MPLS Architecture

MPLS Network Model

MPLS

LSR = Label Switched RouterLER = Label Edge Router

LER

LER

LSR

LER

LSRLSR

IP

MPLS

IP

Internet

LSR

MPLS BenefitsComparing MPLS with existing IP core and

IP/ATM technologies, MPLS has many advantages and benefits:

• The performance characteristics of layer 2 networks

• The connectivity and network services of layer 3 networks

• Improves the price/performance of network layer routing

• Improved scalability

MPLS Benefits (cont’d)

• Improves the possibilities for traffic engineering

• Supports the delivery of services with QoS guarantees

• Avoids need for coordination of IP and ATM address allocation and routing information

Necessity of L3 Forwarding

• For security– To allow packet filtering at firewalls– Requires examination of packet

contents, including the IP header

• For forwarding at the initial router - used when hosts don’t do MPLS

• For Scaling– Forward on a finer granularity than

the labels can provide

MPLS Architecture

• Down stream label assignment for unicast traffic– On demand– Unsolicited

• Path selection– Hop by hop– Explicit

• Ordered vs. independent control• Loop detection and prevention

mechanisms

Label Distribution Protocol (LDP)

• Set of procedures used by LSRs to establish LSPs

• Mapping between network-layer routing information directly to data-link layer switched paths

• LDP peers: – Two LSRs which use LDP to exchange

label/stream mapping – Information exchange known as “LDP Session”

LDP Messages

• Discovery messages – Used to announce and maintain the presence of

an LSR

• Session/Adjacency messages – Used to establish, maintain and terminate

sessions between LDP peers

• Advertisement messages– Used to create, change, and delete label

mappings

• Notification messages– Used to provide advisory information and to

signal error information

Forwarding Equivalence Class (FEC)

• Introduced to denote packet forwarding classes

• Comprises traffic – To a particular destination– To destination with distinct service

requirements

LSP - FEC Mapping

• FEC specified as a set of two elements – IP Address Prefix - any length from 0 – 32– Host Address - 32 bit IP address

• A given packet matches a particular LSP if and only if IP Address Prefix FEC element matches packet’s IP destination address

Label Spaces

• Useful for assignment and distribution of labels

• Two types of label spaces– Per interface label space: Interface-

specific labels used for interfaces that use interface resources for labels

– Per platform label space: Platform-wide incoming labels used for interfaces that can share the same label space

LDP Discovery• A mechanism that enables an LSR to

discover potential LDP peers• Avoids unnecessary explicit configuration

of LSR label switching peers • Two variants of the discovery mechanism

– Basic discovery mechanism: used to discover LSR neighbors that are directly connected at the link level

– Extended discovery mechanism: used to locate LSRs that are not directly connected at the link level

LDP Discovery (Cont’d)• Basic discovery mechanism

– To engage - send LDP Hellos periodically– LDP Hellos sent as UDP packets for all routers

on that subnet

• Extended discovery mechanism– To engage - send LDP targeted Hellos

periodically– Targeted Hellos are sent to a specific address– Targeted LSR decides whether to respond or to

ignore the targeted Hello

Session Establishment

• Exchange of LDP discovery Hellos triggers session establishment

• Two step process– Transport connection establishment

• If LSR1 does not already have a LDP session for the exchange of label spaces LSR1:a and LSR2:b, it attempts to open a TCP connection with LSR2

• LSR1 determines the transport addresses at its end (A1) and LSR2’s end (A2) of the TCP connection

• If A1>A2, LSR1 plays the active role; otherwise it is passive

– Session initialization• Negotiate session parameters by exchanging LDP

initialization messages

Label Distribution and Management

• Two label distribution techniques– Downstream on demand label distribution:

An LSR can distribute a FEC label binding in response to an explicit request

– Downstream Unsolicited label distribution: Allows an LSR to distribute label bindings to LSRs that have not explicitly requested them

• Both can be used in the same network at the same time; however, each LSR must be aware of the distribution method used by its peer

