MPLS

1Telecomm. Dept.Faculty of EEE

Telecom NetworksHCMUT

Telecommunication Networks

Dr. Ing. Vo Que Son

Email: [email protected]



Chapter 5: MPLS NetworksMPLS

DiffServ in MPLS

MPLS TE

FATE schemes

Chapter 6: Traffic EngineeringM/M/1, M/M/c models

M/G/1, M/D/1 models

Queuing networks

Application of queuing theory in Network QoS

Content



Internet core network

Router based core network



Internet core network (cont)

Processing can not meet bandwidth demands

Bottle-neck in software-based routers

Available router interfaces not provide traffic aggregation

Metric-based routing was no longer scalable

Densely connected networks lead to inefficient use of network resources

Destination based routing tends to aggregate all traffic to the same destination: not utilize links




Switch based core network



Internet core network (cont) Faster and simpler forwarding, better traffic aggregation

Fix size cells can be handled in hardware to speed up

Connection-oriented forwarding algorithm improves performance gain: based on short fix length connection identifiers

The ASIC technology : IP packets can be forwarded with high speed. ATM interfaces have even fallen behind the latest increases of optical network (packet over SDH/SONET)

Waste of bandwidth : 5/48 bytes of header.

Complex network management: physical ATM switched infrastructure and logical IP network topology. Each layer uses its own addressing scheme and routing protocol




The n-squared problem : when adding or shutting down any router will create enormous signaling load

IGP stress : intra-domain routing is not conceived for fully meshed topology. With high number of routing peer routers: too much routing information has to be exchanged

Multi-Protocol Label Switching (MPLS) can offer solutions that create a combination of the advantages from both of these worlds



MPLS NGN



MPLS NGN (cont)

Forwarding based on label: speed up processing at node

Forwarding mechanism: label (explicit routing) or IP header (hop-by-hop routing)

Operating on any layer 2 technologies: ATM, Ethernet, FR

Allows for both traffic aggregation and disaggregation

Support VPN : using 64-bit VPN address (total 92 bits)

Allow SPs embed into the IP network : TE and traditional QoSof layer 2 : using DSCP and processing queues based on its packets priority

Easy management and operation



Traditional IP Routing

Choosing the next hopOpen Shortest Path First (OSPF) to populate the routing

table

Route look up based on the IP address

Find the next router to which the packet has to be sent

Replace the layer 2 address

Each router performs these steps



Traditional IP Routing (contd)



Distributing Routing Information

You can reach 145.40 through me

You can reach 125.50 through me

You can reach 125.50 and 145.40 through me

3

1

0125.50

145.40

2

AddressPrefix

AddressPrefix

AddressPrefix

Path Path

Path

125.50

145.40

125.50

145.40

125.50

13

3 0

2



Distributing Routing Information(contd)

3

1

0125.50

145.40

2

AddressPrefix

AddressPrefix

AddressPrefix

Path Path

Path

125.50

145.40

125.50

145.40

125.50

13

3 0

2

Data 125.50.33.85

Data 125.50.33.85



Disadvantages

Header analysis performed at each hop

Increased demand on routers

Utilizes the best available path

Some congested links and some underutilized links!

Degradation of throughput

Long delays

More losses

No QoS

No service differentiation

Not possible with connectionless protocols



Need for MPLS

Rapid growth of Internet

New latency dependent applications

Quality of Service (QoS)

Less time at the routers

Traffic Engineering

Flexibility in routing packets

Connection-oriented forwarding techniques with connectionless IP

Utilizes the IP header information to maintain interoperability with IP based networks

