Multi-Protocol Label Switch (MPLS)

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Outline• Introduction • MPLS Terminology• MPLS Operation

– Label Encapsulation• Label Distribution Protocol (LDP)• Any Transport Service over MPLS• MPLS Applications

– Traffic Engineering– MPLS-based VPN– MPLS and QoS

• Summary

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Why MPLS?• Growth and evolution of the Internet• The need for network convergence to support both voice and

data services on both carrier and enterprise networks• The need for advanced and guaranteed service over the

Internet– The need of virtual circuit but without the complexity of provisioning

and managing virtual circuits.• PVC: too much provisioning and management work• SVC: [signaling] too complex to support and trouble shoot

• The need for an efficient transport mechanism– routing: flexibility– forwarding: price/performance– Can we forward IP packets? Answer: MPLS

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Performance and service of Layer-2 and management of layer-3

Motivation for Carriers• Network convergence

– Single network to support voice and data traffic

• Ease of network management– to provision new services– to support various Service Level Agreements (SLA)

• Ease of Traffic Engineering– To reroute during node failures or network congestion

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Motivation for Enterprises• Network convergence

– Single network for voice and data

• A meshed topology (any-to-any) without the nightmare of cost and management– Confusion with too many Frame Relay PVCs

• Quality of Service (QoS) for intranet– Ease of bandwidth management– Flexibility of bandwidth provisioning

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MPLS History • IP over ATM• IP Switching by Ipsilon• Cell Switching Router (CSR) by Toshiba• Tag switching by Cisco• Aggregate Route-based IP Switching (IBM)• IETF – MPLS

– http://www.ietf.org/html.charters/mpls-charter.html– RFC3031 – MPLS Architecture– RFC2702 – Requirements for TE over MPLS– RFC3036 – LDP Specification– over 113 RFCs related to MPLS

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MPLS and OSI(MPLS is a layer 2.5 protocol)

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Physical

IP

Applications

TCP UDP

PPP FR ATM

MPLS

When a layer is added, no modification is needed on the existing layers.

Ethernet DWDM

MPS

MPLS and OSI(MPLS is a layer 2.5 protocol)

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Label Switching(This is not new!)

• ATM: VPI/VCI• Frame Relay: DLCI• X.25: LCI (logical Channel Identifier)• TDM: the time slot (Circuit Identification

Code)• Ethernet switching: MAC Address

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Label Substitution (swapping)

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Label-A1 Label-B1

Label-A2

Label-A3

Label-A4

Label-B2

Label-B3

Label-B4

MPLS• A protocol to establish an end-to-end path from

source to the destination• A hop-by-hop forwarding mechanism• Use labels to set up the path

– Require a protocol to set up the labels along the path• Support multi-level label transport• It builds a connection-oriented service on the IP

network– Note: ATM and Frame Relay also support connection-

oriented services, but IP does not.

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Terminology• LSR - Routers that support MPLS are called Label Switch

Router • LER - LSR at the edge of the network is called Label Edge

Router (a.k.a Edge LSR)– Ingress LER is responsible for adding labels to unlabeled IP packets.– Egress LER is responsible for removing the labels.

• Label Switch Path (LSP) – the path defined by the labels through LSRs between two LERs.

• Label Forwarding Information Base (LFIB) – a forwarding table (mapping) between labels to outgoing interfaces.

• Forward Equivalent Class (FEC) – All IP packets follow the same path on the MPLS network and receive the same treatment at each node.

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How does it work?

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IP IP #L1 IP #L2 IP #L3 IP

LSR LSRLER LER

IPRouting

IPRouting

LabelSwitching

LabelSwitching

Add label at the ingress LER

remove label at the egress LER

MPLS Operation

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Label Path: R1 => R2 => R3 => R4

Label Forwarding Information Base (LFIB)

Router Incoming Label

Incoming Interface

Destination Network

(FEC)

Outgoing Interface

Outgoing Label

R1 --- E0 172.16.1.0 S1 6

R2 6 S0 172.16.1.0 S2 11

R3 11 S0 172.16.1.0 S3 7

R4 7 S1 172.16.1.0 E0 --

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Note: the label switch path is unidirectional.Q: create LFIB for R4 => R3 => R2 => R1

Label EncapsulationLabel information can be carried in a packet in a variety of ways:• A small, shim label header inserted between the Layer 2 and

network layer headers.• As part of the Layer 2 header, if the Layer 2 header provides

adequate semantics (such as ATM).• As part of the network layer header (future, such as IPv6).

