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1、0Tutorial OutlineLabel EncapsulationsLabel Distribution ProtocolsMPLS & ATMConstraint Based Routing with CR-LDPSummary1“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 stop
2、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 of gett
3、ing from A to B:2Label 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 TURN RIG
4、HT USE LANE#23A 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 - a
5、label is an LCN Proprietary PORS, TAG etc. One day perhaps Frequency substitution where label is a light frequency?4SO WHAT IS MPLS ? Hop-by-hop or source routing to establish labels Uses label native to the media Multi level label substitution transport5ROUTE AT EDGE, SWITCH IN COREIP ForwardingLAB
6、EL SWITCHINGI n t fI nL a b e lI nD e s tI n t fO u t3 0 . 4 04 7 . 1 1I n t fI nL a b e lI nD e s tI n t fO u t3 0 . 4 04 7 . 1 1IP ForwardingIPIP#L1IP#L2IP#L3IP6MPLS: HOW DOES IT WORK UDP-HelloUDP-HelloTCP-openTIMETIMELabel requestIPLabel mapping#L2Initialization(s)7WHY MPLS ?Leverage existing ATM
7、 hardwareUltra fast forwarding IP Traffic EngineeringConstraint-based RoutingVirtual Private NetworksControllable tunneling mechanismVoice/Video on IPDelay variation + QoS constraints8BEST OF BOTH WORLDSPACKETROUTINGCIRCUITSWITCHINGMPLS + IP form a middle ground that combines the best of IP and the
8、best of circuit switching technologies.ATM and Frame Relay cannot easily come to the middle so IP has!MPLS+IPIPATMHYBRID9MPLS TerminologyLDP: Label Distribution Protocol LSP: Label Switched PathFEC: Forwarding Equivalence ClassLSR: Label Switching RouterLER: Label Edge Router (Useful term not in sta
9、ndards)10Forwarding Equivalence ClassesFEC = “A subset of packets that are all treated the same way by a routerThe concept 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 onc
10、e at the network ingressPackets are destined for different address prefixes, but can bemapped to common pathI n t fI nL a b e lI nD e s tI n t fO u t3 0 . 4 04 7 . 1 1I n t fI nL a b e lI nD e s tI n t fO u t3 0 . 4 04 7 . 1 1IP1IP2IP1IP2LSRLSRLERLERLSPIP1#L1IP2#L1IP1#L2IP2#L2IP1#L3IP2#L311MPLS BUIL
11、T ON STANDARD IP47.147.247.3IntfInD estIntfO utLabelO ut347.1.1 21.33347.110.50123D e s tO u t4 7 .114 7 .224 7 .33D e s tO u t4 7 .114 7 .224 7 .33123123 Destination based forwarding tables as built by OSPF, IS-IS, RIP, etc.12IP FORWARDING USED BY HOP-BY-HOP CONTROL47.147.247.3IP 47.1.1.1In tfInD e
12、 s t In tfO u tL a b e lO u t3 4 7 .1 10 .5 012312123NoImage13DestOut47. 1147. 2247. 33MPLS Label Distribution47.147.247.3123121233NoImageMapping: 0.40Request: 47.1Mapping: 0.50Request: 47.114Label Switched Path (LSP)DestOut47. 1147. 2247. 3347.147.247.3123121233NoImageIP 47.1.1.1IP 47.1.1.115DestOu
13、t47. 1147. 2247. 3347.147.247.3123121233IP 47.1.1.1IP 47.1.1.1EXPLICITLY ROUTED LSP ER-LSP16Tutorial OutlineOverviewLabel Distribution ProtocolsMPLS & ATMConstraint Based Routing with CR-LDPSummary17Label EncapsulationATMFREthernetPPPMPLS Encapsulation is specified over various media types. Top labe
14、ls may use existing format, lower label(s) use a new “shim label format.VPI VCIDLCI“Shim LabelL2Label“Shim Label .IP | PAYLOAD18MPLS Link LayersMPLS is intended to run over multiple link layersSpecifications for the following link layers currently exist: ATM: label contained in VCI/VPI field of ATM
15、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-protocol below as well as above19MPLS Encapsulation - ATMATM LSR constrained by the cell format
16、 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 HeaderATM Payload Top 1 or 2 labels are contained in the VPI/VCI fields of ATM header - one in each o
17、r 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 carried in top label in stack, as a proxy for ATM header (that lacks TTL)48 Bytes48 BytesLabelLa
18、belOption 1Option 2Combined LabelOption 3LabelATM VPI (Tunnel)20MPLS Encapsulation - Frame Relayn1DLCIC/READLCIFECNBECNDEEAQ.922HeaderGeneric Encap.(PPP/LAN Format)Layer 3 Header and PacketDLCI Size = 10, 17, 23 Bits Current label value carried in DLCI field of Frame Relay header Can use either 2 or