Label Distribution Control Mode

• Independent Label Distribution Control– Each LSR may advertise label mappings to its

neighbors at any time– Independent Downstream on Demand mode -

LSR answers without waiting for a label mapping from next hop

– Independent Downstream Unsolicited mode - LSR advertises label mapping for a FEC whenever it is prepared

– Consequence: upstream label can be advertised before a downstream label is received

Distribution Control Mode (cont’d)

• Ordered Label Distribution Control– Initiates transmission of label mapping for a

FEC only if it has next FEC next hop or is the egress

– If not, the LSR waits till it gets a label from downstream LSR

– LSR acts as an egress for a particular FEC, if• Next hop router for FEC is outside of label switching

network• FEC elements are reachable by crossing a domain

boundary

Label Retention Mode• Conservative Label Retention Mode

– Advertised label mappings are retained only if they are used for forwarding packets

– Downstream on Demand Mode typically used with Conservative Label Retention Mode

– Advantage: only labels required are maintained– Disadvantage: a change in routing causes

delay

• Liberal Retention Mode– All label mappings are retained regardless of

whether LSR is next hop or not– Faster reaction to routing changes

Label Information Base

• LSR maintains learned labels in Label Information Base (LIB)

• Each entry of LIB associates an FEC with an (LDP Identifier, label) pair

• When next hop changes for a FEC, LSR will retrieve the label for the new next hop from the LIB

Domain #3

Domain #2

Domain #1

Hierarchical Routing in MPLS

C

12 3 4 5

6

D

EBA F

•External Routers A,B,C,D,E,F - Talk BGP

•Internal Routers 1,2,3,4,5,6 - Talk OSPF

Note: Internal routers in domains 1 and 3 not shown

Hierarchical Routing (cont’d)

• When IP packet traverses domain #2, it will contain two labels, encoded as a “label stack”

• Higher level label used between routers C and D, which is encapsulated inside a lower level label used within Domain #2

• Operation at C– C needs to swap BGP label to put label that D expects– C also needs to add an OSPF label that 1 expects– C therefore pushes down the BGP label and adds a

lower level label

Explicit Routing in MPLS• Two options for route selection:

– Hop by hop routing– Explicit routing

• Explicit Routing (Source Routing) is a very powerful technique– With pure datagram routing, overhead of

carrying complete explicit route is prohibitive

– MPLS allows explicit route to be carried only at the time the LSP is setup, and not with each packet

– MPLS makes explicit routing practical

Explicit Routing (Cont’d)

• In an explicitly routed LSP – LSP next hop is not chosen by the

local node– Selected by a single node, usually the

ingress• The sequence of LSRs may be

chosen by– Configuration (e.g., by an operator or

by a centralized server)

Loops and Loop Handling

• Routing protocols used in conjunction with MPLS are based on distributed computation which may contain loops

• Loops handling - 3 categories– Loop Mitigation/Survival– Loop Detection– Loop Prevention

Loop Mitigation

• Minimizes the impact of loops by limiting the amount of resources consumed by the loop

• Method– Based on use of TTL field which is

decremented at each hop– Use of dynamic routing protocol

converging rapidly to non-looping paths

Loop Detection

• Loops may be setup but they are subsequently detected

• The detected loop is then broken by dropping label relationship

• Broken loops now necessitates packets to be forwarded using L3 forwarding

Loop Detection (Cont’d)

• Method is based on transmitting a Loop Detection Control Packet (LDCP) whenever a route changes

• LDCP is forwarded towards the destination until– Last MPLS node along the path is reached– TTL of the LDCP expires– It returns to the node which originated it

Loop Prevention

• Ensures that loops are never set up• Labels are not used until it is sure to be

loop free• Methods

– Labels are propagated starting at the egress switch

– Use source routing to set up label bindings from the egress switch to each ingress switch

QoS in MPLS

Strategy

• To support end-to-end QoS as in IP• MPLS not an end-to-end protocol• Efficient ways of mapping QoS to

