Network Layer - 5.4 Quality of Service

CEN445 – Network Protocols and Algorithms

Chapter 5 – Network Layer

5.4 Quality of Service

Dr.MostafaHassanDahshanDepartmentofComputerEngineering

CollegeofComputerandInformationSciencesKingSaudUniversitymdahshan@ksu.edu.sa

http://faculty.ksu.edu.sa/mdahshan

Quality of Service (QoS)

Withgrowthofmultimedianetworking Ad‐hoccongestioncontrolnotenough NeedprotocolstoguaranteeQoS Matchservicelevelwithapplicationsneeds

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Requirements

Flow:streamofpacketsfromsourcetodest virtualcircuit:flowpacketsusesamepath datagram:mayfollowdifferentpaths

Primaryparametersforflowneeds reliability delay jitter bandwidth

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Requirements

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Requirements

Reliability nobitsmaybedeliveredincorrectly achievedusingchecksum,verifiedatdest damagedpacketsarenotACKed;retransmitted importantforemail,filetransfer,web lessimportantforaudio,video

Delay manyapplicationhavelow,mediumrequired real‐timeaudio,video,strictdelayrequired

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Requirements

Jitter variationindelay problemwithinteractiveapplications

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Requirements

Bandwidth videoisthemostdemandingapplication Maincategories(asinATM)

constantbitrate:telephony real‐timevariablebitrate:videoconferencing nonreal‐timeVBR:watchingmovieonInternet availablebitrate:filetransfer

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Techniques for Achieving Good Quality of Service

Overprovisioning Buffering TrafficShaping TheLeakyBucketAlgorithm TheTokenBucketAlgorithm ResourceReservation AdmissionControl ProportionalRouting PacketScheduling

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Overprovisioning

Providemoreresourcesthanpeakload routercapacity,bufferspace,bandwidth

Expensive Practicalifdesignersknowhowmuchisenough

Usedintelephonesystem

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Buffering

Bufferflowsatreceiverbeforedelivery Noeffectonbandwidth,reliability Delayisincreasedbutjitterissmoothed Buffereddatacanplayedatuniformintervals

Videosteamingsitesusuallybuffer10sec

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Buffering

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Traffic Shaping

Inpreviousexampledatatransmittedwithuniformspacing

Someapplications’dataisnonuniform serverhandlingmultiplestreams allowfastforward,rewindvideostream

Trafficshaping:regulateaveragerate(burstiness)ofdatatransmission

AccordingtoServiceLevelAgreement(SLA)

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Traffic Shaping

13source: Cisco

Traffic Shaping

TrafficPolicing

Whenratereachesmaxexcesstrafficisdropped

Burstsarepropagated Doesn’tqueuepackets Appliestoinboundtraffic

TrafficShaping

Retainpacketsinqueues,scheduleexcessforlaterretransmission

Smoothedoutputrate Requiresmemorytoqueuepackets

Appliestooutboundtraffic RequiresschedulingfunctionFQ,WFQ,CBWFQ,…

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The Leaky Bucket Algorithm

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No matter the rate at which water enters the bucket, the outflow is at a constant rate


Networkinterfacehasfiniteinternalqueue Simplyitisasingleserverqueueingsystemwithconstantservicetime

Smoothbursts,reducecongestionchance Twoapproaches

packet‐counting:forfixed‐sizepackets(ATM) byte‐counting:forvariable‐sizepackets

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Leakybucketconsistsoffinitequeue Whenpacketarrives

ifthereisaroominqueue,appendpacket else,discardpacket

Ateveryclocktic,onepacketistransmitted

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Onevariant:byte‐countingleakybucket Forbyte‐countingleakybucket

ateachtic,counterinitializedton whilesizeofnextpacket<counter

transmitpacket counter=counter– sizeofpacket

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Example

Computersenddata@25MB/s =200Mbps Networkspeedis200Mbps Butrouterscanacceptthisrateonlyforshortintervals,untilbufferfillup

Forlongintervals,theyworkbestat2MB/s Supposedatacomein1MB,40ms burst/s Toreducerateto2MB/s,useleakybucketwithrate =2MB/s&capacityC =1MB

Outputwillbe2MB/sfor500ms =1MB19

Example

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The Token Bucket Algorithm

Leakybuckethasfixedoutputrate Forsomeapplications,bettertoallowoutputspeedupwhenlargeburstarrives

