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Multiprotocol Label Switching (MPLS)

รศ.ดร. อนันต์ ผลเพิ0ม

Asso. Prof. Anan Phonphoem, Ph.D.anan.p@ku.ac.th

http://www.cpe.ku.ac.th/~ananComputer Engineering Department

Kasetsart University, Bangkok, Thailand

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Outline

n Motivationn MPLS Basicsn Operationn Protocol Stack Architecture n Advantages and Disadvantages

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Motivation

n IP n ATMn MPLS positioning

Internet Protocol (IP)

n IP is here and everywheren De facto protocol for global Internetn Disadvantages

n connectionless (e.g. no QoS)n independent forwarding decisions based on IPn large IP header (at least 20 bytes)n routing in Network Layer (Slower than Switching)n Usually shortest path (not concern other metrics)

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Asynchronous Transfer Mode (ATM)

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Packet Sizes in the Network

- traffic unpredictable- Slow and expensive- Delay variation

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Voice Transmission

n If introduce large packet size for voicen Cannot tolerate long delay, large jittern Echo problem

n echo cancellation does not work (long delay)n To support voice

n Small packetn fixed-size packetn Called “Cell” è ATM cell

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Multiplexing using cells

Advantages of cells- Fair delay- high speed and cheap (switching and multiplexing -> HW)

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An ATM cell

VPI: Virtual Path IdentifierVCI: Virtual Circuit Identifier

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TP, VPs, and VCs

TP: Transmission PathVP: Virtual PathVC: Virtual Circuit

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Examples of VPs and VCs

ATM Switching

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Forwarding Table

VP switch (use only VPI)* Most switches

VPC switch (use both VPIs and VCIs)* Boundaries switches

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ATM

n Connection oriented (Supports QoS)n Fast packet switching

n fixed length packets (cells)n Integration of different traffic types

n voice, data, videon Disadvantages

n Complexn Expensiven Not widely adopted

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MPLS Positioning

n Combine the forwarding algorithm used in ATM with IP

IP ATMMPLS

Packet Forwarding Virtual Circuit SwitchingHybrid

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MPLS Overview

n Switch data according to its Label (tag)n look up in tablen determine next hopn substitute new label

n Do not pay attention to n network and transport protocolsn è Multiprotocol

n Switching for IP and non-IPn Signaling protocol based on IP

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Outline

n Motivationn MPLS Basicsn Operationn Protocol Stack Architecture n Advantages and Disadvantages

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MPLS in the protocol stack

n Between Layer 2 and Layer 3

MPLS

SDH: Synchronous Digital Hierarchy (optical fiber)

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MPLS Characteristics

n Flow Managementn Independent of L2 and L3 protocolsn Maps IP-addresses to fixed length labelsn Interfaces to existing routing protocols

(RSVP, OSPF)n Supports ATM, Frame-Relay and Ethernet

RSVP: Resource Reservation Protocol

n Generic label format

Label

19Shim: A thin, often tapered piece of material, such as wood, stone, or metal, used to fill gaps, make something level, or adjust something to fit properly.…http://www.thefreedictionary.com/

bits

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Label Distribution

n Not specify a single method for label distribution

n Routing support for label exchangen BGP and RSVP can piggyback the label information

n IETF defines signal and managementn label distribution protocol (LDP)

n Extension of LDP protocoln support explicit routing based on QoS

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Label InsertionData Link Frame

IEEE 802 MAC Frame

ATM Cell

Frame Relay Frame

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MPLS Terminology

LDP: Label Distribution Protocol

LSP: Label Switched Path

FEC: Forwarding Equivalence Class

LSR: Label Switching Router

LER: Label Edge Router

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Label Edge Router (LER)

n Edge of an MPLS networkn Assigns and removes packet labels n Support multiple ports

n frame relayn ATMn Ethernetn etc.

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Label Switching Router (LSR)

n High speed router in the core on an MPLS network

n ATM switches can be used as LSRn no hardware modificationn label switching is equivalent to VP and VC

switching

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LER and LSR Position

(Label Switched Path)

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Forward Equivalence Class (FEC)

n Represent group of packets n share same requirements for their

transportn Packet Assignment

n assignment to each packetn only one time at entry point

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Label-Switched Path (LSP)

n A path is established before the data transmission starts

n A path is a representation of a Forward Equivalence Class (FEC)

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LSP Setup

n Hop-by-hop routingn each LSR independently selects next hop for a

given FECn Explicit routing

n similar to source routing (sender specify the route of the packet)

n ingress LSR specifies the list of nodes through which the packet traverses

n LSP setup for an FEC is unidirectionaln return traffic must use another LSP

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Label Distribution Protocol (LDP)

n Application layer protocol n for label binding distribution info to LSRsn map FECs to labels (create LSP)n LDP sessions are established between LDP

peers in the MPLS network (not necessarily adjacent).

n Sometimes employs OSPF or BGP

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LDP message types

n Discovery messagesn announce/maintain the presence of an LSR

n Session messagesn establish/maintain/terminate sessions between

LDP peers n Advertisement messages

n create, change, and delete label mappings for FECs

n Notification messagesn provide advisory info and signal error information

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Outline

n Motivationn MPLS Basicsn Operationn Protocol Stack Architecture n Advantages and Disadvantages

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Route at Edge, Switch in Core

IP ForwardingLABEL SWITCHINGIP Forwarding

IP IP #L1 IP #L2 IP #L3 IP

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MPLS: How does it work?

