Internet Routing - MPLS

Semester 1 - 2010

Advanced Telecommunications 143.466 1

Internet Routing - MPLS

By

Richard Harris

Semester 1 - 2010 Advanced Telecommunications 143.466 Slide 2

MPLS Presentation Outline

• Introduction– Problems of Internet size

– Methods for overcoming potential problems

• What is MPLS?– Overview

– MPLS terminology

• MPLS Architecture– The label stack

– Label Switching Protocols

– MPLS Signalling Protocols

• Traffic Management in MPLS


Introduction

• Huge growth in the Internet– Figures vary from 2x to 10x per annum for traffic growth!

• Major ISPs (Internet Service Providers) have copedwith this growth in a variety of different directions,viz:– Network architecture

– Capacity expansion

– Traffic management methods (“Traffic Engineering”)

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Network Architecture

• This approach entails– Development of scalable network architectures

– Defining suitable network topologies and structures

– Identification of abstract objects and classes within thenetwork.


Capacity Expansion

• Most ISPs started with backbonecapacities in the range of 44Mbps(in the USA this is “DS3”) during1996.

• By 1999, the capacities used wereabout 622Mbps

• A recent announcement shows thatISPs are now introducing capacitiesin the region of 9.9Gbps (referred toas OC-192c) and using DWDM(Dense WDM).


Traffic Management/Engineering

• This direction is required because the previous twoapproaches are insufficient to solve the growthproblem mentioned in the introduction!

• Architectures and capacity expansion cannot providethe high quality Internet services that are now beingdemanded by the many different customers of thisnetwork.

• Traffic engineering deals with performanceoptimisation issues and it has been realised howimportant this aspect is for operating such a hugemultipurpose network.

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Traffic Engineering Limitations of theexisting Internet - 1

• Historically difficult to perform effective trafficengineering and management of Internet traffic.

• IP was conceived as a “best effort” service.

• Shortcomings of IP technologies:– Lack of measurement capabilities

– Limitations of intra-domain routing control functions.

– Routing protocols like OSPF, EIGRP, IGP etc work ontopology-driven per-packet progressive connection control.Each router makes independent decisions using a localversion of a routing area link state database.


Traffic Engineering Limitations of theexisting Internet – 2

– Route selection is based on shortest path calculations usingsimple additive link metrics.

• This approach is highly distributed and scalable, but “flawed”,because these protocols do not consider the characteristics ofthe offered traffic and the network capacity constraints whenmaking routing decisions.

• Subsets of network resources become congested.

• Alternate paths become under-utilised.

– This suggests that there is poor resource allocation taking place.

– Traffic engineering is required to fix these problems.

• MPLS to the rescue!


What will MPLS do?

• Multi Protocol Label Switching (MPLS) isexpected to

– improve the price/performance of network layerrouting,

– improve the scalability of the network layer,

– provide greater flexibility in the delivery of (new)routing services (by allowing new routing servicesto be added without a change to the forwardingmechanism).

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History

• MPLS was originally proposed by a group of engineers from IpsilonNetworks, but their "IP Switching“ technology, which was defined onlyto work over ATM, did not achieve market dominance. Cisco introduceda related proposal, not restricted to ATM transmission, called "TagSwitching".

• It was a Cisco proprietary proposal, and was renamed "LabelSwitching". It was handed over to the IETF for open standardization.

• One original motivation was to allow the creation of simple high-speedswitches, (Initially it was impossible to forward IP packets entirely inhardware.)

• The advantages of MPLS primarily revolve around the ability to supportmultiple service models and perform traffic management.

• MPLS also offers a robust recovery framework.



What is MPLS? - 1

• MPLS is a technique for combining the best features of IP andATM, viz:– IP routing– ATM throughput

• It borrows heavily from methods used in ATM to rapidly transfercells across the network.– Supports destination oriented, multicast and hierarchical routing– Supports explicit routing– Label-transport, Label-stack– Quality of Service (QoS)

• MPLS is not a specific layer 3 protocol it appears to lie betweenlayers 2 and 3.

• MPLS supports many other layer 2 technologies (Frame Relay,ATM, Ethernet, Token Ring)


What is MPLS? - 2

• A key concept of MPLS is the separation of an IP router’sfunctions into two parts:

– Forwarding

– Control

• Forwarding:

– Responsible for how data packets are relayed between the networkrouters. It uses label swapping similar to ATM switches that useVirtual Path/Virtual Circuit identifiers – although MPLS may beregarded as more general than VPI/VCI.

• A label is a short fixed length number, independent of the network layer.