Decides on the path of a packet before sending it



What is MPLS? Multi Protocol supports protocols even other than IP

Supports IPv4, IPv6, IPX, AppleTalk at the network layer

Supports Ethernet, Token Ring, FDDI, ATM, Frame Relay, PPP at the link layer

Label short fixed length identifier to determine a route

Labels are added to the top of the IP packet

Labels are assigned when the packet enters the MPLS domain

Switching forwarding a packet

Packets are forwarded based on the label value

NOT on the basis of IP header information



MPLS Background Integration of layer 2 and layer 3

Simplified connection-oriented forwarding of layer 2

Flexibility and scalability of layer 3 routing

MPLS does not replace IP; it supplements IP

Traffic can be marked, classified and explicitly routed

QoS can be achieved through MPLS



IP/MPLS comparison

Routing decisions

IP routing based on destination IP address

Label switching based on labels

Entire IP header analysis

IP routing performed at each hop of the packets path in the network

Label switching performed only at the ingress router

Support for unicast and multicast data

IP routing requires special multicast routing and forwarding algorithms

Label switching requires only one forwarding algorithm



Key Acronyms

MPLS MultiProtocol Label Switching

FEC Forward Equivalence Class

LER Label Edge Router

LSR Label Switching Router

LIB Label Information Base

LSP Label Switched Path

LDP Label Distribution Protocol



Forwarding Equivalence Class (FEC)

A group of packets that require the same forwarding treatment across the same path

Packets are grouped based on any of the following Address prefix

Host address

Quality of Service (QoS)

FEC is encoded as the label



FEC example

Assume packets have the destination address as

124.48.45.20

143.67.25.77

143.67.84.22

124.48.66.90

FEC 1 label x FEC 2 label y

143.67.25.77 124.48.45.20

143.67.84.22 124.48.66.90



FEC example (contd)

- Assume packets have the destination address and QoS requirements as

124.48.45.20 qos = 1

143.67.25.77 qos = 1

143.67.84.22 qos = 3

124.48.66.90 qos = 4

143.67.12.01 qos = 3

FEC 1 label a FEC 2 label b FEC 3 label c FEC 4 label d

143.67.25.77 124.48.45.20 143.67.84.22 124.48.66.90

143.67.12.01



Example of a MPLS network



Label Edge Router (LER)

Can be an ATM switch or a router

Ingress LER performs the following: Receives the packet

Adds label

Forwards the packet into the MPLS domain

Egress LER removes the label and delivers the packet



Label Switching Router (LSR)

A router/switch that supports MPLS

Can be a router

Can be an ATM switch + label switch controller

Label swapping Each LSR examines the label on top of the stack

Uses the Label Information Base (LIB) to decide the outgoing path and the outgoing label

Removes the old label and attaches the new label

Forwards the packet on the predetermined path



Label Switching Router (contd)

Upstream Router (Ru) router that sends packets

Downstream Router(Rd) router that receives packets

Need not be an end router

Rd for one link can be the Ru for the other

Ru Rd Ru Rd



Label Switched Path(LSP)

LSP defines the path through LSRs from ingress to egress router

FEC is determined at the LER-ingress

LSPs are unidirectional

LSP might deviate from the IGP shortest path



LSP

LSP

Label Switched Path(LSP)



Shim Header A short, fixed length identifier (32 bits)

Sent with each packet

Local between two routers

Can have different labels if entering from different routers

One label for one FEC

Decided by the downstream router

LSR binds a label to an FEC

It then informs the upstream LSR of the binding



Shim Header (contd) EXP field

Also known as Class of Service (CoS) bits

Used for experimentation to indicate packets treatment

Queuing as well as scheduling

Different packets can receive different treatment depending on the CoS value

S bit Supports hierarchical label stack

1 if the label is the bottom most label in the label stack

0 for all other labels



Time To Live (TTL) TTL value decremented by 1 when it passes through an LSR

If TTL value = 0 before the destination, discard the packet

Avoids loops may exist because of some misconfigurations

Multicast scoping limit the scope of a packet

Supporting the traceroute command



TTL (contd)

Shim header

Has an explicit TTL field

Initially loaded from the IP header TTL field

At the egress LER, value of TTL is copied into the TTL field of the IP header

Data link layer header (e.g VPI/VCI)

No explicit TTL field

Ingress LER estimates the LSP length

Decrements the TTL count by the LSP length

If initial count of TTL less than the LSP length, discard the packet



Label: AToM ATM

VCI/VPI field of ATM header

Frame Relay

DLCI field of FR header

PPP/LAN

shim header inserted between layer 2 and layer 3



Label stack

MPLS supports hierarchy

A packet can carry a number of labels

Each LSR processes the topmost label Irrespective of the level of hierarchy

If traffic crosses several networks, it can be tunneled across them

Use stacked labels

Advantage reduces the LIB table of each router drastically



Label stack (contd)



