Multi-Protocol Label Switching: Basics and Applications

Multi-Protocol Label Switching:Multi-Protocol Label Switching:Basics & ApplicationsBasics & Applications

Dr. Vishal Sharma Email: [email protected] Web: http://www.metanoia-inc.com

Metanoia, Inc.Critical Systems Thinking™

© Copyright 2002-2005All Rights Reserved

MPLS Seminar, MTNL CETTM, Mumbai, 26th April 2005 2


Copyright 2002-2005All Rights Reserved

The Start: Routing Process at a Router

Destination address (DA) based forwarding

Longest prefix matching

Routing Table

DA=my_add or DA= IP brdcst add. ?

RT entry = complete DA?

RT entry = Destn. n/w id?

Default entry exists?

No

No

No

No

Yes

Yes

Yes

Yes

Deliver datagram to protocol module (TCP/UDP) specified in IP hdr.

Send pkt. to next-hop router or to directly connected interface.

Send pkt. to next-hop router or to directly connected interface.

Send pkt. to next-hop router.

Datagram undeliverable. (Use ICMP to inform source.)

Receive incoming pkt.

DA Next hoprouter

NetworkInterface

Host entry 198.168.7.3 X 2

Host entry 198.168.7.4 X 3

Host entry 198.168.7.1 198.168.7.5 1

Host entry 198.168.7.2 198.168.7.5 1

N/w entry 198.100.x.x 198.100.9.1 4

N/w entry 128.72.x.x 128.72.55.4 5

Default x.x.x.x 128.84.73.1 6




How Routing Works Today

How do routers build their routing tables?

By exchanging information with each other using routing protocols

DA Next hoprouter

N/wInt.

Host entry 198.168.7.3 X 2

Host entry 198.168.7.4 X 3

Host entry 198.168.7.1 198.168.7.5 1

Host entry 198.168.7.2 198.168.7.5 1

N/w entry 198.100.x.x 198.100.9.1 4

N/w entry 128.72.x.x 128.72.55.4 5

Default x.x.x.x 128.84.73.1 6

Routing table (RT) at 198.168.7.6

Longest prefix match gives next hop router as 198.100.9.1 and outgoing interface as 4.

198.100.9.75

198.100.9.75

198.100.9.75

198.100.9.75

198.100.9.75198.168.7.4

198.168.7.3

198.168.7.1

198.168.7.2

198.168.7.5

198.168.7.6

198.100.x.x

198.100.9.1

128.72.x.x

128.72.55.4

128.84.x.x

128.84.73.1

23

4

56

1

198.100.9.75

DA = 198.100.9.75Packet generated




How it Would be with Labels

How do routers learn the labels?

By interpreting routing information and through signaling (as we will learn later)

DA = 198.100.9.75Packet generated Exact matching label

swapping gives outgoing label as and outgoing interface as 4.

Incominglabel

Outgoinglabel

Address prefix N/wInt.

X 2

198.100.x.x 4

128.72.x.x 5

Label Forwarding Table at 198.168.7.6

198.168.7.4

198.168.7.3

198.168.7.1

198.168.7.2

198.168.7.5

198.168.7.6

198.100.x.x

198.100.9.1

128.72.x.x

128.72.55.4

128.84.x.x

128.84.73.1

23

4

56

1

198.100.9.75

Attach label




Shortest-Path Routing: Little Flexibility

Shortest path converges traffic on a few network links

Significant increase in congestion

Unbalanced resource utilization

DA Next hoprouter

N/wInt.

