multiprotocol label switching

Engineering Internet QoS 1

Multiprotocol Label Switching


Overview

L3 Switching vs RoutingWhat is MPLS?MPLS on ATM: IssuesTraffic Engineering Using MPLSMPLS Simulation Results


Introduction

MPLS stands for “Multiprotocol” label switching• techniques applicable to any L3 protocol

A router supporting MPLS is know as Label Switched Router (LSR)

IP routing/control software is combined with label swapping • similar to ATM VCI/VPI swapping


Proprietary Protocols

The Cell Switching Router (CSR)• ftp://ftp.wide.toshiba.co.jp/pub/csr/

Ipsilon’s IP Switching• www.ipsilon.com

Cisco’s Tag Switching• www.cisco.com

IBM’s Aggregate Route-Based IP Switching (ARIS)• www.networking.ibm.com/isr/ip/

IETF MPLS working group• www.ietf.org/html.charters/mpls-charter.html


Why label switching

Scalablity of L3 routing and simplicity of L2 forwarding• high performance• use of ATM hardware• faster than IP lookup• integration of IP and ATM

new Services such as VPNshierarchy of Routing


Terminologies

flow : a single instance of an application to application flow of data

forwarding equivalence class: a group of IP packets which are forwarded along same path (or same treatment)

label: a short fixed length physically contiguous identifier which is used to identify a FEC (local significance)

label Switched Router: an MPLS node which is capable of forwarding L3 packets


MPLS Domain

LSR: label switching router

LER: label edge router

Reprinted with Permission from “Engineering Internet QoS - Jha & Hassan, Artech House Publishing, Norwood, MA, USA. www.artechhouse.com

http://www.artechhouse.com/




Conventional IP Routing

packet travels from one router to the next • an independent forwarding decision made at

each hop

At each hop packets assigned to a Forwarding Equivalence Class (FEC)• packets considered to be in same FEC if the

routing table contains some prefix X such that X is the longest match for each packet’s destination address


MPLS Approach

assignments of packets to FEC done once• as the packet enters the network

FEC to which packet is assigned is encoded with a label

packet is forwarded to next hop with label• no further analysis of packet header at

next hop


Label encoding

MPLS encapsulation should contain • the label stack field• time-to-live (TTL) field• a 3 bit Experimental field (earlier

COS)MPLS uses 32 bits for label encoding

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

label exp s ttl


Reserved Labels

Multiprotocol = IP, IPX, etc. Protocol is inferred from the last label in the stack.

0 = IPv4 Explicit Null Label Pop this last label. Use IPv4 header.

1 = Router Alert Label Looked by the router software.Packet forwarded based on the next label in stackSimilar to “Router Alert Option” in IP Packets

2 = IPv5 Explicit Null Label 3 = Implicit Null Label

Used only for assignment and distributionShould not appear in any label stack in the packets


TTL Handling

At the beginning TTL = TTL from IP headerAt every hop: TTL = TTL -1Drop the packet if TTL=0At the exit: TTL in IP Header = TTL from

labelIn some cases, entire MPLS domain may

be considered one hop, e.g., ATM


MPLS Encapsulation

For ATM VPI/VCI field and for Frame Relay DLCI field used

a “shim” layer between link layer and network layer headers for non-ATM/Frame Relay network• protocol independent• shim layer may consist of sequence of label

stack entries• If PPP used, protocol field may identify

frames that carry labels


Label Processing

label used as index into a table which specifies next hop and a new label• longest match calculation eliminated at

subsequent hops old label replaced with new label and packet

forwarded to next hop label may also contain class of service (COS)

• for scheduling/ discarding packets etc • again saves header processing


Label Distribution

Who assigns labels for communication between A and B?• A, B, or someone else?• Downstream, upstream, ...

Where is the control for the entire path?A, B, ingress or egress LSR?

Separate protocol or existing route distribution mechanisms?• IETF’s Label Distribution Protocol (LDP)

based on Cisco’s TDP and IBM’s ARIS protocol


Sample Network Topology






Routing Tables

17

18

19

Table at R2

162.25.8 1

192.35.10 0Indexof table

Table at R4

162.25.8 1

192.35.10 1






Label Binding Example






Label allocation

Table at R4

162.25.8

In label

?

?

out label

17

18

Addr prefix

192.35.10

Interface

1

1

Table at R2

162.25.8

In label

17

18

out label

19

16

Addr prefix

192.35.10

Interface

0

1






Label Switching






Hierarchy Routing

IGP Routers (R3, R4) in transit domain A maintain all the routes providedby Interdomain routing

- necessary to forward transit traffic to/from domains B & C

BGP: Border Gateway Protocol (inter-domain)IGP: Interior Gateway Protocol (intra-domain)






Hierarchy Routing

BGP entriesinlabel

outlabel

nexthop

R1

R2

R5

R6

….

….

….

….

21

31

4131

41

21 R5

R6

IGP entriesinlabel

outlabel

nexthop

R3

R4

R5

R5

?

?