Tokenbucket leakybucketholdstokens generatedbyaclock1tokenevery sec totransmit1packet,capture&destroy1token canbeusedtoregulatetrafficathosts orsmoothtrafficbetweenrouters

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Essentiallywhatthetokenbucketdoesis:allowbursts,butuptoaregulatedmaximum

length


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Allowsidlehoststosavepermissiontosendlargeburstslater,uptomaxsizeofbucketn leakybucketdoesn’tallowthis

Burstsupton packetscanbesentatonce Throwsawaytokenswhenbucketfillsup Butneverdiscardspackets

leakybucketdiscardswhenbucketfillsup


Aminorvariant,tokenrepresentsk bytesinsteadof1packet

Packetcanbetransmittedifenoughtokensavailabletocoveritssize

Forpacketcountingtokenbucket every ,counter+=1 whenpacketsent,counter‐=1

Forbytecountingtokenbucket every ,counter+=k whenpacketsent,counter‐=packetsize

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Output of TB with C of (c) 250 KB

(d) 500 KB

(e) 750 KB

(f) 500 KB feeding a10 MB/s leaky bucket


Calculatinglengthofmaximumrateburst not1MB/25MB/s.Why? duringburstoutput,moretokensarrive

Definitions S:burstlength(seconds) B:tokencapacity(bytes) :tokenarrivalrate(bytes/s) M:maximumoutputrate(bytes/s)

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Wecanseethat outputburstcontainsmaxof bytes numberofbytesinmax‐speedburstoflengthSisMS bytes

Hencewehave:

TogetS:

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Example

250,000 Bytes 25 10 Bps 2 10 Bps Burstdata=10 Bytes

0.010869 11ms (we’llround tonearestms)

Bytestransmittedinburst= 25 10 0.011 275000 Remainingbyteinburst=10 275000 725000 Timetotransmitremainingbytesusingtokenarrivalrate:

0.3625 363 ms

Totaltimetotransmit1MB=11+363=374ms28

Example

500,000 Bytes 25 10 Bps 2 10 Bps Burstdata=10 B

0.021739 22ms (we’llround tonearestms)


0.225 225 ms

Totaltimetotransmit1MB=22+225=247ms29


Problem:tokenbucketallowslargebursts Frequentlydesirabletoreducepeakrate Oneway:insertleakybucketafterTB LBrate >TBrate and<maxrateMi.e.

Thisproducessmoothertraffic

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Example

500,000 Bytes 10 10 Bps 2 10 Bps Burstdata=10 B

0.0625 62.5ms


0.1875 187.5 ms

Totaltimetotransmit1MB=62.5+187.5=250ms31

Resource Reservation

Needvirtualcircuitofsomethingsimilar Otherwise,hardtoguaranteeanything Oncerouteisfixed,canreserveresources

bandwidth:notoversubscribeoutputlines bufferspace:allocatequeuesp forspecificflow CPUcycles:requiredtoprocesspackets

importanttoensureCPUisnotoverloaded evenwithloadslightlybelowtheoreticalcapacity,queuecanbuildanddelayoccurs

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Resource Reservation

Assumepacketsarriveatrandom meanarrivalrate= packets/second

AssumeCPUtimerequiredisalsorandom meanprocessingrate= packets/second

Botharrival&service:Poissondistribution Fromqueueingtheory,meanpacketdelayT

⁄

seconds

where isCPUutilization33

Servicetimewithnocompetition

slowdownduetocompetingflows

Example

packets/s packets/s

i.e.meanpacketservicetime=

Then,

Meanpacketdelay

, , .

insteadof ,becauseofqueueingdelay ifthereare30routers,> ofqueueingdelay

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Admission Control

Nowwe’reatthepointwheretrafficis wellshaped canpotentiallyfollowsingleroute,inwhich capacitycanbereservedonallroutersonpath

Whensuchflowisoffered,admitorreject? currentcapacity? howmanycommitmentsmadeforotherflows?

Comparecurrent(bandwidth,buffer,cycles)withexcesscapacityinthose3dimensions?

