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Multiprotocol Label Switching (MPLS)Multiprotocol Label Switching (MPLS)and its Applicationsand its Applications

Network ArchitectureNetwork ArchitectureSpring 2009Spring 2009Lecture 17Lecture 17

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Sources for this Material

• MPLS presentation by Philip Matthews, Nortel Networks,April 2000, prepared by Dr. Bilel Jamoussi and Peter Ashwood-Smith

• "Simplified Operations Through Resilient IP Network Design" presented by Hadriel Kaplan, Avici Systems, IPOM 2003 Tutorial

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“Label Substitution” what is it?

Have a friend go to B ahead of you. At every road they reserve a lane just for you. At every intersection they post a big sign that says for a given lane which way to turn and what new lane to take.

Your job in getting to the destination is now easy.

LANE#1

LANE#2

LANE#1 TURN RIGHT USE LANE#2

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Label Switched Path

#7

#99

#9

#3 Right #7

#99 RIGHT #9#7 LEFT #99

#9 LEFT #4072

#3IP

#4072 IP

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Routers Do Both Routing and Switching

• Routing— Deciding the next hop based on

the destination address.

— A Layer 3 (L3) function.

• Switching— Moving a packet from an input

port to an output port and out.

— A layer 2 function.

— Usually a switching decision is a simple table lookup.

INPUT PORTS OUTPUT PORTS

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47.1

47.247.3

IP 47.1.1.1

Dest Out

47.1 147.2 2

47.3 3

1

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Dest Out

47.1 147.2 2

47.3 3

1

2

1

2

3

IP 47.1.1.1

IP 47.1.1.1IP 47.1.1.1

Dest Out

47.1 147.2 2

47.3 3

STANDARD IP

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Label Switched Path (LSP)

Intf In

Label In

Dest Intf Out

3 40 47.1 1

Intf In

Label In

Dest Intf Out

Label Out

3 50 47.1 1 40

47.1

47.247.3

1

2

31

2

1

2

3

3Intf In

Dest Intf Out

Label Out

3 47.1 1 50

IP 47.1.1.1

IP 47.1.1.1

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MPLS: Flexible Forwarding

LSP to IPLABEL SWITCHINGIP to LSP

IP IP #L1 IP #L2 IP #L3 IP

IP DA

IP: Packets are forwarded based on Destination Address (DA). We can call this “destination based routing”.

MPLS:

• Map packets to LSP based on (Source Address, Destination Address, protocol, port, DSCP, interface, etc.)

• Forward packets based on the Label

IP DA IP DA IP DA IP DA

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What is MPLS?

• MPLS = Multi-Protocol Label Switching

• MPLS is an IETF Standardized mechanism for controlling packet routing.

• MPLS Framework and Architecture— Defines the scope, the various components and their interactions

• Encapsulations — Labels are used at the data plane to make forwarding decisions

• Signaling Protocols— Distribute Labels to establish Label Switched Paths

• Routing Protocol Traffic Engineering Extensions— Distribute Bandwidth and other link attributes to make routing

decisions

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MPLS Terminology

• LDP: Label Distribution Protocol

• LSP: Label Switched Path

• LER: Label Edge Router (edge of an area that supports MPLS)

• LSR: Label Switching Router (inside an area that supports MPLS)

• FEC: Forwarding Equivalence Class

— Which packets are on which LSPs – based on

destination, source, QoS, application, UDP or TCP, etc.

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MPLS Encapsulation

Layer 2 Header(eg. PPP, 802.3)

•••Network Layer Header

and Packet (eg. IP)

MPLS ‘Shim’ Headers (1-n)

1n

• Network layer must be inferable from value of bottom label of the stack

MPLS on LANs uses a ‘Shim’ Header Inserted Between Layer 2 and Layer 3 Headers

(other technologies use different approaches)

MPLS on LANs uses a ‘Shim’ Header Inserted Between Layer 2 and Layer 3 Headers

(other technologies use different approaches)

Label Exp. S TTL

Label: Label Value, 20 bits (Values 0 through 16 are reserved)Exp.: Experimental, 3 bits (was Class of Service)S: Bottom of Stack, 1 bit (1 = last entry in label stack)TTL: Time to Live, 8 bits

4 Octets

Label StackEntry Format

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MPLS Turns Routing into Switching

• So we can avoid performing the layer 3 function.— Use labels to decide next hops.

• What benefit does this provide?

• In what situations would this benefit not be very significant?

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Solutions Enabled by MPLS• Virtual Private Networks

— Connect two or more separate sites over the Internet

— Label switched paths can be created to be “virtual links” between routers.

