Chapter 24

Congestion Control andQuality of Service

24.1 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

2424--1 1 DATA TRAFFICDATA TRAFFIC

TheThe mainmain focusfocus ofof congestioncongestion controlcontrol andand qualityquality ofofTheThe mainmain focusfocus ofof congestioncongestion controlcontrol andand qualityquality ofofserviceservice isis datadata traffictraffic.. InIn congestioncongestion controlcontrol wewe trytry totoavoidavoid traffictraffic congestioncongestion InIn qualityquality ofof serviceservice wewe trytry totoavoidavoid traffictraffic congestioncongestion.. InIn qualityquality ofof service,service, wewe trytry totocreatecreate anan appropriateappropriate environmentenvironment forfor thethe traffictraffic.. So,So,beforebefore talkingtalking aboutabout congestioncongestion controlcontrol andand qualityquality ofofbeforebefore talkingtalking aboutabout congestioncongestion controlcontrol andand qualityquality ofofservice,service, wewe discussdiscuss thethe datadata traffictraffic itselfitself..

Topics discussed in this section:Topics discussed in this section:Traffic DescriptorTraffic Profiles

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Figure 24.1 Traffic descriptors

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Figure 24.2 Three traffic profiles

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2424--2 2 CONGESTIONCONGESTION

C tiC ti ii t kt k ifif thth l dl d ththCongestionCongestion inin aa networknetwork maymay occuroccur ifif thethe loadload onon thethenetworknetwork——thethe numbernumber ofof packetspackets sentsent toto thethe networknetwork——ii tt thth thth itit ff thth t kt k ththisis greatergreater thanthan thethe capacitycapacity ofof thethe networknetwork——thethenumbernumber ofof packetspackets aa networknetwork cancan handlehandle.. CongestionCongestion

t lt l ff tt thth h ih i dd t h it h i ttcontrolcontrol refersrefers toto thethe mechanismsmechanisms andand techniquestechniques totocontrolcontrol thethe congestioncongestion andand keepkeep thethe loadload belowbelow thethe

ititcapacitycapacity..

T i di d i thi tiT i di d i thi tiNetwork PerformanceTopics discussed in this section:Topics discussed in this section:

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Figure 24.3 Queues in a router

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Figure Packet delay and throughput as functions of load

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2424--3 3 CONGESTION CONTROLCONGESTION CONTROL

CongestionCongestion controlcontrol refersrefers toto techniquestechniques andandCongestionCongestion controlcontrol refersrefers toto techniquestechniques andandmechanismsmechanisms thatthat cancan eithereither preventprevent congestion,congestion, beforebeforeitit happens,happens, oror removeremove congestion,congestion, afterafter itit hashasitit happens,happens, oror removeremove congestion,congestion, afterafter itit hashashappenedhappened.. InIn general,general, wewe cancan dividedivide congestioncongestioncontrolcontrol mechanismsmechanisms intointo twotwo broadbroad categoriescategories:: openopen--controlcontrol mechanismsmechanisms intointo twotwo broadbroad categoriescategories:: openopenlooploop congestioncongestion controlcontrol (prevention)(prevention) andand closedclosed--looploopcongestioncongestion controlcontrol (removal)(removal)..congestioncongestion controlcontrol (removal)(removal)..

Topics discussed in this section:Topics discussed in this section:Open-Loop Congestion ControlClosed-Loop Congestion Control

Topics discussed in this section:Topics discussed in this section:

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p g

Figure 24.5 Congestion control categories

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Figure 24.6 Backpressure method for alleviating congestion

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Figure 24.7 Choke packet

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2424--4 4 TWO EXAMPLESTWO EXAMPLES

ToTo betterbetter understandunderstand thethe conceptconcept ofof congestioncongestioncontrol,control, letlet usus givegive twotwo examplesexamples:: oneone inin TCPTCP andand thetheotherother inin FrameFrame RelayRelay..

Congestion Control in TCPTopics discussed in this section:Topics discussed in this section:

Congestion Control in Frame Relay

24.12

Figure 24.8 Slow start, exponential increase

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I th l t t l ith th i f

Note

In the slow-start algorithm, the size of the congestion window increases

exponentially until it reaches a threshold.t es o d

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Figure 24.9 Congestion avoidance, additive increase

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I th ti id l ith

Note

In the congestion avoidance algorithm, the size of the congestion window

increases additively until congestion is detected.co gest o s detected

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Note

An implementation reacts to congestion

Note

An implementation reacts to congestion detection in one of the following ways:❏ If detection is by time out a new slow❏ If detection is by time-out, a new slow

start phase starts.❏ If d t ti i b th ACK❏ If detection is by three ACKs, a new

congestion avoidance phase starts.