UDP-Hello

UDP-Hello

TCP-open

TIM

E

Label requestIP

Label mapping#L2

Initialization(s)

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MPLS Operation

n Five Stepsn label creation and distribution n table creation at each router n label-switched path creation n label insertion/table lookup n packet forwarding

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Step 1 :Label creation and distribution

n First, routers bind a label to a specific FECn Then build their tablesn Using LDP

n downstream routers initiate the distribution of labels and the label/FEC binding

n negotiate traffic-related characteristics and MPLS capabilities

n A reliable and ordered transport protocol should be used for the signaling protocol

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Step 2: Table creation

n On receipt of label bindings each LSR creates entries in the label information base (LIB)

n Table specifies the mapping between a label and an FECn mapping between the input port and input label

table to the output port and output label tablen entries are updated whenever renegotiation of

label bindings occurs

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Example of Label Information Base (LIB) Table

Input Port Incoming Port Label Output Port Outgoing

Port Label

1 3 3 6

2 9 1 7

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IntfIn

LabelIn

Dest IntfOut

3 0.40 47.1 1

IntfIn

LabelIn

Dest IntfOut

LabelOut

3 0.50 47.1 1 0.40

MPLS Label Distribution

47.1

47.247.3

12

3

1

2

1

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3Intf In

Dest Intf Out

Label Out

3 47.1 1 0.50

Mapping: 0.40

Request: 47.1

Mapping: 0.50

Request: 47.1

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Label Switched Path (LSP)IntfIn

LabelIn

Dest IntfOut

3 0.40 47.1 1

IntfIn

LabelIn

Dest IntfOut

LabelOut

3 0.50 47.1 1 0.40

47.1

47.247.3

1

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1

2

1

2

3

3IntfIn

Dest IntfOut

LabelOut

3 47.1 1 0.50

IP 47.1.1.1

IP 47.1.1.1

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Step 3: Label switched path creation

n LSPs are created in the reverse direction to the creation of entries in the LIBs.

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MPLS Example

LER 1

LER 2

LER 3

LER 4

Source

DestinationLSR 3

LSR 1

LSR 2

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Step 4: Label insertion/table-lookup

n First router (LER1) uses LIB table to find the next hop and request a label for the specific FEC

n Subsequent routers just use the label to find the next hop

n Once the packet reaches the egress LSR (LER3), the label is removed and the packet is supplied to the destination

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Step 5: Packet forwarding

n For first time packetn LER1 may not have any labels n (In IP) find the longest add match for next hopn Let LSR1 be the next hop for LER1.

n LER1 will initiate a label request toward LSR1n This request will propagate through the

network (green lines)

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MPLS Operation Example

LER 1

LER 2

LER 3

LER 4

Source

DestinationLSR 3

LSR 1

LSR 2

Label Request

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Step 5 (cont.)

n Label downstream (LER3 àLSRà … à LER1)n The LSP setup (blue lines) uses LDP or any

other signaling protocol. n LER1 will insert the label and forward the

packet to LSR1

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MPLS Operation Example

LER 1

LER 2

LER 3

LER 4

Source

DestinationLSR 3

LSR 1

LSR 2

Label RequestLabel Distribution

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Step 5 (cont.)

n Each subsequent LSR (LSR2,LSR3) n examine label in received packetn replace it with outgoing label n forward it

n When reaches LER4, label is removed n leave MPLS domain and deliver to the destination

n Actual data path followed by the packet is the red line

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MPLS Operation Example

LER 1

LER 2

LER 3

LER 4

Source

DestinationLSR 3

LSR 1

LSR 2

Label RequestLabel DistributionData Flow

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Tunneling in MPLS

n Control the entire path of a packet without explicitly specifying the intermediate routers. n Creating tunnels through the intermediary

routers that can span multiple segments. n MPLS based VPNs.

MPLS network application (MPLS L2 VPN)

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http://www.centecnetworks.com/en/SolutionList.asp?ID=77

customer edge (CE) routers provider edge (PE) routers

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Tunneling in MPLS

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Outline

n Motivationn MPLS Basicsn Operationn Protocol Stack Architecturen Advantages and Disadvantages

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MPLS Protocol Stack Architecture

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Outline

n Motivationn Basicsn Operationn Protocol Stack Architecture n Advantages and Disadvantages

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MPLS Advantages

n Improves packet-forwarding performance in the network

n Supports QoS and CoS for service differentiation

n Supports network scalability n Integrates IP and ATM in the network n Builds interoperable networks

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MPLS Disadvantages

n An additional layer is addedn The router has to understand MPLS

Transport Network in Legacy 2G/2.5G

57http://lteuniversity.com/get_trained/expert_opinion1/b/skrishnamurthy/archive/2013/04/01/why-ethernet-backhaul.aspx

Unified Backhaul and Core Network in 2G/2.5G/3G/4G Networks

58http://lteuniversity.com/get_trained/expert_opinion1/b/skrishnamurthy/archive/2013/04/01/why-ethernet-backhaul.aspx

customer edge (CE) routers

customer edge (CE) routers provider edge (PE) routers

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References

n “MPLS Introduction”, Yun Teng, Dept. of Computer Science, UMBC

n “MPLS Tutorial and Operational Experiences”, Peter Ashwood-Smith, Bilel Jamoussi, October, 1999