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SDN Traffic Engineering with Segment Routing
The Next Evolution
Cengiz Alaettinoglu
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Traffic Engineering (TE)
Minimizetheworstlinku1liza1on• Alleviatetrafficconges1on• Be9er/longeruseofequipment/port/fiber
Routetrafficaroundcongestedlinks• Puttrafficonnon-shortestpaths
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Evolution of Traffic Engineering
Offlinetrafficengineering• Op1mal,butnotadap1veOn-devicetrafficengineering• Adap1ve,butnotop1malSo#waredefinednetworking• Bestofbothworlds,yetsimpler;simplicityenabledby:
• Segmentrou1ng• Push-basedtelemetry• SDNTrafficEngineeringapplica1on
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Offline Traffic Engineering
TopologymodelTrafficdemandmatrixOp1miza1onalgorithmcomputesroutessothattheworstlinku1liza1onisminimized• Linearprogramming
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Pros/Cons of Offline Traffic Engineering
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Verygoodlinku1liza1onvalues
NetworkmodelishardtokeepaccurateTrafficdemandmatrixishardtocomputeOp1miza1onalgorithmisveryslow• HourstodaysCannotadapttofailuresToomanytunnels(N2)Somepathsmaybesurprisinglylong
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On-Device Traffic Engineering RSVP-TE/CSPF
RoutersfloodavailablebandwidthoflinksinIGPEachrouter• Setsupone(ormore)tunnelstootherrouters• Monitorstheu1liza1onofthesetunnels(auto-bandwidth)• Triggersre-op1miza1onwhenu1liza1onchanges• UsesCSPF(constraint-basedshortestpathfirst)tocomputethepaths
• SignalsthepathandreservesbandwidthusingRSVP
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Pros/Cons of On-Device Traffic Engineering
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Notsogoodlinku1liza1onvalues• Eachrouterisselfishinop1miza1on
• Nonetwork-wideop1miza1on
NetworkmodelisreadilyavailableTrafficdemandmatrixiseasywithauto-bandwidthCSPFisfastCanadapttofailuresToomanytunnels(N2)SomepathsmaybesurprisinglylongFloodingavailablebandwidthimpactsIGP,par1cularlyconvergence1meRacecondi1onsa\erfailures
• Long-livedFRRRSVP-TEoverheadishighduetoN2tunnels
• Protocolandmanagementoverhead
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Example Deployments
Small Medium Large
Routers 75 450 1,900
Links 300 2,000 8,000
Tunnels 1,600 20,500 132,000
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• Majorityofthetunnelshaveverysmallamountoftraffic
• NoTEneeded
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Link / Tunnel Distribution
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MostlinkscarryasmallnumberoftunnelsSmallnumberoflinkscarryalotoftunnels
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A Tunnel Path - Before, During and After a Link Failure
Awide-arealinkfailsat2015-12-2712:04:05.200490• Itwascarrying327tunnelsfrom22head-endroutersThetunnelabovefailstoop1mize,butwhy?
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2015-12-2712:04:05.200490
2015-12-2712:04:05
2015-12-2712:14:00
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Re-Optimization after Link Failure
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22head-endroutersgetasignalviaRSVP-TEandtrytore-op1mize• RacetoavailablebandwidthEachrouterop1mizesforitself• Itdoesnotknowwhattheother21routersneed• Itdoesn’tevenknowthereareotherrouters/tunnelsinterestedinthis
bandwidth9tunnelsfailtoop1mize• 5head-endrouters• TheexampletunnelisoneoftheunluckyonesThiscouldhavebeenavoidedwithnetwork-wideop:miza:on!
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What Happens to the Traffic?
TrafficnowtakestheIGPpath(greenarrows)Tunnelneeded34MbpswhichisnotavailableanywhereinthenetworkTheIGPpathtoodoesnothavethisbandwidthavailable• Conges1onkicksin
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Another Tunnel is Stuck on its FRR
Whathappenswhenatunnelfailstoop1mizeanditisFRRprotected?• FRRisstuck• Usuallynoreserva1onsaremadeonFRRpaths• Conges1onwillkickin
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N2 Tunnels - Beyond Human Manageability
Itisnotjust9tunnelsthataredown1204downtunnelsistoomanyforanyoperatortofigureouttherootcause• Ifthesetunnelsarefortrafficengineering,canwereallysayweare
successfullydoingtrafficengineering?• Itis1meforso\ware/devopstomanagethenetwork
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ANSDNAPPROACH
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What Do We Really Need?
