Evolution of QoS routing in the...

© O. Bonaventure, 2004

Evolution of QoS routing in the Internet

Olivier Bonaventure

Dept. Computing Science and EngineeringUniversité catholique de Louvain

http://www.info.ucl.ac.be/people/OBO

June 4th, 2004

© O. Bonaventure, 2004Page 2

Agenda

� Routing basics

� The ToS approach

� The Integrated Services approach

� The Traffic Engineering approach

� Issues with QoS routing in the Internet


Architecture of a normal IP router

Routing table

IP packets

Routing protocol

Control

Forwarding

Class.Shap.

Pol

Class.Shap.

Pol

ForwardingTable

IP packets

IP packets

Forwarding decision based on longest matchUpdate of TTL and checksum fields in IP packets

The "best" paths selected from the routing tablebuilt by the routing protocols are installed in theforwarding table


Internet routing

� Exterior Gateway Protocol (EGP)

� Routing of IP packets between domains� Each domain is considered as a blackbox

� Interior Gateway Protocol (IGP)

� Routing of IP packets inside each domain

� Only knows topology of its domain

Domain1

Domain2

Domain3

Domain4


Intradomain routing

� Goal

� Allow routers to transmit IP packets along the best path towards their destination

� best usually means the shortest path� Shortest measured in seconds or as number of hops

� sometimes best means the less loaded path

� Allow to find alternate routes in case of failures

� Behavior

� All routers exchange routing information

� Each domain router can obtain routing information for the whole domain

� The network operator or the routing protocol selects the cost of each link


Three types of Interior Gateway Protocols

� Static routing

Only useful in very small domains

� Distance vector routing

Routing Information Protocol (RIP)

Still widely used in small domains despite its limitations

� Link-state routing

Open Shortest Path First (OSPF)

Widely used in enterprise networks

Intermediate System- Intermediate-System (IS-IS)

Widely used by ISPs


Distance vector routing

� Principle

� Router configuration

Cost associated with each link

� Each router sends periodically a distance vector containing, for each known prefix, :

1. The IP prefix2. The distance between itself and the destination�

The distance vector is a summary of the router's rout ing table

� Each router receives its neighbor's distance vectors and builds its routing table based on those vectors

R2 R5

R3 R4

R1R6

C=5

C=3

C=3

C=1

C=1

C=3

C=6

C=10


Issues with distance vector routing

� How to deal with link failures ?

� Routers should send their distance vector when they detect the failure of one of their links

� How to avoid the count-to-infinity problem ?

� Utilize a non-redundant star shaped network

� Limit the maximum distance between routers

� For RIP, � � 16 !

� Split horizon

� Router A does not advertise to router B the routes for which it sends packets via router B

� Split horizon with Poison reverse


Link state routing

� Principle

� Each router builds link state packet containing its local topology

� Link state packets are created at regular intervals and when the local topology changes

� Link state packet is reliably flooded to all routers inside the domain

� Each router knows the complete domain topology

� Computes routing tables by using Dijkstra

� The best path is the path with the smallest cost

R2 R5

R3 R4

R1R6

C=5

C=3

C=3

C=1

C=1

C=3

C=6

C=10


Agenda

� Routing basics

� The ToS approach

� The Integrated Services approach

� The Traffic Engineering approach

� Issues with QoS routing in the Internet


The ToS byte

� ToS : Type of Service Octet

� defines the relative importance of the IP packet and the type of service required for this packet

� ToS evolution

� definition of ToS Octet changed several times

� Precedence is used in some networks

� ToS field is rarely used

� Today, ToS byte has been replaced by DSCP

0 1 2 3 4 5 6 7

Prec. Type of Service 0

Precedence (relative priority)

1000 minimize delay0100 maximize throughput0010 maximize reliability0001 minimize monetary cost0000 normal service


ToS-based routing

� Principle

� Attach a different IGP cost on each link for each ToS value

� Compute and maintain a forwarding table per ToS value

� Forward the packets based on their ToS

� Example

�

R2 R5

R3 R4

R1R6

d=5, t=10

d=3, t=10

d=3,t=20

d=1,t=10

d=1,t=2

d=3,t=5

d=6,t=9

d=10,t=10

d: delay metrict: throughput metric


What happened to ToS routing ?

� Support was added in

� IS-IS

� for both CLNP and IP

� OSPFv2

� But ISPs did not clearly request this and no major vendor implemented it...