• Label swapping essentially involves a table lookup of a packet’s label todetermine its route and new label value. It is much simpler thanprevious methods.

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What is MPLS? - 3

• A router capable of MPLS is known as a LabelSwitching Router (LSR).

• A set of Label Switching Routers traversed by apacket is called a Label Switched Path (LSP).

• A contiguous set of LSPs under a singleadministration is known as an MPLS domain.

• A packet is transmitted across an MPLS domainbased only on its label.

Packet Label


Some MPLS Terminology

• LDP: Label Distribution Protocol

• LSP: Label Switched Path

• FEC: Forwarding Equivalence Class

• LSR: Label Switching Router

• LER: Label Edge Router


MPLS Label

• The label can:– use an existing layer 2 header label (eg the VPI/VCI field in

the ATM cell header) or

– inserted between the layer 2 and IP headers as a small shimlabel like the following:

20-bits: Label value used by LSR to lookupeither next-hop, operation to perform, or

outgoing data-link encapsulation

Label Value S

8-bits: TTLdecrementedby each LSR

Time-to-Live(TTL)

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

1-bit: "Bottomof Label

Stack" Flag

Type ofservice

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Route at edge, switch in core

IP ForwardingLABEL SWITCHINGIP Forwarding

IP IP #L1 IP #L2 IP #L3 IP


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

2

31

2

1

2

3

3IntfIn

Dest IntfOut

LabelOut

3 47.1 1 0.50

IP 47.1.1.1

IP 47.1.1.1


- ER-LSP follows a route that the source chooses. In

other words, the control message to establish the LSP(label request) is source routed.

#216

#14

#462

#972

#14 #972

A

B

C

Route={A,B,C}

Explicitly routed or ER-LSP

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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

47.1

47.247.3

1

2

3

1

2

1

2

3

3

In t fIn

D e s t In t fO u t

L a b e lO u t

3 4 7 .1 .1 2 1 .3 33 4 7 .1 1 0 .5 0

IP 47.1.1.1

IP 47.1.1.1

Explicitly Routed LSP (ER-LSP)


ER LSP - advantages

• Operator has routing flexibility (policy-based, QoS-based)

• Can use routes other than shortest path

• Can compute routes based on constraints in exactlythe same manner as ATM based ondistributed topology database.(traffic engineering)


MPLS Label Stack

• Stack is LIFO

• Depth “n” stack illustrated below:

Label n

Label n-1

…………

Label 2

Label 1 1 Last label

Push stackPop stack

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MPLS: Label Stack; Hierarchical Routing

S D

Leased Virtual Path

Company Network Company Network


Forwarding Equivalence Classes

• More general than Virtual Circuits in ATM.

• Key features:– All packets divided into subsets called FECs based on:

• IP source address

• IP destination address

• IP protocol

• TCP/UDP source/destination ports

• Time to Live (TTL)

• Type of service fields

– Granularity can be varied if desired for packet forwardingcriteria.

– Mapping of FEC done only once – at entry to MPLS domain.


Packet Processing

• Forwarding strictly according to labels!

• Label is removed at the egress LSR.

• Next Hop Label Forwarding Entries NHLFEs:– Outgoing interface (next hop)

– Outgoing label (push/pop label stack)

– Data link encapsulation (optional)

– Information about resources (optional)

– Packet handling policies (optional)

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Quality of Service Routing

• The control part of MPLS consists of– Network layer routing protocols to distribute routing

information between LSRs;

– Label binding procedures for converting this routinginformation into the forwarding tables needed for labelswitching.

• MPLS is designed to work with existing routingprotocols like OSPF, BGP etc.

• Major interest in QoS routing instead of just shortestpath routing.

• Signalling is required to establish the routes.


Signalling Methods in MPLS

• There are two standardized protocols for managingMPLS paths:– CR-LDP (Constraint-based Routing Label Distribution Protocol) and

– RSVP-TE, an extension of the RSVP protocol for traffic engineering.

• An MPLS header does not identify the type of datacarried inside the MPLS path.– If we need to carry two different types of traffic between the

same two routers, with different treatment from the corerouters for each type, we must establish a separate MPLSpath for each type of traffic.

MPLS Signalling Protocols

• The signalling protocol informs the switches alongthe route which labels and links to use for each LSP.This information is used to program the switchingfabric.– RSVP-TE is used where traffic engineering is required.

– LDP is used when traffic engineering is not required, as itneeds less management.