LSPs• Traffic Engineering key to QoS

QoS Models

• Best effort – Original IP service

• Int-serv.– Fist IP effort to support QoS

• Diff-serv.– Simple, scalable

• Future– Int+ Diff+ TE with e2e SLAs

CISCO QoS Framework

PR

OV

ISIO

NIN

G &

MO

NIT

OR

ING

PR

OV

ISIO

NIN

G &

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VPNsVPNsMultimediaVideo Conference,

Collaborative Computing

MultimediaVideo Conference,

Collaborative Computing

Mission Critical Services

Mission Critical ServicesVoIPVoIP

HybridHybridMPLSMPLSDiffServDiffServIntServIntServ

Signaling Techniques (RSVP, DSCP*, ATM (UNI/NNI))Signaling Techniques (RSVP, DSCP*, ATM (UNI/NNI))

Link Efficiency Mechanisms (Compression, Fragmentation)Link Efficiency Mechanisms (Compression, Fragmentation)

Congestion Avoidance Techniques (WRED)Congestion Avoidance Techniques (WRED)

Congestion Management Techniques (WFQ, CBWFQ, LLQ)Congestion Management Techniques (WFQ, CBWFQ, LLQ)

Classification & Marking Techniques (DSCP, MPLS EXP, NBAR, etc.)Classification & Marking Techniques (DSCP, MPLS EXP, NBAR, etc.)

FrameRelay

FrameRelay

PPPHDLC

PPPHDLC SDLC

SDLCATM, POSATM, POS FE,Gig.E

10GE

FE,Gig.E 10GE

WirelessFixed,Mobile

WirelessFixed,Mobile

BroadBandCable,xDSL

BroadBandCable,xDSL

PO

LIC

Y-B

AS

ED

NETW

OR

KIN

GP

OLIC

Y-B

AS

ED

NETW

OR

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Traffic Conditioners (Policing, Shaping)Traffic Conditioners (Policing, Shaping)

Support of RSVP

• Very similar to tag switching• Bind labels to reserved flows

– Label object inside the RESV message– Labels propagate upstream

• Only the edge router need to know the packet to flow mapping– Can aggregate flows instead of micro-

flows

RSVP Scalability

• Aggregation• Refresh reduction

– Use acknowledgements for refresh– Once received, increase the refresh

time– Summary refresh

Diff-Serv Support

– E-LSP

– “Queue” inferred from Label and EXP field

– “Drop priority” inferred from label and EXP field

– L-LSP

– Queue” inferred exclusively from Label

– “Drop priority” inferred from EXP field

E-LSP

E-LSP

LSRLDP/RSVP LDP/RSVP

EF

AF1

•E-LSPs established by various label binding protocols (LDP or RSVP)

•no new Signalling needed.

•EF and AF1 on a single E-LSP

•EF and AF1 packets travel on single LSP (single label) but are enqueued in different queues (different EXP values)

•Queue & Drop Precedence is selected based on EXP

E-LSP

VersionLengthVersionLength

ToSToS1 Byte1 ByteToSToS

1 Byte1 Byte LenLen

Standard IPV4: Bits 0-2 Called IP Precedence (Three MSB)(DiffServ Uses Six ToS bits…: Bits 0-5, with Two Reserved)

IDID offsetoffset TTLTTL ProtoProto FCSFCS IP-SAIP-SA IP-DAIP-DA DataData

Referred to as Packet Classification or Coloring

IP Precedence to Label EXP

E-LSP vs. L-LSP

• PHB from EXP• No additional

signaling• EXP->PHB

configured• Shim header

required• Up to 8 PHBs per LSP

• PHB from label + Exp/CLP

• Signaled at LSP setup

• Label->PHB mapped • Shim or link layer

header used• Arbitrarily large

Explicit Congestion Notification(ECN)

• TCP approach – based on packet drop– May not reflect the status– Resources could have been wasted

• Early notification– Mark packets– Receiver conveys information to sender

• Two bits used to deal with deployment disparity (CE & ECT)

