Outline Introduction Applications of MPLS Fundamental concepts - - PDF document

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MPLS – Multiprotocol Label Switching

Datakommunikation, påbyggnad DK2-VT01 Jerry Eriksson

Outline

• Introduction
• Fundamental concepts
• Predecessor switching

techniques

– IP switching – Tag switching

• MPLS Core Protocols
• Applications of MPLS

– Constraint-based Routing – Virtual Private Networks (VPN) – (RSVP)

Introduction

• Router architectures
• Background

– Growth of Internet – Price and performance – Integration of IP over ATM – Extending routing functionality

• A brief history

– IP over ATM – IP switching – Tag switching – MPLS Working group

• Summary of

introduction

Definitions

• Forwarding

– Receive a packet on an input – determine where it needs to go by examine some fields in the packet – send it to an appropriate output

• Label

– a short, fixed-length identifier

Questions

• What is MPLS?
• How does it works?
• What benefits does it

provide?

• Label swapping

technique

– IP switching – Tag switching – ARIS (IBM)

• Standardisation needed

– IETF promote MPLS

How did we get there?

• Need of fast cheap IP routers is one reason
• More important

– Scalability – Price/performance

• Switches cheaper
• router more powerful

– IP over ATM

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IP over ATM

• Mapping of ATM and IP involves

considerable complexity

• IP and ATM were developed with no regard
• f each other
• BROADCAST: Go everywhere, stop when you get to B, never

ask for directions.

• HOP BY HOP ROUTING: Continually ask who’s closer to B

go there, repeat … stop when you get to B. “Going to B? You’d better go to X, its on the way”.

• SOURCE ROUTING : Ask for a list (that you carry with you)
• f places to go that eventually lead you to B.

“Going to B? Go straight 5 blocks, take the next left, 6 more blocks and take a right at the lights”.

Many ways of getting from A to B:

ROUTE AT EDGE, SWITCH IN CORE

IP Forwarding LABEL SWITCHING IP Forwarding

IP IP #L1 IP #L2 IP #L3 IP

The fish picture

Router Under utilized Over utilized Consider destination, only

Comparison - Hop-by-Hop vs. Explicit Routing

Hop-by-Hop Routing Explicit Routing

• Source routing of control traffic
• Builds a path from source to dest
• Requires manual provisioning, or

automated creation mechanisms.

• LSPs can be ranked so some

reroute very quickly and/or backup paths may be pre-provisioned for rapid restoration

• Operator has routing flexibility

(policy-based, QoS-based,

• Adapts well to traffic engineering
• Distributes routing of control traffic
• Builds a set of trees either fragment

by fragment like a random fill, or backwards, or forwards in organized manner.

• Reroute on failure impacted by

convergence time of routing protocol

• Existing routing protocols are

destination prefix based

• Difficult to perform traffic

engineering, QoS-based routing Explicit routing shows great promise for traffic engineering Explicit routing shows great promise for traffic engineering

Explicit Routing - MPLS vs. Traditional Routing

• Connectionless nature of IP implies that routing is based on information in

each packet header

• Source routing is possible, but path must be contained in each IP

header

• Lengthy paths increase size of IP header, make it variable size,

increase overhead

• Some gigabit routers require ‘slow path’ option-based routing of IP packets
• Source routing has not been widely adopted in IP and is seen as impractical
• Some network operators may filter source routed packets for security

reasons

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#216 #14 #462 #972 #14 #972

A B C Route= {A,B,C}

EXPLICITLY ROUTED

Summary of introduction

• The need to evolve the routing architecture
• f IP networks
• The need for greater price/performance in

routers

• Complexity of mapping IP to ATM
• Scalability
• The need to add new routing functionality

Fundamental Concepts

• Network layer routing functional components:

control and forwarding

• Label switching: The forwarding component
• Label switching: The control component
• Edge devices
• Relationship between label switching and network

layer addressing and routing

(MPLS) Terminology

• LDP: Label Distribution Protocol
• LSP: Label Switched Path
• FEC: Forwarding Equivalence Class
• LSR: Label Switching Router
• LER: Label Edge Router (Useful term not in

standards)

Control and forwarding

• Two basic components

– Control component: responsible for construction and maintenance of the forwarding table – Forwarding component: responsible for the actual forwarding of packets from input to output across a switch

• uses forwarding table and information in the packet
• Each router implements both components

