p00555-3-1 packets and circuits: chris cooper feb 2005 mpls topics: introduction to mpls tutorial...

P00555-3-1Packets and Circuits: Chris Cooper Feb 2005

MPLSMPLS

Topics:

Introduction to MPLS

Tutorial Questions and Recommended Reading


Routing Without MPLSRouting Without MPLS Large organizational networks are heavily subnetted for

structuring purposes◦ Packets crossing such a network could transit many

routers Connectionless nature of IP poses two challenges

◦ Per-packet processing◦ Distributed routing (Per hop behaviour PHB)

Core routers needs to forward tens of millions of packets per second◦ Must process each packet in a few nanoseconds

Potential for congestion in router forwarder (packet switch) Packets follow the best path according to the routing

table in each router◦ No opportunity for setting end-to-end path◦ Could override with static routes

But this approach doesn’t scale


Normal IP Transit NetworkNormal IP Transit Network

Connecting widely separated parts of an enterprise network◦ ‘Enterprise’: a large (national, international) company,

organization◦ a number of sites (campuses, branches, offices)

Enterprise Network

Enterprise Network

Service Provider’sTransit Network

BGP Routersto Service Provider

Normal Subnet Routers


IP vs. MPLS Transit NetworksIP vs. MPLS Transit Networks

Enterprise network(routed normally)


MPLS transit network

Enterprise Network

Enterprise Network

Service Provider’sTransit Network

Routed Path

Label Switched Path

MPLS-enabledrouters

NormalIP routers

IP

MPLS


Benefits of MPLSBenefits of MPLSLower transit delay

◦ Core routers switch not routeTraffic engineering

◦ Packets can take non-standard path(OSPF path)

Scalability◦ Labels can be nested

to facilitate networkhierarchy

Flexibility◦ Can be used over

LANs PPP tunnels [ATM & Frame Relay backbones]


MPLS Goal: Dynamic Switched MPLS Goal: Dynamic Switched FlowsFlows

Original goal: bypass routing table lookup for as many packets as possible◦ Dynamically detect packet flows

Identified by unique pairs of IP addresses and port numbers◦ Switch, rather than route, packets on known flows

Cisco called this “route once, switch many” Original approach based on two TCP-related

assumptions◦ Majority of IP packets belong to TCP sessions

Rather than UDP datagram streams◦ TCP sessions have (relatively) long duration

File transfers, conferencing Increasing popularity of Web browsing undermined this

goal◦ Uses short-duration sessions

Per-flow path setup doesn’t scale


MPLS Goal: Dynamic Switched MPLS Goal: Dynamic Switched FlowsFlows(continued)(continued)



MPLS transit network

Packet flowfollowing switched path

MPLS-enabledrouters

Flow-detecting MPLS routers


MPLS Goal: Traffic MPLS Goal: Traffic EngineeringEngineering Determine end-to-end path for given packet flows

◦ Override routing protocol decision where administratively appropriate

Allows routing policy to be set◦ Reflect service offerings

Low-delay path for voice traffic More secure path for certain customers

Now seen as most important reason for using MPLSMPLS transit network

MPLS routers

Predetermined path


Multiprotocol Label Multiprotocol Label SwitchingSwitching

Overview

Label Switching

Operation


Label SwitchingLabel Switching Labels packets for faster switching through network

◦ Connection-oriented protocols use virtual circuit ID Frame relay DLCI ATM VPI/VCI

◦ Connectionless protocols need to add label VLAN identifier (802.1Q trunking) MPLS label (added to Ethernet and PPP)

Switches set up paths as required◦ Associate labels with paths◦ Use label as route-table lookup

Labels often have only link-by-link significance◦ Allows switch to differentiate incoming flows◦ Each switch maps label values predictably for outgoing

flows

DLCI = data-link connection identifierVCI = virtual channel identifierVPI = virtual path identifier


Label Switching RoutersLabel Switching Routers MPLS-enabled devices are called Label Switching

Routers (LSRs)◦ Multilayer switches enhanced with MPLS protocols

MPLS identifies two roles for LSRs◦ Edge LSRs and Core LSRs

Edge LSRs often calledLabel Edge Routers (LERs)

Edge LSRs◦ Determine packet path and perform flow classification◦ Assign unique labels to each flow

Core LSRs◦ Use label values to switch packets over cut-through

paths◦ Layer 2 forwarding bypasses normal routing function

Edge LSR

Edge LSR

Core LSRs


Multiprotocol Label Multiprotocol Label SwitchingSwitching

Overview

Label Switching

Operation


Operational Overview IOperational Overview IIdentify groups of packets of packets

travelling over a common path◦For example, towards the same destination

network or host◦Called a forwarding equivalence class

Assume they have common forwarding requirements and assign a label to each group◦Encapsulate with label header carrying same label

value◦Communicate label settings to downstream router◦Downstream router assigns label to outgoing FEC

and communicates downstream And so on


Operational Overview IIOperational Overview IIOnce LSP has been set up routers process FEC

accordingly

Ingress LSR (ingress LER) adds label to packet

Core LSRs match incoming labels to route table, which gives output port◦ Outgoing label map applies downstream label value

As previously communicated to downstream router◦ Bypassing conventional packet-by-packet, hop-by-hop L3

processing

Egress LSR (egress LER) removes it

Set of label mappings for a group constitutes the label switched path (LSP) for that FEC


Label SwitchingLabel Switching

Port 5label map

Incoming label

Outgoing port

a 8

b 5

c 5

d 7

Incoming port

Incoming label

Outgoing label

4 a p

1 b q

1 c r

3 z s

Port 1route table

Port 5label map

1

2

3

4

5

6

7

8

c

x

y z

a

bb c

x

y z

a

rqsp

mm

ss

bb


Label Switched Path (LSP)Label Switched Path (LSP) MPLS identifies two types of label switched path

◦ Destination- based(follow the OSPF path)◦ Explicitly routed( follow the way u determined)

(Cisco terminology; descriptive)

Destination-based LSP follows conventional forwarding path◦ As determined by IP routing table◦ Originally set up from destination LER source LER

(Why is that?)

