Multiprotocol Label Switching(MPLS)

Sookyoung Lee

Agenda

Problems of traditional IP routing Motivations for MPLS Objectives of MPLS What is MPLS?

Label, FEC, LIB, LER, LSR, and LSP How MPLS works?

LDP, CR-LEP, TE-RSVP Main capabilities of MPLS

Connection-oriented QoS Support, Traffic Engineering, VPN support, Multiprotocol Support

References

Problems of traditional IP routing

Problem of Traditional IP Routing Longish latency at every hop

Header analysis Routing table lookup based on the IP address Replace the layer 2 address

No assurance how a packet will travel Some congested links and some underutilized links

No QoS - all packets are treated equally No capability to prioritize packets between different h

osts and of different applications

Motivation for MPLS

Rapid growth of Internet Increase in traffic volumesVoice and data convergence on a single

network infrastructureNew latency dependent applicationsEver-increasing number of ISP networksStill IP protocol suite popular – the most

predominant networking technology

Objectives of MPLS

Speed up IP packet forwarding By cutting down on the amount of processing at

every intermediate routerPrioritize IP packet forwarding

By providing ability to engineer traffic flow and assure differential QoS

Without losing on the flexibility of IP based network

What is MPLS?

A technology to switch (forward) a packet at a high speed at layer 2 using fixed length labels obtained from layer 3 routing information. Integration of layer 2 and layer 3 IP supplements

MPLS and ISO model MPLS Architecture MPLS terminology MPLS Cloud

MPLS and ISO model

No modification needed on the existing layers when MPLS layer is added.

MPLS must be backward compatible.

PPP

Physical (Optical - Electrical) 1

2

IP 34

Applications7to5

FrameRelay ATM (*)

TCP UDP

PPP FR ATMMPLS

MPLS Architecture

Routing protocol OSPF OSPF OSPF

Attributes Precedence

Local tableLabel table Local table Local table

LSP Label swapping Label removalClassificationLabel assignment

IngressNode

CoreNode

EgressNode

Label SwitchLayer 2

Layer 1

Layer 2

Layer 1

Layer 2

Layer 1

Layer 2

Layer 1

Layer 2

Layer 1

FEC table Local table Local table Local table

MPLS terminology

Label and Label Stack

FEC – Forward Equivalence Class LIB – Label Information Base

LER – Label Edge Router LSR – Label Switching Router

LDP – Label Distribution Protocol LSP – Label Switched Path

Label A short, fixed length identifier (32

bits) Sent with each packet Local between two routers Can have different labels if entering

from different routers One label for one FEC Decided by the downstream router

LSR binds a label to an FEC It then informs the upstream LSR

of the binding Different depending on layer 2

technology ATM: VCI/VPI field of ATM header Frame Relay: DLCI field of FR

header PPP/LAN: ‘shim’ header inserted

between layer 2 and layer 3

ATM-MPLS label

S: bottom of stack bitExp: Experimental

Label EXP S TTL

20bits 3bits 8bits132bits

DLL header label stack entry NL header L3 data1 to many

PTIVCIVPIGFC CLP HEC DATA

Label

Label StackLayer 2 Header Label 3 IP PacketLabel 2 Label 1

MPLS Domain 1

MPLS Domain 2

MPLS Domain 3

Slide by ByTamrat Bayle, Reiji Aibara, Kouji Nishimura

MPLS supports hierarchy. Each LSR processes the

topmost label. If traffic crosses several

networks, it can be tunneled across them

Advantage – reduces the LIB table of each router drastically

FEC (Forward Equivalence Class) A group of packets that require the same forwarding treatment acros

s the same path grouped based on

Address prefix Host address QoS

FEC is encoded as a label

Assume packets have the destination address and QoS requirements as 124.48.45.20 qos = 1 143.67.25.77 qos = 1 143.67.84.22 qos = 3 124.48.66.90 qos = 4 143.67.12.01 qos = 3

FEC –1 label a FEC – 2 label b FEC – 3 label c FEC – 4 label d 143.67.25.77 124.48.45.20 143.67.84.22 124.48.66.90 143.67.12.01

MPLS Cloud

LSR

Egress LER

LSR

Ingress LER

IP PacketIP Packet w/ Label

L3 RoutingL3 Routing

Label SwappingLabel Swapping

LER

LERLER

L3 RoutingL3 Routing

L3 Routing

LSP

LER Can be an ATM switch or a router Ingress LER performs the following:

Receives the packet Adds label Forwards the packet into the MPLS domain

Egress LER removes the label and delivers the packet

LSR A router/switch that supports MPLS Can be a router Can be an ATM switch + label switch controller Label swapping

Each LSR examines the label on top of the stack Uses LIB to decide the outgoing path and the outgoing label Removes the old label and attaches the new label Forwards the packet on the predetermined path

LSP LSP defines the path through LSRs from ingress to egress router FEC is determined at the LER-ingress LSPs are unidirectional LSP might deviate from the IGP shortest path

LSR

OrdinaryIP Router

Label Distribution Protocol (LDP)

LDP is the set of procedures and messages For LSRs to establish LSPs through a network by mapping network-layer routing information directly to

data-link layer switched paths. associates a FEC with each LSP it creates.

