This teaching material is a part of e-Photon/ONe Master study in Optical Communications and Networks Course and module: Author(s): This tutorial is licensed

This teaching material is a part of e-Photon/ONe Master study in Optical Communications and Networks

Course and module:

Author(s):

This tutorial is licensed under the Creative Commonscreativecommons.org/licenses/by-nc-sa/3.0/

http://www.e-photon-one.org

Optical Core Networks

MPLS - basics

Piero Castoldi, Scuola Superiore Sant’Anna, [email protected]

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Authors: Piero Castoldi Course: Optical Core NetworksModule: MPLS basics

Outline

• MPLS fundamentals

• Label Encapsulation

• Label Distribution methods

CREDIT: some figures are taken from the presentation “MPLS tutorial” by Peter Ashwood-Smith Bilel N. Jamoussi

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What is MPLS?

MPLS stands for “Multi-Protocol Label Switching”

MPLS is an IETF–specified framework that provides for the efficient control of traffic flows through the network regardless of transport media.

MPLS controls the way of mapping Layer 3 data flow onto Layer 2 traffic between adjacent network nodes without concern how Layer 2 or Layer 3 traffic is transported (That’s why it called ‘Multiple Protocol’)

MPLS supports the IP, ATM, and frame-relay Layer-2 protocols, even though it is appreciate as a more effective means of deploying IP networks across ATM-based WAN backbones.

MPLS incorporate best properties in both packet routing (IP) and circuit switching (ATM)

Packet Routing Hybrid Circuit switching

IP ATMMPLS + IP

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Multi Protocol Label Switching

(MPLS) fundamentals

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“Label Substitution”, what is it? (1)

• 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, it is on the way”.

• SOURCE ROUTING: Ask for a list (that you carry with you) of 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”.

One of the many ways of getting from A to B:

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Label Substitution, what is it? (2)• Have a friend go to B ahead of you using one of the last two techniques. At every road (link) he reserves a lane just for you. At every intersection (node) they post a big sign that says for a given lane which way to turn and what new lane to take.

LANE#1

LANE#2

LANE#1 TURN RIGHT USE LANE#2

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A label by any other name ...

There are many examples of label substitution protocols already in existence.

• ATM - label is called VPI/VCI and travels with cell.

• Frame Relay - label is called a DLCI and travels with frame.

• TDM - label is called a timeslot its implied, like a lane.

• X25 - a label is an LCN

• Proprietary TAG etc..

• GMPLS allows to use a “color substitution” where label is a light frequency (color) ..

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What is a “LABEL”?

A property that uniquely identifies a flow on a logical or physical interface

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Label Switched Path (LSP)

#7

#99

#9

#3 Right #7

#99 RIGHT #9#7 LEFT #99

#9 LEFT #4072

#3IP

#4072 IP

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Optical or Generalized Label Switched Path (G-LSP)

RED RIGHT BLUE

WHITE RIGHT ORANGEBLUE LEFT WHITE

ORANGE LEFT RED

IP

IP

RED

BLUE

WHITE

ORANGE

RED

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Label concept

Value: Label value 20 bitsExp: Experimental Use, 3 bitsS: Bottom of stack, 1 bitTTL: Time To Live, 8 bits

Value: Label value 20 bitsExp: Experimental Use, 3 bitsS: Bottom of stack, 1 bitTTL: Time To Live, 8 bits

Total: 32 bit = 4 byte

Value Exp S TTL IP packetMPLS label

MPLS generates a short fixed-length label that acts as a shorthand representation of an IP packet’s header

The label is attached in front of a IP packet.

! Packets are switched, not routed, based on labels

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Basic operation

LER LERLSR LSR

Relative meaning of label (only within the link):

each MPLS-capable router (LSR) changes the packet label

LSR: Label Switching Router

LER: Label Edge Router (Useful

term not in standards) Ingress

Router and Egress Router

LSR: Label Switching Router

LER: Label Edge Router (Useful

term not in standards) Ingress

Router and Egress Router

IP forwarding IP forwardingLabel Switching

IP1 IP1#L1 IP1#L2 IP1#L3 IP1

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FEC Forwarding Equivalence Class

IP1IP1#L1

• FEC = “A subset of packets that are all treated the same way by an edge router”

• 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 bemapped to common pathPackets are destined for different address prefixes, but can bemapped to common path

