination address and source address

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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 new label followed by pushing another new label on top

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

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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, mostly used.

RSVP ( Resource reservation Protocol ) ,CR-LDP ( Constrained based Label Distribution Protocol )-used for traffic engineering and resource reservation

Protocol-independent multicast (PIM)-used for multicast states label mapping

Border Gateway Protocol -external labels (VPN)

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Issues Concerning Label Distribution

Label allocation mode DoD : downstream-on-demand DU: downstream unsolicited

Label control mode Ordered Independent

Label hold mode Liberal retention mode : upon receiving a label, if there is no

route destined for the corresponding FEC, hold the label for later use

Conservative mode: upon receiving a label, if there is no route destined for corresponding FEC, discard the label

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Label Allocation Mode: DoD

Upstream Downstream

The upstream LSR sends a label request (containing FEC description information) to the downstream LSR.

The downstream LSR allocates a label to this FEC and feeds back the bound label to the upstream LSR via the label mapping message.

171.68.10/24

LSR1 LSR2 LSR3

171.68.10/24

Requesting labels destined for 171.68.10/24

Requesting labels destined for 171.68.10/24

171.68.10/24 20

Label 20 is allocated to 171.68.10/24

171.68.10/24 18

Label 18 is allocated to 171.68.10/24

Route triggering for 171.68.10/24

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Pop at Last Hop But One (PHP)

The label at the outmost layer is not needed at the last hop. If only one layer of label is used, the last hop performs IP forwarding else, it will perform internal label forwarding.

Parse IP header FEC bound with LSP

Parse IP header distribute to destination

Ingress LER

LSR LSR

Egress LER

Label operation: push

Label operation : swap

Label operation: PoP

PHP

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MPLS Forwarding Example

1 2Ingress Routing Table

Destination Next Hop134.5/16

200.3.2/24

(2, 84)

(3, 99)

MPLS TableIn Out

(2, 84) (6, 0)

MPLS TableIn Out

(1, 99) (2, 56)

MPLS TableIn Out

(3, 56) (5, 0)

3 5

2

3

2 6134.5.1.5

200.3.2.7200.3.2.1

134.5.6.1

DestinationEgress Routing Table

Next Hop

134.5/16

200.3.2/24

134.5.6.1

200.3.2.1

200.3.2.7

200.3.2.7

9999200.3.2.7 00200.3.2.7

200.3.2.75656200.3.2.7

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Summarizing MPLS

Combines the benefits of IP and Circuit Switching Assigns LABELS on Packets Router makes forwarding decision based on LABELS LABELS are exchanged between Routers Paths are build using LABELS Predominantly used for VPN services and Traffic

Engineering application.

Multi Protocol Label Switching

Label Switching - Label packets, and replace IP forwarding with label switching

Multi-Protocol -Support multiple Layer-3 /2 protocols

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