19、 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 value21MPLS Encapsulation - PPP & LAN Data LinksLabelExp.STTLLabel: Label Value, 20 bits (0-16 reserved)Exp.: Experimental, 3 b
20、its (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)1n Network layer must be inferable from value of bottom label of the stack TTL must be set to
21、the value of the IP TTL field when packet is first labelled When last label is popped off stack, MPLS TTL to be copied to IP TTL field Pushing multiple labels may cause length of frame to exceed layer-2 MTU - LSR must support “Max. IP Datagram Size for Labelling parameter - any unlabelled datagram g
22、reater 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 Format22Tutorial OutlineOverviewLabel EncapsulationsMPLS & ATMIETF StatusNortels ActivitySummary23Label Distribution ProtocolsOverview of Hop
23、-by-hop & ExplicitLabel Distribution Protocol (LDP)Constraint-based Routing LDP (CR-LDP)Extensions to RSVPExtensions to BGP24Comparison - Hop-by-Hop vs. Explicit RoutingHop-by-Hop RoutingExplicit RoutingSource routing of control trafficBuilds a path from source to destRequires manual provisioning, o
24、r 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-based, Adapts well to traffic engineeringDistributes routing of control trafficBuilds a set of trees eit
25、her 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 destination prefix basedDifficult to perform traffic engineering, QoS-based routingExplicit routing shows great
26、 promise for traffic engineering25Explicit Routing - MPLS vs. Traditional RoutingConnectionless nature of IP implies that routing is based on information in each packet header Source routing is possible, but path must be contained in each IP headerLengthy paths increase size of IP header, make it va
27、riable 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 operators may filter source routed packets for security reasonsMPLSs enables the use of source routing by it
28、s 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 makes the use of source routing in the Internet practical26Label Distribution ProtocolsOverview of Hop-
29、by-hop & ExplicitLabel Distribution Protocol (LDP)Constraint-based Routing LDP (CR-LDP)Extensions to RSVPExtensions to BGP27Label Distribution Protocol (LDP) - PurposeNoImageNoImageLabel distribution ensures that adjacent routers havea common view of FEC label bindingsRouting Table:Addr-prefix Next
30、HopLSR1LSR2NoImageLSR3IP 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 valueStep 2: LSR communicatesbinding to adjacent LSRStep 3: LSR inserts labelvalue i
31、nto 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 created28Label Distribution - MethodsNoImageNoImageLSR1LSR2Label Distribution can take
32、 place using one of two possible methodsDownstream Label DistributionLabel-FEC Binding LSR2 and LSR1 are said to have an “LDP adjacency (LSR2 being the downstream LSR) LSR2 discovers a next hop for a particular FEC LSR2 generates a label for the FEC and communicates the binding to LSR1 LSR1 inserts
33、the binding into its forwarding tables If LSR2 is the next hop for the FEC, LSR1 can use that label knowing that its meaning is understoodNoImageNoImageLSR1LSR2Downstream-on-Demand Label DistributionLabel-FEC Binding LSR1 recognizes LSR2 as its next-hop for an FEC A request is made to LSR2 for a bin
34、ding between the FEC and a label If LSR2 recognizes the FEC and has a next hop for it, it creates a binding and replies to LSR1 Both LSRs then have a common understandingRequest for BindingBoth methods are supported, even in the same network at the same timeFor any single adjacency, LDP negotiation
35、must agree on a common method29I n t fI nL a b e lI nD e s tI n t fO u t3 0 . 4 04 7 . 1 1I n t fI nL a b e lI nD e s tI n t fO u t3 0 . 4 04 7 . 1 1I n t fI nL a b e lI nD e s tI n t fO u t3 0 . 4 04 7 . 1 1I n t fI nL a b e lI nD e s tI n t fO u t3 0 . 4 04 7 . 1 1I n t fI nL a b e lI nD e s tI n
