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Optical Core Networks MPLS - basics. Piero Castoldi, Scuola Superiore Sant’Anna, [email protected] 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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Optical core networks mpls basics

Optical Core NetworksMPLS - basics

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


Outline

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


What is mpls

What is MPLS?

Hybrid

Packet Routing

Circuit switching

MPLS + IP

ATM

IP

  • 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)


Optical core networks mpls basics

Multi Protocol Label Switching

(MPLS) fundamentals


Label substitution what is it 1

“Label Substitution”, what is it? (1)

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

  • 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”.


Label substitution what is it 2

Label Substitution, what is it? (2)

LANE#1 TURN RIGHT USE LANE#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


A label by any other name

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) ..


Optical core networks mpls basics

What is a “LABEL”?

A property that uniquely identifies a flowon a logical or physical interface


Optical core networks mpls basics

#3

IP

#7

#99

#9

#4072

IP

Label Switched Path (LSP)

#3 Right #7

#7 LEFT #99

#99 RIGHT #9

#9 LEFT #4072


Optical core networks mpls basics

IP

IP

Optical or Generalized Label Switched Path (G-LSP)

RED RIGHT BLUE

RED

BLUE

BLUE LEFT WHITE

WHITE RIGHT ORANGE

WHITE

ORANGE

ORANGE LEFT RED

RED


Label concept

Label concept

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.

Value

Exp

S

TTL

IP packet

MPLS label

Value:Label value 20 bits

Exp:Experimental Use, 3 bits

S:Bottom of stack, 1 bit

TTL:Time To Live, 8 bits

Total: 32 bit = 4 byte

! Packets are switched, not routed, based on labels


Basic operation

Basic operation

#L2

#L3

IP1

IP1

LER

LER

LSR

LSR

IP1

#L1

IP1

IP1

IP forwarding

IP forwarding

Label Switching

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


Fec forwarding equivalence class

FEC Forwarding Equivalence Class

IP1

#L2

#L3

IP1

IP1

#L2

#L3

IP2

IP2

IP2

LSR

LER

LER

LSR

IP1

#L1

IP1

#L1

IP2

IP2

Packets are destined for different address prefixes, but can be

mapped to common path

  • 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


Label stacking

Label stacking

L3

L2

L1

IP

  • 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

MPLS Domain 1

MPLS Domain 2

MPLS Domain 3


Mpls features

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 stacking

    • Increase the scalability

  • Constraint-based routing

    • Traffic Engineering

    • Fast reroute

  • Facilitate the virtual private networks (VPNs)

  • Enables Traffic Engineering and QoS

    • Provides an opportunity for mapping DiffServ fields onto an MPLS label

  • Facilitate the elimination of multiple layers

    • Resolve the problems of IP over ATM, in particular: Complexity of control and management and scalability issues


So what is mpls

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


Routers do both routing and switching

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?


Mpls flexible forwarding

MPLS: Flexible Forwarding

IP

IP

IP

IP

IP

DA

DA

DA

DA

DA

IP

#L1

IP

#L2

IP

#L3

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

IP

IP to LSP

LSP to IP

LABEL SWITCHING


Mpls based solutions

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


Mpls terminology

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


Label switched path lsp

Label Switched Path (LSP)

#14

#311

#216

#99

#311

#963

#311

#963

#14

#612

#462

#311

#99

#5

- 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.


Topology dissemination in standard ip

Topology dissemination in standard IP

3

47.1

1

2

1

3

2

1

47.2

3

47.3

2

  • Destination based forwarding tables

  • as built by OSPF, IS-IS, RIP, etc.


Ip forwarding using hop by hop control

IP forwarding using hop-by-hop control

IP 47.1.1.1

47.1

1

IP 47.1.1.1

2

IP 47.1.1.1

1

3

2

IP 47.1.1.1

1

47.2

3

47.3

2


Mpls label distribution use case

MPLS Label Distribution use-case

Request: 47.1

Request: 47.1

Mapping: 0.50

Mapping: 0.40

1

47.1

3

3

2

1

1

2

47.3

3

47.2

2


Label switched path lsp1

Label Switched Path (LSP)

IP 47.1.1.1

IP 47.1.1.1

1

47.1

3

3

2

1

1

2

47.3

3

47.2

2


Benefits and limitations

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


Explicited routed lsp or er lsp 1

Explicited RoutedLSP or ER-LSP (1)

Route={A,B,C}

#972

#14

#216

#14

#972

#462

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

B

C

A


Optical core networks mpls basics

IP 47.1.1.1

IP 47.1.1.1

Explicited RoutedLSP or ER-LSP (2)

1

47.1

3

3

2

1

1

2

47.3

3

47.2

2


Er lsp advantages

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)


Er lsp discord

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.


Optical core networks mpls basics

Label encapsulation


Label encapsulation

Label Encapsulation

IP or other non-IP PAYLOAD

“Shim Label” …….

