Tải bản đầy đủ - 0 (trang)
Chapter 6. Ensuring Content Delivery with Quality of Service

Chapter 6. Ensuring Content Delivery with Quality of Service

Tải bản đầy đủ - 0trang

periodicallybasedonprioritybeforecongestionoccurs.Signalingallocates

bandwidthandQoSfortrafficflowsbeforesendingtheapplicationtraffic.



CiscoQualityofService(QoS)ensuresthatroutersgive

appropriatenetworkservicestoreal-timeapplicationsthatare

sensitivetodelay,jitter,packetloss,andbandwidth.Your

applicationsmaybesensitivetoanyoneofthefollowing

network-relatedconstraints:

PacketDelayCongestioncausesdelayduetoanoverload

oftrafficonthenetwork.Routersthatstorepacketsin

queuesforexcessdurationspriortoforwardingthemtothe

networkmaycausepacketstodelay.

JitterJitterreferstoavariationinpacketdelay,resultingin

differingpacketinter-arrivaltimesorout-of-sequence

packetsorboth.

PacketLossPacketlossresultsfromnetworkerrorsdueto

congestionorotherfaults.YoulearnedinChapter2,

"ExploringtheNetworkLayers,"howTCPhosts

automaticallydetectandretransmitlostpackets.



Note

Delayandjittermaycauseout-of-sequence

packets,whichareamajorproblemwhenUDPis

usedasthetransport.Applicationsthatrelyon

TCPfortransportdealwithout-of-orderpacket

deliverybybufferingandreorderingTCP

segmentsastheyarrive.TCPalsocandealwith

packetlossbyresendingmissingpackets.



BandwidthApplicationsoftenrequirefixedbandwidthin

ordertodelivercontent,suchasstreamingaudio-video.

Thequalityofreal-timeapplicationsinvariablysuffersfrom

delay,jitter,packetloss,andinsufficientbandwidth.Thebasis

forQoSstemsfromtheneedtoaddressthesecommonnetwork

issues.QoSisdividedintothefollowingcomponents:

ClassificationandMarkingPacketforwardingpriorities

fordifferentgroupsofapplications.

CongestionManagementBasedonpacketforwarding

priorities,switchpacketswithvariousqueuingmechanism

andapplyshapingorpolicing.Congestionmanagementalso

dictatesthetypeoftrafficthatcanbedelayed.Thistraffic

caneitherwithstandsomedelayoristheleastimportant

typeoftraffic,makingitacceptablefordropping.

CongestionAvoidanceBasedonclass,youcanconfigure

theroutertorandomlydropTCPsegmentsperiodically

beforecongestionoccurs.Youcanalsoconfigurebandwidth

shapingandpolicingtoensurethatyourbulkyapplications

donotfloodyourlinks.Additionally,PolicySignalingcanbe

usedtoallocatebandwidthandQoSfortrafficflowspriorto

sendingtheapplicationtraffic.



ClassificationandMarking

Classificationinvolvesdifferentiatingservicesattheedgeof

yournetwork.Whenyouclassifyapplications,theroutermarks

individualpacketsaccordingtothetypeofapplicationthey

belongto.Whenconfiguringmarking,youshouldconsideran

application'soverallpriorityascomparedtootherapplications

inyournetwork.Anapplication'soverallprioritytypically

dependsonthebusinesscriticalityortimesensitivityofthe

applicationforminimalandconsistentlatencyandjitter.You

canassigneachapplicationinyournetworktoadifferent

priorityvalue,or,dependingonthenumberofapplications

runninginyournetwork,youcangroupapplicationsintoclasses

oftraffic.Oncetheroutermarksapacket,youcanconfigure

downstreamrouterstoapplycongestionmanagementand

congestionavoidancetechniquestoit.

Youcanusethefollowingtechnologiestoclassifyandmark

yourapplicationsinyournetwork:

Class-BasedPacketMarkingClass-basedpacketmarking

isanenhancedclassificationmechanism.Youcanuseitto

markapacket'sIPPrecedence,DifferentiatedServicesCode

Point(DSCP),Layer2ClassofService(CoS),orQoSgroup

valuesinordertodifferentiateyourapplications.

Network-BasedApplicationRecognition(NBAR)You

canuseNBARtoclassifytrafficattheapplicationlayer.

NBARclassifiesbasedontrafficbehaviortodetermine

applicationsthatusedynamicports.

