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MPLS Architecture

SMUCSE 8344 MPLS Architecture. SMUCSE 8344 MPLS Network Model MPLS LSR = Label Switched Router LER = Label Edge Router LER LSR LER LSR IP MPLS IP Internet

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Text of SMUCSE 8344 MPLS Architecture. SMUCSE 8344 MPLS Network Model MPLS LSR = Label Switched Router LER =...

  • MPLS Architecture

  • MPLS Network ModelMPLSLSR = Label Switched RouterLER = Label Edge Router

  • MPLS BenefitsComparing MPLS with existing IP core and IP/ATM technologies, MPLS has many advantages and benefits:The performance characteristics of layer 2 networksThe connectivity and network services of layer 3 networksImproves the price/performance of network layer routingImproved scalability

  • MPLS Benefits (contd)Improves the possibilities for traffic engineeringSupports the delivery of services with QoS guaranteesAvoids need for coordination of IP and ATM address allocation and routing information

  • Necessity of L3 ForwardingFor securityTo allow packet filtering at firewallsRequires examination of packet contents, including the IP headerFor forwarding at the initial router - used when hosts dont do MPLSFor ScalingForward on a finer granularity than the labels can provide

  • MPLS ArchitectureDown stream label assignment for unicast trafficOn demandUnsolicitedPath selectionHop by hopExplicitOrdered vs. independent controlLoop detection and prevention mechanisms

  • Label Distribution Protocol (LDP)Set of procedures used by LSRs to establish LSPsMapping between network-layer routing information directly to data-link layer switched pathsLDP peers: Two LSRs which use LDP to exchange label/stream mapping Information exchange known as LDP Session

  • LDP MessagesDiscovery messages Used to announce and maintain the presence of an LSRSession/Adjacency messages Used to establish, maintain and terminate sessions between LDP peersAdvertisement messagesUsed to create, change, and delete label mappingsNotification messagesUsed to provide advisory information and to signal error information

  • Forwarding Equivalence Class (FEC)Introduced to denote packet forwarding classesComprises traffic To a particular destinationTo destination with distinct service requirements

  • LSP - FEC MappingFEC specified as a set of two elements IP Address Prefix - any length from 0 32Host Address - 32 bit IP addressA given packet matches a particular LSP if and only if IP Address Prefix FEC element matches packets IP destination address

  • Label SpacesUseful for assignment and distribution of labelsTwo types of label spacesPer interface label space: Interface-specific labels used for interfaces that use interface resources for labelsPer platform label space: Platform-wide incoming labels used for interfaces that can share the same label space

  • LDP DiscoveryA mechanism that enables an LSR to discover potential LDP peersAvoids unnecessary explicit configuration of LSR label switching peers Two variants of the discovery mechanismBasic discovery mechanism: used to discover LSR neighbors that are directly connected at the link levelExtended discovery mechanism: used to locate LSRs that are not directly connected at the link level

  • LDP Discovery (Contd)Basic discovery mechanismTo engage - send LDP Hellos periodicallyLDP Hellos sent as UDP packets for all routers on that subnetExtended discovery mechanismTo engage - send LDP targeted Hellos periodicallyTargeted Hellos are sent to a specific addressTargeted LSR decides whether to respond or to ignore the targeted Hello

  • Session EstablishmentExchange of LDP discovery Hellos triggers session establishmentTwo step processTransport connection establishment If LSR1 does not already have a LDP session for the exchange of label spaces LSR1:a and LSR2:b, it attempts to open a TCP connection with LSR2LSR1 determines the transport addresses at its end (A1) and LSR2s end (A2) of the TCP connectionIf A1>A2, LSR1 plays the active role; otherwise it is passiveSession initializationNegotiate session parameters by exchanging LDP initialization messages

  • Label Distribution and ManagementTwo label distribution techniquesDownstream on demand label distribution: An LSR can distribute a FEC label binding in response to an explicit requestDownstream Unsolicited label distribution: Allows an LSR to distribute label bindings to LSRs that have not explicitly requested themBoth can be used in the same network at the same time; however, each LSR must be aware of the distribution method used by its peer

  • Label Distribution Control ModeIndependent Label Distribution ControlEach LSR may advertise label mappings to its neighbors at any timeIndependent Downstream on Demand mode - LSR answers without waiting for a label mapping from next hopIndependent Downstream Unsolicited mode - LSR advertises label mapping for a FEC whenever it is preparedConsequence: upstream label can be advertised before a downstream label is received

  • Distribution Control Mode (contd)Ordered Label Distribution ControlInitiates transmission of label mapping for a FEC only if it has next FEC next hop or is the egressIf not, the LSR waits till it gets a label from downstream LSRLSR acts as an egress for a particular FEC, ifNext hop router for FEC is outside of label switching networkFEC elements are reachable by crossing a domain boundary