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Admission Control

Decisionisnotthatsimplebecause fewappsknowtheirCPUrequirement,unlikebw

differentwayneededtodescribeflow

someappsmoretolerantinoccasionalmisseddeadlinethanothers

someappsmayhaggleaboutparameters,othersmaynot movieviewercandropfrom30fpsto25fps similarly,pixels/frame,audiobpsmaybeadjustable

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Admission Control

Manypartiesinvolvedinflownegotiation sender,receiver,allroutersalongpath

Flowsmustbedescribedaccurately specificparametersthatcanbenegotiated calledflowspecification

Senderproducesflowspecification Routerscheckandmodifyasneeded Atotherend,parameterscanbeestablished

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Admission Control

Parameter Unit Notes

Token bucketrate Bytes/second Max sustainedratesendermaytransmitaveragedoverlonginterval

Token bucketsize Bytes If TBrate=1Mbps,TBsizeis500KB,buckettake4stofill,afterthattokensarelost

Peakdatarate Bytes/second Mustnotexceed,evenforshortintervals

Minimumpacketsize Bytes Importantbecauseprocessingeachpackettakessomefixedtime,nomatterhowshort

Maximum packetsize Bytes Importantduetointernalnetworklimitationsthatmaynotbeexceeded 38

Example

Howarouterturnsaflowspecificationintoasetofspecificresourcereservations? implementationspecific,notstandardized

Supposeroutercanprocess100,000packets/s Offeredflow1MB/s( bytes/s) Min=maxpacketsize=512bytes

Thus,packetrate= =2048packets/s

Routermustreserve2%CPUforthatflow Ifpolicyisnotallocatemorethan50%CPU Routerwith49%willrejectthe2%flow 39

Example

Tighterflowspecificationismoreuseful IfspecstatesitneedsTBrate5MB/sbutpacketsvarybetween50and1500bytes

packetrateformax= packets/s

packetrateformin= packets/s

Routerwillrejectbecauseoflatternumber Ifminpacketsizewas1000,mightaccept

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Proportional Routing

Mostroutingalgorithmstryfindbestpath Sendalltrafficoverbestpath ToprovidehigherQoS,splittrafficforeachdestinationovermultiplepaths

Routersdon’thavecompleteoverviewofnetworktraffic

Dividetrafficequallyorinproportiontocapacityofoutgoinglinks

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Packet Scheduling

1flowmayhogcapacity,starveotherflows Processingpacketsinorderofarrivalmeansaggressivesendercancapturemostcapacity

Topreventthis,usepacketscheduling fairqueueing weightedfairqueueing …

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Packet Scheduling

Fairqueueing(packet‐by‐packetRR) separatequeuesforeachoutputline onequeueforeachflow whenlineisidle,scanqueuesinroundrobin takefirstpacketonnextqueue helpsinpreventingaggressiveflows problem:flowswithlargepacketstakemoreBW

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Packet Scheduling

Fairqueueing(byte‐by‐byteRR) solvestheproblemwithpacket‐by‐packetRR simulatebyte‐by‐byteround‐robin scanthequeuesrepeatedly,byte‐for‐byte findtickonwhicheachpacketwillbefinished packetsarethensorted sentinorderoftheirfinishing

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Packet Scheduling

Fairqueueing(byte‐by‐byteRR) calculatingfinishtime forpacketi

:arrivaltime :packetsize

iscalculatedindependentlyforeachqueue

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Example – Fair Queue

Packet Arrival Queue Length Finish (Calc) Finish Out order

A 0 1 8 Max(0, 0)+8 8 1

B 5 2 6 Max(5, 0)+6 11 2

C 5 3 10 Max(5, 0)+10 15 3

D 8 2 9 Max(8, 11)+9 20 5

E 8 3 8 Max(8, 15)+8 23 6

F 10 1 6 Max(10, 8)+6 16 4

G 11 3 10 Max(11, 23)+10 33 8

H 20 2 8 Max(20, 20)+8 28 7

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A B C F D E H G

Packet Scheduling

Fairqueueinggivesallflowssamepriority Manytimes,weneedtogivesomeflowsmorebandwidth(e.g.video)

WeightedFairQueueing(WFQ) onequeueforeachflow eachflowhasdifferentweight

where istheweightofthequeueofflowi

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Example – Weighted Fair Queue

