8/7/2019 17_MPLS_TE

1/44

Introduction to MPLS

and TrafficEngineering

8/7/2019 17_MPLS_TE

2/44

2

Outline

Traditional IP Routing Forwarding and routing Problems with IP routing

Motivations behind MPLS MPLS Terminology and Operation

MPLS Label, LSR and LSP, LFIB Vs FIB Transport of an IP packet over MPLS More MPLS terminology

Traffic Engineering [with MPLS] Nomenclature Requirements Examples

8/7/2019 17_MPLS_TE

3/44

3

Outline

Traditional IP Routing Forwarding and routing Problems with IP routing

Motivations behind MPLS MPLS Terminology and Operation

MPLS Label, LSR and LSP, LFIB Vs FIB Transport of an IP packet over MPLS More MPLS terminology

Traffic Engineering [with MPLS] Nomenclature Requirements Examples

8/7/2019 17_MPLS_TE

4/44

8/7/2019 17_MPLS_TE

5/44

5

IP versus MPLS routing

IP routing Each IP datagram is routed independently Routing and forwarding is destination-based

Routers look at the destination addresses May lead to congestion in parts of the network

MPLS routing A path is computed in advance and a virtual

circuit is established from ingress to egress An MPLS path from ingress to egress node is

called a label switched path (LSP)

8/7/2019 17_MPLS_TE

6/44

6

How IP routing works

Searching

Longest

Prefix Match

in FIB (Too

Slow)

8/7/2019 17_MPLS_TE

7/44

7

Problems with IP routing

Too slow IP lookup (longest prefix matching) was a

major bottleneck in high performance routers This was made worse by the fact that IP

forwarding requires complex lookup operationat every hop along the path

Too rigid no flexibility Routing decisions are destination-based

Not scalable in some desirable applications When mapping IP traffic onto ATM

8/7/2019 17_MPLS_TE

8/44

8

IP routing rigidity example

Packet 1: Destination A Packet 2: Destination B S computes shortest paths to A and B; finds D as next hop Both packets will follow the same path

Leads to IP hotspots!

Solution? Try to divert the traffic onto alternate paths

1 1

1 2

A B

C

A

B

S

D

8/7/2019 17_MPLS_TE

9/44

9

IP routing rigidity example

Increase the cost of link DA from 1 to 4

Traffic is diverted away from node D

A new IP hotspot is created!

Solution(?): Network Engineering Put more bandwidth where the traffic is!

Leads to underutilized links; not suitable for large networks

1 4

1 2

A B

C

S

A

B

D

8/7/2019 17_MPLS_TE

10/44

10

Motivations behind MPLS

Avoid [slow] IP lookup Led to the development of IP switching in 1996

Provide some scalability for IP over ATM

Evolve routing functionality Control was too closely tied to forwarding

Evolution of routing functionality led to someother benefits Explicit path routing

Provision of service differentiation (QoS)

8/7/2019 17_MPLS_TE

11/44

11

IP routing versus MPLS

routing

Traditional IP RoutingMultiprotocol Label Switching (MPLS)

S D

543

21

MPLS allows overriding shortest paths!

8/7/2019 17_MPLS_TE

12/44

12

Outline

Traditional IP Routing Forwarding and routing Problems with IP routing Motivations behind MPLS

MPLS Terminology and Operation MPLS Label, LSR and LSP, LFIB Vs FIB Transport of an IP packet over MPLS More MPLS terminology

Traffic Engineering [with MPLS] Nomenclature Requirements Examples

8/7/2019 17_MPLS_TE

13/44

13

MPLS label

To avoid IP lookup MPLS packets carryextra information called Label

Packet forwarding decision is made usinglabel-based lookups

Labels have local significance only!

How routing along explicit path works?

IP DatagramLabel

8/7/2019 17_MPLS_TE

14/44

14

Routing along explicit paths

Idea: Let the source make the complete routingdecision

How is this accomplished? Let the ingress attach a label to the IP packet and let

intermediate routers make forwarding decisions only

On what basis should you choose different pathsfor different flows? Define some constraints and hope that the constraints

will take some traffic away from the hotspot!