VPI

VCI

DLCI

“Shim Label”

λ

Label

ATM

FR

Ethernet

PPP

Optical

Medium

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.


Mpls link layers

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 above


Mpls encapsulation ppp lan data links

MPLS Encapsulation - PPP & LAN Data Links

MPLS ‘Shim’ Headers (1-n)

•••

n

1

Network Layer Header

and Packet (eg. IP)

Layer 2 Header

(eg. PPP, 802.3)

4 Octets

Label Stack

Entry Format

TTL

Label

Exp.

S

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

  • 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 encapsulation atm

MPLS Encapsulation -> ATM

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

5 Octets

ATM Header

Format

VPI

VCI

PT

HEC

CLP

Label

Option 1

Label

Combined Label

Option 2

Option 3

ATM VPI (Tunnel)

Label

AAL-5 PDU Frame (nx48 bytes)

•••

n

1

Network Layer Header

and Packet (eg. IP)

Generic Label Encap.

(PPP/LAN format)

AAL-5 Trailer

ATM

SAR

48 Bytes

48 Bytes

ATM Header

• • •

ATM Payload

  • 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


Mpls encapsulation frame relay

MPLS Encapsulation -> Frame Relay

Generic Encap.

(PPP/LAN Format)

Q.922

Header

Layer 3 Header and Packet

•••

n

1

C/

R

FE

CN

E

A

BE

CN

D

E

E

A

DLCI Size = 10, 17, 23 Bits

DLCI

DLCI

  • 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


Optical core networks mpls basics

Label distribution

methods


Label distribution protocol ldp purpose

Label Distribution Protocol (LDP) - Purpose

Label distribution ensures that adjacent routers have

a common view of FEC <-> label bindings

Routing Table:

Addr-prefix Next Hop

47.0.0.0/8 LSR3

Routing Table:

Addr-prefix Next Hop

47.0.0.0/8 LSR2

LSR1

LSR3

LSR2

IP Packet

47.80.55.3

Label Information Base:

Label-In FEC Label-Out

XX 47.0.0.0/8 17

For 47.0.0.0/8

use label ‘17’

Label Information Base:

Label-In FEC Label-Out

17 47.0.0.0/8 XX

Step 2: LSR communicates

binding to adjacent LSR

Step 3: LSR inserts label

value into forwarding base

Step 1: LSR creates binding

between FEC and label value

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 methods

Label Distribution - Methods

Label Distribution can take place using one of two possible methods

Downstream-on-Demand Label Distribution

Downstream (unsolicited) Label Distribution

LSR2

LSR1

LSR2

LSR1

Label-FEC Binding

Request for 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

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

Both methods are supported, even in the same network at the same time

For any single adjacency, LDP negotiation must agree on a common method


Downstream unsolicited label distribution

Downstream (unsolicited) Label Distribution

#14

#311

#216

#99

#311

#963

#311

D

#963

#14

#612

D

#462

D

D

D

#311

#99

#5

D

D

D


Downstream on demand label distribution

Downstream on-demand Label Distribution

#14

#311

#216

#99

#311

#963

#311

D

D?

D?

#963

#14

D?

D?

#612

D

D?

#462

D

D?

D

D

#311

#99

#5

D

D

D

D?

D?


Distribution control ordered v s independent

Distribution Control: Ordered vs. Independent

Next Hop

(for FEC)

MPLS path forms, as associations

are made between FEC next-hops

and incoming and outgoing labels

Incoming

Label

Outgoing

Label

Independent LSP Control

Ordered LSP Control

  • 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

Features

  • 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

  • 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

  • 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

Comparison

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


Independent mode

Independent mode

#14

#311

#216

#99

#311

#963

#311

D

#963

#14

#612

D

#462

D

D

D

#311

#99

#5

D

D

D


Label retention methods

Label Retention Methods

Binding

for LSR5

LSR2

An LSR may receive label

bindings from multiple LSRs

Some bindings may come

from LSRs that are not the

valid next-hop for that FEC

LSR1

LSR5

Binding for LSR5

LSR3

Binding

for LSR5

LSR4

Conservative Label Retention

Liberal Label Retention

LSR2

LSR2

Label Bindings

for LSR5

Label Bindings

for LSR5

LSR1

LSR1

LSR3

LSR3

LSR4’s Label

LSR3’s Label

LSR2’s Label

LSR4’s Label

LSR3’s Label

LSR2’s Label

LSR4

LSR4

Valid

Next Hop

Valid

Next Hop

  • 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


Liberal retention mode

Liberal retention mode

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

#216

D

D

#963

#14

#622

#612

D

#462

D

D

D

D

#311

#422

#99

#5

D

D

D


Conservative retention mode

Conservative retention mode

These labels are released the moment they are received.

#216

D

D

#963

#14

#622

#612

D

#462

D

D

D

D

#311

#422

#99

#5

D

D

D


Suggested reading

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.


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