Host entry 198.168.7.4 X 3

Host entry 198.168.7.1 198.168.7.5 1

Host entry 198.168.7.2 198.168.7.5 1

N/w entry 198.101.x.x 198.168.7.4 3

N/w entry 198.100.x.x 198.100.9.1 4

N/w entry 128.72.x.x 128.72.55.4 5

Default x.x.x.x 128.84.73.1 6

Routing table (RT) at 198.168.7.6

198.168.7.4198.168.7.1

198.168.7.2

198.168.7.5

198.168.7.6

198.100.x.x

198.100.9.1

128.72.x.x

128.72.55.4

128.84.x.x

128.84.73.1

3

4

56

1

198.100.9.75

198.101.84.21

R1

R2

R3

R4




Labels De-couple Routing and Forwarding: Much more Flexibility

Labels enable:

Differentiation based on criteria other than shortest path

Permit policy routing

198.168.7.4198.168.7.1

198.168.7.2

198.168.7.5

198.168.7.6

198.100.x.x

198.100.9.1

128.72.x.x

128.72.55.4

128.84.x.x

128.84.73.1

3

4

56

1

198.100.9.75

198.101.84.21

Incominglabel

Outgoinglabel

Address Prefix N/wInt.

X 2

198.101.x.x 4

198.101.x.x 3

Label Forwarding Table at 198.168.7.6

R3

R2

R1

R4




Basic Concept of MPLS

Routing fills routing table

Signaling fills label forwarding table

DA Next hop router

N/w Int.

128.89.10.x 198.168.7.6 1

179.69.x.x 198.168.7.6 1

128.89.10.x

1

179.69.x.x

21

128.89.10.12

179.69.42.3

198.168.7.6

Inlabel

Outlabel


Advertises binding<5, 128.89.10.x>

Advertises binding<7, 179.69.x.x>

128.89.10.x 5 1

179.69.x.x 7 2

Advertises bindings<3, 128.89.10.x> <4, 179.69.x.x>

128.89.10.x 3 1

179.69.x.x 4 1

3

4

X

X

DA Next hop router

N/w Int.

128.89.10.x 128.89.10.1 1

179.69.x.x 179.69.42.3 2

Routing Table

Inlabel

Outlabel


Label Table

R1 R2

R3

R4




Basic Concept of MPLS

128.89.10.x

1

179.69.x.x

21

128.89.10.12

179.69.42.3

198.168.7.6

Inlabel

Outlabel


Inlabel

Outlabel


128.89.10.x 5 1

179.69.x.x 7 2128.89.10.x 3 1

179.69.x.x 4 1

3

4

X

X

3

5

Packet arrives DA=128.89.10.25

3Push Label

5Pop label

Forward packet

553

Swap Label

R2R1

R3

R3 R4




A Word on Network Layer Routing

Control Plane

Forwarding/Data Plane

Control ComponentResponsible for construction and maintenanceof forwarding table. Consists of:• Routing protocols for exchange of routing info.

• Algorithms to convert this into forwarding table

Forwarding/data ComponentAlgos. used to make forwarding decision on packet

The algorithms define: • Information from packet used to find an entry in the forwarding table

• Exact procedures used to find that entry For unicast routing … Information = Network layer (IP) address Procedure = Longest prefix matching




So What about MPLS Control and Forwarding?

Superset of conventional router control

Distribute routing info. via network layer routing (OSPF, BGP, etc.)

Algos. to convert routing info. into forwarding table for fwding component

Create binding from FEC (derived from routing info.) --> label

Assign and distribute labels to peer LSRs via signaling

Uses a label switching forwarding table (or LIB), looking as:

Forwarding algorithm = label swapping, independent of control component (implementable in optimized hardware or software)

ControlComponent

ForwardingComponent

First Subentry Second Subentry(for multicast or load balancing)

Incoming Label Map

Next hop label forwarding entry (NHFLE)

Outgoing labelOutgoing inf.Next hop address

Outgoing labelOutgoing inf.Next hop address

Incoming Label




What does a label represent? The issue of label granularity

Packets treated identically by participating routers form Forwarding Equivalence Class (FEC

Assigned the same label

Membership of a FEC must be determinable from IP header

Info. that ingress router has about the packet

Entities grouped into a FEC are flexible, and could involve A connection between two IP ports on two hosts