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MPLS Protocols

MPLS Forwarding

OSPF/ ISISRouting

CR LDP/RSVP-TE

LDP

Link StateInformation

Physical

IP Forwarding

UDPTCP

LDP Protocol - 1

• MPLS LDP provides the means for LSRs to request, distribute,and release label prefix binding information to peer routers in anetwork.

• LDP enables LSRs to discover potential peers and to establishLDP sessions with those peers for the purpose of exchanginglabel binding information.

• MPLS LDP enables one LSR to inform another LSR of the labelbindings it has made.

• Once a pair of routers communicates the LDP parameters, thenthey establish a label-switched path (LSP). MPLS LDP enablesLSRs to distribute labels along normally routed paths to supportMPLS forwarding.

– This method of label distribution is also called hop-by-hop forwarding.


LDP Protocol - 2

• With IP forwarding, when a packet arrives at a routerthe router looks at the destination address in the IPheader, performs a route lookup, and forwards thepacket to the next hop.

• With MPLS forwarding, when a packet arrives at arouter the router looks at the incoming label, looks upthe label in a table, and then forwards the packet tothe next hop.


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Label Merging

The following edited extract fromhttp://opalsoft.net/qos/MPLS-26.htm

explains the process of label merging (this is essentiallythe aggregation process).– Label Merging is the capability for forwarding two different

packets belonging to the same FEC, but arriving withdifferent labels, with the same outgoing label. An LSR is labelmerging capable if it can receive two packets from differentincoming interfaces, and/or with different labels, and sendboth packets out the same outgoing interface with the samelabel. Once transmitted, the information that they arrive fromdifferent interfaces and/or with different labels is lost.


RSVP-TE Protocol

• RSVP-TE is used to establish MPLS LSPs when there are trafficengineering requirements. It is mainly used to provide QoS andload balancing across the network core, and includes the abilityto control all-optical networks.

• RSVP allows the use of source routing where the ingress routerdetermines the complete path through the network.

• The ingress router can use a Constrained Shortest Path First(CSPF) calculator to determine a path to the destination,ensuring that any QoS and Shared Risk Link Group (SRLG)requirements are met.

• The resulting path is then used to establish the LSP.


Label Merging


Router #1Router #1

Router #2Router #2

Router #3Router #3

ASBR GatewayASBR Gateway

FEC #1

FEC #1

FEC #1

Label L2

Label L2

FEC #1

Label L3

FEC #1

Label L3

FEC #1

Gateway supporting

FEC#1

Cannot distinguish

between incoming

packets from R1 and R2

as they have the same

label

Gateway supporting

FEC#1

Cannot distinguish

between incoming

packets from R1 and R2

as they have the same

label

LDP Tunnel #1LDP Tunnel #1

LDP Tunnel #2LDP Tunnel #2

Next Generation Network Service Resiliency Project

Network: LDP subnetwork

Author: Richard Harris Date: 02/12/2009

Company: Massey University

Packets eminating from R1 and R2 towards the ASBR are part

of FEC #1 as they are destined for the same prefix. At R3, their

incoming labels (Label L2) are swapped for Label L3 giving

the appearance of being a single LSP to the ASBR Gateway.

LDP subnetwork

Richard Harris 02/12/2009

Massey University

Packets eminating from R1 and R2 towards the ASBR are part

of FEC #1 as they are destined for the same prefix. At R3, their

incoming labels (Label L2) are swapped for Label L3 giving

the appearance of being a single LSP to the ASBR Gateway.

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ER-LSP setup using RSVP

LSR B LSR C LER DLER A

1. Path message. It containsER path < B,C,D>

2. New path state. Pathmessage sent to next node

3. Resv message originates.Contain the label to use and the

required traffic/QoS para.

4. New reservation state.Resv message propagated

upstream

5. When LER A receivesResv, the ERestablished.

Per-hop Path andResv refresh unless

suppressed


suppressed


suppressed


• CR = “Constraint” based “Routing”

• eg: USE: (links with sufficient resources AND

(links of type “someColor”) AND

(links that have delay less than 200 ms)

&&

=

Constraint Routing – LDP (CR-LDP)


1) A topology database that knows about link attributes.

2) A label distribution protocol that goes where it’s told.

z

{a,b,c}

ANSWER: OSPF/ISIS + attribs{a,b,c}

zmyx

ANSWER: LDP + Explicit Route{x,y,m,z}

z

{a,b,c}

Requirements for Constraint Based Routing

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Traffic Engineering

A

B C

D

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

Demand

NetworkTopology

Traffic engineering key to optimizing cost/performance

Constrained Shortest Path First (CSPF)

• CSPF is an extension of shortest path algorithms.