� ToS byte was seldom used by applications

� Main usage was Precedence=6,7 for routing packets

� ToS routing will be removed from the OSPF and IS-IS specification


Agenda

Routing basics

The ToS approach

The Integrated Services approach

The Traffic Engineering approach

Issues with QoS routing in the Internet


Integrated servicesThe "hard" approach for QoS

! Basic hypothesis

" Some specific applications require QoS

# delay guarantees

# bandwidth guarantees

" Three types of services

# Guaranteed Service

# Controlled Load

# Null

" QoS will be provided to layer 4 flows

# Each layer 4 (TCP or UDP) flow will inform the network about its QoS requirements

# The network will accept or reject the flow based on its requirements and the current state of the network

" QoS should support unicast and multicast

" QoS flows should be allowed to coexist with best-effort flows in the same network


Integrated services network

$ Provision of integrated services in a network

R6

R1

R3

R5

R4 R2

QoS Response

Applications utilize signaling protocol to indicate their QoS requirements

Routers rely on per layer 4 flow scheduling to provide the required QoS

Routers perform connection admission control to reserve ressources for each flow

Normal IP routing is used to select path towards destination. IP routing is not changed to support Integrated Services

QoS Request


An Integrated services router

% Model of an Integrated Services router

RoutingRSVPAdmission

Control

Resourcereservation

Layer-4Classification

+traffic contractenforcement

IP packets withreservat ion

IP packets without reservation

Data IP packetsData IP Packets

Integrated Servicescapable router

RSVP messagesRouting protocol

Control

Forwarding

Scheduler


RSVP

& RSVP : Resource Reservation Protocol

& Objectives

' Support the establishment of unidirectional flows in IP networks

( different types of flows) init ially layer 4 f lows

' Suitable for IP unicast

( should take into account the possibility of route changes without requiring any modification to routing protocols

' Suitable for IP multicast

( should fully support the IP multicast service model including dynamic groups


RSVP (2)

* Principles of operation

* Two important RSVP messages

+ PATH

, used by sender to inform routers and receivers of the new flow and its required resources- no resources are reserved due to reception of PATH

+ RESV

, used by receiver to actually reserve resources for the flow specified in the PATH message- resources are reserved for the IP packets sent by the sender

towards the receiver along the path taken by the PATH message

* RSVP messages are sent inside IP packets


RSVP : example

. Unicast flow establishment with RSVP

S1 R1 R2 R3 D1

PATH specifies flowand announces amountof traffic to send

PATH

Routing findsnext hop=R2nothing is reserved

PATH

PATH

PATH

D1 knows requirementsfor this new flow

RESV

D1 asks R3 to reserveresources on R3-D1link for this flow

RESV

R3 can reserve enoughresources for D1's flow

RESVRESV

S1 is asked to reserveresources on S1-R1

RESVconfOptional confirmation for D1will be sent to D1

RSVP messages are sent unreliably as simple datagrams

RESV messages must follow same route as PATH messages


Constrained routing

/ What should be added to traditional routing algorithms ?

0 a way to distribute information about current network state

1 routers must know load of remote links to choose paths meeting constraints for flows with QoS guarantees

0 a way to compute a path subject to constraints

1 current routing algorithms find shortest path

1 how can we find a path with2 minimum hop count2 at least 10 Mbps2 at most 10 msec of delay


Distributing load information

3 Distance vector routing protocols [RIP,BGP]

4 routers conspire to distribute routing table

4 difficult to inform routers of load on remote links

4 difficult to support constrained routing

3 Link state routing protocols [OSPF, IS-IS]

4 routers conspire to distribute network map

5 simple to add information about network load

4 routers distribute link state packets with load info

5 delay is already distributed as the IGP metric

5 bandwidth/link load is main information to distribute

5 tradeoff between frequent distribution (accurate information) and rare distribution (avoid network overload)

4 each router knows topology and load of each link and can find constrained paths


Distributing load information (2)

6 Potential problem

7 Link load information is not distributed immediately

7 routers must establish flows based on partial information about the current load in the network

1. new flow [B=4] is created between R4 and R62. before information about load changes, R3 wants to

create a new flow [B=2] towards R68 R3 believes that R3-R4-R6 is the best path

R2 R5

R3 R4

R1R6

D=5B=8

D=3B=6

D=3B=2

D=1B=6

D=1B=6

D=3B=2

D=6B=5

D=10B=8

1

2


Constraints

9 Three types of constraints on path selection

: Additive constraint

; find path minimizing < example< hop count< l ink delay or cost

: Multiplicative constraint

; find path minimizing < example< loss rate

: Concave constraint

; find path containing links whose characteristic is always above a given constraint< example< bandwidth< resource class or color

= > d1 , d2 , ... , dn ?

@ > d1 , d 2 , ... , dn ?