All traffic between two IP hosts

All traffic headed for a particular network with same TOS bits

All destination networks with a certain prefix

All traffic headed to a particular router (e.g. an egress)

A manually configured connection … and many others




Let’s Recap: Elements of MPLS Label Forwarding

Use data link addressing, e.g. ATM VPI/VCI, FR DLCI Put “shim” header between data link and IP header

Label Creation and Binding

Label Assignment and Distribution Ride piggyback on routing protocols, where possible (BGP) Use separate label distribution protocol – RSVP, LDP/CR-LDP

Reliability: TCP or separate ACK/NACK

Variable

L2 header L3 IP header MPLS “shim” header

Higher Layers

4 bytes 20 bytes

Label CoS TTL S

20 bits 3 bits 8 bits

Data Plane

Control Plane

EXP/

1 bit




Benefits over Conventional Routing

MPLS forwarding possible by: Switches incapable of analyzing network layer headers Unable to do so at adequate speeds

Ingress can use any info. about packet to assign to FEC/LSP Conventional forwarding only considers info. in the packet

Forwarding decisions can depend on ingress router Conventional routing, identity of ingress router does not travel with packet

Packet FEC assignment can use complex decision process No impact on forwarding of labeled packets!

Explicit routing packet need not carry encoding of entire route Unlike “source routing” in conventional IP forwarding




MPLS Header over POS or IEEE 802.3

Label(20 bits)

TTL(8 bits)

EXP(3 bits)

S(1 bit)

4 octets

MPLSShim

Header

IPHeader

IP PayloadLayer 2 Hdr(e.g., PPPor 802.3)

For labeled packets, Layer 2 header indicates whether it is MPLSunicast packet or MPLS multicast packet

The label stack: sequence of 4-octet label stack entries (no limiton stack depth)

Network layer packet immediately follows the label stack entry thathas the S bit set. Label values 0 -->16 are reserved




MPLS Header over ATM

Top stack of shim carries placeholder label value of 0. VPI/VCI value in headerrepresent actual label value (no SNAP/LLC encapsulation used)

Upstream LSR connected to first ATM-LSR adjusts TTL value based on howmany ATM-LSRs are consecutively connected downstream (learnt via LDP)

For ATM LSRs, UNI gives 24-bit VPI/VCI label, NNI gives 28-bit VPI/VCI label If two ATM-LSRs connected via VPC through ATM cloud, 16-bit VCI label used

Label=0(20 bits)

TTL(8 bits)

EXP(3 bits)

S(1 bit)

4 octets

MPLSShim

Header

IPHeader

IP PayloadAAL5 Trailer(length, CRC-

32, ...)

ATMHeader

ATMPayload

ATMHeader

ATMPayload

48 octets 48 octets




Label Assignment and Distribution (Control Component)

Downstream Upstream

Ordered Solicited (On Demand)Unsolicited

SolicitedUnsolicited

Independent Solicited (On Demand)Unsolicited

SolicitedUnsolicited

Direction from which labels flow

Refers to whether LSR distributes labels on demand or voluntarily

Whether LSR waits to hear from its upstream/downstream nbrs. before responding to a requestfor label(s)

Label Retention: Liberal or Conservative

Whether LSR keeps labels from a neighbor who is not currently the next hop for a FEC

Labels

Data

Labels

Data




Example Label Assignment and Distribution Modes

4

33’

Edge LSR

Edge LSR

Downstream-on-demand with Independent Control

1 Requests

2

2’Assignments

Edge LSR

2

35

6

Edge LSR

Downstream-on-demand with Ordered Control

1 Requests

4

Assignments




Comparison of ATM Switch, IP Router, LSR, and Optical X-connect

ATM switch

IP router LSR OXC

Control Plane

Dynamic routing protocol for route exchange

PNNI BGP, OSPF, IS-IS, RIP

BGP, OSPF, IS-IS, RIP

OSPF, IS-IS

Signaling protocols

UNI, PNNI RSVP LDP/CR-LDP, extended RSVP

LDP/CR-LDP, extended RSVP

Data Plane

Forwarding “Engine”

ASICs Software, ASICs

Software, ASICs

ASICs

Switched entity ATM SVC, PVC.