• In general, MPLS can be implemented over any media type, including point-to-point, Ethernet, Frame Relay, and ATM links. The label-forwarding component is independent of the network layer protocol.

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Shim Header The Label (Shim Header) is represented as a

sequence of Label stack entries Each Label is 4 bytes (32 bits) 20 Bits is reserved for the Label Identifier

Label Identifier(20 bits)

Exp(3 bits)

S(1 bit)

TTL(8bits)

Label Identifier : Label value (0 to 15 are reserved)Exp : Experimental UseS : Bottom of Stack (set to 1 for the last entry in the label)TTL : Time To Live

MPLS and TTL• TTL: Time to Live• In IP, TTL is used to prevent packets to travel

indefinitely in the network.• MPLS uses the same mechanism as IP.• Why do we need TTL?

– MPLS may interwork with non-MPLS network.

• TTL is in the label header of PPP and Ethernet (shim header)

• Not supported in ATM.

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Ingress Label FEC Egress Label

6 138.120.6.0/24 9

When an unlabeled packet arrives at an ingress router, a label has to be applied. A packet can be mapped to a particular FEC based on the following criteria:

• destination IP address• source IP address• TCP/UDP port• class of service (CoS) or type of service (ToS)• application used• any combination of the previous criteria.

Forward Equivalent Class (FEC) Classification

Label Distribution Protocol (LDP)

• Labels are distributed between LERs and LSRs using LDP

• LSRs regularly exchange label and reachability information with each other using standardized procedures

• Used to build a picture of the network that can be used to forward packets

• Label Switch Paths are created by network operators – similar to PVC and VPN

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MPLS over ATM/Frame Relay/Ethernet

• A majority of MPLS examples are used to carry IP traffic over Ethernet links

• But MPLS can also carry IP traffic over ATM and frame relay links

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MPLS and ATM

Most carriers’ backbone are ATM-based. What is the process of

migrating from ATM to MPLS-based backbone?

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MPLS and ATM• The label information of MPLS can be carried in the ATM VCI field. If two

levels of labeling are needed, then the ATM VPI field could be used. The VCI field, however, is adequate for most applications.

• Implementing MPLS on an ATM switch would simplify integration of ATM switches and routers. An ATM switch capable of MPLS would appear as a router to an adjacent router. This approach avoids the issues of ATM addressing, routing, and signaling schemes.

• Implementing MPLS on an ATM switch does not preclude the capability to support a traditional ATM control plane (such as PNNI) on the same switch.

• The two components, MPLS and the ATM control plane, would operate independently with VPI/VCI space and other resources partitioned so that the components would not interact.

• Two implementation schemes: VC-merging vs. non-merging

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ATM LSR - Non-Merging(note: this is MPLS over ATM)

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171.68 IPPacket

IPPacket

ATMcell

5

ATMcell

8

ATMcell

8

ATMcell

8

ATMcell

5

ATMcell

3

ATMcell

4

ATMcell

4

ATMcell

4

ATMcell

3

Each ATM VC has its own MPLS label.

1

2

0

In/Intf In/Label Destination Out/Intf Out/Label

1 5 171.68 0 3

2 8 171.68 0 4

… … … …

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ATM LSR - VC-Merging

171.68

IPPacket

IPPacket

ATMcell

5

ATMcell

8

ATMcell

8

ATMcell

8

ATMcell

5

ATMcell

3

ATMcell

3

ATMcell

3

ATMcell

3

ATMcell

3

Multiple AMT VCs with the same destination and QoS are merged on the MPLS network with the same label.

1

2

0

In/Intf In/Label Destination Out/Intf Out/Label

1 5 171.68 0 3

2 8 171.68 0 3

… … … …

Frame Relay over MPLS Example

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R1R2

Ra Rb Rc

Lo: loopback interface. Why is it needed?R1 and R2: legacy frame relay routerRa and Rb: Label Switch Edge Router. Label path is between Ra and Rb.

Frame Relay over MPLS (cont.)

