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1、Tutorial OutlineOverviewLabel EncapsulationsLabel Distribution ProtocolsMPLS & ATMConstraint Based Routing with CR-LDPSummary“Label Substitution what is it?BROADCAST: Go everywhere, stop when you get to B, never ask for directions.HOP BY HOP ROUTING: Continually ask whos closer to B go there, repeat

2、 stop when you get to B. “Going to B? Youd better go to X, its on the way.SOURCE ROUTING: Ask for a list (that you carry with you) of places to go that eventually lead you to B. “Going to B? Go straight 5 blocks, take the next left, 6 more blocks and take a right at the lights.One of the many ways o

3、f getting from A to B:Label SubstitutionHave a friend go to B ahead of you using one of the previous two techniques. At every road they reserve a lane just for you. At ever intersection they post a big sign that says for a given lane which way to turn and what new lane to take.LANE#1LANE#2LANE#1 TUR

4、N RIGHT USE LANE#2A label by any other name .There are many examples of label substitution protocols already in existence. ATM - label is called VPI/VCI and travels with cell. Frame Relay - label is called a DLCI and travels with frame. TDM - label is called a timeslot its implied, like a lane. X25

5、- a label is an LCN Proprietary PORS, TAG etc. One day perhaps Frequency substitution where label is a light frequency?SO WHAT IS MPLS ? Hop-by-hop or source routing to establish labels Uses label native to the media Multi level label substitution transportROUTE AT EDGE, SWITCH IN COREIP ForwardingL

6、ABEL SWITCHINGIP ForwardingIPIP#L1IP#L2IP#L3IPMPLS: HOW DOES IT WORK UDP-HelloUDP-HelloTCP-openTIMETIMELabel requestIPLabel mapping#L2Initialization(s)WHY MPLS ?Leverage existing ATM hardwareUltra fast forwarding IP Traffic EngineeringConstraint-based RoutingVirtual Private NetworksControllable tunn

7、eling mechanismVoice/Video on IPDelay variation + QoS constraintsBEST OF BOTH WORLDSPACKETROUTINGCIRCUITSWITCHINGMPLS + IP form a middle ground that combines the best of IP and the best of circuit switching technologies.ATM and Frame Relay cannot easily come to the middle so IP has!MPLS+IPIPATMHYBRI

8、DMPLS TerminologyLDP: Label Distribution Protocol LSP: Label Switched PathFEC: Forwarding Equivalence ClassLSR: Label Switching RouterLER: Label Edge Router (Useful term not in standards)Forwarding Equivalence ClassesFEC = “A subset of packets that are all treated the same way by a routerThe concept

9、 of FECs provides for a great deal of flexibility and scalabilityIn conventional routing, a packet is assigned to a FEC at each hop (i.e. L3 look-up), in MPLS it is only done once at the network ingressPackets are destined for different address prefixes, but can bemapped to common pathIP1IP2IP1IP2LS

10、RLSRLERLERLSPIP1#L1IP2#L1IP1#L2IP2#L2IP1#L3IP2#L3MPLS BUILT ON STANDARD IP47.147.247.3123123123 Destination based forwarding tables as built by OSPF, IS-IS, RIP, etc.IP FORWARDING USED BY HOP-BY-HOP CONTROL47.147.247.3IP 47.1.1.112312123MPLS Label Distribution47.147.247.3123121233Mapping: 0.40Reques

11、t: 47.1Mapping: 0.50Request: 47.1Label Switched Path (LSP)47.147.247.3123121233IP 47.1.1.1IP 47.1.1.147.147.247.3123121233IP 47.1.1.1IP 47.1.1.1EXPLICITLY ROUTED LSP ER-LSPTutorial OutlineOverviewLabel EncapsulationsLabel Distribution ProtocolsMPLS & ATMConstraint Based Routing with CR-LDPSummaryLab

