Introduction to MPLS and Traffic Engineering Zartash Afzal Uzmi

Introduction to MPLS and Traffic

Engineering

Zartash Afzal Uzmi

Jan 11, 2006 Lahore University of Management Sciences 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







Forwarding and routing

Forwarding: Passing a packet to the next hop router

Routing: Computing the “best” path to the destination

IP routing – includes routing and forwarding Each router makes the forwarding decision Each router makes the routing decision

MPLS routing Only one router (source) makes the routing decision Intermediate router make the forwarding decision


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)


How IP routing works

Searching Longest Prefix Match in FIB (Too Slow)


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 operation at 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


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


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

SA

B

D


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 some other benefits Explicit path routing Provision of service differentiation (QoS)


IP routing versus MPLS routing

Traditional IP RoutingMultiprotocol Label Switching (MPLS)

S D

543

21

MPLS allows overriding shortest paths!







MPLS label

To avoid IP lookup MPLS packets carry extra information called “Label”

Packet forwarding decision is made using label-based lookups

Labels have local significance only! How routing along explicit path works?

IP DatagramLabel


Routing along explicit paths

Idea: Let the source make the complete routing decision

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

paths for 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)


Label, LSP and LSR Label

Router that supports MPLS is known as label switching router (LSR)

An “Edge” LSR is also known as LER (edge router)

Path which is followed using labels is called LSP

Label = 20 bits Exp = Experimental, 3 bits S = Bottom of stack, 1bitTTL = 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


LFIB versus FIB

Labels are searched in LFIB whereas normal IP Routing uses FIB to search longest prefix match for a destination IP address

Why switching based on labels is faster? LFIB has fewer entries Routing table FIB has very large number of entries

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


Mpls Flow Progress

LSR1

LSR2

LSR3

LSR5

LSR6

R1 R2LSR4D

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

R1 and R2 areregular routers

D

destination


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


Mpls Flow Progress

LSR1

LSR2

LSR3

LSR5

LSR6

R1 R2LSR4

D

3 – LSR1 establishes a path to LSR6 and “PUSHES” a label(Makes a routing as well as a forwarding decision)

D

destination

31


Mpls Flow Progress

LSR1

LSR2

LSR3

LSR5

LSR6

R1 R2LSR4

D

4 – LSR3 just looks at the incoming labelLSR3 “SWAPS” with another label before forwarding

D

destination

17

Labels have localsignifacance!


MPLS Flow Progress

LSR1

LSR2

LSR3

LSR5

LSR6

R1 R2LSR4

D

5 – LSR6 looks at the incoming labelLSR6 “POPS” the label before forwarding to R2

D

destination

17

Path within MPLS cloudis pre-established:LSP (label-switched path)


MPLS and explicit routing recap

Who establishes the LSPs in advance? Ingress routers

How do ingress routers decide not to always take the 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


CSPF

What is the mechanism? 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


More MPLS terminology

172.68.10/24

LSR1 LSR2

Upstream Downstream

Data


Label advertisement

Always downstream to upstream label advertisement and distribution

171.68.32/24

LSR1LSR2

Use label 5 for destination 171.68.32/24

MPLS Data Packet

with label 5 travels

Upstream Downstream


Label advertisement

Label advertisement can be downstream unsolicited 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


Label distribution Label distribution can be ordered or unordered First we see an example of ordered label distribution

Ingress LSR

Egress LSR

Label


Label distribution Label distribution can be ordered or unordered Next we see an example of unordered label distribution

Ingress LSR

Egress LSR

Label

Label


Label retention modes Label retention can be conservative or liberal

LSR1

Destination

Label

Label

?


Label operations

Advertisement Downstream unsolicited Downstream on-demand

Distribution Ordered Unordered

Retention Liberal Conservative






Traffic Engineering

Traffic Engineering with MPLS(Application of CSPF)


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


Traffic engineering Recall the IP hotspot problem

The ability to move traffic away from the shortest path calculated by the IGP (such as OSPF) 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


MPLS-TE: How to do it?

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

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


MPLS TE: How we actually do it?

MPLS TE Requires: Enhancements to routing protocols

OSPF-TE ISIS-TE

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

Constraint based routing Explicit route selection Recovery mechanisms defined


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 distribution and path reservation – use any one of them!


RSVP-TE

Basic flow of LSP set-up using RSVP


RSVP-TE PATH Message

PATH message is used to establish state and request label assignment

R1 transmits a PATH message addressed to R9


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 transmits RESV with inbound label to upstream LSR

R1 binds label to forwarding equivalence class (FEC)


Rerouting LSP tunnels

When a more “optimal” route/path becomes available

When a failure of a resource occurs along a TE LSP

Make-before-break mechanism Adaptive, smooth rerouting and traffic

transfer before tearing down the old LSP Not disruptive to traffic


Recovering LSP tunnels

LSP Set-up


Protection LSP set up


Protection LSP


References

RFC 2702 “Requirements for Traffic Engineering Over MPLS”

RFC 3031 “Multiprotocol Label Switching Architecture”

RFC 3272 “Overview and Principles of Internet Traffic Engineering”

RFC 3346 “Applicability Statement for Traffic Engineering with MPLS”

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

Documents

Introduction to MPLS and Traffic Engineering Zartash Afzal Uzmi