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CMU CS 15441 - lecture

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Page ‹#›1Lecture 8Virtual Circuits, ATM, MPLSDavid AndersenSchool of Computer ScienceCarnegie Mellon University15-441 Networking, Fall 2006http://www.cs.cmu.edu/~srini/15-441/F06/2Outline Exam discussion Layering review (bridges, routers, etc.)» Exam section C. Circuit switching refresher Virtual Circuits - general» Why virtual circuits?» How virtual circuits? -- tag switching! Two modern implementations» ATM - teleco-style virtual circuits» MPLS - IP-style virtual circuitsPage ‹#›3Exam statsMax/avg/min: 90 / 63 / 2068.6%58.3%74.8%57.6%11.012.817.919.6DCBA4Common Exam Problems Routing: No one big problem; many smallmisunderstandings. Please check yourscores. Short answer: Many incorrect round-triptimes 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 serverdoes. Could run on clients, but usually doesn’t. Routing and bridging and addressing…Page ‹#›5Packet Switching Source sends information as self-containedpackets that have an address.» Source may have to break up single message in multiple Each packet travels independently to thedestination host.» Routers and switches use the address in the packet todetermine how to forward the packets Destination recreates the message. Analogy: a letter in surface mail.6Circuit Switching Source first establishes a connection (circuit)to the destination.» Each router or switch along the way may reserve somebandwidth for the data flow Source sends the data over the circuit.» No need to include the destination address with the datasince the routers know the path The connection is torn down. Example: telephone network.Page ‹#›7Circuit SwitchingDiscussion Traditional circuits: on each hop, the circuithas a dedicated wire or slice of bandwidth.» Physical connection - clearly no need to includeaddresses with the data Advantages, relative to packet switching:» Implies guaranteed bandwidth, predictable performance» Simple switch design: only remembers connectioninformation, no longest-prefix destination address lookup Disadvantages:» Inefficient for bursty traffic (wastes bandwidth)» Delay associated with establishing a circuit Can we get the advantages without (all) thedisadvantages?8Virtual 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 theVC 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 circuitPage ‹#›9Packet Switching andVirtual Circuits: Similarities “Store and forward” communication based on anaddress.» 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 areinterleaved without a fixed pattern» Reservations: some flows are guaranteed to get a certainnumber of “slots”AB ACBD10Virtual Circuits VersusPacket Switching Circuit switching:» Uses short connection identifiers to forward packets» Switches know about the connections so they can moreeasily implement features such as quality of service» Virtual circuits form basis for traffic engineering: VCidentifies 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 aconnection first» Switches are stateless: easier to recover from failures» Adding QoS is hard» Traffic engineering is hard: too many packets!Page ‹#›11Circuit SwitchingInputPortsOutputPortsSwitchConnects (electrons or bits) ports to ports12Packet switched vs. VCABR2R1R3R4R1 packetforwardingtable:Dst R2R1 VC table:VC 1 R2VC 2 R3Different paths tosame destination!(useful for trafficengineering!)VCIPayloadDstPayloadDst1234333111224442Page ‹#›13Virtual CircuitABR2R1R3R4R1 VC table:VC 5 R2VCIPayloadPayloadDst1234333111224442R2 VC table:VC 5 R4Challenges: - How to set up path? - How to assign IDs??14Connections 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 pathPage ‹#›15Virtual 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 9R2: 2 9 4 2R4: 1 2 3 5ABR2R1R3R4 Dst123433311122444216Label (“tag”) Swapping Result: Signalling protocol must only findper-link unused VCIs.» “Link-local scope”» Connection setup can proceed hop-by-hop.– Good news for our setup protocols!Page ‹#›17PVC 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 up18SVC Connection Setupcallingpartynetwork calledpartySETUPSETUPCONNECTACKCONNECTACKCONNECTCONNECTPage ‹#›19Virtual 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.


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