1 Cisco Systems MPLS Tutorial ETSI June 99 Francois Le Faucheur Systems Architect Cisco Systems [email protected]

Cisco Systems1

MPLS TutorialETSI June 99

Francois Le Faucheur

Systems ArchitectCisco [email protected]

Cisco Systems2

Agenda

• Label Switching Technology Overview– History & Motivation

– Destination-Based Routing

– Label Distribution Protocol(s)

– Encapsulation

– MPLS Over ATM

• Applications– Quality of Service

– Traffic Engineering

– VPNs

• Conclusion: Gbit Routing or MPLS?

Cisco Systems3

Label Switching Motivation

• Address major network evolution problems:–Throughput

–Scaling–Number of nodes, flows, routes

–Traffic engineering (explicit routes)

• Permit graceful evolution of routing– Flexibility, new applications

• Simplify integration of ATM and IP

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Label Switching Basics

• Combines Layer 3 routing with label-swapping forwarding– Simplicity of Layer 2 forwarding offers high performance

– Layer 3 routing has proven scalability

• Clean separation of Forwarding and Control/Routing– Forwarding component: Simple label-swapping paradigm

– Control component: Collection of modules to maintain and distribute label bindings

– Separation leads to graceful evolution of control paradigm

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Label Switching DevicesLabel Switching Routers (LSRs)

(ATM Switch or Router)

Label Edge Routers

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Forwarding Component

• Label Forwarding Information Base (LFIB)– Each entry consists of:

– Incoming labelIncoming label

– One or more sub-entries:

Outgoing labelOutgoing label, outgoing interface, outgoing MAC address

– LFIB is indexed by incoming label

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Forwarding Component (Cont.)

• Forwarding algorithm:– Extract label from a packet

– Find LFIB entry withincoming label = label from packet

– Replace label in packet with outgoing label(s)

– Send packet on outgoing interface(s)

• Observation: Observation: forwarding algorithm is– Network Layer-independent

– independent of how labels have been assigned (ie by Control module)

Label Switching Example

171.69

128.89.10

i/f 0

i/f 1i/f 1

AddressPrefix Interface

...

128.89.10

1

0

171.69

...

128.89.10

1

1

171.69

Advertises Reachabilityto 128.89.10

Advertises Reachability to 128.89.10 and 171.69

Destination-Based Routing Module


10Cisco Systems Confidential0675_03F7_c3

Advertises Reachabilityto 171.69

Label Switching Example (Cont.)

171.69

128.89.10

i/f 0

i/f 1i/f 1


...

128.89.10

1

0

171.69

...

128.89.10

1

1

171.69

Advertises Binding<7,171.69> Using LDP

Advertises Binding<5,128.89.10> Using LDP

Advertises Bindings<3,128.89.10><4,171.69> Using LDP


11

Cisco Systems10

Label Switching Example (Cont.)

171.69

128.89.10

0

1

1

171.69.12.1 data 171.69.12.1 data171.69.12.1 data

4 7

...

128.89.10

1

0

171.69

3

4

5

7

...

128.89.10

1

1

171.69

LocalLabel

RemoteLabel


x

x

3

4

‘Edge’ Router DoesLongest Match, Adds Label

Subsequent RoutersForward on Label Only

LocalLabel

RemoteLabel


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Label Distribution Protocol (LDP)

• Used to distribute <label,prefix> bindings

• Incremental updates over reliabletransport

• One of several label-binding mechanisms

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Frame Encapsulation

S = Bottom of StackTTL = Time to LiveEXP = Experimental (formerly COS = Class of Service)

• Can be used over Ethernet, 802.3, or PPP links

• new Ethertypes/PPP PIDs

• Contains everything needed at forwarding time

• MPLS Headers can be used “recursively”

0 1 2 30 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| Label | Exp |S| TTL |

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Label Switching and ATM

• label switching forwarding:– Make decision using fixed length label

– Rewrite the label with a new value

– Sounds like ATM

• label switching control:– Based on L3 protocols

– used to set-up/maintain ATM VCs (instead of traditional ATM Control plane protocols UNI/PNNI)

– Resolves ‘impedance mismatch’

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LocalLabel

RemoteLabel

...

128.89

0

0

128.89

AddressPrefix

Outputi/f

5

6

7

8

Inputi/f

1

2

Label Distribution for ATMDownstream on Demand

Requests a labelfor 128.89

Requests a labelfor 128.89

Requests Two Labels for 128.89

Returns a Label to Each Requester

128.89

Label Switching = ATM switchingbecause labels copied in VCI

Cisco Systems15

MPLS ATM Impedance Mismatch

• Downstream on Demand – label conservation

• VC-Merge – Cell Interleave

• Loop prevention– Hop count fields in request and response

– Per-VC queuing to limit damage

– loop detection

– optional loop prevention

• TTL semantics– Decrement by hop count on ingress

– Use ‘router alert’ to handle traceroute

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Scaling in L2/L3 Networks

Problem: Huge Number of Routing AdjacenciesImpacts Routing Performance

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Scaling in MPLS Networks

Solution: Only Neighbor-Neighbor Routing Adjacencies

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Agenda




– Encapsulation

– MPLS Over ATM



– VPNs


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MPLS QoS

• MPLS targets support of existing IETF QoS models (does not reinvent a new QoS model):– Diff-Serv over MPLS

