Lecture 8 Virtual Circuits, ATM, MPLSOutlineExam statsCommon Exam ProblemsPacket SwitchingCircuit SwitchingCircuit Switching DiscussionVirtual CircuitsPacket Switching and Virtual Circuits: SimilaritiesVirtual Circuits Versus Packet SwitchingSlide 11Packet switched vs. VCVirtual CircuitConnections and SignalingVirtual Circuit Switching: Label (“tag”) SwappingLabel (“tag”) SwappingPVC connection setupSVC Connection SetupVirtual Circuits In PracticeAsynchronous Transfer Mode: ATMCell SwitchingATM FeaturesWhy 53 Bytes?ATM Adaptation LayersAAL5 Adaptation LayerATM Packet Shredder EffectIP over ATMIP over ATM Static VCsATM DiscussionMulti Protocol Label Switching - MPLSMPLS + IPMPLS core, IP interfaceMPLS use case #1: VPNsMPLS use case #2: Reduced State CoreMPLS use case #3: Traffic EngineeringMPLS MechanismsMPLS DiscussionTake Home Points--- Extra Slides ---ATM Traffic ClassesLAN EmulationFurther reading - MPLSIP SwitchingIP Switching ExampleSlide 45Slide 46Another ViewIP Switching DiscussionAn Alternative Tag SwitchingIP Switching versus Tag SwitchingPackets over SONET1Lecture 8Virtual Circuits, ATM, MPLSDavid AndersenSchool of Computer ScienceCarnegie Mellon University15-441 Networking, Fall 2006http://www.cs.cmu.edu/~srini/15-441/F06/2OutlineExam discussionLayering review (bridges, routers, etc.)»Exam section C.Circuit switching refresherVirtual Circuits - general»Why virtual circuits?»How virtual circuits? -- tag switching!Two modern implementations»ATM - teleco-style virtual circuits»MPLS - IP-style virtual circuits3Exam statsMax/avg/min: 90 / 63 / 20A B C D19.6 17.9 12.8 11.057.6% 74.8% 58.3% 68.6%4Common Exam ProblemsRouting: No one big problem; many small misunderstandings. Please check your scores.Short answer: Many incorrect round-trip times vs. one-way times.DNS»Always sends the full query! (e.g. “ra1.streaming.npr.org”, not just “npr.org”)»Clients don’t recurse; the local recursive DNS server does. Could run on clients, but usually doesn’t.Routing and bridging and addressing…5Packet SwitchingSource sends information as self-contained packets that have an address.»Source may have to break up single message in multipleEach packet travels independently to the destination host.»Routers and switches use the address in the packet to determine how to forward the packetsDestination recreates the message.Analogy: a letter in surface mail.6Circuit SwitchingSource first establishes a connection (circuit) to the destination.»Each router or switch along the way may reserve some bandwidth for the data flowSource sends the data over the circuit.»No need to include the destination address with the data since the routers know the pathThe connection is torn down.Example: telephone network.7Circuit Switching DiscussionTraditional circuits: on each hop, the circuit has a dedicated wire or slice of bandwidth.»Physical connection - clearly no need to include addresses with the dataAdvantages, relative to packet switching:»Implies guaranteed bandwidth, predictable performance »Simple switch design: only remembers connection information, no longest-prefix destination address look upDisadvantages:»Inefficient for bursty traffic (wastes bandwidth)»Delay associated with establishing a circuitCan we get the advantages without (all) the disadvantages?8Virtual CircuitsEach wire carries many “virtual” circuits. »Forwarding based on virtual circuit (VC) identifier– IP header: src, dst, etc.–Virtual circuit header: just “VC”»A path through the network is determined for each VC when the VC is established»Use statistical multiplexing for efficiencyCan support wide range of quality of service.»No guarantees: best effort service»Weak guarantees: delay < 300 msec, …»Strong guarantees: e.g. equivalent of physical circuit9Packet Switching andVirtual Circuits: Similarities“Store and forward” communication based on an address.»Address is either the destination address or a VC identifierMust have buffer space to temporarily store packets.»E.g. multiple packets for some destination arrive simultaneouslyMultiplexing on a link is similar to time sharing.»No reservations: multiplexing is statistical, i.e. packets are interleaved without a fixed pattern»Reservations: some flows are guaranteed to get a certain number of “slots”AB ACBD10Virtual Circuits Versus Packet Switching Circuit switching:»Uses short connection identifiers to forward packets»Switches know about the connections so they can more easily implement features such as quality of service»Virtual circuits form basis for traffic engineering: VC identifies long-lived stream of data that can be scheduledPacket switching:»Use full destination addresses for forwarding packets»Can send data right away: no need to establish a connection first»Switches are stateless: easier to recover from failures»Adding QoS is hard»Traffic engineering is hard: too many packets!11Circuit SwitchingInput PortsOutput PortsSwitchConnects (electrons or bits) ports to ports12Packet switched vs. VCABR2R1R3R4R1 packet forwarding table:Dst R2R1 VC table:VC 1 R2VC 2 R3Different paths to same destination!(useful for traffic engineering!)VCIVCIPayloadDstDstPayloadDst123433311122444213Virtual CircuitABR2R1R3R4R1 VC table:VC 5 R2VCIVCIPayloadPayloadDst1234333111224442R2 VC table:VC 5 R4Challenges: - How to set up path? - How to assign IDs??14Connections and SignalingPermanent vs. switched virtual connections (PVCs, SVCs)»static vs. dynamic. PVCs last “a long time”–E.g., connect two bank locations with a PVC that looks like a circuit–SVCs are more like a phone call»PVCs administratively configured (but not “manually”)»SVCs dynamically set up on a “per-call” basisTopology»point to point»point to multipoint»multipoint to multipointChallenges:»How to configure these things?–What VCI to use?–Setting up the path15Virtual Circuit Switching:Label (“tag”) SwappingGlobal VC ID allocation -- ICK! Solution: Per-link uniqueness. Change VCI each hop. Input Port Input VCI Output Port Output VCI R1: 1 5 3 9 R2: 2 9 4 2R4: 1 2 3 5ABR2R1R3R4 Dst123433311122444216Label (“tag”) SwappingResult: Signalling protocol must only find per-link unused VCIs.»“Link-local scope”»Connection
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