Finding a constrained path

A Single additive or multiplicative constraintsB

apply Dijkstra's algorithmC

example

A 2 or more additive/ multiplicative constraintsB

unfortunately problem is NP hardC

need to evaluate all possible paths to find exact solutionC

several heuristics have been proposed in literature to find acceptable solutions

R2 R5

R3 R4

R1R6

C=5

C=3

C=3

C=1

C=1

C=3

C=6

C=10


Finding a constrained path (2)

D Concave constraintsE

fortunately easy to handleF

remove from the network map all links that do not satisfy the constraintF

utilize Dijkstra's algorithm on the reduced mapF

exampleG

f ind shortest 3 Mbps from R3 to R6

G

utilize only links with some kind of protection

R2 R5

R3 R4

R1R6

C=5B=8

C=3B=6

C=3B=2

C=1B=6

C=1B=1

C=3B=10

C=6B=1

C=10B=8


Constrained routing in IP networks

H Several solutions proposed by researchersI

Lessons learnedJ

Constrained routing should be applied to flows and not on a per packet basis

J

Bandwidth and delay are key constraintsK

delay jitter is less important and diff icult to efficiently supportJ

Path selection should be performed by the source K

the source of a flow selects an explicit route K

at least inside a single areaK

if the path crosses areas of domains, the explicit route will be loosK

intermediate nodes perform connection admission control but do not perform any constrained routing decisionK

path select ion algorithm does not need to be standardizedK

if the new flow is acceptable, establishment continues otherwise the source will have to compute another path


Constrained routing and Intserv

L Integrated services was developped at the same time as ATM-Traffic ManagementM

Guaranteed Service <-> CBR, rt-VBRM

Controlled Load <-> VBR, GFR

L Scalability problems pledged both end-to-end ATM and InterServ

L Constrained routing was used for ATMM

PNNI was defined and implemented

L For Integrated services, IETF was relunctant to change anything to IP routingM

IETF defined is a framework for QoS routing, but no protocolN

although OSPF extensions were proposed by many researchers and prototypes were implemented


A critique of Integrated services

O AdvantagesP

provides per layer 4 flow QoS guaranteesP

GS with delay/ bandwidth guaranteesP

CL with bandwidth guarantees

O DrawbacksP

Requires each intermediate router to perform some operations for each layer 4 flowQ

RSVP message processingQ

per layer 4 flow classificationR

classif ication can become complex for multicast !Q

per layer 4 flow policing/queuing/scheduling R

a backbone router may see thousands of simultaneous f lows !P

Not all applications are able to express precisely their traffic and QoS requirements


Scaling issues with integrated services

R6

R1

R3

R5

R4 R2

PATH

RESV

2. State information required inside each routerS

Ident if ication of layer 4 f low (IP dest, Protocol, Port)S

Ident if ication of previous hop (to forward RESV)S

Reservat ion stateS

Reserved resourcesS

Ident if ication of the senders that benefit from reservations

3. Per data packet processingS

f ind layer 4 flow stateS

check traffic contractS

check reservation

1. RSVP signaling overheadS

one RSVP PATH/RESVS

per layer 4 f lowS

per refresh period


Agenda

T Routing basics

T The ToS approach

T The Integrated Services approach

T The Traffic Engineering approach

T Issues with QoS routing in the Internet


MPLS Integrating label swapping and IP

Rd

Rb

Flow 1

Flow 2 LSR1

Flow 3

LSR3

LSR2

RdRa

Ingress Edge RouterU

inserts labels on packets sent through backbone

Core LSR : Label-Switching RouterU

packet forwarding based only on labels

Egress Edge routerU

removes labels before sendingpackets outside MPLS network

Pure IP packetsPure IP packets

Labeled packets


Label-swapping example

Crd

Cra Routing tabledefault via BR1

Label tableFlow1 : L1 via BR1Flow2 : L2 via BR1

Crb

Flow 1 Flow 2

Routing tabledefault via BR1

Label tableFlow3 : L1 via BR1

BR1

Flow 3 BR1 Routing tabledefault via BR3BR2, Cra, Crb : direct

Label forwarding tableInport Inlabel Outport OutlabelWest L1 North L2West L2 East L1S-W L1 East L2

BR2 Routing tabledefault via BR3BR1, BR3 : directCra, Crb via BR1Label forwarding tableInport Inlabel Outport OutlabelSouth L2 S- E L1

BR3 Routing tabledefault -> InternetBR1, BR2, CRd : directCra, Crb via BR1

BR3 Label forwarding tableInport Inlabel Outport OutlabelWest L1 East L1West L2 East L3N-W L1 North L1

BR3

BR2

Cra


Architecture of a core LSR

Labeled packets

Control

Forwarding

Class.Shap.