– Int-Serv over MPLS

• targeted result is end-to-end IP QoS through MPLS clouds indistinguishable from IP QoS in non-MPLS network

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Diff-serv on ATM-LSRs

• Challenges:– No DS field in header

– Re-ordering constraints of Diffserv

– Different drop algorithms in switches (ie no RED/WRED)

• Solution approach:– Use parallel LSPs to one destination (FEC)

– Each LSP represents a group of PHBs (ie the PHBs with ordering constraint)eg. EF, Default, AF1x, AF2x, AF3x, AF4x

--> one LSP per <FEC, PHB group>

– CLP to indicate drop preference within PHB group

Cisco Systems21

Parallel LSPs

• PHB Group (ie EF, AF1x, AF2x,..) signaled at label establishment time

• Switch performs scheduling based on PHB Group :– eg. all AF1x labels into the same queue

– eg. Switches perform per-class WFQ (not per-VC)

• Switch performs “drop precedence” based on CLP bit

EFAF1

AF2

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Diff-Serv on PPP LSR

• Two complementary approaches pursued and allowed simultaneously

• Similar to Diff-Serv over ATM LSR – ie Parallel LSPs

– PHB Group is signaled at LSP set-up

– use MPLS Shim Header EXP field to convey Drop Precedence

• use MPLS EXP field exactly as DSCP is used for IP– takes advantage of fact that MPLS EXP field is seen at every

PPP LSR hop

– use MPLS EXP field to indicate the PHB Group as well as the Drop Precedence

– limit to total 8 PHBs

Cisco Systems23

Int-Serv over MPLS

• Each RSVP session has dedicated label– label binding carried in RSVP RESV and

PATH messages

– Enables simple flow classification (label vs. src and dest address and port)

– Note: this is for establishment of a label per RSVP flow (as opposed to using RSVP to set up labels for fat aggregates for Traffic Engineering)

• Stable I-D but not high priority of MPLS group

Cisco Systems24

Agenda




– Encapsulation

– MPLS Over ATM



– VPNs


R8

R2

R6

R3

R4

R7

R5

R1

IP Routing & “the Fish”

IP (Mostly) Uses Destination-Based Least-Cost RoutingIP (Mostly) Uses Destination-Based Least-Cost RoutingFlows from R8 and R1 Merge at R2 and Become IndistinguishableFlows from R8 and R1 Merge at R2 and Become IndistinguishableFrom R2, Traffic to R3, R4, R5 Use Upper RouteFrom R2, Traffic to R3, R4, R5 Use Upper Route

Alternate Path Under-UtilizedAlternate Path Under-Utilized

6

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MPLS Traffic Engineering

• MPLS TE is not about offering additional QoS services visible by end-user

• MPLS TE is about reducing cost of providing end-user services (eg Diff-Serv) through better use of given resources

• May improve QoS

• MPLS TE takes advantage of “connection-like” nature of MPLS to distribute traffic based on Bandwidth demand/use

• like current Voice Traffic Engineering

R8

R2

R6

R3

R4

R7

R5

R1

MPLS TE Tunnel

60401_10F8_c1NW97_EMEA_504

Normal Route: R1->R2->R3->R4->R5

TE Tunnel: R1->R2->R6->R7->R4->R5

Labels, like ATM VCs can be used to establish virtual circuits which are “Qos Routed”

Cisco Systems28

MPLS TE

• TE Tunnels need be “automatically” routed

• performs Constraint Based Routing where constraints include:– Bandwidth need of a tunnel versus bandwidth

available on all links

– Policy constraint configurable by Operator (eg that sort of Tunnel must not use that sort of links)

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TE Example DeploymentFind route & set-up tunnel for 20 Mb/s from POP1 to POP4

Find route & set-up tunnel for 10 Mb/s from POP2 to POP4

POP4

POP

POP

POP

POP2

POP1

WAN area

Cisco Systems30

MPLS TE Components (1)

• Link state IGP protocols enhanced to advertise “unreserved capacity” per link

• SPF computation enhanced to route a TE tunnel (Constraint based Routing): – first prune the links which do not satisfy a

constraint from the topology

– Pick shortest path on the remaining topology

Cisco Systems31


• Tunnel set-up (ie label binding) along the route computed by Constraint Base Routing:– via RSVP with extensions (eg Explicit Route

Object), Note: RSVP state applies to a large aggregate of flows (i.e. a tunnel), rather than to a single flow or

– via CR-LDP (ie extensions over LDP such as Explicit Route TLV)

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• MPLS LFIB handles the forwarding “as usual”– only LFIB has been populated by another