36、t fO u t3 0 . 4 04 7 . 1 1I n t fI nL a b e lI nD e s tI n t fO u t3 0 . 4 04 7 . 1 1I n t fI nL a b e lI nD e s tI n t fO u t3 0 . 4 04 7 . 1 1I n t fI nL a b e lI nD e s tI n t fO u t3 0 . 4 04 7 . 1 1#963#14#99#311#311#311DOWNSTREAM ON DEMAND MAKING SPF TREE COPY IN H/W#462D#311D#963D#14D#99D#216
37、D#612D#5DD?D?D?D?D?D?D?D?30Label Distribution ProtocolsOverview of Hop-by-hop & ExplicitLabel Distribution Protocol (LDP)Constraint-based Routing LDP (CR-LDP)Extensions to RSVP31Constraint-based LSP Setup using LDPUses LDP Messages (request, map, notify)Shares TCP/IP connection with LDPCan coexist w
38、ith 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 ER, and other “Constraints 32ER-LSP Setup using CR-LDPLSR BLSR CLER DLER AER Label Switched PathIngressEgressI n t fI nL a b e lI nD e s tI n t fO u t3 0 . 4 0
39、4 7 . 1 1I n t fI nL a b e lI nD e s tI n t fO u t3 0 . 4 04 7 . 1 1I n t fI nL a b e lI nD e s tI n t fO u t3 0 . 4 04 7 . 1 1I n t fI nL a b e lI nD e s tI n t fO u t3 0 . 4 04 7 . 1 14. Label mapping message originates.3. Request message terminates.2. Request message processed and next node deter
40、mined. Path list modified to 1. Label Request message. It contains ER path In tfInL a b e lInD e s tIn tfO u tL a b e lO u t30 .5 04 7 .110 .4 05. LSR C receives label to use for sending data to LER D. Label table updated6. When LER A receives label mapping, the ER established.33CR-LDP PREEMPTIONA C
41、R-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. 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
42、LSPs.These are called the setupPriority and a holdingPriority and 8 levels are provided.34CR-LDP PREEMPTIONWhen an LSP is established its setupPriority is compared with the holdingPriority of existing LSPs, any with lower holdingPriority may be bumped to obtain their resources. This process may cont
43、inue in a domino fashion until the lowest holdingPriority LSPs either clear or are on the worst routes.35ER-LSP setup using RSVPLSR BLSR CLER DLER AI n t fI nL a b e lI nD e s tI n t fO u t3 0 . 4 04 7 . 1 1I n t fI nL a b e lI nD e s tI n t fO u t3 0 . 4 04 7 . 1 1I n t fI nL a b e lI nD e s tI n t
44、 fO u t3 0 . 4 04 7 . 1 1I n t fI nL a b e lI nD e s tI n t fO u t3 0 . 4 04 7 . 1 11. Path message. It contains ER path In tfInL a b e lInD e s tIn tfO u tL a b e lO u t30 .5 04 7 .110 .4 02. New path state. Path message sent to next node3. Resv message originates. Contain the label to use and the
45、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 refresh unless suppressedPer-hop Path and Resv refresh unless suppressedPer-hop Path and Resv refresh unless suppressed36Tutorial OutlineOverviewL
46、abel EncapsulationsLabel Distribution ProtocolsMPLS & ATMConstraint Based Routing with CR-LDP37Traffic EngineeringNoImageNoImageNoImageNoImageABCDTraffic engineering is the process of mapping traffic demand onto a networkDemandNetworkTopologyPurpose of traffic engineering: Maximize utilization of li
47、nks and nodes throughout the network Engineer links to achieve required delay, grade-of-service Spread the network traffic across network links, minimize impact of single failure Ensure available spare link capacity for re-routing traffic on failure Meet policy requirements imposed by the network op
48、eratorTraffic engineering key to optimizing cost/performance38Traffic Engineering AlternativesCurrent methods of traffic engineering:Manipulating routing metricsUse PVCs over an ATM backboneOver-provision bandwidthDifficult to manageNot scalableNot economicalMPLS combines benefits of ATM and IP-laye
49、r 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)Ingress nodeexplicitly routestraffic over uncongested pathPotential benefits of MPLS for traffic engineering: - a
50、llows explicitly routed paths - no “n-squared problem - per FEC traffic monitoring - backup paths may be configured operator controlscalable granularity of feedback redundancy/restorationCongested Node39MPLS Traffic Engineering Methods MPLS can use the source routing capability to steer traffic on d