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Figure 24.10 TCP congestion policy summary

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Figure 24.11 Congestion example

24.19

Figure 24.12 BECN

24.20

Figure 24.13 FECN

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Figure 24.14 Four cases of congestion

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2424--5 5 QUALITY OF SERVICEQUALITY OF SERVICEQQ

QualityQuality ofof serviceservice (QoS)(QoS) isis anan internetworkinginternetworking issueissueQualityQuality ofof serviceservice (QoS)(QoS) isis anan internetworkinginternetworking issueissuethatthat hashas beenbeen discusseddiscussed moremore thanthan defineddefined.. WeWe cancaninformallyinformally definedefine qualityquality ofof serviceservice asas somethingsomething aainformallyinformally definedefine qualityquality ofof serviceservice asas somethingsomething aaflowflow seeksseeks toto attainattain..

Flow CharacteristicsTopics discussed in this section:Topics discussed in this section:

Flow Classes

24.23

Figure 24.15 Flow characteristics

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2424--6 6 TECHNIQUES TO IMPROVE QoSTECHNIQUES TO IMPROVE QoSQ QQ Q

InIn SectionSection 2424 55 wewe triedtried toto definedefine QoSQoS inin termsterms ofof itsitsInIn SectionSection 2424..55 wewe triedtried toto definedefine QoSQoS inin termsterms ofof itsitscharacteristicscharacteristics.. InIn thisthis section,section, wewe discussdiscuss somesometechniquestechniques thatthat cancan bebe usedused toto improveimprove thethe qualityquality ofoftechniquestechniques thatthat cancan bebe usedused toto improveimprove thethe qualityquality ofofserviceservice.. WeWe brieflybriefly discussdiscuss fourfour commoncommon methodsmethods::schedulingscheduling traffictraffic shapingshaping admissionadmission controlcontrol andandscheduling,scheduling, traffictraffic shaping,shaping, admissionadmission control,control, andandresourceresource reservationreservation..

SchedulingTopics discussed in this section:Topics discussed in this section:

Traffic ShapingResource ReservationAdmission Control

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Admission Control

Figure 24.16 FIFO queue

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Figure 24.17 Priority queuing

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Figure 24.18 Weighted fair queuing

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Figure 24.19 Leaky bucket

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Figure 24.20 Leaky bucket implementation

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Note

A leaky bucket algorithm shapes bursty

Note

A leaky bucket algorithm shapes bursty traffic into fixed-rate traffic by averaging the data rate It ma drop the packets ifthe data rate. It may drop the packets if

the bucket is full.

24.31

Note

The token bucket allows bursty traffic at l t d i ta regulated maximum rate.

24.32

Figure 24.21 Token bucket

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2424--7 7 INTEGRATED SERVICESINTEGRATED SERVICES

TT d ld l hh bb d i dd i d tt idid litlit ffTwoTwo modelsmodels havehave beenbeen designeddesigned toto provideprovide qualityquality ofofserviceservice inin thethe InternetInternet:: IntegratedIntegrated ServicesServices andandDiff ti t dDiff ti t d S iS i WW didi thth fi tfi t d ld lDifferentiatedDifferentiated ServicesServices.. WeWe discussdiscuss thethe firstfirst modelmodelherehere..

SignalingTopics discussed in this section:Topics discussed in this section:SignalingFlow SpecificationAdmissionService ClassesRSVP

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Problems with Integrated Services

Note

Integrated Services is a flow-based QoS d l d i d f IPmodel designed for IP.

24.35

Figure 24.22 Path messages

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Figure 24.23 Resv messages

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Figure 24.24 Reservation merging

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Figure 24.25 Reservation styles

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2424--8 8 DIFFERENTIATED SERVICESDIFFERENTIATED SERVICES

DifferentiatedDifferentiated ServicesServices (DS(DS oror Diffserv)Diffserv) waswasDifferentiatedDifferentiated ServicesServices (DS(DS oror Diffserv)Diffserv) waswasintroducedintroduced byby thethe IETFIETF (Internet(Internet EngineeringEngineering TaskTaskForce)Force) toto handlehandle thethe shortcomingsshortcomings ofof IntegratedIntegratedForce)Force) toto handlehandle thethe shortcomingsshortcomings ofof IntegratedIntegratedServicesServices..

DS FieldTopics discussed in this section:Topics discussed in this section:

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Note

Differentiated Services is a class-based Q S d l d i d f IPQoS model designed for IP.

24.41

Figure 24.26 DS field

24.42

Figure 24.27 Traffic conditioner

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2424--9 9 QoS IN SWITCHED NETWORKSQoS IN SWITCHED NETWORKSQQ

LetLet usus nownow discussdiscuss QoSQoS asas usedused inin twotwo switchedswitchednetworksnetworks:: FrameFrame RelayRelay andand ATMATM.. TheseThese twotwo networksnetworksareare virtualvirtual--circuitcircuit networksnetworks thatthat needneed aa signalingsignalingprotocolprotocol suchsuch asas RSVPRSVP..

QoS in Frame Relay

Topics discussed in this section:Topics discussed in this section:QoS e e yQoS in ATM

24.44

Figure 24.28 Relationship between traffic control attributes

24.45

Figure 24.29 User rate in relation to Bc and Bc + Be

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Figure 24.30 Service classes

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Figure 24.31 Relationship of service classes to the total capacity of the network

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