Policy-BasedRouting(PBR)PBRistraditionallyafeature

forextendingnormalroutingfeatures.YoucanusePBRin

yournetworktospecifycustomroutingpathsfortraffic



flows,usingcriteriasuchasIPaddress,TCP/UDPports,

MACaddresses,andpacketsize.However,youcanalsouse

PBRtomarkapacket'sIPPrecedencevalue.

Class-BasedPolicingandCommittedAccessRate

(CAR)Bothclass-basedpolicingandCARarepolicing

featuresthatyoucanusetorate-limittrafficenteringyour

network.Youalsocanusethemtoclassifypacketswhen

bandwidthlevelsexceedconfiguredthresholds.Routers

withinyournetworkapplycongestionmanagementand

avoidanceQoSfeaturestothemarkedpackets.Youwill

learnaboutpolicinglaterinthisChapter.

QoSPolicyPropagationviaBGPWiththisfeature,you

canclassifytrafficbasedoncriteria,includingBGP

communities,ASpaths,andaccesslists,inyourrouting

domains.Forexample,aBGProutermayadvertisethree

routesassignedtothreedifferentcommunities.Whenits

peersreceivethethreeBGProutes,theytranslatethe

communityvaluesintothreelevelsofIPPrecedence.When

itspeersreceivepacketsdestinedtotheBGProutes,they

assigntherespectiveIPPrecedencebeforeroutingthe

packet.

QoSforVirtualPrivateNetworks(VPN)QoSforVPNs

classifypacketsbeforetheyenterVPNtunnelstoallowfor

tunnelrouterstoapplyQoSpolicies.



MarkingPacketsatLayer3

ThethreevaluesthatyoucanusetomarkpacketsatLayer3

are

IPPrecedence



IPDSCP

QoSgroupvalue



MarkingaPacket'sIPPrecedenceValue

AsyoulearnedinChapter2theIPPrecedenceusesthreebits

ofthe8-bittypeofservice(ToS)fieldintheIPpacketheader,

asdefinedintheInternetProtocolRFC(RFC791).Theeight

possiblevaluesofIPPrecedencearegivennamesinRFC791,

butyoucanusethesevaluestodifferentiatewhateverclasses

oftrafficyouhaveinyournetwork.IPPrecedencevalues6and

7arereservedfornetworkcontrolinformation,suchasrouting

updatesandlinkkeep-alives,leavingsixvaluesfrom0to5for

useinyournetwork.Table6-1givestheseIPPrecedencevalues

andtheirrespectivenamesasspecifiedinRFC791.

Table6-1.IPPrecedenceValues

IPPrecedenceValue



Name



0



routine



1



priority



2



immediate



3



flash



4



flash-override



5



critical



6



internet



7



network



Forexample,youshoulduseIPPrecedence0forapplications

thatdonotrequireQoSguarantees,andIPPrecedence5for

themostcriticalapplicationsinyournetwork.Youcanmark

packetswithIPPrecedenceusinganyofthefollowingmarking

toolsinCiscoIOS:

Policy-BasedRouting

QoSPolicyPropagationviaBorderGatewayProtocol

CommittedAccessRateandClass-BasedPolicing

Network-BasedApplicationRecognition

Class-BasedPacketMarking

Example6-1givesaLayer3markingconfigurationtomark

packetsenteringanetworkfromahigh-prioritysourcesubnet

10.1.1.0/24withIPPrecedence5,usingpolicy-basedrouting

(PBR).Packetsfromthelesscriticalsubnet10.1.2.0/24are

markedwithIPPrecedence0.



Example6-1.ConfiguringLayer3PacketMarking

UsingPolicy-BasedRouting



ipcef

access-list1permit10.1.1.0255.255.255.0

access-list2permit10.1.2.0255.255.255.0

interfacefastethernet0/1

ippolicyroute-mapmarkprec

!

route-mapmarkprecpermit10



matchipaddress1

setipprecedence5

route-mapmarkprecpermit20

matchipaddress2

setipprecedence0



Note

Bydefault,routersswitchpacketstreatedwithPBR

byusingCEFswitchingwhenyouenableCEFon

yourrouter.However,youalsocanswitchPBRwith

theprocessorfastswitchingpath.



MarkingaPacket'sIPDifferentiatedServicesCodeField

TomarkpacketsusingtheIPDSCPdefinition,youmustuse

class-basedpacketmarking.AsyoulearnedinChapter2,there

aresixavailablebitstoclassifytraffic,giving64possibletraffic

classes.Table6-2liststhesupportedIPDSCPvalues.

