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CMU 15441 Computer Networking - Lecture

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Lecture 13 Virtual Circuits, ATMPacket SwitchingCircuit SwitchingCircuit Switching DiscussionVirtual CircuitsPacket Switching and Virtual Circuits: SimilaritiesVirtual Circuits Versus Packet SwitchingSlide 8Packet 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 DiscussionATM + IPIP SwitchingIP Switching ExampleSlide 30Slide 31Another ViewIP Switching DiscussionAn Alternative Tag SwitchingIP Switching versus Tag Switching1Lecture 13Virtual Circuits, ATM15-441 Networking, Fall 2007These slides swiped, with pride, from Dave Anderson, 15-441 Spring 20062Packet SwitchingSource sends information as self-contained packets that have an address.»Source may have to break up single message in multipleEach packet travels independently to the destination host.»Routers and switches use the address in the packet to determine how to forward the packetsDestination recreates the message.Analogy: a letter in surface mail.3Circuit SwitchingSource first establishes a connection (circuit) to the destination.»Each router or switch along the way may reserve some bandwidth for the data flowSource sends the data over the circuit.»No need to include the destination address with the data since the routers know the pathThe connection is torn down.Example: telephone network.4Circuit Switching DiscussionTraditional circuits: on each hop, the circuit has a dedicated wire or slice of bandwidth.»Physical connection - clearly no need to include addresses with the dataAdvantages, relative to packet switching:»Implies guaranteed bandwidth, predictable performance »Simple switch design: only remembers connection information, no longest-prefix destination address look upDisadvantages:»Inefficient for bursty traffic (wastes bandwidth)»Delay associated with establishing a circuitCan we get the advantages without (all) the disadvantages?5Virtual CircuitsEach 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 efficiencyCan support wide range of quality of service.»No guarantees: best effort service»Weak guarantees: delay < 300 msec, …»Strong guarantees: e.g. equivalent of physical circuit6Packet Switching andVirtual Circuits: Similarities“Store and forward” communication based on an address.»Address is either the destination address or a VC identifierMust have buffer space to temporarily store packets.»E.g. multiple packets for some destination arrive simultaneouslyMultiplexing 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 ACBD7Virtual 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 scheduledPacket 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!8Circuit SwitchingInput PortsOutput PortsSwitchConnects (electrons or bits) ports to ports9Packet switched vs. VCABR2R1R3R4R1 packet forwarding table:Dst R2R1 VC table:VC 1 R2VC 2 R3Different paths to same destination!(useful for traffic engineering!)VCIVCIPayloadDstDstPayloadDst123433311122444210Virtual CircuitABR2R1R3R4R1 VC table:VC 5 R2VCIVCIPayloadPayloadDst1234333111224442R2 VC table:VC 5 R4Challenges: - How to set up path? - How to assign IDs??11Connections and SignalingPermanent 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” basisTopology»point to point»point to multipoint»multipoint to multipointChallenges:»How to configure these things?–What VCI to use?–Setting up the path12Virtual Circuit Switching:Label (“tag”) SwappingGlobal 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 Dst123433311122444213Label (“tag”) SwappingResult: Signalling protocol must only find per-link unused VCIs.»“Link-local scope”»Connection setup can proceed hop-by-hop.–Good news for our setup protocols!14PVC connection setupManual?»Configure each switch by hand. Ugh.Dedicated signalling protocol»E.g., what ATM usesPiggyback on routing protocols»Used in MPLS. E.g., use BGP to set up15SVC Connection Setupcallingpartynetwork calledpartySETUPSETUPCONNECTACKCONNECTACKCONNECTCONNECT16Virtual Circuits In PracticeATM: Teleco approach»Kitchen sink. Based on voice, support file transfer, video, etc., etc.»Intended as IP replacement. That didn’t happen. :)»Today: Underlying network protocol in many teleco networks. E.g., DSL speaks ATM. IP over ATM in some cases.MPLS: The “IP Heads” answer to ATM»Stole good ideas from ATM»Integrates well with IP»Today: Used inside some networks to provide VPN support, traffic engineering, simplify core.Other nets just run IP.Older tech: Frame Relay»Only provided PVCs. Used for quasi-dedicated 56k/T1 links between offices, etc. Slower, less flexible than ATM.17Asynchronous Transfer Mode: ATMConnection-oriented, packet-switched»(e.g., virtual circuits).Teleco-driven. Goals:»Handle voice, data, multimedia»Support both PVCs and


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