IP2IP2#L1

IP1#L2

IP2#L2

IP1#L3

IP2#L3

LER LERLSR LSR

IP1

IP2

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Label stacking

Hierarchical use of the labels

Only outer label is used to forward packets

Creation of tunnel between non-neighbouring router => MPLS Domain

Scalability: the expansion of the network doesn’t increase the number of labels => This drastically reduces the size of routing tables in LSRs

IPL1L2L3…

MPLS Domain 1

MPLS Domain 2

MPLS Domain 3

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MPLS features Label swapping:

Bring the speed of layer 2 switching to layer 3

Separation of forwarding plane and control plane

Forwarding hierarchy via Label stackingIncrease the scalability

Constraint-based routingTraffic Engineering

Fast reroute

Facilitate the virtual private networks (VPNs)

Enables Traffic Engineering and QoSProvides an opportunity for mapping DiffServ fields onto an MPLS label

Facilitate the elimination of multiple layersResolve the problems of IP over ATM, in particular: Complexity of control and management and scalability issues

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So what is MPLS?

• Hop-by-hop or source routing to establish labels

• Possible use of labels native to the media (colors)

• Multi level label substitution transport

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Routers Do Both Routing and Switching

• Routing Deciding the next hop based on the

destination address. A Layer 3 (L3) function.

• Switching Moving a packet from an input port to an

output port and out. A layer 2 function. INPUT PORTS OUTPUT PORTS

• So we can avoid performing the layer 3 function.• What benefit does this provide?• In what situations would this benefit not be very significant?

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MPLS: Flexible Forwarding

LSP to IPLABEL SWITCHINGIP to LSP

IP IP #L1 IP #L2 IP #L3 IP

IP DA

IP: Packets are forwarded based on Destination Address (DA)

MPLS: Route at edge and switch in core• Map packets to LSP based on (Source Address, Destination Address, protocol, port, DSCP,

interface, etc.) and forward packets based Label

IP DA IP DA IP DA IP DA

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MPLS-based Solutions

• IP Traffic Engineering Constraint-based Routing making routing adapt to latest network loading

• Virtual Private Networks Controllable tunneling mechanism

• L2/L3 Integration Easy software implementation in current routers

• L1/L3 Integration Use of MPLS to control Optical Cross Connects (OXC) -> GMPLS

• Enable QoS in IP Networks Support IP Diffserv + ATM-style QoS

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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), can be

Ingress Router, Egress Router, Transit Router

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#216

#612

#5#311

#14

#99

#963

#462

- An LSP is actually part of a tree from every source to that destination (unidirectional).

- LDP builds that tree using existing IP forwarding tables to route the control messages.

#963

#14

#99

#311

#311

#311


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Topology dissemination in standard IP

47.1

47.247.3

Dest Out

47.1 147.2 2

47.3 3

1

23

Dest Out

47.1 147.2 2

47.3 3

Dest Out

47.1 147.2 2

47.3 3

1

23

1

2

3

• Destination based forwarding tables as built by OSPF, IS-IS, RIP, etc.

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IP forwarding using hop-by-hop control

47.1

47.247.3

IP 47.1.1.1

Dest Out

47.1 147.2 2

47.3 3

1

23

Dest Out

47.1 147.2 2

47.3 3

1

2

1

2

3

IP 47.1.1.1

IP 47.1.1.1IP 47.1.1.1

Dest Out

47.1 147.2 2

47.3 3

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IntfIn

LabelIn

Dest IntfOut

3 0.40 47.1 1

IntfIn

LabelIn

Dest IntfOut

LabelOut

3 0.50 47.1 1 0.40

MPLS Label Distribution use-case

47.1

47.247.3

12

31

2

1

2

3

3IntfIn

Dest IntfOut

LabelOut

3 47.1 1 0.50 Mapping: 0.40

Request: 47.1

Mapping: 0.50

Request: 47.1

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IntfIn

LabelIn

Dest IntfOut

3 0.40 47.1 1

IntfIn

LabelIn

Dest IntfOut

LabelOut

3 0.50 47.1 1 0.40

47.1

47.247.3

1

2

31

2

1

2

3

3IntfIn

Dest IntfOut

LabelOut

3 47.1 1 0.50

IP 47.1.1.1

IP 47.1.1.1

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Benefits and Limitations

• Why might this approach be better than normal IP forwarding that does not use MPLS? Remember, all packets still travel the same paths.