• The path computed using CSPF is a shortest path that satisfiesa set of constraints. It runs a shortest path algorithm afterpruning those links that violate a given set of constraintrequirements.

– A constraint could be the minimum bandwidth required per link(also known as bandwidth guaranteed constraint), end-to-end delay,maximum number of links traversed, or a set of include/excludenodes.

• CSPF is widely used in MPLS Traffic Engineering.

– The routing using CSPF is known as Constraint Based Routing(CBR).


Comment on Routes obtained with CSPF

• Note:– The path computed using CSPF could be exactly same as

that of computed from OSPF and IS-IS, or it could becompletely different depending on the set of constraints tobe met.


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Example of CSPF

• Cost is 1 – hop count

• Constrained to 50Mbps: 1 2 3

• Constrained to 55Mbps: 1 4 5 3

• Constrained to 90Mbps: 1 4 5 6 3

• Using hop count, OSPF/ISIS always gives 1 2 3


Router #1Router #1

Router #2Router #2

Router #3Router #3

Router #4Router #4

Router #5Router #5

Router #6Router #6

50Mbps50Mbps 100Mbps100Mbps

10

0M

bp

s1

00

Mb

ps

100Mbps100Mbps

60Mbps

60Mbps

100Mbps100Mbps

10

0M

bp

s1

00

Mb

ps

Router #1Router #1

Router #2Router #2

Router #3Router #3

Router #4Router #4

Router #5Router #5

Router #6Router #6


10

0M

bp

s1

00

Mb

ps

100Mbps100Mbps

60Mbps

60Mbps

100Mbps100Mbps

10

0M

bp

s1

00

Mb

ps

Router #1Router #1

Router #2Router #2

Router #3Router #3

Router #4Router #4

Router #5Router #5

Router #6Router #6


10

0M

bp

s1

00

Mb

ps

100Mbps100Mbps

60Mbps

60Mbps

100Mbps100Mbps

10

0M

bp

s1

00

Mb

ps

Router #1Router #1

Router #2Router #2

Router #3Router #3

Router #4Router #4

Router #5Router #5

Router #6Router #6


10

0M

bp

s1

00

Mb

ps

100Mbps100Mbps

60Mbps

60Mbps

100Mbps100Mbps

10

0M

bp

s1

00

Mb

ps

CSPF Path Selection - 1

Juniper documentation summarises the path selection process asfollows:

1. Computes LSPs one at a time, beginning with the highest priorityLSP (the one with the lowest setup priority value). Among LSPs ofequal priority, CSPF starts with those that have the highestbandwidth requirement.

2. Prunes all the links that are not full duplex and do not havesufficient reservable bandwidth.

3. If the LSP configuration includes the include statement, prunes alllinks that do not share any included colours.

4. If the LSP configuration includes the exclude statement, prunes alllinks that contain excluded colours. If the link does not have acolour, it is accepted.


CSPF Path Selection - 2

5. Finds the shortest path toward the LSP’s egress router, taking intoaccount explicit-path constraints. For example, if the path mustpass through Router A, two separate SPFs are computed, one fromthe ingress router to Router A, the other from Router A to theegress router.

6. If several paths have equal cost, chooses the one whose last-hopaddress is the same as the LSP’s destination.

7. If several equal-cost paths remain, selects the one with the fewestnumber of hops.

8. If several equal-cost paths remain, applies the CSPF load-balancing rule configured on the LSP (least fill, most fill, orrandom).


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Purpose of traffic engineering

• Maximize utilization of links and nodes throughoutthe network

• Engineer links to achieve required delay, grade-of-service

• Spread the network traffic across network links,minimize impact of single failure

• Ensure available spare link capacity for re-routingtraffic on failure

• Meet policy requirements imposed by the networkoperator

:


MPLS Traffic Engineering Methods

• MPLS can use the source routing capability to steer traffic on desiredpath

• Operator may manually configure these in each LSR along thedesired path - analogous to setting up PVCs in ATM switches

• Ingress LSR may be configured with the path, RSVP used to set upLSP - some vendors have extended RSVP for MPLS path set-up

• Ingress LSR may be configured with the path, LDP used to set upLSP - many vendors believe RSVP is not suitable.

• Ingress LSR may be configured with one or more LSRs along thedesired path, hop-by-hop routing may be used to set up the rest ofthe path - loose source routing, less configuration required.

• If desired for control, routes discovered by hop-by-hop routing canbe frozen (“route pinning”)

• In the future, constraint-based routing will off-load traffic engineeringtasks from the operator to the network itself.

Documents

Internet Routing - MPLS