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IP

RFC2427

Frame Relay

PHY

FR

PHY

RFC 4619

MPLS

L2

PHY

MPLS

L2

PHY

MPLS

L2

PHY

FR

PHY

RFC 4619

IP

RFC2427

Frame Relay

PHY

IP IP IP

68.68.68.168.68.68.2

lo: 3.1.1.2

192.168.34.2

192.168.34.5

192.168.34.6

DLCI514 DLCI

513

Note: The IP addresses for MPLS configuration and Frame Relay are on different IP subnets.

lo: 3.1.1.1

192.168.34.1

MPLS Applications

• Traffic Engineering• Virtual Private Network (VPN)• Quality of Service (QoS)

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Traffic Engineering• Traffic engineering allows a network administrator to select the path

between two notes and bypass the normal routed hop-by-hop paths. An administrator may elect to explicitly define the path between nodes to ensure QoS or have the traffic follow a specified path to avoid traffic congestion at certain hops.

• The network administrator can reduce congestion by forcing the frame to travel around the overloaded segments. Traffic engineering, then, enables an administrator to define a policy for forwarding frames rather than depending upon dynamic routing protocols.

• Traffic engineering is similar to source-routing in that an explicit path is defined for the frame to travel. However, unlike source-routing, the hop-by-hop definition is not carried with every frame. Rather, the hops are configured in the LSRs ahead of time along with the appropriate label values.

• The administrator could be a centrally located program.• Traffic engineering is an important tool for network management. It is

NOT a customer service. (So you will not see it on a carrier’s web site.)

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MPLS – Traffic Engineering

End-to-End forwarding decision determined by ingress node.

Enables Traffic Engineering

LER 1

LSR 2 LSR 3

LER 4

LIP

Forward toLSR 2LSR 3LSR 4LSR X

Overload !!

Overload !!IPIP

LIP

LIP

MPLS-based VPN• One of most popular MPLS applications is the

implementation of VPN.• The basic concept is the same as ATM transparent

LAN.• Using label (instead of IP address) to interconnect

multiple sites over a carrier’s network. Each site has its own private IP address space.

• Different VPNs may use the same IP address space.

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MPLS VPN - Example

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192.168.1.0 192.168.2.0

192.168.3.0 192.168.4.0-- E1 10 E3-- E2 20 E3

10 E1 30 E220 E1 40 E2

30 E3 -- E140 E3 -- E2

E1

E2

E1

E2

-- E1 30 E3-- E2 40 E3

30 E2 10 E140 E2 20 E1

10 E3 -- E120 E3 -- E1

LSP

LSP

E3E1 E2

E3

uni-direction

uni-direction

MPLS VPN Connection Model

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VPN_A

VPN_A

VPN_B

10.3.0.0

10.1.0.0

11.5.0.0

VPN_A

VPN_B

VPN_B

10.1.0.0

10.2.0.0

11.6.0.0

VPN_A

10.2.0.0

MPLS Core

VPN_A: 10.2.0.0/24, 11.5.0.0/24, 11.6.0.0/24, 10.1.0.0/24VPN_B: 10.2.0.0/24, 10.1.0.0/24, 10.3.0.0/24Q: For a meshed connection, how many label paths are needed?

MPLSEdge

MPLSEdge

Case Study (I) AT&T MPLS Private Transport Network Services

Features and Benefits• Advanced Management options • MPLS-based security • Meshed topology for any-to-

any connectivity • Traffic prioritization - 4 Classes

of Services (CoS) • Service Level Agreements

(SLAs) • Web-based reporting

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Case Study (II)Verizon Private IP Service (MPLS)

History: • MCI (Verizon) adopted MPLS on a large scale in 1998 as a traffic engineering

technology on its public Internet backbone

Features and Benefits:• Exceptional Service. 24-hour monitoring customer service, and service level

agreements (SLAs).• Any-to-Any Connectivity. Multiple locations are connected (meshed topology).

You no longer need PVCs to communicate between sites; rather• Cost-Effective Solution. Private IP Service utilizes existing network infrastructure

without building and operating a private VPN.• Intranets and Extranets. Private IP Service captures the enhanced networking

efficiencies associated with an IP-based WAN, bringing together all the elements to support e-business applications within the company or between companies.

• MPLS Technology. Private IP Service provides varying Class of Services (CoS) and flexible IP routing that optimize network’s performance.

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