Labels scope and uniquenessLabels are local between two LSRs

Rd might give label L1 for FEC F and distribute it to Ru1

At the same time, it might give a label L2 to FEC F and distribute it to Ru2

L1 might not necessarily be equal to L2

Can there be a same label for different FECs? Generally, NO

BUT no such specification

LSR must have different label spaces to accommodate both

SHIM header specifies that different label spaces used for unicast packets and multicast packets



Invalid labelsWhat should be done if an LSR receives an invalid

label?

Should it be forwarded as an unlabeled IP packet?

Should it be discarded?

MUST be discarded!

Forwarding it can cause a loop

Same treatment if there is no valid outgoing label



Route selectionRefers to the method of selecting an LSP for a

particular FEC

Done by LDP Set of procedures and messages

Messages exchanged between LSRs to establish an LSP

LSRs associate an FEC with each LSP created

Two types of LDP Hop by hop routing

Explicit routing



Route selection (contd)Hop by Hop

Allows each LSR to individually choose the next hop

This is the usual mode today in existing IP networks

No overhead processing as compared to IP

Explicit routing A single router, generally the ingress LER,specifies several

or all of the LSRs in the LSP

Provides functionality for traffic engineering and QoSo Several: loosely explicitly routed

o All: strictly explicitly routed

E.g. CR-LDP, TE-RSVP



Label Information Base (LIB)Table maintained by the LSRs

Contents of the table Incoming label

Outgoing label

Outgoing path

Address prefix Incoming label Address PrefixOutgoing

Path

Outgoing

label



MPLS forwarding

Existing routing protocols establish routes

LDP establishes label to route mappings

LDP creates LIB entries for each LSR

Ingress LER receives packet,adds a label

LSRs forward labeled packets using label swapping

Egress LER removes the label and delivers the packet



FEC in MPLS



MPLS forwarding (contd)

Use label 8 for 145.40

Use label 9 for 125.50

Use label 2 for 125.50 andlabel 1 for 145.40

3

1

0125.50

145.40

2

AddressPrefix

AddressPrefix

Out Path

In Label

Out Path

Out Path

In Label

In Label

Out Label

Out Label

Out Label

125.50 125.50

125.50

145.40 145.40

3

3

2

1

2

1 8

9

9

0

1

2Address

Prefix



MPLS forwarding (contd)

3

1

0125.50

145.40

2

AddressPrefix

AddressPrefix

AddressPrefix

Out Path

In Label

Out Path

Out Path

In Label

In Label

Out Label

Out Label

Out Label

125.50 125.50

125.50

145.40 145.40

3

3

2

1

2

1 8

9

9

0

1

2

Data 125.50.33.85 2

Data 125.50.33.85 9



DiffServ & MPLS

46



DiffServ Architecture



The IETF DiffServ ModelUse 6 bits in IP header to sort traffic into

Behavior AggregatesAKA Classes!

Defines a number of Per Hop Behaviors - PHBs Two-Ingredient Recipe:

Condition the Traffic at the Edges

Invoke the PHBs in the Core

Use PHBs to Construct Services such as Virtual Leased Line!



Defined PHBs

Expedited Forwarding (EF): RFC2598 dedicated low delay queue Comparable to Guaranteed B/W in IntServ

Assured Forwarding (AF): RFC25974 queues 3 drop preferencesComparable to Controlled Load in IntServ

Class Selector: Compat. with IP PrecDefault (best effort)



AF PHB Group Definition

4 independently-forwarded AF classes

Within each AF class, 3 levels of drop priority! This is very useful to protect conforming to a purchased, guarantee rate, while increasing chances of packets exceeding contracted rate being dropped if congestion is experienced in the core.