12、el EncapsulationATMFREthernetPPPMPLS Encapsulation is specified over various media types. Top labels may use existing format, lower label(s) use a new “shim label format.VPIVCIDLCI“Shim LabelL2Label“Shim Label .IP | PAYLOADMPLS Link LayersMPLS is intended to run over multiple link layersSpecificatio

13、ns for the following link layers currently exist: ATM: label contained in VCI/VPI field of ATM header Frame Relay: label contained in DLCI field in FR header PPP/LAN: uses shim header inserted between L2 and L3 headersTranslation between link layers types must be supportedMPLS intended to be “multi-

14、protocol below as well as aboveMPLS Encapsulation - ATMATM LSR constrained by the cell format imposed by existing ATM standardsVPIPTCLPHEC5 OctetsATM HeaderFormatVCIAAL5 TrailerNetwork Layer Headerand Packet (eg. IP)1nAAL 5 PDU Frame (nx48 bytes)Generic Label Encap.(PPP/LAN format)ATMSARATM HeaderAT

15、M Payload Top 1 or 2 labels are contained in the VPI/VCI fields of ATM header - one in each or single label in combined field, negotiated by LDP Further fields in stack are encoded with shim header in PPP/LAN format- must be at least one, with bottom label distinguished with explicit NULL TTL is car

16、ried in top label in stack, as a proxy for ATM header (that lacks TTL)48 Bytes48 BytesLabelLabelOption 1Option 2Combined LabelOption 3LabelATM VPI (Tunnel)MPLS Encapsulation - Frame Relayn1DLCIC/READLCIFECNBECNDEEAQ.922HeaderGeneric Encap.(PPP/LAN Format)Layer 3 Header and PacketDLCI Size = 10, 17,

17、23 BitsCurrent label value carried in DLCI field of Frame Relay headerCan use either 2 or 4 octet Q.922 Address (10, 17, 23 bytes)Generic encapsulation contains n labels for stack of depth n - top label contains TTL (which FR header lacks), explicit NULL label valueMPLS Encapsulation - PPP & LAN Dat

18、a LinksLabelExp.STTLLabel: Label Value, 20 bits (0-16 reserved)Exp.: Experimental, 3 bits (was Class of Service)S:Bottom of Stack, 1 bit (1 = last entry in label stack)TTL:Time to Live, 8 bitsLayer 2 Header(eg. PPP, 802.3)Network Layer Headerand Packet (eg. IP)4 OctetsMPLS Shim Headers (1-n)1nNetwor

19、k layer must be inferable from value of bottom label of the stackTTL must be set to the value of the IP TTL field when packet is first labelledWhen last label is popped off stack, MPLS TTL to be copied to IP TTL fieldPushing multiple labels may cause length of frame to exceed layer-2 MTU - LSR must

20、support “Max. IP Datagram Size for Labelling parameter - any unlabelled datagram greater in size than this parameter is to be fragmentedMPLS on PPP links and LANs uses Shim Header Inserted Between Layer 2 and Layer 3 HeadersLabel StackEntry FormatTutorial OutlineOverviewLabel EncapsulationsLabel Dis

21、tribution ProtocolsMPLS & ATMIETF StatusNortels ActivitySummaryLabel Distribution ProtocolsOverview of Hop-by-hop & ExplicitLabel Distribution Protocol (LDP)Constraint-based Routing LDP (CR-LDP)Extensions to RSVPExtensions to BGPComparison - Hop-by-Hop vs. Explicit RoutingHop-by-Hop RoutingExplicit

22、RoutingSource routing of control trafficBuilds a path from source to destRequires manual provisioning, or automated creation mechanisms.LSPs can be ranked so some reroute very quickly and/or backup paths may be pre-provisioned for rapid restorationOperator has routing flexibility (policy-based, QoS-

23、based, Adapts well to traffic engineeringDistributes routing of control trafficBuilds a set of trees either fragment by fragment like a random fill, or backwards, or forwards in organized manner.Reroute on failure impacted by convergence time of routing protocolExisting routing protocols are destina

