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MPLSMulti-protocol label switching

Mario BaldiPolitecnico di Torino

(Technical University of Torino)http://staff.polito.it/mario.baldi


MPLS is the enabling technology for the New Broadband (IP) Public

Network

From MPLS Forum Documents


IP

Leased Line

Frame Relay

ATM CircuitEmulation

IP over DWDM

IP over ATM Circuit Emulation

IP over ATM

IP over SONET/SDH

IP over Frame Relay

Optical switching

The “onion” that makes telecos “cry”


IPIP router

with LAN or WAN

interface

IP router with T3 interfacePABX with T1 interfaceIP router with LAN (GE) or WAN (PoS) Interface

IP/MPLS

Optical switching

The future: WDM, IP, MPLS


The idea

Instead of the IPdestination address

Forwarding packets according to a label

Label IP packet


Why?

The label may be used as an index

Faster lookup

Traffic engineering

Instead of longest prefix matching


MPLS introduces a connection-oriented

paradigm in IP networks


Network architecture

Label edge routerIngress/egress LSR

Label switch router (LSR)

MPLS networkMPLS cloud


Egress LSR

Ingress LSR

Label Switched Path (LSP)

A.k.aLSP tunnel


Label switching

D

D

1234

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ADX

001

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In Out221

In Out01

In Out01

In Out01

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Forwarding table


MPLS “header” Contains the label

MPLS key elements

Enhanced routing protocols Constraints in choosing paths

Protocols for label distribution Signaling


MPLS history

Tag SwitchingIP over ATM No problems with resolution of

addressesSimpler signalingOnly one control plan

ATM with IPReusing ATM switching

hardware


MPLS historyMPλS (Multi-Protocol Lambda

Switching)G-MPLS (Generalized MPLS)Packet switchingCell switchingCircuit switching (SONET/SDH)Lambda switchingAnything switching

Unifying control plane


MPLS “header”


Shim headerMPLS header

Level 2 IP packet

Label Exp S TTL

20 bit 3 bit 8 bit1

TTL: Time to live

Exp: Experimental bits (CoS)S: Bottom of stack


VCI/VPI (ATM)

MPLS labels in layer 2 header

Connection oriented layer 2 protocols

ATM and frame relay

DLCI (Frame relay)


LSP setup:label and path selection

A glimpse into the control plane


Forwarding equivalence class (FEC)

Packets that

Are treated the same wayby each LSR

Follow the same pathin MPLS network

Receive the same label


Three Key Actions

Label bindingLabel mappingLabel distribution

Taken by LSRs


Label BindingAn LSR determines the label thatshould be prepended to packets

belonging to a given FECDownstream bindingLSR on receiving end of a linkPackets of the FEC shall be

received with chosen labelUpstream node to be notified

UnsolicitedOn-demand


How can the LSR know it?

Label MappingChosen by the considered LSR

Chosen by downstream LSR

Based on routing

Association betweenInput labelOutput labelOutput port/next hop

Actual creation of an LSP


Label Distribution

Notification of the label chosen for a given FEC

Follows label bindingAn LSR that has made a

binding, distributes it to neighboring LSRsOr at least to the upstream

one


Putting it all together: Sample LSP Setup

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ABX

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Label for destination D

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for D

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Putting it all together: Sample LSP Setup

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for D

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Binding

Distrib

ution


Static label binding (and mapping)

Through management

Non-scalable

Equivalent to PVC ATM

No interoperability among managing systems

Impossible to have LSPs through different networks


Dynamic Label Binding

Data/traffic drivenTriggered by data packets

Control drivenTriggered by control

messageSignalingRouting


Control Driven Label Binding

Topology based

Explicit creation of LSPs

The creation of LSPs is linked to the discovery of routes towards destinations

Explicit signaling Initiated by label edge routers


Routing protocol: BGP Only topology based

Designed for allocation in integrated service networks

Resource reservation protocol (RSVP)

Label distribution protocols

Three alternatives (incompatible)

Label distribution protocol (LDP) Designed for the purpose


Routing protocols

Used to determine LSP routing

Impact label mapping phase

Indirectly determine packetrouting


carry topological information

Routing protocols

Existing protocols

IS-IS

OSPF

BGP-4

In MPLS context they are enhanced to...


Routing protocols

...carry information to constraint routing decisions (constraint data)

Capacity of linksLink utilization

Dependencies among linksUsed for fault recovery


Enhanced routing protocols

Constraint based routingis fundamental to support

traffic engineering

IS-IS–TE

OSPF–TE


Hop-by-hop Routing

Each LSR decides the next LSR of the LSP path

E.g., Based on IP routing tableSame principle and outcome

(route) as in traditional IP routing


Hop-by-hop Routing ProcedurePick label for upstream linkLabel binding

Map it to Next hopAddress of connected

interface of next LSRLabel announced by next LSR


Hop-by-hop Routing Example

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In Out12

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Routing table

X → 3D → 2


Routing table

A → 3D → 1



The choice is constraint based Constraint based routing

Explicit constraint based routing

For example, ingress LSR

A single switch chooses the pathof a whole LSP

Explicit routing


Constraint Based Routing

Distributed operation impossibleNo unique route selection

criteriaConflicting constraints

Hard to maintain constraint information synchronizedIt changes more frequently

than topology information


CR-LDP Constraint-based routing LDP

Should be modified

Label distribution protocols

RSVP-TE RSVP for Traffic Engineering

To be used withOSPF-TE and IS-IS-TE


New possibilities

Traffic engineering

Per-class traffic engineering

Guaranteed quality of service

Fast fault recovery

Not yet supported

In less than 50 ms

Synergy with DiffServ


Traffic Engineering

Motivation and enablement


Traditional IP routingTraffic for D on optimal path

D

Aggregation!