Table6-2.IPDSCPValues

DSCPValue



ServiceName



IETFDescription/Cisco

IOS"set"Command

Code



0



Besteffort



Besteffort

(default)



8



Class1



Class1(CS1)



10



Class1,gold



AF11



12



Class1,silver



AF12



14



Class1,bronze



AF13



16



Class2



Class2(CS2)



18



Class2,gold



AF21



20



Class2,silver



AF22



22



Class2,bronze



AF23



24



Class3



Class3(CS3)



26



Class3,gold



AF31



28



Class3,silver



AF32



30



Class3,bronze



AF33



32



Class4



Class4(CS4)



34



Class4,gold



AF41



36



Class4,silver



AF42



38



Class4,bronze



AF43



40



Expressforwarding



Expressforwarding(CS5)



46



Expeditedforwarding



Expeditedforwarding(EF)



48



Control



Control(CS6)



56



Control



Control(CS7)



Table6-3mapstheIPPrecedencevaluestothesupportedIP

DSCPvalues.

Table6-3.IPDSCP-to-PrecedenceMappings

DSCPValue



IPPrecedence



Purpose



0



0



Besteffort



0



1



Class1



8,10



2



Class2



16,18



3



Class3



24,36



4



Class4



32,34



5



Expressforwarding



48



6



Control



56



7



Control



Example6-2givesaLayer3markingconfigurationtomark

packetsenteringanetworkfromthehigh-prioritysourcesubnet

10.1.1.0/24withIPDCSP40(CS5)andfromalesscritical

sourcesubnet10.1.2.0/24withIPPrecedence0(default).You

canperformthisexampleusingclass-basedpacketmarking,

configurablewithintheCiscoModularQoSCLI(MQC).



Note

TheMQCenablesyoutocreatetrafficpoliciesand



attachthesepoliciestorouterinterfaces.You

configuretrafficpolicieswiththepolicy-map

command.Thepolicycontainstrafficclassesthat

youconfigurewithclass-mapcommands,andone

ormoreQoSfeatures.AnexampleQoSfeature

wouldbetomarkapacket'sDSCPvaluewiththe

prioritycommand.Youusetrafficclassestoclassify

traffic,whiletheQoSfeaturesinthetrafficpolicy

determinehowtomarktheclassifiedtraffic.



Example6-2.ConfiguringLayer3PacketMarking

UsingClass-BasedPacketMarking



ipcef

access-list1permit10.1.1.0255.255.255.0

access-list2permit10.1.2.0255.255.255.0

class-mapmatch-allsubnet1

matchaccess-group1

class-mapmatch-allsubnet2

matchaccess-group2

policy-mapprioritize_subnets

classsubnet1

setipdscpcs5

classsubnet2

setipdscpdefault

interfacefastethernet0/1

service-policyinputprioritize_subnets



InExample6-2,twoaccesslistsarecreatedtoidentifytwo

subnets.YouthenentertheMQCwiththepolicy-map

command.Thepolicynamed"prioritize_subnets"marksthetwo



subnetswiththeirrespectiveDSCPvalues,usingthesetip

dscpcommand.Therouterclassifiesthesubnetswithin

individualclass-mapmodules,assignedthenames"subnet1"

and"subnet2."



Note

Class-basedpacketmarkingrequiresthatyoufirst

enableCEFonyourrouters.



IPQoSGroupValue

MarkingapacketwithaQoSgroupvalueaffectsthepacket

onlywithinthelocalrouterthatassignsthegroupvalue.That

is,therouterdoesnotmodifythepacketheaderToSfield.You

canassignupto100QoSgroupvaluestoyourapplications.You

shouldusetheIPQoSgroupvalueifchangingtheIP

PrecedenceorDSCPforthepacketisnotdesirableinyour

network.YoucanassignQoSgroupsbasedonprefix,BGP

autonomoussystem,orBGPcommunitystring.TosettheQoS

groupvalueinapolicy,usethesetqos-grouppolicy-map

classcommand.



MarkingFrames,Cells,andTrunksatLayer2

FormarkingatLayer2,youcanuseoneofthefollowing:

AsynchronousTransferMode(ATM)CellLossPriority

(CLP)bitmarkingAnATMcellheadercontainsa1-bitCLP

fieldindicatingthat,ifyousettothevalue1,thecellshould



Tài liệu bạn tìm kiếm đã sẵn sàng tải về

Chapter 6. Ensuring Content Delivery with Quality of Service

Tải bản đầy đủ ngay(0 tr)

×