24、tion prefix basedDifficult to perform traffic engineering, QoS-based routingExplicit routing shows great promise for traffic engineeringExplicit Routing - MPLS vs. Traditional RoutingConnectionless nature of IP implies that routing is based on information in each packet header Source routing is poss

25、ible, but path must be contained in each IP headerLengthy paths increase size of IP header, make it variable size, increase overheadSome gigabit routers require slow path option-based routing of IP packets Source routing has not been widely adopted in IP and is seen as impracticalSome network operat

26、ors may filter source routed packets for security reasonsMPLSs enables the use of source routing by its connection-oriented capabilities - paths can be explicitly set up through the network - the label can now represent the explicitly routed pathLoose and strict source routing can be supportedMPLS m

27、akes the use of source routing in the Internet practicalLabel Distribution ProtocolsOverview of Hop-by-hop & ExplicitLabel Distribution Protocol (LDP)Constraint-based Routing LDP (CR-LDP)Extensions to RSVPExtensions to BGPLabel Distribution Protocol (LDP) - PurposeLabel distribution ensures that adj

28、acent routers havea common view of FEC label bindingsRouting Table:Addr-prefix Next HopLSR1LSR2LSR3IP PacketRouting Table:Addr-prefix Next Hopuse label 17Label Information Base:Label-In FEC Label-OutLabel Information Base:Label-In FEC Label-OutStep 1: LSR creates bindingbetween FEC and label valueSt

29、ep 2: LSR communicatesbinding to adjacent LSRStep 3: LSR inserts labelvalue into forwarding baseCommon understanding of which FEC the label is referring to!Label distribution can either piggyback on top of an existing routing protocol,or a dedicated label distribution protocol (LDP) can be createdLa

30、bel Distribution - MethodsLSR1LSR2Label Distribution can take place using one of two possible methodsDownstream Label DistributionLabel-FEC BindingLSR2 and LSR1 are said to have an “LDP adjacency (LSR2 being the downstream LSR)LSR2 discovers a next hop for a particular FECLSR2 generates a label for

31、the FEC and communicates the binding to LSR1LSR1 inserts the binding into its forwarding tablesIf LSR2 is the next hop for the FEC, LSR1 can use that label knowing that its meaning is understoodLSR1LSR2Downstream-on-Demand Label DistributionLabel-FEC BindingLSR1 recognizes LSR2 as its next-hop for a

32、n FECA request is made to LSR2 for a binding between the FEC and a labelIf LSR2 recognizes the FEC and has a next hop for it, it creates a binding and replies to LSR1Both LSRs then have a common understandingRequest for BindingBoth methods are supported, even in the same network at the same timeFor

33、any single adjacency, LDP negotiation must agree on a common method#963#14#99#311#311#311DOWNSTREAM ON DEMAND MAKING SPF TREE COPY IN H/W#462D#311D#963D#14D#99D#216D#612D#5DD?D?D?D?D?D?D?D?Label Distribution ProtocolsOverview of Hop-by-hop & ExplicitLabel Distribution Protocol (LDP)Constraint-based

34、Routing LDP (CR-LDP)Extensions to RSVPConstraint-based LSP Setup using LDPUses LDP Messages (request, map, notify)Shares TCP/IP connection with LDPCan coexist with vanilla LDP and inter-work with it, or can exist as an entity on its ownIntroduces additional data to the vanilla LDP messages to signal

35、 ER, and other “Constraints ER-LSP Setup using CR-LDPLSR BLSR CLER DLER AER Label Switched PathIngressEgress4. Label mapping message originates.3. Request message terminates.2. Request message processed and next node determined. Path list modified to 1. Label Request message. It contains ER path 5.