AF Class 1: 001dd0

AF Class 2: 010dd0

AF Class 3: 011dd0

AF Class 4: 100dd0

Eg. AF12 = Class 1, Drop 2, thus 001100

dd = drop preference



DiffServ Scalability via Aggregation

DiffServ scalability comes from:- aggregation of traffic on Edge- processing of Aggregate only in Core

1000s of flows

Diff-Serv:

Aggregated Processing in Core

Scheduling/Dropping (PHB) based on DSCP

Diff-Serv:

Aggregation on Edge

Many flows associated with a Class (marked with DSCP)



MPLS Scalability via Aggregation

MPLS scalability comes from:- aggregation of traffic on Edge- processing of Aggregate only in Core

1000s of flows

MPLS:

Aggregated Processing in Core

Forwarding based on label

MPLS:

Aggregation on Edge

Many flows associated with a Forwarding Equivalent Class (marked with label)



MPLS & DiffServ - The Perfect Match!

Because of same scalability goals, both models do:- aggregation of traffic on Edge- processing of Aggregate only in Core

1000s of flows

MPLS: flows associated with FEC, mapped into one label

MPLS: Switching based on Label

DS: Scheduling/Dropping based on DSCP

DS: flows associated with Class, mapped to DSCP



DSCP field is not directly visible to MPLS Label Switch Routers (they forward based on MPLS Header)

Information on DiffServ must be made visible to LSR in MPLS Header (using EXP field / Label)

0 1 2 3

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 |

DSCP

IPv4 Packet MPLS Header

Non-MPLS Diff-Serv Domain

MPLS Diff-Serv Domain

DSCP

MPLS:Whats New? The Shim Header



DS o MPLS : Coloring MPLS Frames

This describes how DiffServ information is conveyed to LSRs in MPLS Header

Two methods: E-LSP {{ Cisco IOS 12.1(5)T, 12.0(11)ST }}

Queue inferred from Label and EXP fieldDrop priority inferred from label and EXP

field

L-LSP {{ Planned, once an RFC }}Queue inferred exclusively from Label Drop priority inferred from EXP field



E-LSPs can be established by various label binding protocols (LDP or RSVP)no new Signalling Needed.

Example above illustrates support of EF and AF1 on a single E-LSP (Note: This is the plain old LSP established for MPLS Switching)

Note: EF and AF1 packets travel on single LSP (single label) but are enqueued in different queues (different EXP values)

Queue & Drop Precedence is selected based on EXP

E-LSP

LSRLDP/RSVP LDP/RSVP

EF

AF1

The E-LSP Story...



L-LSPs can be established by various label binding protocols (LDP or RSVP)EXTENSIONS REQUIRED!

Example above illustrates support of EF and AF1 on separate L-LSPs

EF and AF1 packets travel on separate LSPs and are enqueued in different queues (different label values)

Queue selected based on Label, Drop Precedence Selected with Optional EXP field.

L-LSPs

LSR

LDP/RSVP LDP/RSVP

L- LSP Supporting 64 Classes!



MPLS Traffic Engineering

58



The Fish ProblemR8

R1

R5

R2

R3R4

R7R6

IP Uses Shortest Path Destination-Based Routing

Shortest Path May Not Be the only path Alternate Paths May Be under-Utilized while the

shortest Path Is over-Utilized



An LSP Tunnel: (A Constrained MPLS LSP)

R8

R1

R5

R2

R3R4

R7R6

Labels, Like VCIs (ATM) Can Be Used to Establish Virtual Circuits

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

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



LSP Tunnel Setup

22

4917

R8

R2

R6

R3

R4

R7

R1R5

R9

Setup: Path (R1->R2->R6->R7->R4->R9) Tunnel ID 5, Path ID 1

Reply: Communicates Labels and Label OperationsReserves Bandwidth on Each Link

Pop

32



Real-World MPLS TE Use!

POP4

POP

POPPOP

POP2

POP1

WAN area

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

Find route & set-up tunnel for 20 Mb/s from POP1 to POP4



MPLS TE & QoS The Relationship

MPLS TE designed as tool to improve backbone efficiency independently of core QoS techniques: MPLS TE compute routes for aggregates across all PHBs.