— This can create something that looks like a network for a customer.

— Key Features: Security, control over performance, management ability.

• Enable QoS in IP Networks— Support Diffserv using connection-oriented QoS

— “Connections” can be flows or large aggregates

• IP Traffic Engineering— Use constraint-based routing to adapt to latest network loading and

QoS performance

• L2/L3 Integration— Integrate with L1 and L2 technologies like Optical Cross Connects

(OXC’s) and ATM

• Resilient Network Design— Automatic Failover and Backup

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BEST OF BOTH WORLDS

PACKETForwarding

CIRCUITSWITCHING

• MPLS + IP forms a middle ground that combines the best of IP and the best of circuit switching

technologies.

• MPLS + IP forms a middle ground that combines the best of IP and the best of circuit switching

technologies.

MPLS+IP

IP ATM

HYBRID

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Intf In

Label In

Dest Intf Out

3 40 47.1 1

Intf In

Label In

Dest Intf Out

Label Out

3 50 47.1 1 40

47.1

47.247.3

1

2

3

1

2

1

2

3

3

Intf In

Dest Intf Out

Label Out

3 47.1.1 2 33 3 47.1 1 50

IP 47.1.1.1

IP 47.1.1.1

EXPLICITLY ROUTED LSP ER-LSP

Explicitly Routing LSP that does not follow the standard IP path.

Explicitly Routed LSP that does not follow the standard IP path.

This entry gives the longest prefix match.

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ER LSP - Advantages

• Operator has routing flexibility

— Can establish LSP’s based on policy, QoS, etc.

— Can have pre-established LSP’s that can be used in case of failures.

• Can use routes other than the shortest path

• Can compute routes based on dynamic constraints (available bandwidth, delay, etc.) based on a distributed topology database.(traffic engineering)

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Traffic Engineering

A

B C

D

Traffic engineering is the process of mapping traffic demand onto a networkTraffic engineering is the process of mapping traffic demand onto a network

Demand

NetworkTopology

Purpose of traffic engineering:• Maximize utilization of links and nodes throughout the network• Engineer links to achieve required delay, grade-of-service• Spread the network traffic across network links to minimize impact of failure• Ensure available spare link capacity for re-routing traffic on failure• Meet policy requirements imposed by the network operator

Traffic engineering is key to optimizing cost/performance

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The need for MPLS protection

• Layer 3 recovery is too slow.— OSPF, RIP, etc. require a redistribution of updated link status

information in response to a fault.

— Then routers must recompute their routes.

— Takes on the order of seconds.

— Can have looping and lost packets in the meantime.

• Other technologies are very fast.— SONET can establish an alternate route around a failure within

50 milliseconds.

— By having active backup resources immediately available.

• It would be good to have millisecond failovers with MPLS.

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Pre-signaled Standby LSP’s

• Planning occurs before failure— Then LSP ingress learns of the failure

— Moves traffic to use standby LSP

• Ingress must first know about the failure— Must receive failure notifications.

— The farther away from the failure, the longer it will take to start the reroute.

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MPLS Fast Reroute

— A merge node joins traffic back onto the primary LSP.

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Summary of Motivations for MPLS

• Simplified forwarding based on an exact match of a fixed length label

— Initial driver for MPLS was based on the existence of cheap, fast switches from previous ATM technology

• Separation of routing and forwarding in IP networks— Facilitates evolution of routing techniques by fixing the forwarding

method

— New routing functionality can be deployed without changing the forwarding techniques of every router in the Internet

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Summary of Motivations for MPLS

• Enables the use of explicit routing/source routing in IP networks— Can easily be used for such things as traffic management, QoS

routing

• Promotes the partitioning of functionality within the network— Moves detailed processing of packets to the edge; restricts core to

simple packet forwarding

— Assists in maintaining scalability of IP protocols in large networks

• MPLS can enable fast restoration from failures.

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• Applicability to multiple layers— Can be deployed at Layer 2 on Ethernet, Wireless, or legacy ATM and

Frame Relay technologies.

— Can be deployed at Layer 1 for Fiber, Wireless, etc.

• But MPLS is much more complex than traditional IP forwarding— Routers need to be able to forward based on labels (in addition to their

normal functions).

— LSP’s must be signalled and maintained.

— Some ISP’s have said they are not using MPLS and do not plan to.– This will continue to be true if overprovisioning remains effective.– But some of these ISP’s are realizing that their customers want

MPLS to provide more assurance about their IP-based services.

Summary of Motivations for MPLS