Pol

Class.Shap.

Pol

LabelForwarding

Table

Labeledpackets

Labeledpackets

IP Routing tableIP Routing protocol

Label table

Label Distribution


Architecture of an ingress edge LSR

IPpackets

Control

Forwarding

Class.Shap.

Pol

Class.Shap.

Pol

IdentifyFEC

Attach Label

Labeledpackets

Labeledpackets

IP Routing tableIP Routing protocol

Label table

Label Distribution


The Traffic Engineering problem

V ProblemW

Shortest path chosen by IP routing does not always lead to a good network utilizationX

fish problem

W

How to better optimize the network utilization ?W

How to react to changes in traffic conditions

R1

R9

R3

R4

R5 R6

R7

R8

R2Shortest path treerooted on R3

many packetstowards R8

many packetstowards R9


MPLS-based traffic engineering

Y PrincipleZ

In each POP, routers establish MPLS tunnels with bandwidth reservations towards remote routers[

MPLS's label swapping will ensure that packets will follow chosen path[

full mesh of MPLS tunnels

Y IssuesZ

How to select the router-router path ?[

Back to constrained routing with some adaptationsZ

How to establish the MPLS tunnel[

Slightly change RSVP to distribute MPLS labels\

path is computed by the source and specified as an explicit route in the RSVP Path message\

Labels are allocated by the RESV messagesZ

How to change bandwidth or route a LSP ?[

Make-before-break in RSVP-TE


Constrained routing andMPLS-based traffic engineering

] What should we add to OSPF for MPLS-TE ?

^ Bandwidth information for each link_

OSPF-TE will attach unreserved bandwidth information to each link`

one amount of unreserved bandwidth for each of the 8 preemption levels `

preemption levels are used to indicate “importance” of LSPs

^ Maximum bandwidth of each link_

This is the physically available bandwidth on the link

^ Maximum reservable bandwidth_

Maximum amount of bandwidth that can be reserved by MPLS tunnels`

Could be higher than, equal to or lower than maximum bandwidth


Constrained routing andMPLS-based traffic engineering (2)

a What should we add to OSPF for MPLS-TE ?

b Traffic engineering metricc

Could be different from the normal IGP metric which usually is set based on rerouting and bandwidth requirementsd

Common usage of TE metric is to indicate delay to establish tunnels supporting VoIP or tunnels carrying EF trafficd

We're back to ToS routing, but on a per-tunnel basis !

b Resource class or colorc

Allows to identify all the links with the same characteristic, essential to establish protection LSPsd

all satellite links are in blued

all l inks provided by FT are in redd

all l inks provided by Sprint are in yellow


Agenda

e Routing basics

e The ToS approach

e The Integrated Services approach

e The Traffic Engineering approach

e Issues with QoS routing in the Internet


Dynamics of QoS routing with MPLS-TE

f How stable is an MPLS-TE network wheng

link failures occurh

Sprint shows that failures are common eventsh

each failure may cause the rerouting of some MPLS tunnels which will change the bandwidth availableh

dampening mechanisms on LSP generationi

threshold-based LSP generationi

holding time on LSP generationg

Routers are upgradedh

Most ISPs have 2 hours maintenance windown during the nighti

many MPLS tunnels will need to be reroutedg

Change in MPLS tunnels bandwidth reservations h

some vendors propose to measure per tunnel bandwidth and adapt reservation on hourly basish

some vendors propose to reoptimize MPLS-tunnel paths on a regular basis to preserve network optimality


How to extend QoS routing across areas ?

j IGPs used by large ISPsk

ISIS : mainly single area deploymentsk

OSPF : often multi-area deployments

j How to establish interarea MPLS tunnels ?k

interarea IP routing is not as clean as PNNIk

Tunnel headend does not know all topology to select best path to reach destinationl

What kind of TE information should be leaked across areas ?k

Possible solutions includel

headend selects loose route and area border router updates the routel

headend contacts path computation server to select the route, based on full area knowledge


How to extend QoS routing across domains ?

m Several large ISPs want to use MPLS tunnels between ASesn

large company with several ASesn

research networks (GEANT, I2, ...)

m What is required to establish those tunnels ?n

RSVP-TE allows to specify interdomain routesn

Buto

BGP is a path-vector protocol that does not provide any TE informationp

no metricp

no bandwidth attributep

no topological informationn

Headend should define loose route that will be improved by AS border routerso

optimality of interdomain paths is unknown

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