SDN-P4原版完整課件

P4:ProgrammingProtocol-IndependentPacketProcessorsP.Bosshart,D.Daly,G.Gibb,M.Izzard,N.McKeown,J.Rexford,C.Schlesinger,D.Talayco,A.Vahdat,G.Varghese,D.WalkerSIGCOMMCCR,2014PresentedbyYeTianforCourseCS05112OverviewMotivationAbstractForwardingModelAProgrammingLanguageAnExampleReviewMotivationOverthepastfiveyears,OpenFlowhasgrownincreasinglymorecomplicatedTheproliferationofnewheaderfieldsshowsnosignsofstopping.MotivationRatherthanrepeatedlyextendingtheOpenFlowspecification,wearguethatfutureswitchesshouldsupportflexiblemechanismsforparsingpacketsandmatchingheaderfields,allowingcontrollerapplicationstoleveragethesecapabilitiesthroughacommon,openinterface(i.e.,anew“OpenFlow2.0”API).Suchageneral,extensibleapproachwouldbesimpler,moreelegant,andmorefuture-proofthantoday'sOpenFlow1.xstandard.MotivationP4:ahigher-levellanguageforProgrammingProtocol-independentPacketProcessorsConfigureaswitch,

tellingithow

packetsaretobe

processedPopulatethe

forwardingtablesin

fixedfunctionswitchesThreegoalsReconfigurableRedefinethepacketparsingand

processinginthefield.Protocolindependentthecontrollershouldbeabletospecifyapacketparserforextractingheader

fieldswithparticularnamesandtypesandacollectionoftypedmatch+actiontablesthatprocesstheseheaderTargetindependentCompilershouldtaketheswitch'scapabilitiesinto

accountwhenturningatarget-independentdescription(writteninP4)intoatarget-dependentprogramOverviewMotivationAbstractForwardingModelAProgrammingLanguageAnExampleReviewAbstractForwardingModelAprogrammingparserAllownewheaderstobedefinedMultiplestagesof

match+actionInseries,parallel,or

combinationofbothComparewithOpenFlowFixedparserFixedseriesofactionsAbstractForwardingModelTwotypesofoperations:Configureoperationsprogramtheparser,settheorderofmatch+actionstages,andspecifytheheaderfieldsprocessedbyeachstage.Static,offlinewayPopulateoperationsadd(andremove)entriestothematch+actiontablesthatwerespecifiedduringconfiguration.RuntimeAbstractForwardingModelArrivingpacketsarefirsthandledbytheparser.RecognizeandextractfieldsfromtheheaderTheextractedheaderfieldsarethenpassedtothematch+actiontables.Ingressmatch+actiontable:determinestheegressport(s)anddeterminesthequeueintowhichthepacketisplaced.Thepacketmaybeforwarded,replicated,dropped,ortriggerflowcontrol.Egressmatch+actiontable:performsper-instancemodificationstothepacketheaderAbstractForwardingModelPacketscancarryadditionalinformationbetweenstages,calledmetadata,whichistreatedidenticallytopacketheaderfields.Example:inport,timestamp,…,etcQueueing:anactionmapsapackettoaqueueOverviewMotivationAbstractForwardingModelAProgrammingLanguageAnExampleReviewAProgrammingLanguageDependenciesbetweenthefieldsDeterminewhichtablecanbeexecutedinparallelTableDependencyGraphs(TDG)TDGnodesmapdirectlytomatch+actiontables,andadependencyanalysisidentifieswhereeachtablemayresideinthepipeline.anexampletabledependencygraphforanL2/L3switch.AProgrammingLanguageTwo-stepcompilationAtthehighestlevel,programmersexpresspacketprocessingprogramsusinganimperativelanguagerepresentingthecontrolflow(P4);Belowthis,acompilertranslatestheP4representationtoTDGstofacilitatedependencyanalysisandthenmapstheTDGtoaspecificswitchtarget.OverviewMotivationAbstractForwardingModelAProgrammingLanguageAnExampleReviewAnExampleConsideranexampleL2networkdeploymentwithtop-of-rack(ToR)switchesattheedgeconnectedbyatwo-tiercore.mTag:acombinationofPortLandandMPLS.Theroutesthroughthecoreareencodedbya32-bittagcomposedoffoursingle-bytefields.Eachcoreswitchneedonlyexamineonebyteofthetagandswitchonthatinformation.Thetagisaddedbythefirstedgeswitch.P4ConceptsAP4programcontainsthefollowingkeycomponentsHeaders:describesthesequenceandstructureofaseriesoffields.Parsers:specifyhowtoidentifyheadersandvalidheadersequenceswithinpackets.Tables:Match+actiontablesarethemechanismforperformingpacketprocessing.Actions:Constructionofcomplexactionsfromsimplerprotocol-independentprimitives.ControlPrograms:determinetheorderofmatch+actiontablesthatareappliedtoapacket.HeaderFormatsAnorderedlistofnamestogetherwiththeirwidthEthernetandVLANHeaderFormatsThemTagheadercanbeaddedwithoutalteringexistingdeclarations.Eachcoreswitchisprogrammed