Real-1memodel• Alleviateconges1on,especiallya\eralinkfailureCreateasfewtunnelsasnecessary• Verysmallsignalingoverhead• VerysmallIGPoverhead
• DonotwantIGPdynamicsduetoavailablebandwidthchanges
Network-wideop1miza1onSimpletodeployandoperate
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SDN Promises a Solution
Segmentrou1ng(SR)replacesRSVP• Providesuncompromisedfunc1onality• SimplecontrolplanewithverylowoverheadPush-basedtelemetryfortrafficmatrices• YANGmodelbased• FreesIGPSDNcontrollerispartofthenetworkcontrolplane• Hasreal-1metopology• Enablesmanipula1ngpathsonthedevicesusingstandardsouthbound
protocolsTrafficEngineeringbecomesanSDNapplica1on• Op1mizespathsnetwork-wide• Asfewtunnelsasnecessary
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Segment Routing
Segmentrou1ngsimplifiesIP/MPLScontrolplane• NoneedtorunLDPorRSVP-TEFunc1onalityisnotcompromised• Canforwardtrafficonnon-shortestpathsfortrafficengineering
• Detour,bypassFRR(fastre-route),andIPLFAprotec1on• Secondarypaths• SLA-conformingservicespecificpaths(e.g.L2/L3VPNs)• SDNprogrammability
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TE Needs Shortest and Non-Shortest Paths; SR Can Encode Any Path
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A B
Z
DC
V W YX
1Segment(shortestIGPpath)• GotoZonshortestpath(nodesegment)
A B
Z
DC
V W YX5Segments• GotoBonshortestpath• GotoWonshortestpath• GotoYonshortestpath• GotoDonshortestpath• GotoZonshortestpath
A B
Z
DC
V W YX
3Segments• GotoConshortestpath• GotoXonlink3(adjacencysegment)• GotoZonshortestpath
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Push-Based Telemetry Eases Traffic Demand Matrix Generation
HowmuchcustomertrafficentersthenetworkinHanoiandisdes1nedforTokyo?• Demanddoesnotchangebasedoninternalrou1ngTradi1onally,NetFlowisusedforthisandcans1llbeusedPush-andmodel-basedtelemetryhaveverypromisingfeatures,includingreal-1metrafficvisibility
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YANG Model Pushed by Ingress Routers
SimilarcontenttoNetFlowfortrafficmatrixgenera1on• Missesport/protoleveldetailPushedfromtherouters• FewsecondstominutesEfficienttransferofdata• BinaryencodedusingProtoBuf
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+--rotraffic-collector+--roafs+--roaf*[af-name]+--rocounters+--roprefixes+--roprefix*+--roipaddr?string+--romask?string+--rolabel?Tc-oper-local-label+--robase-counter-sta1s1cs|+--rotransmit-packets-per-second-switched?uint64|+--rotransmit-bytes-per-second-switched?uint64|+--rocount-history*|+--roevent-start-1mestamp?uint64|+--roevent-end-1mestamp?uint64|+--rotransmit-number-of-packets-switched?uint64|+--rotransmit-number-of-bytes-switched?uint64|+--rois-valid?boolean+--rotraffic-matrix-counter-sta1s1cs|+--rotransmit-packets-per-second-switched?uint64|+--rotransmit-bytes-per-second-switched?uint64|+--rocount-history*|+--roevent-start-1mestamp?uint64|+--roevent-end-1mestamp?uint64|+--rotransmit-number-of-packets-switched?uint64|+--rotransmit-number-of-bytes-switched?uint64|+--rois-valid?boolean+--roprefix?string
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Traffic Engineering as an SDN App.
SDNapplica1onmanagestrafficdemand• Currentaswellasfuturereserva1ons• IGPdoesnothavetosignalavailablebandwidth• Push-basedtelemetryorNetFlow-basedmatricescanbegeneratedSDNapplica1oncomputespathsandallocatesbandwidth• Centraliza1onyieldsnetwork-wideresourceop1miza1on• CreatesthefewesttunnelsnecessarySDNapplica1onadaptstofailuresandrepairs• SDNcontrollerprovidesreal-1metopologyview• Noracecondi1onsa\erfailuresandrepairsSegmentrou1ngcanbeusedfornetworksimplifica1on• SDNcontrollermakesthisanabstrac1onfortheapplica1on• RSVP-TEcans1llbeusedwhereSRisnotavailable
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Reducing the N2 Tunnels
Onlygeneratestunnelsfortrafficgoingovercongestedlinks• Tunnelsnolongerneedtobeconfiguredaprioriattherouters
• Onlycreatethemiftheywillhaveaposi1veimpact• Specialcase:
• Undernormalcondi1onsdon’tgenerateanytunnels• Underfailurecondi1onsgenerateenoughtoalleviateconges1on
DonotcreatetunnelswhenIGPpathsa1sfiestheconstraintsEasytoimplementinso\warewithaglobalview,buthardtodoonedeviceata1me
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Illustration
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1GbpslinksTwoelephantflows• 850Mbpswesttoeast• 500MbpsnorthtosouthLotsofmiceflows
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Concluding Remarks
SDNsimplifiesrunningatrafficengineerednetwork• Applica1onistheSDNrevolu1onSDNapplica1onneedsenablersfromtheinfrastructure• Controller• Segmentrou1ng(orRSVP-TEwhennotavailable)• Push-basedtelemetry• NETCONF/YANG,PCEP,andothersouthboundprotocols
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