Forwarding Equivalence Classes

• Partitioning the set of all possible packets into a

finite number of disjoint subsets

• Packets with the same FEC are treated equally

– i.e., sent to the same next hop

• Examples

– unicast packets with same destination address – packets with the same type-of-service

• Granularity - very important

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Forwarding Equivalence Classes

• The concept of FECs provides for a great deal of flexibility and scalability
• In conventional routing, a packet is assigned to a FEC at each hop (i.e. L3

look-up), in MPLS it is only done once at the network ingress

Packets are destined for different address prefixes, but can be mapped to common path Packets are destined for different address prefixes, but can be mapped to common path

IP1 IP2 IP1 IP2

LSR LSR LER LER

LSP IP1 #L1 IP2 #L1 IP1 #L2 IP2 #L2 IP1 #L3 IP2 #L3

The forwarding component

• Label switching forwarding table

Incoming label First subentry Second subentry

Incoming label Outgoing label Outgoing interface Next hop address Outgoing label Outgoing interface Next hop address

Link layer header “shim” layer header Network layer header Network layer data

• Label in a packet ?

– “Shim” label header – VCI/VPI for ATM

Label switching forwarding algorithm

• LSR receives a packet
• Router extracts the label

– uses it as an index in the forwarding table

• For each subentry found the router replaces the label in the

packet with the outgoing label

• Sends the packet over the interface specified by the next

hop address

Single switching forwarding algorithm

Routing function Unicast routing

Unicast routing with types of service Forwarding algorithm Longest match

• n destination address

Longest match on destination + exact match

• n Type of service

Multicast routing Longest match on source address + exact match

• n source address,

destination address, and incoming interface

Routing function Unicast routing

Unicast routing with types of service Forwarding algorithm Multicast routing

Common forwarding (label swapping)

The forwarding component (cont)

• Multiprotocol: both above and below

IPv6, IPv4, AppleTalk Label switching Ethernet, FDDI, ATM

The forwarding component: summary

• Uses a single forwarding algorithm

– based on label switching

• The label is a short, fixed-length unstructured

entity – both forwarding and reservation semantics

• No constraints on the granularity that can

associated with a label

• Support many network and link layer protocols

5 The control component

• Responsible for

– distributing routing information to other LSRs

• as in conventional routing

– Algorithms to maintain forwarding tables

The control component

• a) Create bindings between labels and FECs
• b) Inform other LSRs of the binding it

creates

• Utilize both a) and b) to construct and

maintain the forwarding table

– used by the the label switching component

The label switching control component

Network Layer routing protocols (e.g., OSPF, BGP, PIM) Procedure for creating binding between labels and FECs Procedure for distributing information about created label bindings Maintenance of forwarding table

Construction of a label switching forwarding table

Network Layer routing protocols (e.g., OSPF, BGP, PIM) Procedure for creating binding between labels and FECs Procedure for distributing information about created label bindings Label switching forwarding table (label to next hop mapping) FEC to label mapping FEC to next hop mapping

Binding labels

• Local versus remote binding
• Upstream versus downstream binding

Label Distribution - Methods

Label Distribution can take place using one of two possible methods Label Distribution can take place using one of two possible methods

LSR1 LSR2

Downstream Label Distribution

Label-FEC Binding

• LSR2 and LSR1 are said to have an “LDP

adjacency” (LSR2 being the downstream LSR)

• LSR2 discovers a ‘next hop’ for a particular FEC
• LSR2 generates a label for the FEC and

communicates the binding to LSR1

• LSR1 inserts the binding into its forwarding tables
• If LSR2 is the next hop for the FEC, LSR1 can use

that label knowing that its meaning is understood

Downstream-on-Demand Label Distribution

• LSR1 recognizes LSR2 as its next-hop for an FEC
• A request is made to LSR2 for a binding between

the FEC and a label

• If LSR2 recognizes the FEC and has a next hop for

it, it creates a binding and replies to LSR1

• Both LSRs then have a common understanding

LSR1 LSR2 Label-FEC Binding Request for Binding

Both methods are supported, even in the same network at the same time For any single adjacency, LDP negotiation must agree on a common method

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Creating and destroying label binding:Data-driven