withrulestoexamineoneof

thesebytesdeterminedbyits

locationinthehierarchyand

thedirectionoftravel.Outputportonfirstandsecond

levelsupswitchOutputportonfirstandsecond

levelsdownswitchThePacketParserP4assumestheunderlyingswitchcanimplementastatemachinethattraversespacketheadersfromstarttofinish,extractingfieldvaluesasitgoes.P4describesthisstatemachinedirectlyasthesetoftransitionsfromoneheadertothenext.ThePacketParserStartsinthestartstateandproceedsuntilanexplicitstopstateisreachedoranunhandledcaseisencounteredTableSpecificationTheedgeswitchmatchesontheL2destinationandVLANID,andselectsanmTagtoaddtotheheader.Thereadsattributedeclareswhichfieldstomatch,qualifiedbythematchtype(exact,ternary,etc).Theactionsattributeliststhepossibleactionswhichmaybeappliedtoapacketbythetable.Themaxsizeattributespecifieshowmanyentriesthetableshouldsupport.Thetablespecificationallowsacompilertodecidehowmuchmemoryitneeds,andthememorytype(e.g.,TCAMorSRAM)toimplementthetable.NotethatthisisNOTtherun-timepacketprocessinglogic.TableSpecificationBriefdenfitionsofothertablesBriefdefitionsofothertablesActionSpecificationP4definesacollectionofprimitiveactionsfromwhichmorecomplicatedactionsarebuilt.ActionSpecificationIfanactionneedsparameters(e.g.,theup1valueforthemTag),itissuppliedfromthematchtableatruntime.P4'sprimitiveactionsinclude:setfield:Setaspecificfieldinaheadertoavalue.copyfield:Copyonefieldtoanother.addheader:Setaspecificheaderinstance(andallitsfields)asvalid.removeheader:Delete(“pop”)aheader(andallitsfields)fromapacket.increment:Incrementordecrementthevalueinafield.checksum:Calculateachecksumoversomesetofheaderfields(e.g.,anIPv4checksum).TheControlProgramSpecifytheflowofcontrolfromonetabletothenext.TheControlProgramThesourcechecktableverifiesconsistencybetweenthereceivedpacketandtheingressport.ItalsostripsmTagsfromthepacket,recordingwhetherthepackethadanmTaginmetadata.Thelocalswitchingtableisthenexecuted.Ifthistable“misses“,itindicatesthatthepacketisnotdestinedforalocallyconnectedhost.ThemTagtableisappliedtothepacket.BothlocalandcoreforwardingcontrolcanbeprocessedbytheegresschecktablewhichhandlesthecaseofanunknowndestinationbysendinganotificationuptheSDNcontrolstack.TheControlProgramCompilingpacketparserThecompilertranslatestheparserdescriptionintoaparsingstatemachineCompilingcontrolprogramControlprogram:notexplicitlycalloutdependenciesbetweentablesoropportunitiesforconcurrency.Employacompilertoanalyzethecontrolprogramtoidentifydependenciesandlookforopportunitiestoprocessheaderfieldsinparallel.Finally,thecompilergeneratesthetargetconfigurationfortheswitch.CompilingcontrolprogramHowmTagcancompiledondifferenttargetsSoftwareswitch:ThecompilerdirectlymapsthemTagtablegraph