Currently, several protocols used as LDP are available: CR-LDP, RSVP-TE: Provides functionality for traffic engi

neering and QoS Multiprotocol extentions of BGP-4

LDP messages

Discovery Messages - UDP Used to announce and maintain the presence of an LSR in a net

work LSRs multicast these messges periodically to 224.0.0.2 and all r

outers listen to this group Session Messages - TCP

used to establish, maintain and terminate sessions between LDP peers

Advertisement Messages - TCP create, change and delete label mappings for FECs

Notification Messages - TCP Used to provide advisory information and to signal error informati

on

Label Distribution Methods

Unsolicited Downstream Label Distribution

Rd discovers a ‘next hop’ for a particular FEC

Rd generates a label for the FEC and communicates the binding to Ru

Ru inserts the binding into its forwarding tables

Downstream on Demand Label Distribution

Ru recognizes Rd as its next-hop for an FEC

A request is made to Rd for a binding between the FEC and a label

If Rd recognizes the FEC and has a next hop for it, it creates a binding and replies to Ru

Label-FEC Binding Label-FEC Binding

Request for BindingRu Rd RdRu

Rd and Ru are said to have LDP adjacencyLDP adjacency

Unsolicited Downstream

MPLS switch

MPLS switch

MPLS switch

MPLS switch1

2

3

1 2

3

1

2

3

41

2

3

138.120

192.168127.20

Mapping 12Mapping 5

5 1212

5

IngressInterface

IngressLabel

FEC EgressInterface

EgressLabel

1 138.120 3

IngressInterface

IngressLabel

FEC EgressInterface

EgressLabel

1 138.120 x4

3

IngressInterface

IngressLabel

FEC EgressInterface

EgressLabel

1 x 138.120

The downstream node defines the label and advertises it to the upstream node.

Downstream stream on demand

Mapping 12Mapping 5

5 12

IngressInterface

IngressLabel

FEC EgressInterface

EgressLabel

1 138.120 312

IngressInterface

IngressLabel

FEC EgressInterface

EgressLabel

1 138.120 x4

53

IngressInterface

IngressLabel

FEC EgressInterface

EgressLabel

1 x 138.120

Request 138.120Request 138.120

MPLS switch

MPLS switch

MPLS switch

MPLS switch1

2

3

1 2

3

1

2

3

41

2

3

138.120

192.168127.20

The label is requested by the upstream node and the downstream node defines the label used.

Label Distribution and Management Label Distribution Control Mode

Independent LSP control: Each LSR makes independent decision on when to generate labels and communicate them to upstream peers

Ordered LSP control Label-FEC binding is communicated to peers if:

• LSR is the ‘egress’ LSR to particular FEC• Label binding has been received from upstream LSR

Used for explicit routing

Label Retention Mode Conservative – LSR maintains only valid bindings Liberal - LSR maintains bindings other than the valid next hop, more

label, quick adaptation for routing change

Label Advertisement Mode Downstream allocation Downstream-on-Demand allocation

Label Information Base (LIB)

Table maintained by the LSRs Contents of the table

Incoming label Outgoing label Outgoing path Address prefix

Incoming label

Address Prefix Outgoing Path

Outgoing label

MPLS forwarding example

128.89.25.4

128.89.*.*

171.69.*.*

data

InLabel

Address P refix

OutIntf

1

1

0

OutLabel

X 128.89 1 4X 171.69 1 5... ... ...

InLabel

Address P refix

OutIntf

OutLabel

4 128.89 0 95 171.69 1 7... ... ... ...

InLabel

Address P refix

OutIntf

OutLabel

9 128.89 0 XX 171.69 2 5... ... ... ...

2 0

...