Packet Arrival Queue Length Finish (Calc) Finish Out order

A 0 1 8 Max(0, 0)+8 8 1

B 5 2 6 Max(5, 0)+6 11 3

C 5 3 10 Max(5, 0)+10/2 10 2

D 8 2 9 Max(8, 11)+9 20 7

E 8 3 8 Max(8, 10)+8/2 14 4

F 10 1 6 Max(10, 8)+6 16 5

G 11 3 10 Max(11, 14)+10/2 19 6

H 20 2 8 Max(20, 20)+8 28 8

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A C B E F G D H

Integrated Services

Flow‐basedalgorithms,integratedservices ResultofalotofIETFeffortstodeviseanarchitectureforstreamingmultimedia

Aimedatbothunicastandmulticasttraffic unicast:singleuserstreamingvideofromsite multicast:collectionofdigitaltelevisionstationsbroadcastingprogramsasstreamsofIPpacketstomanyreceiversatvariouslocations

notsuitablefordynamicgroupmembershipbecausebandwidthisreservedinadvance 49

RSVP – Resource reSerVation Protocol

MainpartofIntServ architecture DescribedinRFCs2205‐2210 Usedformakingreservations Allowsmultiplesenderstotransfertomultiplegroupsofreceivers

Individualreceiverscanswitchchannels Optimizesbandwidthusewhileeliminatingcongestion

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Multicastroutingusingspanningtrees Eachgroupisassignedgroupaddress Senderputsgroupaddressinitspackets MCroutingalgorithmbuildsSTforgroup RoutingalgorithmnotpartofRSVP

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Toreducecong,anyreceiveringroupcansendareserv messageupthetreetosender

Messageispropagatedusingreversepathforwarding(discussedearlier)

Eachhop,routerreservesnecessaryBW Ifinsufficientbandwidth,reportfailure Whenmessagegetsbacktosource,BWisreservedallthewayfromsendertoreceiver

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RSVP – Example

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R3 reserves flow from S1

R3 reserves flow from S2

R5 reserves flow from S1; merged with R3 at H

CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011

Capacityreservedmustbelargeenoughtosatisfythe

greediestreceiver


Receivercanspecifymultiplesources Canspecifyifchoicesarefixedordynamic RouteruseinfofortooptimizeBWplanning

tworeceiversareonlysetuptoshareapathiftheybothagreenottochangesourceslateron

BWreserv isdecoupledfromsourcechoice OnceBWreserved,canswitchsource,keepportionofreservedpathfornewsource

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Differentiated Services

Flow‐basedprotocols(IntServ)offergoodQoS becauseneededresourcesarereserved

Thehavedownside: requireadvancesetup don’tscalewellforlargenetworkswM’sflows maintainper‐flowstate,vulnerableroutercrash requiresubstantialchangetoroutercode

Thus,notwidelyimplemented

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SimplerapproachtoQoS devisedbyIETF Canbelargelyandeasilyimplemented Class‐based insteadofflow‐based QoS DiffServ orDS,RFCs2474,2475andothers Advantages

noadvancesetup noresourcereservation notime‐consumingend‐to‐endnegotiationperflowasIntServ

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Canbeofferedbysetofroutersforminganadministrativedomain(ISPorTelco)

Admindefinesetofserviceclasses CustomersignsupforDS PacketscarryTypeofService(DSCP)field Betterserviceforsome(premium)classes Trafficinclassmayberequiredtoconformtospecificshape(e.g.LB)

Extrachargeif>Npremiumpackets/month58

Diffserv Architecture

From: Computer Networking: A Top Down Approach 5th edition. Jim Kurose, Keith Ross,

Addison-Wesley, April 2009. 59

Edge router: per-flow traffic management marks packets as in-profile

and out-profile

Core router: per class traffic management buffering and scheduling based

on marking at edge preference given to in-profile

packets

scheduling

...

r

b

marking


Per‐Hop‐Behavior:PHB DefaultPHP:besteffort ExpeditedForwardingPHB

highestpriority reservedBW≥required

AssuredForwardingPHB fourpriorityclasses threediscardprobabilitiesforeachclass

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Expedited Forwarding

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Assured Forwarding

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Sending host or Edge router

1. classify packets

2. Mark packets according to class

3. Pass packets to shaper/filter

External References

ComparingTrafficPolicingandTrafficShapingforBandwidthLimiting,www.cisco.com/en/US/tech/tk543/tk545/technologies_tech_note09186a00800a3a25.shtml

CoursenotesofDr.MohammedArafah FairQueueing,

http://webpages.cs.luc.edu/~pld/courses/netmgmt/spr08/fair_queuing.html

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Network Layer - 5.4 Quality of Service

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