MPLS Support of ECN

• Could use two bits as before– May not be available– Usually 1 bit available– LSRs should have the understanding

on mapping

Traffic Engineering in MPLS

Traffic Engineering Objectives

• Traffic Engineering (TE) concerned with performance optimization

• The key performance objectives – traffic oriented e.g. minimization of

packet loss – resource oriented - optimization of

resource utilization e.g. efficient management of bandwidth

Objectives (cont’d)

• Minimizing congestion is a major traffic and resource oriented performance objective

• Congestion manifest under two scenarios– Network resources insufficient or

inadequate• Solved by capacity expansion or classical

congestion control techniques

– Inefficient mapping of traffic streams onto available resources

• Reduced by adopting load balancing policies

MPLS and Traffic Engineering

• Main components used– Traffic Trunk - aggregation of traffic

flows of the same class which are placed inside a Label Switched Path

– Induced MPLS Graph • Analogous to a virtual topology in an

overlay model• Logically mapped onto the physical

network • Set of LSRs as nodes of the graph • Set of LSPs providing logical point to

point connectivity between LSRs as edges

Constraint Based Routing (CBR)

• Associate each path with set of constraints– Performance, administrative– Local information

• Routing algorithms – Optimizes various metrics– Ensures that the constraints are not

violated

Can IP Routing Do CBR?

• Plain IP routing cannot– CBR has to be source based – each

source may have different constraint to same destination

– Link attributes need to be distributed– Need explicit routing instead of

“destination-based”• Can be augmented to support CBR

– Usually a combination is used

CBR Components

• Mechanism for source based path computing

• Mechanism to collect necessary information– Constraints (local), attributes, topology

• Support forwarding along the computed paths

• Notification of residual resources after allocation

Constrain-Based SPF

24

7

53

1

6

150

45

150

150

150150

150

CSPF• Uses the following inputs

– Link attributes– Topology state information– Path constraints

• Basic approach– Prune resources that do not meet the

constraints– Run a shortest path algorithm on the

residual graph

MPLS for Forwarding

• Ideal to use MPLS explicit routing capability

• Once the path is computed – Need to establish forwarding state along the

path– Reserve resources along the path

• Two approaches– RSVP extensions– CR-LDP

CBR (cont’d)

• Strict & Loose Explicit Routes– Constraint Based LSP (CRLSP) is

calculated at one point at the edge of the network based on certain criteria

– special char. such as assigning certain bandwidth can be supported

– The route is encoded as a series of Explicit routed hops contained in a CR based route TLV

CBR (cont’d)

• Comparison of RSVP and CR-LDP– Scalability– Signaling mechanism– Qos Models

Application of CR in TE

• IP?• ATM• Overlay• MPLS

TE in MPLS - II

Fish Network

R8

R1

R5

R2

R3R4

R7R6

150

150

150

150

150

Is Plain IP Enough?

R8

R1

R5

R2

R3R4

R7R6

150

150

150

150

150

Under utilized

Why IP Routing Fails

• Based only on metric optimization– Shortest path– Administrative optimization– Split paths

• Per link constraints not taken into consideration

TE in MPLS Using CBR • Define traffic trunks

– Collection of micro-flows that share same path and class of service

– These are not end-to-end paths, rather paths within a single service provider

• No. of trunks dependent only on the topology

• Forwarding table does not grow with the traffic

• Rerouting– RSVP, CR-LDP, or IGP

Forwarding Packets

R1

R5

R2

R3R4

R7R6

150

150

150

150

150

Fast Rerouting

• Total restoration time after failure– Failure detection time– Propagation– Computation of new path

• Usually the 2nd and 3rd steps are significantly slow

Is FR possible with IP?

R1

R4R3

R2

R5

Even if the traffic is rerouted to R3, it will that back to R1 since R3 is not aware of the failure

X

FR using CBR

• Compute protection LSP for every link

• When a failure happens– Traffic rerouted to the protection LSP– Use label stacking for the transit

within the protection LSP– Beyond the end-nodes labels original

labels remain in tact