IP packets or flows

LSPs SONET channels, Wavelengths, fibers




More on the MPLS Control Plane: Hop-by-hop Routed LSPs

LSPs whose routes are determined by IP routing protocols

Shortest path, based on destination IP address of packet

Effectively creates labels for each route in forwarding table

Label distribution for hop-by-hop routed LSPs

LDP (Label Distribution Protocol)

Defined by IETF MPLS Working Group

LDP messages: Notification, Hello, Initialization, KeepAlive, Address, Address Withdraw, Label

Mapping, Label Request, Label Withdraw, Label Release

Peer discovery msgs. over UDP, rest over TCP for reliability

Piggyback on existing IP routing protocols

Example: Add label information to BGP

Not all IP interfaces may be enabled for dynamic routing protocols




Hop-by-hop Routed LSP Setup using LDP

Edge LSR

Edge LSR

Label Req.

Label Req.

Label Req.

Label Mapping.

Label Mapping.

Label Mapping.

LSR1 learns new IP network prefix 1.1.1.0/24 via dynamic IP routing

• Each LSR forwards Label Req. along hop-by-hop routed path to 1.1.1.0/24

• Path established via a dynamic IP routing protocol

• When next hop to 1.1.1.0/24 changes in LSR2 (e.g. due to topology or link metric change)

• LSR2 releases original LSP• Starts setting up new LSP from that point on

• Several other options available

1.1.1.0/24.




ER-LSPs: Explicitly Routed LSPs

Routes determined by operators or n/w management apps

Based on specific TE policy, QoS, or VPN membership

Significantly more efficient than conventional IP source routing

Label distribution for ER-LSPs

Extended RSVP (significantly different from original RSVP)

Associates labels with RSVP flows, supports aggregate flows

Control messages run on raw IP transport, requiring refreshes

CR-LDP (Constraint-based Routed LDP)

Now mostly of historical value




Strict ER-LSP Setup using CR-LDP

Edge LSR

Edge LSR

Label Req.<1.1.1.2, 2.2.2.2, 3.3.3.2>

Label Req.<2.2.2.2, 3.3.3.2>

Label Req.<3.3.3.2>

Label Mapping

Label Mapping

Label Mapping

Network operator or network management creates ER-LSP request with path and traffic parameters

• Traffic parameter TLV contains: • Frequency, weight • Peak data rate, Peak burst rate• Committed data rate, committed burst rate, excess burst size

• Frequency specifies granularity at which CDR is made available

• Weight determines excess bandwidth possible above CDR

1.1.1.2 2.2.2.2

3.3.3.2




Loose ER-LSP Setup using CR-LDP

Edge LSR

Edge LSR

Label Req.<as100, 3.3.3.2> Label Req.

<as100, 3.3.3.2>

Label Req.<3.3.3.2>

Label Mapping.

Label Mapping.

Label Mapping.

Network operator or network management creates ER-LSP request with path and traffic parameters

4.4.5.6 4.4.5.7

3.3.3.2

AS100




Are there any implications for hardware/ASICS?

Label stacking depth (if any) supported depends on hardware processing capabilities and speeds

Hardware engine needs ability to examine both EXP bits and LABEL, and map it to any control hardware used for scheduling MPLS packets

Ability to push and/or pop labels determines whether switch can be an edge LSR, or only a core LSR (doing only swapping)

Number of queues in the switch/router determines per-label queueing or per-class queueing ability

Label merging capability determined by ability to re-assemble packets from interleaved cells




Advantages of MPLS

Original justification was fast, amortized, ATM hardware

Eliminated by hardware forwarding engines at multi-gigabit rates

Current justifications include:

Separates forwarding from control, enabling

Evolution of routing functionality independently of forwarding algorithm (which can continue to be label swapping)