128.89.25.4 data4128.89.25.4 data9

128.89.25.4 data

MPLS Protocol Stack

IP

PHY

Layer2

IP

Layer2

PHYPHY

Layer2

PHY

Layer2

PHY

Layer2

IP

Layer2

PHYPHY

Layer2

IP

PHY

Layer2

End System End System

CoreMPLS

Network

LER LER

Core LSR

MPLS Network

IP

TCP/UDPRoutingProtocol

IP

TCP/UDPRoutingProtocol

IP

TCP/UDPRoutingProtocol

MPLS Control protocol Stack Architecture

MPLS Data Protocol Stack Architecture

MPLS Interworking Architecture

LER LER

LER Core LSR LER

LDP LDP LDP

MPLSMPLSMPLS

End System

End System

Four main capabilities of MPLS

Connection-oriented QoS Support

Traffic Engineering

VPN support

Multiprotocol Support

Connection-oriented QoS Support

Connection-oriented network has powerful traffic management and QoS capabilities.

MPLS imposes a connection-oriented framework on a connecti

onless IP-based Internet providing the foundation for sophisticated and reliable QoS traffic contracts.

Flow-by-flow QoS (End-to-end) not packet-by-packet QoS (Hop-by-hop)

Traffic Engineering (TE)

What is TE? Dynamically define routes Maximize Bandwidth Utilization by spreading the network traffic

across network Ensure available spare link capacity for re-routing traffic on failur

e Meet policy requirements imposed by the network operator

MPLS has a primitive form of automated TE. is aware of flows of packet not just individual packets With MPLS, Routes are changed on a flow-by-flow basis (Explici

t routing), instead of simply changing the route on a packet-by-packet basis

Constrained-Based Routed LDP (CR-LDP) Modified LDP to set up the “Explicit Routing (ER-LSP)” Strict ER-LSP: Specifies list of nodes using actual address of each n

ode to traverse. Loose ER-LSP: Specifies list of nodes to act as one of the ‘abstract’

nodes to traverse. It can co-exist with the pure LDP. Introduces additional constraints (new parameters) for traffic regulati

on

LER1 LSR2 LSR3 LER4

Advantages of Explicit Routing Can use routes other than shortest path Operator has routing flexibility Traffic engineeringTraffic engineering

Explicitly Routed LSP

End-to-End forwarding decision determined by ingress node. Enables Traffic Engineering

LER 1

LSR 2 LSR 3

LER 4

Forward toLSR 2LSR 3LSR 4LSR X

Overload !!

Overload !!

CR-LDP Traffic Engineering

QoS and Traffic parameters Path Preemption Path Re-optimization Failure Notification Loop Detection LengthU F Traffic Para TLV

0 311 15

Excess Burst Size

Committed Burst Size

Committed Data Rate

Peak Burst Size

Peak Data Rate

Flags Frequency Reserved Weight

Peak Rate – Maximum rate at which traffic should be sent to CR-LDP

Committed Rate – The rate that the MPLS domain commits to be available to the CRLSP

Excess Burst Size – Measures the extent by which the traffic sent on CR-LSP exceeds the committed rate

Frequency – constraints delay

TE-RSVP QoS and Traffic parameters Failure Notification Loop Detection Multi Protocol Support Path Preemption

Slide by ByTamrat Bayle, Reiji Aibara, Kouji Nishimura

VPN support With VPN, the traffic of a given enterprises or group passes transparently through

the Internet in a way that effectively segregates that traffic from other packets on the Internet.

MPLS provides an efficient mechanism for supporting VPNs proving performance guarantees and security.

VPN A

VPN A

VPN B

VPN AVPN B

VPN

LDPVPN

LDPVPN

LDPVPN

P1

P2

P3

P4

P5

LSP - Label Switched Path

LDP

Multiprotocol Support

MPLS can be used on many networking technologies. MPLS supports IPv4, IPv6, IPX, AppleTalk at the network layer. MPLS supports Ethernet, Token Ring, FDDI, ATM, FR, PPP at t

he link layer.

Universal nature of MPLS MPLS enabled routers can coexist with ordinary IP routers. MPLS-enabled ATM switches and MPLS-enabled FR switches c

an be configured to co-exist wit ordinary ATM or FR switches.

MPLS is a good solution to optimize resources and expand QoS support over mixed network technologies.

References

MPLS Charter: http://www.ietf.org/html.charters/mpls-charter.html

MPLS Resource Center: http://www.mplsrc.com MPLS Forum: http://www.mplsforum.org

Basic RFCs RFC 3031/3032 MPLS Forwarding/Architecture RFC 3036 MPLS LDP Specification RFC 3215 LDP State Machine RFC 2205 MPLS Signaling RSVP RFC 3209 MPLS Signaling RSVP-TE