ANSWER: The label look-up allows ultra-fast forwarding of FEC

• What else might we be able to do with MPLS that could be even more powerful? See next two slides

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#216

#14

#462

- ER-LSP follows route that source chooses. In other words, the control message to establish the LSP (label request) is source routed.

#972

#14 #972

A

B

C

Route={A,B,C}

Explicited Routed LSP or ER-LSP (1)

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IntfIn

LabelIn

Dest IntfOut

3 0.40 47.1 1

IntfIn

LabelIn

Dest IntfOut

LabelOut

3 0.50 47.1 1 0.40

47.1

47.247.3

1

2

3

1

2

1

2

3

3

IntfIn

Dest IntfOut

LabelOut

3 47.1.1 2 1.333 47.1 1 0.50

IP 47.1.1.1

IP 47.1.1.1

Explicited Routed LSP or ER-LSP (2)

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ER LSP - advantages

•Operator has routing flexibility (policy-based, QoS-based)•Can use routes other than shortest path•Can compute routes based on constraints in exactly the same manner as ATM based on distributed topology database (traffic engineering)

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ER LSP - discord!

• Two signaling options proposed in the standards: CR-LDP, RSVP extensions:

— CR-LDP = LDP + Explicit Route— RSVP ext = Traditional RSVP + Explicit Route + Scalability Extensions

• Little difference in mechanisms, but RSVP is the winner (in terms of market).

• Survival of the fittest not such a bad thing.

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Label encapsulation

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Label Encapsulation

MPLS Encapsulation is specified over various media types. Outermost labels may use existing format (VPI/VCI, etc.), while inner label(s) use a new “shim” label format.

ATM FR Ethernet PPP

VPI VCI DLCI “Shim Label”

Medium

Label

“Shim Label” …….

IP or other non-IP PAYLOAD

Optical

λ

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MPLS Link Layers

•MPLS is intended to run over multiple link layers

•Specifications for the following link layers currently exist:

• PPP/LAN: uses ‘shim’ header inserted between L2 and L3 headers

• ATM: label contained in VCI/VPI field of ATM header

• Frame Relay: label contained in DLCI field in FR header

Translation between link layers types must be supported

MPLS intended to be “multi-protocol” below as well as aboveMPLS intended to be “multi-protocol” below as well as above

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MPLS Encapsulation - PPP & LAN Data Links

Label Exp. S TTL

Label: Label Value, 20 bits (0-16 reserved)Exp.: Experimental, 3 bits (was Class of Service)S: Bottom of Stack, 1 bit (1 = last entry in label stack)TTL: Time to Live, 8 bits

Layer 2 Header(eg. PPP, 802.3)

•••Network Layer Header

and Packet (eg. IP)

4 Octets

MPLS ‘Shim’ Headers (1-n)

1n

•Network layer must be inferable from value of bottom label of the stack•Note: The label at the bottom of the stack is the “top” label.

MPLS on PPP links and LANs uses ‘Shim’ Header Inserted Between Layer 2 and Layer 3 Headers

MPLS on PPP links and LANs uses ‘Shim’ Header Inserted Between Layer 2 and Layer 3 Headers

Label StackEntry Format

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MPLS Encapsulation -> ATM

ATM LSR constrained by the cell format imposed by existing ATM standardsATM LSR constrained by the cell format imposed by existing ATM standards

VPI PT CLP HEC

5 Octets

ATM HeaderFormat VCI

AAL-5 Trailer

•••Network Layer Header

and Packet (eg. IP)

1n

AAL-5 PDU Frame (nx48 bytes)

Generic Label Encap.(PPP/LAN format)

ATMSAR

ATM HeaderATM Payload • • •

48 Bytes

48 Bytes

Label LabelOption 1

Option 2 Combined Label

Option 3 LabelATM VPI (Tunnel)

• Top 1 or 2 labels are contained in the VPI/VCI fields of ATM header - Option 1 uses two labels.