Use of MPLS to control non-packet technologies like SONET/SDH channels or optical light-paths

Facilitates scalable hierarchical routing (via label stacking)

Scalability by reducing number of IP peers/neighbors

Provides explicit, manageable IP routes: enables policy routing and traffic engineering (can setup routes different than default shortest-path)




Reducing number of IP Peers

• VCs between routers connected over ATM network

• O(n^2) VCs for full adjacency

• O(n^4) routing info. exchange

overwhelms routers and network

LSR (runsIP routing)

IP routing peers

• Interior switches participate in IP routing protocols minimizes IP nbrs.

• Eliminates full VC mesh for adjacency, as LSRs run IP routing protocols

Router

ATM Switch

IP routing peers

ATM Network




Hierarchical Label Stacking/Switching

Inside transit AS each interior router must keep track of all networks reachable through it

With hierarchical labels, an arrangement is possible where only Border Routers need to know what networks might eventually be reached through them

All transit traffic can tunneled through interior routers of the AS using LSPs with stacked labels




Utility of Hierarchical Label Switching

Interior LSRs

Border LSRs

Swap and Push Pop

Swap




Explicit Manageable Routes -- Policy Routing, Traffic Engineering

Carriers want certain traffic to go over certain routes This type of network engineering:

Keeps network loads balanced

Enhances network stability and reliability

Enables better QoS and performance assurances

Allows carriers to meet SLAs

Constraint-based routing + MPLS

Allows carriers to bind specific traffic to an LSP

Place (or route) LSP over a desired sequence of LSRs




Constraint Based Routing

A class of routing systems that computes routes through a network subject to a set of constraints and requirements

QoS-based Routing

Path of flows determined by

Knowledge of resource availability in network

QoS requirements of flows

Policy-based Routing

Path/routing decision based on administrative policy

Can be on-line or off-line




CB Routing System

Inputs

Flow/path attributes: required b/w, hop count, ...

Resource attributes: properties of nodes/links

Network topology & state

Outputs

Computed feasible path

Explicit route of the path

Constraint-BasedRouting Process

Attributes

Resources

Topology

Feasible PathERO {1,3,4,5}

1

3

4

5

2




TE Topology versus Regular Routed Topology

A

B

C D

E

Network Diagram

Regular Routed Topology

A

B

CD

E

4

11

1

3

2

3

Link weights

A

B

CD

E

Traffic EngineeringTopology

OC-3

OC-12

OC-192OC-12

DS3

Best effort shortestpath from D to E

TE Path from D Eavoiding green links

with at least STS-3 b/w




LSP ID = L2

Automatic Reroute Using MPLS RSVP-TE

Rerouting is done when

A better path is available

Upon failure along LSP

Use SESSION Obj. & SE style

Tunnel uniquely identified by

Destination IP address

Tunnel ID

Ingress IP address

Tunnel ingress made to appear as 2 different senders to the RSVP session (via LSP ID)

Src

Rcvr

LSP ID = L1

On these links theLSPs share resources

Tunnel ID inSession Obj

Originates LSPswith IDs 1 and 2

Here they are treated as differentLSPs within the same Session

LSPs 1 and 2 have a common SESSION Obj, buta new LSP ID in the SENDER_TEMPLATE and adifferent ERO (with possibly common hops)




So what did we look at? Let’s summarize … Looked at conventional IP routing and its limitations

Saw how labels decouple data plane from control plane

Examined basics of MPLS

Control and forwarding components

Label granularity (forwarding equivalence class, FEC)

Benefits over conventional routing

Label assignment and distribution methods

Downstream-on-demand, with ordered or independent control

Hop-by-hop routed LSPs, strict- and loosely explicitly-routed LSPs

Advantages of MPLS – efficient hierarchical routing, reduces number of IP peers, facilitates explicit routing

Use of MPLS for traffic engineering, protection, automatic rerouting

Business

Multi-Protocol Label Switching: Basics and Applications