Use CSPF instead of SPF (shortest path first)

8/7/2019 17_MPLS_TE

15/44

15

Label, LSP and LSR

Label

Router that supports MPLS is known as labelswitching router (LSR)

Path which is followed using labels is called LSP

Label = 20 bits

Exp = Experimental, 3 bits

S = Bottom of stack, 1bit

TTL = Time to live, 8 bits

0 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

8/7/2019 17_MPLS_TE

16/44

16

LFIB versus FIB

Labels are searched in LFIB whereas normal IPRouting uses FIB to search longest prefix matchfor a destination IP address

Why switching based on labels is faster? LFIB has fewer entries

Routing table FIB has larger number of entries???

In LFIB, label is an exact match In FIB, IP is longest prefix match

8/7/2019 17_MPLS_TE

17/44

17

MPLS Flow Progress

LSR1

LSR2

LSR3

LSR5

LSR6

R1 R2LSR4D

1 - R1 receives a packet for destination D connected to R2

R1 and R2 are

regular routers

D

destination

8/7/2019 17_MPLS_TE

18/44

18

MPLS Flow Progress

LSR1

LSR2

LSR3

LSR5

LSR6

R1 R2LSR4D

2 - R1 determines the next hop as LSR1 and forwards the packet

(Makes a routing as well as a forwarding decision)

D

destination

8/7/2019 17_MPLS_TE

19/44

19

MPLS Flow Progress

LSR1

LSR2

LSR3

LSR5

LSR6

R1 R2LSR4

D

3 LSR1establishes a path to LSR6 and PUSHES/ATTACHES a label

(Makes a routing as well as a forwarding decision)

D

destination

31

8/7/2019 17_MPLS_TE

20/44

20

MPLS Flow Progress

LSR1

LSR2

LSR3

LSR5

LSR6

R1 R2LSR4

D

4 LSR3 just looks at the incoming label

LSR3 SWAPS with another label before forwarding

D

destination

17

Labels have local

signifacance!

8/7/2019 17_MPLS_TE

21/44

21

MPLS Flow Progress

LSR1

LSR2

LSR3

LSR5

LSR6

R1 R2LSR4

D

5 LSR6 looks at the incoming label

LSR6 POPS/REMOVES the label before forwarding to R2

D

destination

17

Path within MPLS cloudis pre-established:

LSP (label-switched path)

8/7/2019 17_MPLS_TE

22/44

22

MPLS and explicit routing

recap

Who establishes the LSPs in advance? Ingress routers

How do ingress routers decide not to always takethe shortest path? Ingress routers use CSPF (constrained shortest path

first) instead of SPF

Examples of constraints:

Do not use links left with less than 7Mb/s bandwidth Do not use blue-colored links for this request

Use a path with delay less than 130ms

8/7/2019 17_MPLS_TE

23/44

23

CSPF

What is the mechanism? (in typical cases!) First prune all links not fulfilling constrains

Now find shortest path on the rest of the topology

Requires some reservation mechanism Changing state of the network must also be

recorded and propagated For example, ingress needs to know how much

bandwidth is left on links The information is propagated by means of routing

protocols and their extensions

8/7/2019 17_MPLS_TE

24/44

8/7/2019 17_MPLS_TE

25/44

25

Label advertisement

Always downstream to upstream labeladvertisement and distribution

171.68.32/24

LSR1

LSR2

Use label 5 for destination

171.68.32/24

MPLS Data Packet

with label 5 travels

Upstream Downstream

8/7/2019 17_MPLS_TE

26/44

26

Label advertisement

Label advertisement can be downstreamunsolicited or downstream on-demand

171.68.32/24

LSR1 LSR2

Sends label

Without any Request

Upstream Downstream

171.68.32/24

LSR1 LSR2

Sends label ONLY after

receiving request

Request For label

Upstream Downstream

8/7/2019 17_MPLS_TE

27/44

27

Label distribution

Label distribution can be ordered or unordered

First we see an example of ordered label distribution

Ingress LSREgress LSR

Label

8/7/2019 17_MPLS_TE

28/44

28

Label distribution

Label distribution can be ordered or unordered

Next we see an example of unordered label distribution

Ingress LSREgress LSR

Label

Label

8/7/2019 17_MPLS_TE

29/44

29

Label operations

Advertisement Downstream unsolicited Downstream on-demand

Distribution Ordered Unordered

8/7/2019 17_MPLS_TE

30/44

30

Outline

Traditional IP Routing Forwarding and routing Problems with IP routing Motivations behind MPLS