• Label bindnings are created when user data

packets arrive

• + created only when there is a traffic flow
• - control traffic is directly proportional to

the number of traffic flows

Creating and destroying label binding:Control-driven

• Label bindnings are created when control

information arrive

• +Assignment before arrival of traffic
• + scalability
• + Every packet is label switched (not only

the tail-end)

Distributing label binding information

• Piggybacking on top of routing protocols
• Label distribution protocol

Label Distribution Protocol (LDP) - Purpose

Label distribution ensures that adjacent routers have a common view of FEC <-> label bindings

Routing Table: Addr-prefix Next Hop 47.0.0.0/8 LSR2 Routing Table: Addr-prefix Next Hop 47.0.0.0/8 LSR2

LSR1 LSR2 LSR3 IP Packet

47.80.55.3 Routing Table: Addr-prefix Next Hop 47.0.0.0/8 LSR3 Routing Table: Addr-prefix Next Hop 47.0.0.0/8 LSR3

For 47.0.0.0/8 use label ‘17’

Label Information Base: Label-In FEC Label-Out 17 47.0.0.0/8 XX Label Information Base: Label-In FEC Label-Out 17 47.0.0.0/8 XX Label Information Base: Label-In FEC Label-Out XX 47.0.0.0/8 17 Label Information Base: Label-In FEC Label-Out XX 47.0.0.0/8 17

Step 1: LSR creates binding between FEC and label value Step 2: LSR communicates binding to adjacent LSR Step 3: LSR inserts label value into forwarding base Common understanding of which FEC the label is referring to!

Intf In Label In Dest Intf Out 3 0.40 47.1 1 Intf In Label In Dest Intf Out Label Out 3 0.50 47.1 1 0.40

MPLS Label Distribution

47.1 47.2 47.3 1 2 3 1 2 1 2 3 3

Intf In Dest Intf Out Label Out 3 47.1 1 0.50

Mapping: 0.40 Request: 47.1 Mapping: 0.50 Request: 47.1

Label Switched Path (LSP)

Intf In Label In Dest Intf Out 3 0.40 47.1 1 Intf In Label In Dest Intf Out Label Out 3 0.50 47.1 1 0.40

47.1 47.2 47.3 1 2 3 1 2 1 2 3 3

Intf In Dest Intf Out Label Out 3 47.1 1 0.50

IP 47.1.1.1 IP 47.1.1.1

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Relations between label switching and network layer addressing and routing

• Label switching assumes the use of existing

protocols (OSPF, BGP, etc)

– i.e. Doesn’t replace the need for establishment and maintaining of routing information

IP switching

• IP switching overview
• Ipsilon Flow management protocol
• (General switch management protocol)
• Implementations

IP switching overview

IP PNNI Q.2931 ATM Hardware ATM ARP MARS NHRP IP ATM Hardware IFMP IP over standard ATM IP switching Goals: high-speed routing and simplicity

Label-Controlled ATM

IP Packet 17 IP Packet 05

B A D C

Forwarding Table B 17 C 05

• Port

Label Switching Router

Forwarding Table Network Layer Routing (eg. OSPF, BGP4)

Label Packets forwarded by swapping short, fixed length labels (I.e. ATM technique) Packets forwarded by swapping short, fixed length labels (I.e. ATM technique) Switched path topology formed using network layer routing (I.e. TCP/IP technique) Switched path topology formed using network layer routing (I.e. TCP/IP technique) Label

ATM Label Switching is the combination of L3 routing and L2 ATM switching ATM Label Switching is the combination of L3 routing and L2 ATM switching

IP switching overview

Fig 3.2 på tavlan

Tag switching

• Tag switching overview

– Focused on adding functionality

• Tag switching link layer independent

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Improving routing scalability

• Hierarchy of routing knowledge
• Multiple tags
• Pop/push
• Example: Inter/Intra domain routing

– + scalability – + inter-domain router doesn’t need to maintain inter-domain routing information – + faster convergence and fault isolation

Summary of Tag switching

• Tag stacks
• IP/ATM integration

– (most popular over non-ATM link)

• Handles multicast (PIM-SM)
• Mostly control-driven
• Both piggy-backing and own protocols

MPLS Core protocol

• Working group origins and charter
• The MPLS architecture
• Encapsulation
• Label distribution
• ATM issues
• Multicast
• Summary

Working group origins

• December (1996): BOF-meeting

– 800 people (record at that time)

• Co-chairs, IBM and Cisco
• Specify label maintaince, distribution protocol

supporting unicast, multicast, a hiearchy of routing knowledge and explicit paths.