  • Label Retention ModeConservative Label Retention ModeAdvertised label mappings are retained only if they are used for forwarding packetsDownstream on Demand Mode typically used with Conservative Label Retention ModeAdvantage: only labels required are maintainedDisadvantage: a change in routing causes delayLiberal Retention ModeAll label mappings are retained regardless of whether LSR is next hop or notFaster reaction to routing changes

  • Label Information BaseLSR maintains learned labels in Label Information Base (LIB)Each entry of LIB associates an FEC with an (LDP Identifier, label) pairWhen next hop changes for a FEC, LSR will retrieve the label for the new next hop from the LIB

  • Hierarchical Routing in MPLS

    Domain #3Domain #2

    Domain #1C123456DEBAFExternal Routers A,B,C,D,E,F - Talk BGPInternal Routers 1,2,3,4,5,6 - Talk OSPFNote: Internal routers in domains 1 and 3 not shown

  • Hierarchical Routing (contd)When IP packet traverses domain #2, it will contain two labels, encoded as a label stackHigher level label used between routers C and D, which is encapsulated inside a lower level label used within Domain #2Operation at CC needs to swap BGP label to put label that D expectsC also needs to add an OSPF label that 1 expectsC therefore pushes down the BGP label and adds a lower level label

  • Explicit Routing in MPLSTwo options for route selection:Hop by hop routingExplicit routingExplicit Routing (Source Routing) is a very powerful techniqueWith pure datagram routing, overhead of carrying complete explicit route is prohibitiveMPLS allows explicit route to be carried only at the time the LSP is setup, and not with each packetMPLS makes explicit routing practical

  • Explicit Routing (Contd)In an explicitly routed LSP LSP next hop is not chosen by the local nodeSelected by a single node, usually the ingressThe sequence of LSRs may be chosen byConfiguration (e.g., by an operator or by a centralized server)

  • Loops and Loop HandlingRouting protocols used in conjunction with MPLS are based on distributed computation which may contain loopsLoops handling - 3 categoriesLoop Mitigation/SurvivalLoop DetectionLoop Prevention

  • Loop MitigationMinimizes the impact of loops by limiting the amount of resources consumed by the loopMethodBased on use of TTL field which is decremented at each hopUse of dynamic routing protocol converging rapidly to non-looping paths

  • Loop DetectionLoops may be setup but they are subsequently detected The detected loop is then broken by dropping label relationshipBroken loops now necessitates packets to be forwarded using L3 forwarding

  • Loop Detection (Contd)Method is based on transmitting a Loop Detection Control Packet (LDCP) whenever a route changesLDCP is forwarded towards the destination untilLast MPLS node along the path is reachedTTL of the LDCP expiresIt returns to the node which originated it

  • Loop PreventionEnsures that loops are never set upLabels are not used until it is sure to be loop freeMethodsLabels are propagated starting at the egress switchUse source routing to set up label bindings from the egress switch to each ingress switch

  • QoS in MPLS

  • StrategyTo support end-to-end QoS as in IPMPLS not an end-to-end protocolEfficient ways of mapping QoS to LSPsTraffic Engineering key to QoS

  • QoS ModelsBest effort Original IP serviceInt-serv.Fist IP effort to support QoSDiff-serv.Simple, scalableFutureInt+ Diff+ TE with e2e SLAs

  • CISCO QoS FrameworkPROVISIONING & MONITORINGSignaling Techniques (RSVP, DSCP*, ATM (UNI/NNI))Link Efficiency Mechanisms (Compression, Fragmentation)Congestion Avoidance Techniques (WRED)Congestion Management Techniques (WFQ, CBWFQ, LLQ)Classification & Marking Techniques (DSCP, MPLS EXP, NBAR, etc.)POLICY-BASED NETWORKINGTraffic Conditioners (Policing, Shaping)

  • Support of RSVPVery similar to tag switchingBind labels to reserved flowsLabel object inside the RESV messageLabels propagate upstreamOnly the edge router need to know the packet to flow mappingCan aggregate flows instead of micro-flows

  • RSVP ScalabilityAggregationRefresh reductionUse acknowledgements for refreshOnce received, increase the refresh timeSummary refresh

  • Diff-Serv SupportE-LSPQueue inferred from Label and EXP fieldDrop priority inferred from label and EXP fieldL-LSPQueue inferred exclusively from Label Drop priority inferred from EXP field

  • E-LSPE-LSPs established by various label binding protocols (LDP or RSVP)no new Signalling needed.EF and AF1 on a single E-LSPEF and AF1 packets travel on single LSP (single label) but are enqueued in different queues (different EXP values)Queue & Drop Precedence is selected based on EXP