Explicitly routed LSP use source-specified path (source routing)◦ Path set up from source LER destination LER◦ Useful for overriding normal route selection based on least

cost path E.g. for enforcing route selection (‘routing policy’)


LSP Set-UpLSP Set-Up LSR is essentially an MPLS-enabled multi-layer switch

Routing database constructed in normal way◦ Using, for example, OSPF◦ Then made available to MPLS switching engine

Switching engine assigns labels to forwarding paths◦ Sends route/label mappings to next-hop neighbour using

a/the Label Distribution Protocol (LDP)( the path are renewed periodically)

Once LSP set up, packets are label-switched(packets can flow)

Conventional, in most cases, for router prior to egress LSR to remove label◦ Avoids processing load on LSR◦ Called penultimate hop-popping


How MPLS WorksHow MPLS Works(continued)(continued)

NormalIP packet

NormalIP packet

Labelled packet

Edge LSR

Edge LSR

Label Switching Path (LSP)Core LSRs

May pop label


MPLS HeaderMPLS HeaderHeader is 32 bits (4 octets)

◦ Label field is 20 bits◦ Three-bit EXPerimental field can be used to carry

(some) DiffServ markings through MPLS network◦ S = 1 indicates bottom of stack◦ Time To Live (TTL) is decremented by LSRs to

maintain usual packet hop count

Number of bits 20 3 1 8

Label EXP S TTL


Label Encapsulation Label Encapsulation SchemesSchemes

Format: 32 bits added to beginning of packet (prepended)◦ 20-bit label◦ Remaining bits used for

variety of purposes Two ways of inserting label

◦ Add as protocol shim to existing header PPP and switched LANs usual method

◦ Map onto existing virtual circuit identifier ATM or Frame Relay rare: included for completeness

Once the label is in place, established schemes can be used◦ E.g. label multiplexing and

switching

Label inserted as additional header

IP payloadCRC

LAN/PPPheader

Label in FR DLCI field

IP payloadCRC

Label in ATM VPI/VCI fields

IP part-payload


Label Stacking ILabel Stacking I Can add further label in front of (‘on top of’) the existing

one(s)◦ Nested labels treated as stack

Hence use of term ‘pop’◦ Network switches on top-most label

Allows several LSPs to be grouped for forwarding purposes◦ Provided they can be treated as a single FEC

E.g. all heading to same edge-point

Can continue the process, grouping groups together into a further group, with a new label

Each LSP marked by pair of label edge routers and a label in the stack◦ Ingress LER pushes new label onto stack◦ Egress LER pops label off the stack


Label Stacking IILabel Stacking II

LERs

LERs

LSP

LSP


MPLS Summary IMPLS Summary I Multiprotocol Label Switching (MPLS)

◦ Applies label switching to IP networks◦ Facilitates

Unequal cost load balancing Setting routing policies (traffic engineering) Virtual Private Networks (VPNs)

◦ Bypasses potential bottlenecks causes by large route table look-ups

◦ Allows provider network nesting though label stacking Allows label mapping to be communicated in variety of ways How do u communicate label route information across the

network◦ LDP(label distribution protocol)◦ OSPF and BGP enhancements◦ RSVP

Details of how to recover from link failure still being finalized Generalised MPLS: paths over SONET/SDH & wavelengths

(‘s’) in WDM networks


Tutorial topics: MPLSTutorial topics: MPLS1. What are the similarities between MPLS

label switching and FR/ATM switching?

2. What (if any) are the differences?

3. Look up “penultimate hop popping”. What is it and what does it achieve?

4. Why is a ‘destination-based’ MPLS path set up from destination LSR back towards source LSR?

1. Hint Remember path is unidirectional: think about label mapping


STUDY QUESTIONSTUDY QUESTIONQ1(i) What does the term label switching

mean? Explain, using appropriate diagrams, how MPLS applies this principle to IP packets and the main advantages it gives. [5]

(ii) Figure 1 shows part of an OSPF network. The network administrator notices that traffic from the remote site LANs frequently congests the route to Head Office. Explain why this is, and describe, with examples, how MPLS could be used to overcome this problem. [5]


FIGURE 1FIGURE 1


Q 2. Explain what is meant by a Forwarding Equivalence Class and a Label Switched Path in MPLS. What is meant by 'DiffServ-enabled MPLS'? Explain briefly what MPLS labelling arrangements you would expect to see for a set of enterprise VPNs, where each VPN supports its own two DiffServ per-hop behaviours (PHBs). Would this change, and if so, how, if within each VPN, an enterprise also used MPLS to traffic engineer the routing of traffic with a different per-hop behaviour? Explain your answer. [6]

p00555-3-1 packets and circuits: chris cooper feb 2005 mpls topics: introduction to mpls tutorial...

Documents