Traditional IP routing

D

Traffic for D on optimal path4 overloaded linksCongestion!


Traditional IP routingTraffic for D on optimal path4 overloaded linksUnderutilization!

9 underutilized links

D


D

Traffic Engineering

Enables traffic distribution


Traffic Engineering

D

No conge-stion

Uniform use


Can you do it with traditional IP routing? Traffic for D on

optimal path4 overloaded links9 underutilized links

D


Control plane and data plane

Data plane

Control plane

RIP IGRP

OSPF

BGPIS-IS

Routing database

Routingtable

Continuous updating

IPR

o

u

t

e


It would be possible to choosepaths according to load

Traffic for D on optimal pathoverloaded linksunderutilized links

D


In such way, unloaded links are loadedand viceversa ...

Traffic for D on optimal pathoverloaded linksunderutilized links

D


... and routing tables change ... Traffic for D on

optimal pathoverloaded linksunderutilized links

D


…so does the link load...Traffic for D on optimal pathoverloaded linksunderutilized links

D


... and the paths change again!!! Traffic for D on

optimal pathoverloaded linksunderutilized links

D

Instability!


Control plane and data plane

Dataplane

Control plane

RIPIGRP

OSPF

BGP

IS-IS

Routing database

Forwarding table

MPLS

LDPRSVP

LSP setup

Signaling (LSP setup)

RoutingtableL

S

R


Traffic Engineering with MPLS

D


ATM has so far been used for traffic engineering on IP networks

Two control plansRouters are ATM-unaware

Great number of adjacencies Limitated scalability

Traffic Engineering without MPLS


Only one control plan operating on physical topology

SimplerGreater scalability

MPLS is IP-aware

Traffic Engineering with MPLS


MPλS (MPLambdaS)MPLS control plans in optical

networksGMPLS (Generalized MPLS)MPLS control plane in any

network Packet, circuit, optics,

etc…

MPLS extensions


Adding CoS and QoS

Explicit support is required in LSR data plan and control

plan

Resources and service modes may be associated to a FEC at

LSP setup


Class of service (CoS)

Relative priority among different FECs

It does offer absolute guarantiesSupport of DiffServ modelPer-hop behaviorEF (expedite forwarding), AF

(assured forwarding)Per class traffic engineeringDS-aware traffic engineering


Quality of service (QoS)

Specific guaranties onBandwidthDelayBurst size


Advantages of QoS in MPLSUnified network supporting all types of

services(marketing message)

Support of QoS and real-time services on IP (e.g., voice) is not ready

A lot of multi-service networks now have a paradigm “Ships-in-the-night” ATM protocols are ATM typical

servicesMPLS control plan for IP services


Fast Fault Recovery

Edge-to-edge re-routing

LSP

Link re-routing

Re-routing node

Re-routing node


Header Layer 2 IP packetLabel 3

MPLS Domain 3

Label 2

MPLS Domain 2

Label 1

MPLS Domain 1

Label Stackhierarchy and scalability

• Routing table reduction

• Forwarding table reduction


MPLS and scalability

MPLS labels introduce hierarchyVarious hierarchical levels, as

needed for the necessary scalability

Routing tables of transit routers do not have to be comprehensiveLSP between edge routers

(Simpler and faster exact match of labels rather than longest prefix matching)


Penultimate Hop Popping (PHP)

The last but one node on the path of an LSP pops the labelThe LER routes the packet based

on IP address (or next label in stack)

Distribution of label 3 indicates (implicit) PHP

Explicit PHP: label 0 is swappedWhen shim header is neededE.g., EXP bits


VPN: Virtual Private Network

Services similar to those of a private network, but provided on a public infrastructure (IP)

Privacy and securityOverlapping addressing spaces

(private) Non-unique addresses

Class of Service (CoS) and Quality of Service (QoS)


VPN based on MPLS

Labels “hide” IP addresses of users on the public network Allow for ovelapping address

spacesService flexibility FEC


VPN based on MPLS

MPLS allows a scalable solution for VPN servicesOther approaches require

explicit setting (manual) of tunnels between each pair of websites

Number of VPNsNumber of VPN members


Standardization

IETF –Internet Engineering Task Force

FR/MPLS Alliance

MPLS working group

Consortium of vendors Foster deployment Aspects omitted by IETF VoMPLS, ADSL