Page 115-441: NetworkingVirtual Circuits, ATM, MPLS1Peter SteenkisteCarnegie Mellon UniversityLecture Feb 23, 2010Outlinez Circuit switching refresherz Virtual Circuits - general» Why virtual circuits?» How virtual circuits? -- tag switching!2ggz Two modern implementations» ATM - teleco-style virtual circuits» MPLS - IP-style virtual circuitsPacket Switchingz Source sends information as self-contained packets that have an address.» Source may have to break up single message in multiplezEach packet travels independently to the destination host.3» Routers and switches use the address in the packet to determine how to forward the packetszDestination recreates the message.z Analogy: a letter in surface mail.Circuit Switchingz Source first establishes a connection (circuit) to the destination.» Each router or switch along the way may reserve some bandwidth for the data flowzSource sends the data over the circuit.4» No need to include the destination address with the data since the routers know the pathzThe connection is torn down.z Example: telephone network.Circuit Switching Discussionz 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 datazAdvantages, relative to packet switching:»Implies guaranteed bandwidth predictable performance5»Implies guaranteed bandwidth, predictable performance » Simple switch design: only remembers connection information, no longest-prefix destination address look upzDisadvantages:» Inefficient for bursty traffic (wastes bandwidth)» Delay associated with establishing a circuitzCan we get the advantages without (all) the disadvantages?Virtual Circuitsz 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 established6» Use statistical multiplexing for efficiencyz 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 2Packet Switching andVirtual Circuits: Similaritiesz “Store and forward” communication based on an address.» Address is either the destination address or a VC identifierz Must have buffer space to temporarily store packets.» E.g. multiple packets for some destination arrive simultaneouslyzMultiplexing on a link is similar to time sharing7zMultiplexing 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 ACBDVirtual Circuits Versus Packet Switching z 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 ff8identifies long-lived stream of data that can be scheduledzPacket 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!Circuit SwitchingSwitch9Input PortsOutput PortsConnects (electrons, light, or bits) ports to portsPacket switched vs. VCAR2R1 packet forwarding tblVCIPayloadDstPayload1333114210BR1R3R4table:Dst R2R1 VC table:VC 1 R2VC 2 R3Different paths to same destination!(useful for traffic engineering!)Dst2433112244Virtual CircuitAR2VCIPayloadPayload1333114211BR1R3R4R1 VC table:VC 5 R2Dst2433112244R2 VC table:VC 5 R4Challenges:- How to set up path?- How to assign IDs??Connections and Signalingz Permanent vs. switched virtual connections (PVCs, SVCs)» static vs. dynamic. PVCs last “a long time”– E.g., connect two bank locations with a PVC– SVCs are more like a phone call12» PVCs administratively configured (but not “manually”)» SVCs dynamically set up on a “per-call” basiszTopology» point to point» point to multipoint» multipoint to multipointzChallenges: How to configure these things?– What VCI to use?– Setting up the pathPage 3Virtual Circuit Switching:Label (“tag”) SwappingABR2R1R3R4 Dst123433311122444213z Global VC ID allocation -- ICK! Solution: Per-link uniqueness. Change VCI each hop.Input Port Input VCI Output Port Output VCIR1: 1 5 3 9R2: 2 9 4 2R4: 1 2 3 524Label (“tag”) Swappingz Result: Signalling protocol must only find per-link unused VCIs.» “Link-local scope”» Connection setup can proceed hop-by-hop.14–Good news for our setup protocols!PVC connection setupz Manual?» Configure each switch by hand. Ugh.zDedicated signalling protocol» E.g., what ATM uses15gz Piggyback on routing protocols» Used in MPLS. E.g., use BGP to set up SVC Connection Setupcallingpartynetwork calledpartySETUPSETUP16CONNECTACKCONNECTACKCONNECTCONNECTVirtual Circuits In Practicez 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.17g, pz 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.z Other nets just run IP.z Older tech: Frame Relay» Only provided PVCs. Used for quasi-dedicated 56k/T1 links between offices, etc. Slower, less flexible than ATM.Asynchronous Transfer Mode: ATMz Connection-oriented, packet-switched» (e.g., virtual circuits).zTeleco-driven. Goals:» Handle voice, data, multimedia18» Support both PVCs and SVCs» Replace IP. (didn’t happen…)zImportant feature: Cell switchingPage 4Cell Switchingz Small, fixed-size cells[Fixed-length data][header]z Why?»Efficiency: All packets the same19»Efficiency: All packets the same– Easier hardware parallelism, implementation» Switching efficiency:– Lookups are easy -- table index.» Result: Very high cell switching rates.» Initial ATM was 155Mbit/s. Ethernet was 10Mbit/s at the same time. (!)z How do
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