A Single Chunk of Bandwidth requested for the Tunnel

MPLS TE performs admission control over a global b/w pool. Un-aware of bandwidth allocated to each Class / PHB

MPLS TE and MPLS DiffServ: Can run simultaneously in a network. Can provide their own individual benefits

TE distributes aggregate load

DiffServ provides differentiation)

Are unaware of each other



LDP: Label Distribution Protocol

Label Distribution Protocol



LDP



Traffic Aggregate in IP networks



Traffic Aggregate in MPLS networks



TE Fast Reroute - Tunneling



TE Global Fast Reroute (Makam)



TE Region Fast Reroute



TE Local Fast Reroute



TE Haskin Fast Reroute



LDP - Advantages Explicit routing

Set up a LSP between Ingress Router and Egress Router

Label request for each hop on down-stream

Label mapping : up-stream

Errors occur: router sends a alarm message to neighbors or operating routers to re-direct for current LSP

Less resources (compared with RSVP)



LDP - Disadvantages Slow error recovery

Not support dynamically re-optimization of traffic flows

Transient periods: efficiency of Resource Location could be influenced by routing traffic.

Require means to restore the LSP to the original routes once congestion has subsided

FATE, FATE+ : using dynamic reroute mechanism

FATE++ : combination of 2 above algorithms with improvements



Weighted Fair Queue Monitoring packets

Classifier

10-6

10-4

Scheduler

Output linkIP LIP L

IP L

Input link

IP L

Output buffer

Unsuccessful insertion. Destroy packet.

Successfully inserted packets

per-class buffering

Template

Management

CCCDCCCDCCCD

Buffer ID

C

D



FATE - Fast Acting Traffic Engineering

At each LSR:

If packet loss: calculate packet loss in buffer

If this over pre-defined threshold, generate a Congestion Indicator (CIN) message to Ingress Router on upstream

Set up a timer for CIN regeneration

CIN: Congestion Indication Notification



FATE Mechanism (cont)

At other LSRs on up-stream: when receiving CIN

If timer not expired, forward CIN without appending infor

Otherwise, add its congested info before forwarding

At Ingress LSR: when receiving CIN message

Decide that the packet loss it is currently experiencing remains sufficiently low for it to continue to meet its SLA requirements, allowing no further action to be taken

Renegotiate for a new requirements along existing LSP (higher priority buffer along same path, within same LSR)

Negotiate for a new quality requirements along an alternative LSP



FATE CIN information

Identity of the LSP that is experiencing congestion

The LSRs this loss is occurring in

The current loss in the buffers the LSP is traversing

The current loss of the LSP



FATE Scalability

How to monitor thousands of LSPs ?

How a MPLS network applies congestion control mechanism in a situation of numerous flows?

How can SPs ensure the customers SLA is met while traversing this network?

Solutions

In a single domain: exchanging messages can be easily handled under the control of Ingress and Egress LERs

In a Generalized MPLS domain : assign Virtual Source/Virtual Destination (VS/VD) of each domain using label stacking



FATE Label stacking



FATE Advantages/Disadvantages Fast acting mechanism

Allow individual LSPs to be dynamically remapped to QoS buffers

Provide a notification from congested LSR to Ingress LER

Can not make decision about the current LSP

Suitable in strictly routed LSPs



FATE+ - Extension of FATE Remove generation of CIN message

Redistributing traffic flows while fully utilizing all available LSRs and links

Ensure the customers SLAs are met

Suitable in loosely routed LSPs

Scale well in large WANs

Permit the congested LSR to make decisions :

Transfer the CR-LSP on to a higher buffer stream

Re-route the CR-LSP via an alternative downstream LSR

Re-route the CR-LSP via an alternative upstream LSR



FATE+ - Transfer LSP to a higher buffer stream



FATE+ - Reroute via a alternative downstream LSR



FATE+ - Reroute via a alternative upstream LSR



FATE+ - Disadvantages

Loop packets

Can not reroute when:

Finding a new LSP via downstream LER but no resources

Congested LER and its neighbors can not reroute but other LERs can



FATE+ - Loop packets



FATE+ - No resources



FATE++ - Dual FATE and FATE+

MPLS

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