- One in each or single label in combined field, negotiated by LDP• Further fields in stack are encoded with ‘shim’ header in PPP/LAN format

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MPLS Encapsulation -> Frame Relay

•••n 1

DLCIC/R

EA

DLCIFECN

BECN

DE

EA

Q.922Header

Generic Encap.(PPP/LAN Format) Layer 3 Header and Packet

DLCI Size = 10, 17, 23 Bits

• Current label value carried in DLCI field of Frame Relay header

• Can use either 2 or 4 octet Q.922 Address (10, 17, 23 bytes)

• Generic encapsulation contains n labels for stack of depth n - top label contains TTL (which FR header lacks), ‘explicit NULL’ label value

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Label distribution

methods

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Label Distribution Protocol (LDP) - Purpose

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

Routing Table:

Addr-prefix Next Hop47.0.0.0/8 LSR2

Routing Table:


LSR1 LSR2 LSR3

IP Packet 47.80.55.3

Routing Table:


Routing Table:


For 47.0.0.0/8use label ‘17’

Label Information Base:

Label-In FEC Label-Out17 47.0.0.0/8 XX


Label-In FEC Label-Out17 47.0.0.0/8 XX


Label-In FEC Label-OutXX 47.0.0.0/8 17


Label-In FEC Label-OutXX 47.0.0.0/8 17

Step 1: LSR creates bindingbetween FEC and label value

Step 2: LSR communicatesbinding to adjacent LSR

Step 3: LSR inserts labelvalue into forwarding base

Common understanding of which FEC the label is referring to!

Label distribution can either piggyback on top of an existing routing protocol,or a dedicated label distribution protocol (LDP) can be created

Label distribution can either piggyback on top of an existing routing protocol,or a dedicated label distribution protocol (LDP) can be created

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Label Distribution - Methods

LSR1 LSR2

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

Downstream (unsolicited) 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

LSR1 LSR2

Downstream-on-Demand Label Distribution

Label-FEC Binding

• 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

Request for Binding

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

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#963

#14

#99

#311

#311

#311

Downstream (unsolicited) Label Distribution

#462

D

#311

D

#963D

#14 D

#99D

#216

D

#612 D

#5 D

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#963

#14

#99

#311

#311

#311

Downstream on-demand Label Distribution

#462

D

#311

D

#963D#14 D

#99D

#216

D

#612 D

#5 D

D?

D? D?D?

D?

D?

D?

D?

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Distribution Control: Ordered vs. Independent

Independent LSP ControlIndependent LSP Control Ordered LSP ControlOrdered LSP Control

Next Hop(for FEC)

OutgoingLabel

IncomingLabel

MPLS path forms, as associationsare made between FEC next-hopsand 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

FeaturesFeatures

ComparisonComparison

• 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

• 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

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

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#963

#14

#99

#311

#311

#311

Independent mode

#462

D

#311

D

#963D

#14 D

#99D

#216

D

#612 D

#5 D

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Label Retention Methods

LSR1

LSR2

LSR3

LSR4

LSR5

Bindingfor LSR5

Binding for LSR5

Bindingfor LSR5An LSR may receive label

bindings from multiple LSRs

Some bindings may comefrom LSRs that are not thevalid next-hop for that FEC

Liberal Label Retention Conservative Label Retention

LSR1

LSR2

LSR3

LSR4

Label Bindingsfor LSR5

Valid Next Hop

LSR4’s LabelLSR3’s LabelLSR2’s Label

LSR1

LSR2

LSR3

LSR4

Label Bindingsfor LSR5

Valid Next Hop

LSR4’s LabelLSR3’s LabelLSR2’s Label

• LSR maintains bindings received from LSRs other than the valid next hop

• If the next-hop changes, it may begin using these bindings immediately

• May allow more rapid adaptation to routing changes

• Requires an LSR to maintain many more labels

• LSR only maintains bindings received from valid next hop

• If the next-hop changes, binding must be requested from new next hop

• Restricts adaptation to changes in routing

• Fewer labels must be maintained by LSR

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Liberal retention mode

#462

D

#311

D

#963D

#14 D

#99D

#216

D

#612 D

#5 D

#422D

#622 D

These labels are kept incase they are needed after a failure.

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Conservative retention mode

#462

D

#311

D

#963D

#14 D

#99D

#216

D

#612 D

#5 D

#422D

#622 D

These labels are released the moment they are received.

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Suggested reading

• B. Davie, Y. Rekhter, “MPLS – Technology and Applications”, Morgan Kaufmann, 2000, ISBN 1-55860-656-4.

• E. Gray, “MPLS: Implementing the Technology”, Addison-Wesley, Reading, MA, 2001, ISBN 0-201-65762-7.

Documents

This teaching material is a part of e-Photon/ONe Master study in Optical Communications and Networks Course and module: Author(s): This tutorial is licensed