Control module than Destination Based LDP)

• IGP enhanced on tunnel Head-ends to “route” IP packets “into” TE tunnels

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Traffic Engineering Summary

• Connection-like aspects of MPLS allow traffic engineering for IP

• Addresses limitations of connectionless routing

• Avoids drawbacks of overlay (L2/L3) model

• Combination with constraint-based routing provides automatic tunnel setup which maximises usage of existing resources and re-optimization on topology change

• Underlying mechanism to achieve IP QoS more efficiently

• In core, uses unmodified label switching Forwarding component

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Agenda




– Encapsulation

– MPLS Over ATM



– VPNs


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Scalability issues of Layer 2 VPNs

• Complexity of provisioning n2 VCs per VPN, along with QOS for each VC

• Complexity of designing routing system for each VPN over full VC mesh

• Poor routing performance over mesh of adjacencies

• Poor bandwidth efficiency if mesh is not used

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Why MPLS VPNs?

• MPLS combines L3 routing and L2 forwarding

• L3 routing provides– improved scalability by eliminating mesh of

connections from CPE-to-CPE

• L2 (label-based) forwarding provides– comparable security to L2 approaches

– hiding of non-registered addresses

• Hierarchical labels (label stack) further enhance scalability

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VPN - example

VPN A/Site 1

VPN A/Site 2

VPN A/Site 3

VPN B/Site 2

VPN B/Site 1

VPN B/Site 3

CEA1

CEB3

CEA3

CEB2

CEA2CE1B1

CE2B1

PE1

PE2

PE3

P1

P2

P3

MPLS

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Basic ingredients:

• Constrained distribution of routing information w/ BGP

• VPN-IP addresses

• Multiprotocol Label Switching (MPLS)– in backbone, LFIB Forwarding “as usual”

• Peer Model

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VPN - example

VPN A/Site 1

VPN A/Site 2

VPN A/Site 3

VPN B/Site 2

VPN B/Site 1

VPN B/Site 3

CEA1 CEB3

CEA3

CEB2

CEA2CE1B1

CE2B1

PE1

PE2

PE3

P1

P2MPLSiBGP (VPN-IPv4 @)

LDP

Two-levellabelled packets

SingleRoutingAdjacencyVPN<-->Cloud

Cisco Systems40

Agenda




– Encapsulation

– MPLS Over ATM



– VPNs


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A Perception Problem

• A lot of people think label switching is all about forwarding performance– ATM switches used to be faster than routers

– Plenty of label switching marketing reinforced this

• This causes Gbit router implementors to say `Ha! Label Switching is useless’ as routers catch up

• If standard IP forwarding at Gbit speeds is the only requirement, Gbit routers are the solution

Cisco Systems42

The value of label switching

• Label switching adds value to Gbit routers– Traffic engineering support

– VPNs

– Ease of evolution

• Label switching enables better IP/ATM integration– only relevant if ATM core was chosen for some

reason, e.g. service integration

• Not too hard to add label switching to Gbit routers

Cisco Systems43

References

• Diffserv– RFC 2474. Definition of the Differentiated Services Field (DS Field) in the IPv4

and IPv6 Headers. K. Nichols et al.

– RFC 2475. An Architecture for Differentiated Service. S. Blake et al.

• MPLS Basics– draft-ietf-mpls-arch-04.txt

– draft-ietf-mpls-atm-01.txt

– draft-ietf-mpls-ldp-03.txt

• MPLS Traffic Engineering & DiffServ– draft-ietf-mpls-rsvp-lsp-tunnel-02.txt

– draft-ietf-mpls-traffic-eng-00.txt

– draft-ietf-mpls-cr-ldp-01.txt

– draft-ietf-mpls-diff-ext-00.txt

– draft-davari-mpls-diff-ppp-00.txt

• MPLS VPNs– RFC 2547. BGP/MPLS VPNs. E. Rosen, Y. Rekhter. March 1999.

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References

• Gigabit routers– Partridge et al. “A 50-Gb/s IP router," IEEE/ACM Transactions on

Networking, vol. 6, June 1998.

• Fast Routing Lookups– Brodnik et al. “Small Forwarding Tables for Fast Routing Lookups”,

Sigcomm ‘97.

– Waldvogel et al. “Scalable High Speed IP Routing Lookups”, Sigcomm ‘97.

– Srinivasan et al. “Fast Scalable Level 4 Switching”, Sigcomm '98.

– Lakshman and Stiliadis, "High Speed Policy Based-Packet forwarding...", Sigcomm '98.

• MPLS– Davie et al. “Switching in IP Networks”, Morgan Kaufmann Publishers, May

1998.

– Rekhter et al. “Tag Switching Architecture Overview”, IEEE Proceedings, vol 85, No. 12, Dec 1997.

Documents

1 Cisco Systems MPLS Tutorial ETSI June 99 Francois Le Faucheur Systems Architect Cisco Systems [email protected]