36、LSR C receives label to use for sending data to LER D. Label table updated6. When LER A receives label mapping, the ER established.CR-LDP PREEMPTIONA CR-LSP carries an LSP priority. This priority can be used to allow new LSPs to bump existing LSPs of lower priority in order to steal their resources.

37、 This is especially useful during times of failure and allows you to rank the LSPs such that the most important obtain resources before less important LSPs.These are called the setupPriority and a holdingPriority and 8 levels are provided.CR-LDP PREEMPTIONWhen an LSP is established its setupPriority

38、 is compared with the holdingPriority of existing LSPs, any with lower holdingPriority may be bumped to obtain their resources. This process may continue in a domino fashion until the lowest holdingPriority LSPs either clear or are on the worst routes.ER-LSP setup using RSVPLSR BLSR CLER DLER A1. Pa

39、th message. It contains ER path 2. New path state. Path message sent to next node3. Resv message originates. Contain the label to use and the required traffic/QoS para.4. New reservation state. Resv message propagated upstream5. When LER A receives Resv, the ER established.Per-hop Path and Resv refr

40、esh unless suppressedPer-hop Path and Resv refresh unless suppressedPer-hop Path and Resv refresh unless suppressedTutorial OutlineOverviewLabel EncapsulationsLabel Distribution ProtocolsMPLS & ATMConstraint Based Routing with CR-LDPSummaryTraffic EngineeringABCDTraffic engineering is the process of

41、 mapping traffic demand onto a networkDemandNetworkTopologyPurpose of traffic engineering:Maximize utilization of links and nodes throughout the networkEngineer links to achieve required delay, grade-of-serviceSpread the network traffic across network links, minimize impact of single failureEnsure a

42、vailable spare link capacity for re-routing traffic on failureMeet policy requirements imposed by the network operatorTraffic engineering key to optimizing cost/performanceTraffic Engineering AlternativesCurrent methods of traffic engineering:Manipulating routing metricsUse PVCs over an ATM backbone

43、Over-provision bandwidthDifficult to manageNot scalableNot economicalMPLS combines benefits of ATM and IP-layer traffic engineering Chosen by routing protocol(least cost)Chosen by Traffic Eng.(least congestion)Example Network:MPLS provides a new method to do traffic engineering (traffic steering)Ing

44、ress nodeexplicitly routestraffic over uncongested pathPotential benefits of MPLS for traffic engineering: - allows explicitly routed paths - no “n-squared problem - per FEC traffic monitoring - backup paths may be configured operator controlscalable granularity of feedback redundancy/restorationCon

45、gested NodeMPLS Traffic Engineering MethodsMPLS can use the source routing capability to steer traffic on desired pathOperator may manually configure these in each LSR along the desired path - analogous to setting up PVCs in ATM switchesIngress LSR may be configured with the path, RSVP used to set u

46、p LSP - some vendors have extended RSVP for MPLS path set-upIngress LSR may be configured with the path, LDP used to set up LSP - many vendors believe RSVP not suitedIngress LSR may be configured with one or more LSRs along the desired path, hop-by-hop routing may be used to set up the rest of the p

47、ath - a.k.a loose source routing, less configuration requiredIf desired for control, route discovered by hop-by-hop routing can be frozen - a.k.a “route pinningIn the future, constraint-based routing will offload traffic engineering tasks from the operator to the network itselfMPLS: Scalability Thro

48、ugh Routing HierarchyBR1BR2BR3BR4TR1TR2TR3TR4AS1AS2AS3Border routers BR1-4 run an EGP, providing inter-domain routingInterior transit routers TR1-4 run an IGP, providing intra-domain routingNormal layer 3 forwarding requires interior routers to carry full routing tables - transit router must be able

49、 to identify the correct destination ASBR (BR1-4)Carrying full routing tables in all routers limits scalability of interior routing - slower convergence, larger routing tables, poorer fault isolationMPLS enables ingress node to identify egress router, label packet based on interior routeInterior LSRs would only require enough information to forward packet to e