MPLS Terminology and Operation MPLS Label, LSR and LSP, LFIB Vs FIB Transport of an IP packet over MPLS More MPLS terminology

Traffic Engineering [with MPLS] Nomenclature Requirements Examples

8/7/2019 17_MPLS_TE

31/44

Traffic Engineering

Traffic Engineering with MPLS

(Application of CSPF)

8/7/2019 17_MPLS_TE

32/44

32

What is traffic engineering?

Performance optimization of operational networks optimizing resource utilization optimizing traffic performance reliable network operation

How is traffic engineered? measurement, modeling, characterization, and

control of Internet traffic

Why? high cost of network assets service differentiation

8/7/2019 17_MPLS_TE

33/44

33

Traffic engineering

Recall the IP hotspot problem

The ability to move traffic away from the

shortest path calculated by the IGP (such asOSPF) to a less congested path

IP: changing a metric will cause ALL the traffic

to divert to the less congested path MPLS: allows explicit routing (using CSPF) and

setup of such explicitly computed LSPs

8/7/2019 17_MPLS_TE

34/44

34

MPLS-TE: How to do it?

LSPs are set up by LSRs based on informationthey learn from routing protocols (IGPs)

This defeats the purpose! If we were to use shortest path, IGP was okay

8/7/2019 17_MPLS_TE

35/44

35

MPLS TE: How we actually do

it?

MPLS TE Requires: Enhancements to routing protocols

OSPF-TE ISIS-TE

Enhancement to signaling protocols to allowexplicit constraint based routing RSVP-TE and CR-LDP

Constraint based routing Explicit route selection Recovery mechanisms defined

8/7/2019 17_MPLS_TE

36/44

36

Signaling mechanisms

RSVP-TE Extensions to RSVP for traffic engineering

BGP-4 Carrying label information in BGP-4

CR-LDP A label distribution protocol that distributes labels

determined based on constraint based routing

RSVP-TE and CR-LDP both do label distributionand path reservation use any one of them!

8/7/2019 17_MPLS_TE

37/44

37

RSVP-TE

Basic flow of LSP set-up using RSVP

8/7/2019 17_MPLS_TE

38/44

38

RSVP-TE PATH Message

PATH message is used to establish state andrequest label assignment

R1 transmits a PATH message addressed to R9

8/7/2019 17_MPLS_TE

39/44

39

RSVP-TE RESV Message

RESV is used to distribute labels after reserving resources

R9 transmits a RESV message, with label=3, to R8 R8 and R4 store outbound label and allocate an inbound label.

They also transmit RESV with inbound label to upstream LSR R1 binds label to forwarding equivalence class (FEC)

8/7/2019 17_MPLS_TE

40/44

40

Rerouting LSP tunnels

When a more optimal route/pathbecomes available

When a failure of a resource occurs alonga TE LSP

Make-before-break mechanism

Adaptive, smooth rerouting and traffictransfer beforetearing down the old LSP Not disruptive to traffic

8/7/2019 17_MPLS_TE

41/44

41

Recovering LSP tunnels

LSP Set-up

8/7/2019 17_MPLS_TE

42/44

42

Protection LSP set up

8/7/2019 17_MPLS_TE

43/44

43

Protection LSP

8/7/2019 17_MPLS_TE

44/44

44

References

http://www.cisco.com/univercd/cc/td/doc/product/software/ios120/120newft/120limit/120s/120s5/mpls_te.htm

RFC 2702 Requirements for Traffic EngineeringOver MPLS

RFC 3031 Multiprotocol Label SwitchingArchitecture

RFC 3272 Overview and Principles of Internet

Traffic Engineering RFC 3346 Applicability Statement for Traffic

Engineering with MPLS MPLS Forum (http://www.mplsforum.org)