52

52 62

7262

72

R2

R4

R3

R5






Stack of labels

Packets carry several labels organised as a label stack packet forwarded from one domain to other contains one label packet forwarded through a transit domain contains two labels LSRs use label from top of stack

• labels pushed (ingress) and poped (egress) at domain boundaries


push/pop on stack

BGP entriesinlabel

outlabel

nexthop

R1

R2

R5

R6

….

….

….

….

21

31

4131

41

21 R5

R6

push

IGP entriesinlabel

outlabel

nexthop

R3

R4

R5

R5

?

?

52

52 62

7262

72

R2

R4

R3

R5

pop

swap label 21 to 31R5 not directly connectedFind direct conn. and labelpush label52 and send to R3

next hop R5 (self)pop off the top label of the stackswap this label 31 to 41 (BGP)






MPLS over ATM

ATM switches performing label switching are called ATM-LSRs

labels need to be encoded in VCI/VPI fields MPLS forwarding similar to “label swapping” in

ATM • same ATM user plane may be used• control plane to be changed (MPLS)

protocols BGP, OSPF .. replaces UNI, PNNI


MPLS over ATM

Packets have a shim with Label=0. Actual label is encoded in VPI/VCI.

No TTL decrementn-to-1 merge or n-to-n multipoint not

supportedVCI = 0 through 32 should not be used in

labelsIf LSRs are connected via SVCs, VCI/VPIs

cannot be used as labels. LSRs exchange VCIDs.


Encoding labels in ATM header

SVC Encoding• VPI/VCI field to encode the label from top of

stack• ATM signaling can be used

SVP Encoding• VPI encodes top label• VCI encodes 2nd label

SVP Multipoint Encoding


Cell Interleave Problem






Request Separate labels

R1

R2

ATM R3

162.25

inlabel

outlabel

addrprefix interface

162.25

162.25

51

41

21

21

1

1

0

2

ininterface

if0

if2

get a label for 162.25

get a label for 162.25

request separate labels for R1 and R2 from R3

Predefined VPI/VCIused for Label binding

if1


VC - Merge

Delay cells from one frame(AAL5 EOF marker)






MPLS on ATM: Issues

VCI field is sufficient for one level taggingVPI may be used for the 2nd level

LSR switches need to participate in network layer routing protocols (OSPF, BGP)

Multiple tags per destination may be used to avoid frame merging

VPI/VCI space may be segmented for label switching and normal ATM switching


Traffic Engineering Using MPLS

Traffic Engineering • Performance Optimization• Efficient resource allocation• Constrained routing / Load balancing• Maximum throughput, Min delay, min

loss Quality of service

• Meet policy requirements of operators


Shortest-Path Routing

A

B

C

MPLS Backbone

R3

R1 R5

R2

R4 R6

R2->R5->R6 link 45MbpsR2->R4->R6 link 15Mbps






Constrained Routed LSP

Hard to do non-shortest path routing with connectionless forwarding

Constrained Routed label-switched paths allows selective non-shortest path routing• load balancing across path possible• Signaling protocols for such path

establishment being developed by IETF M-RSVP is based on Resource Reservation

Protocol RSVP (soft-state model) CR-LDP defines hard-state signaling protocols


CR-LSP Example

A

B

C

MPLS Backbone

R1

R2

R5

R6R4

R3






Traffic Trunk

In MPLS networks: “Traffic Trunks” = SVCsA set of traffic parameters can be specified to

determine the “Forwarding Equivalence Class (FEC)” or set of packets assigned to that trunk

Features of trunks • Multiple trunks can be used in parallel to the same

egress.• Traffic trunks are routable entities like VCs• Each traffic trunk can have a set of associated

characteristics, e.g., priority, preemption, policing, overbooking


Traffic Trunks Features

Traffic Trunk Features• Trunk paths are setup based on policies or

specified resource availability.• A traffic trunk can have alternate sets of paths

in case of failure of the main path. Trunks can be rerouted.

• Some trunks may preempt other trunks. A trunk can be preemptor, non-preemptor, preemptable, or non-preemptable.

• Each trunk can have its own overbooking rate


Flows, Trunks, LSPs, and Links

Label Switched Path (LSP): All packets with the same label

Trunk: Same Label+Exp Flow: Same MPLS+IP+TCP headers

DL Label Exp S TTL IP TCP






MPLS Simulation Results

Total network throughput improves significantly with proper traffic engineering

Congestion-unresponsive flows affect congestion- responsive flows• Separate trunks for different types of

flowsTrunks should be end-to-end

• Trunk + No Trunk = No Trunk


Summary

Simplified forwarding based on exact match of fixed length label

Separation of routing and forwarding in IP networks

Facilitates the integration of ATM and IPEnables the use of explicit routing/source

routing in IP networksImproved routing scalability (hierarchy)

through stacking of labelsTraffic Engineering Benefits

multiprotocol label switching

Documents

label encodinglabel

label value

label stackttl

router alert label

label stack fieldtime

implicit null label

ip lookupintegration

label edge routerreprinted