• Independent of link layer

– Attention to ATM

The MPLS architecture

• Flexible assignment of labels

– Globally unique, locally unique, etc

• Label stack
• Hop-by-hop and explicit routing
• Control-driven

– Not mandatory

Distribution Control: Ordered v. Independent

Next Hop (for FEC) Definition Definition Comparison Comparison

• Requires more delay before packets can be

forwarded along the LSP

• Depends on availability of egress node
• Mechanism for consistent granularity and freedom

from loops

• Used for explicit routing and multicast

Independent LSP Control Independent LSP Control Ordered LSP Control Ordered LSP Control Outgoing Label Incoming Label MPLS path forms as associations are made between FEC next-hops and incoming and outgoing labels

• Each LSR makes independent decision on when to

generate labels and communicate them to upstream peers

• Communicate label-FEC binding to peers once

next-hop has been recognized

• LSP is formed as incoming and outgoing labels are

spliced together

• Label-FEC binding is communicated to peers if:
• LSR is the ‘egress’ LSR to particular FEC
• label binding has been received from

upstream LSR

• LSP formation ‘flows’ from egress to ingress
• Labels can be exchanged with less delay
• Does not depend on availability of egress node
• Granularity may not be consistent across the nodes

at the start

• May require separate loop detection/mitigation

method

Both methods are supported in the standard and can be fully interoperable

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Encapsulation

• Label, 20 bits
• Exp, Expremential, (Class-of-service) 3 bits
• Stack, 1 bit
• TTL, 8 bit, as in IP

Label Exp Stack TTL

Label distribution protocol

• LSR Neighbour discovery
• Reliable transport
• LDP message
• Label distribution modes

Label Distribution - Methods

Label Distribution can take place using one of two possible methods Label Distribution can take place using one of two possible methods

LSR1 LSR2

Downstream Label Distribution

Label-FEC Binding

• LSR2 and LSR1 are said to have an “LDP

adjacency” (LSR2 being the downstream LSR)

• LSR2 discovers a ‘next hop’ for a particular FEC
• LSR2 generates a label for the FEC and

communicates the binding to LSR1

• LSR1 inserts the binding into its forwarding tables
• If LSR2 is the next hop for the FEC, LSR1 can use

that label knowing that its meaning is understood

Downstream-on-Demand Label Distribution

• LSR1 recognizes LSR2 as its next-hop for an FEC
• A request is made to LSR2 for a binding between

the FEC and a label

• If LSR2 recognizes the FEC and has a next hop for

it, it creates a binding and replies to LSR1

• Both LSRs then have a common understanding

LSR1 LSR2 Label-FEC Binding Request for Binding

Both methods are supported, even in the same network at the same time For any single adjacency, LDP negotiation must agree on a common method

ATM issues

• Encapsulation of labeled packets on ATM

links

– VC-Merge

• Looping and TTL adjustments

Summary

• Handles multicast
• Still elvolving
• Multiprotocol label switching

Constraint-based routing

• What is it?
• Components
• Application to

– traffic engineering – fast re-routing – QoS

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What is it

• Routing: optimize a certain metric subjet to

not voilate a some constraints

• Example: minimum available bandwitdh,

adminstrative.

• Plain IP can’t implemented this. Routers

have not sufficient knowledge

Components

• We need the ability

– 1) to compute a path at the source – 2) to distribute the information about the network topology – 3) to establish a route for a particular set of traffic may require reservation along the route

Constrained shortest path first

• is optimal with respect to some scalar

metric

• does not violate a set of constraints
• Example: different capacity of links.

Traffic engineering: ATM solution

• Overlay model:

– Fig 7.4 – Fig 7.5

• Drawback

– Extra devices, ATM – Additional network – ”Cell tax”

Traffic engineering: IP solution

• Consider only destination of packets.