  • E-LSPVersionLengthToS1 ByteLenStandard IPV4: Bits 0-2 Called IP Precedence (Three MSB)(DiffServ Uses Six ToS bits: Bits 0-5, with Two Reserved)IDoffsetTTLProtoFCSIP-SAIP-DADataReferred to as Packet Classification or Coloring

  • IP Precedence to Label EXP

  • E-LSP vs. L-LSPPHB from EXPNo additional signalingEXP->PHB configuredShim header requiredUp to 8 PHBs per LSP

    PHB from label + Exp/CLPSignaled at LSP setupLabel->PHB mapped Shim or link layer header usedArbitrarily large

  • Explicit Congestion Notification(ECN)TCP approach based on packet dropMay not reflect the statusResources could have been wastedEarly notificationMark packetsReceiver conveys information to senderTwo bits used to deal with deployment disparity (CE & ECT)

  • MPLS Support of ECNCould use two bits as beforeMay not be availableUsually 1 bit availableLSRs should have the understanding on mapping

  • Traffic Engineering in MPLS

  • Traffic Engineering ObjectivesTraffic Engineering (TE) concerned with performance optimization The key performance objectives traffic oriented e.g. minimization of packet loss resource oriented - optimization of resource utilization e.g. efficient management of bandwidth

  • Objectives (contd)Minimizing congestion is a major traffic and resource oriented performance objectiveCongestion manifest under two scenariosNetwork resources insufficient or inadequateSolved by capacity expansion or classical congestion control techniquesInefficient mapping of traffic streams onto available resourcesReduced by adopting load balancing policies

  • MPLS and Traffic EngineeringMain components usedTraffic Trunk - aggregation of traffic flows of the same class which are placed inside a Label Switched PathInduced MPLS Graph Analogous to a virtual topology in an overlay modelLogically mapped onto the physical network Set of LSRs as nodes of the graph Set of LSPs providing logical point to point connectivity between LSRs as edges

  • Constraint Based Routing (CBR)Associate each path with set of constraintsPerformance, administrativeLocal informationRouting algorithms Optimizes various metricsEnsures that the constraints are not violated

  • Can IP Routing Do CBR?Plain IP routing cannotCBR has to be source based each source may have different constraint to same destinationLink attributes need to be distributedNeed explicit routing instead of destination-basedCan be augmented to support CBRUsually a combination is used

  • CBR ComponentsMechanism for source based path computingMechanism to collect necessary informationConstraints (local), attributes, topologySupport forwarding along the computed pathsNotification of residual resources after allocation

  • Constrain-Based SPF247531615045150150150150150

  • CSPFUses the following inputsLink attributesTopology state informationPath constraintsBasic approachPrune resources that do not meet the constraintsRun a shortest path algorithm on the residual graph

  • MPLS for ForwardingIdeal to use MPLS explicit routing capabilityOnce the path is computed Need to establish forwarding state along the pathReserve resources along the pathTwo approachesRSVP extensionsCR-LDP

  • CBR (contd)Strict & Loose Explicit RoutesConstraint Based LSP (CRLSP) is calculated at one point at the edge of the network based on certain criteriaspecial char. such as assigning certain bandwidth can be supportedThe route is encoded as a series of Explicit routed hops contained in a CR based route TLV

  • CBR (contd)Comparison of RSVP and CR-LDPScalabilitySignaling mechanismQos Models

  • Application of CR in TEIP?ATMOverlayMPLS

  • TE in MPLS - II

  • Fish NetworkR8R1R5R2R3R4R7R6150150150150150

  • Is Plain IP Enough?R8R1R5R2R3R4R7R6150150150150150Under utilized

  • Why IP Routing Fails Based only on metric optimizationShortest pathAdministrative optimizationSplit pathsPer link constraints not taken into consideration

  • TE in MPLS Using CBR Define traffic trunksCollection of micro-flows that share same path and class of serviceThese are not end-to-end paths, rather paths within a single service providerNo. of trunks dependent only on the topology Forwarding table does not grow with the traffic ReroutingRSVP, CR-LDP, or IGP

  • Forwarding PacketsR1R5R2R3R4R7R6150150150150150

  • Fast ReroutingTotal restoration time after failureFailure detection timePropagationComputation of new pathUsually the 2nd and 3rd steps are significantly slow

  • Is FR possible with IP?R1R4R3R2R5Even if the traffic is rerouted to R3, it will that back to R1 since R3 is not aware of the failureX

  • FR using CBRCompute protection LSP for every linkWhen a failure happensTraffic rerouted to the protection LSPUse label stacking for the transit within the protection LSPBeyond the end-nodes labels original labels remain in tact