– Changed metrics

• Many theoretical attemps have been made

– No success, some parts always congested

• Basic problem: doesn’t known the availble

bandwidth on the links

Traffic engineering: MPLS solution

• Traffic trunks – a set of microflows
• Support for explicit paths
• Output statistics can be used for planning
• Meet certain specific requirements
• May run over packet-switches networks

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Traffic Engineering Alternatives

MPLS combines benefits of ATM and IP-layer traffic engineering

Chosen by routing protocol (least cost) Chosen by Traffic Eng. (least congestion) Example Network:

MPLS provides a new method to do traffic engineering (traffic steering)

Ingress node explicitly routes traffic over uncongested path

Potential benefits of MPLS for traffic engineering:

• allows explicitly routed paths
• no “n-squared” problem
• per FEC traffic monitoring
• backup paths may be configured
• perator control

scalable granularity of feedback redundancy/restoration

Congested Node

Traffic Engineering Alternatives

Current methods of traffic engineering: Manipulating routing metrics Use PVCs over an ATM backbone Over-provision bandwidth Difficult to manage Not scalable Not economical

Application to fast re-routing

• Uses label stack
• Handles failure of links.

– Purposes: just for a short while

• QoS, possibly as well

– Intserv, diffserv

Virtual private networks

• What is it?
• Overlay model
• Peer model

What is a VPN?

A set of sites that can communicate with each other under adminstrative policies that control both connectivety and QoS among the sites.

Overlay model –”virtual backbone”

• Each site has a router that is connected via

point-to-point links to routers in other sites.

– ATM may be used as point-to-point links

• Dominates today

12 Overlay model: Drawbacks

• Experties needed: IP QoS, Layer2QoS, and

their mapping and routing.

• Scalability, N-1 routing peerings
• Configuration changes
• Large-scale VPN not that promising

The Peer Model

• Aims at enabling VPN service to very large-

scale VPN.

• Constrained distribution of routing

information

• Multiple forwarding tables
• Use of a new type of addresses, VPN-IP

addresses

• MPLS

Abbreventaions

• CE = Customer Edge Router
• PE = Provider Edge router (sevice provider)
• P = Provider Router

The Peer Model

• A CE router maintains routing peering only

with its directly connected PE.

– Basic attribute of the peering model.

• Constant and independent of the number of nodes in

the VPN.

• Scalablity in the area of configuration

management

• A PE router only needs to keep information
• f the CE directly connected to it.

Distribution of routing information: 5 steps

• From CE to PE (RIP, OSPF etc)
• At the ingress PE router, this information is

exported into the provider’s BGP.

• This information is distributed within the service

provider among the PE outers using BGP.

• At egress PE router, the routing information is

imported from the provider’s BGP.

• From Egress PE router to a CE router

VPN-IP addresses

• BGP assumes that IP addresses are unique

– Incorrect assumption for VPNs

• Turn non-unique address into unique
• Concatenate a Route Distinguisher with the

plain IP address => VPN/IP

– RD consists of AS number, Assigned number

• VPN/IP not used in headers, just in routers
• How to manage this?

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MPLS as a forwarding mechanism

• MPLS decouples the information used for

packet forwarding (label) from the information carries in the IP header

• Bind LSPs to VPN-IP routes and then

forward IP packets along these routes by using MPLS as the forwarding mechanism

Scalability Through Routing Hierarchy

• Border routers BR1-4 run an EGP, providing inter-domain

routing

• TR1-4 run an IGP, providing intra-domain routing
• Normal layer 3 forwarding requires interior routers to carry

full routing tables, transit router must be able to identify the correct destination ASBR (BR1-4)

• Carrying full routing tables in all routers limits scalability of

interior routing, slower convergence, larger routing tables MPLS enables ingress node to identify egress router, label packet based on interior route

• MPLS increases scalability by partitioning exterior routing

from Interior routing

Scalability Through Routing Hierarchy

AS1

BR1 BR2 BR3 BR4 TR1 TR2 TR3 TR4

AS2 AS3

Ingress router receives packet Ingress router receives packet Packet labelled based on egress router Packet labelled based on egress router Forwarding in the interior based on IGP route Forwarding in the interior based on IGP route Egress border router pops label and fwds. Egress border router pops label and fwds.

RSVP

• Resource resevation protocol

– End-to-end guaraantees – Per flow reservation possible – Scalability problems with per flow – However, per FEC (a set of flows) is easilly accomplished with

• Class of services also possible within an

LSP (bits in packet)

ER-LSP setup using RSVP

LSR B LSR C LER D LER A

• 1. Path message. It contains

ER path < B,C,D>

• 2. New path state. Path

message 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

receives Resv, the ER established. Per-hop Path and Resv refresh unless suppressed Per-hop Path and Resv refresh unless suppressed Per-hop Path and Resv refresh unless suppressed