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CMU 15441 Computer Networking - Lecture

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15-441 Computer NetworkingOutlineHistoryOne BISDN: STMMore Flexible Solution: ATMATM FeaturesATM Standard Protocol LayersThe ATM Cell (UNI)Why 53 Bytes?Virtual Circuit SwitchingVirtual PathsVirtual Path TrunkingATM Adaptation LayersAAL3/4 Adaptation Layer (Telco)SEAL (AAL5) Adaptation Layer (computer mfr.)AAL Relative MeritsATM Traffic ClassesUBR ChallengesABR: Max-Min Fair SharingMax-Min Fair Sharing ExampleConnections and SignalingConnection SetupQ.SAAL: Signaling ATM Adaptation LayerIP over ATM and SONETIP over ATMLAN EmulationATM ARPIP over ATM (2)Packets over SONETATM DiscussionIP SwitchingIP Switching ExampleSlide 33Slide 34IP Switching DiscussionAn Alternative: Tag SwitchingIP Switching versus Tag SwitchingMulti-Protocol Label Switching MPLSMPLS Mechanisms15-441 Computer NetworkingATM and Label SwitchingLecture #19: 11-08-01 2Outline•ATM.•IP over ATM.•Label switching.Lecture #19: 11-08-01 3History•Telephone companies supported voice telephony: 4 kHz analog, 64 kbps digital.•They already provided lines for data networking.•ISDN: 64 + 64 + 16 kbps•T1 (1.544 Mbps)•T3 (44.736 Mbps)•They wanted to become the primary service provider for data networking services.•file transfer: bursty, many Mbps peak•database access: bursty, low latency•Multimedia: synchronized •Video: 6 MHz analog, 1.2-200 Mbps digital•How?Lecture #19: 11-08-01 4One BISDN: STM•Synchronous Transfer Mode•Provide multirate frame structure: iH4 + jH3 + kH2 + lH1 + mH0 + nB + DProblems»complex channel assignment/subdivision»poor support for bursty connectionsH4H4H3H3H3H3H2H2H2H2H2H2H1H1H1H1H1H1H1H1H0H0H0H0H0H0H0H0BBH0H0BBBBBBBBBBBBBBBBBBH0H0BBDDe.g.(Broadband Integrated Services Digital Network)Lecture #19: 11-08-01 5More Flexible Solution: ATM•Asynchronous Transfer Mode•Instead of predefined TDM slots, tag each slot with a virtual connection ID.Bandwidth can change dynamically•Small packets allow good real time behavior.•Fixed sized packets (cells) support fast switchingVCIVCIdatadataLecture #19: 11-08-01 6ATM Features•Fixed size cells (53 bytes).•Virtual circuit technology using hierarchical virtual circuits (VP,VC).•PHY (physical layer) processing delineates cells by frame structure, cell header error check.•Support for multiple traffic classes by adaptation layer.•E.g. voice channels, data traffic•Elaborate signaling stack.•Backwards compatible with respect to the telephone standards•Standards defined by ATM Forum.•Organization of manufacturers, providers, usersLecture #19: 11-08-01 7ATM Standard Protocol LayersUpper Layer ProtocolsUpper Layer ProtocolsATM adaptation layerPhysical medium dependentCSCSSARSARAALAALATMATMPMDPMDTCTCPHYPHYTransmission convergenceSegmentation and reassemblyConvergence sublayerLecture #19: 11-08-01 8GFCGFCVPIVPIVPIVPIVCIVCIVCIVCIVCIVCIPTPTCLPCLPHECHECpayloadpayloadThe ATM Cell (UNI)5 bytes48 byteshdrhdrpldpld(proportional)Lecture #19: 11-08-01 9Why 53 Bytes?•Small cells favored by voice applications•delays of more than about 10 ms require echo cancellation•each payload byte consumes 125 s (8000 samples/sec)•Large cells favored by data applications•Five bytes of each cell are overhead•France favored 32 bytes•32 bytes = 4 ms•France is 3 ms wide•USA, Australia favored 64 bytes•64 bytes = 8 ms•USA is 16 ms wide•CompromiseLecture #19: 11-08-01 10Virtual Circuit Switching•Signaling establishes mapping from (Portin, VCIin) to (Portout, VCIout) at each switch on path.•VCI remapping•Cells in a VC arrive in order.swswswswswswswswP0P12P3P7P6P12P1P5VCI=5VCI=3VCI=27VCI=3 VCI=16Lecture #19: 11-08-01 11Virtual Paths•Virtual path is a bundle of virtual circuits.•VCs in a virtual path follow the same route•Benefits:•route and rerouting at the virtual path level•fast connection set up•bandwidth managementswswswswswswswswP0P12P3P7P6P12P1P5VPI=5VCI=7VPI=3VCI=7VPI=27VCI=7VPI=3VCI=7VPI=16VCI=7Lecture #19: 11-08-01 12swswswswswswswswVirtual Path Trunking•Allows aggregated resource management and fault recovery.swswLecture #19: 11-08-01 13ATM Adaptation Layerssynchronous asynchronousconstant variable bit rateconnection-oriented connectionless1 2 3 4 5AAL 1: audio, uncompressed videoAAL 2: compressed videoAAL 3: long term connectionsAAL 4/5: data trafficLecture #19: 11-08-01 14AAL3/4 Adaptation Layer (Telco)headerheaderdatadatatrailertrailerATMheaderATMheaderSAR headerSAR header(SAR: segment and reassembly)type, seq#, MID (message identifier)length, CRC. . .payload(44 bytes)payload(44 bytes)SARtrailerSARtrailerincludeslength predictionLecture #19: 11-08-01 15SEAL (AAL5) Adaptation Layer (computer mfr.)datadataATMheaderATMheader. . .padpadpayload(48 bytes)payload(48 bytes)includes EOF flagctlctllenlenCRCCRCLecture #19: 11-08-01 16AAL Relative Merits•AAL3/4•cell by cell data integrity promotes pipelined processing•packet multiplexing within VC supported•length prediction makes smart buffer allocation possible•AAL5•48 byte cell makes better use of bursts on host buses, e.g. 32+16 vs. 32+8+4•cell processing simpler•CRC32 more robust (?)•lost cell means lost packet – very significantLecture #19: 11-08-01 17ATM Traffic Classes•Constant Bit Rate (CBR) and Variable Bit Rate (VBR).•Guaranteed traffic classes for different traffic types.•Unspecified Bit Rate (UBR).•Pure best effort with no help from the network•Available Bit Rate (ABR).•Best effort, but network provides support for congestion control and fairness•Congestion control is based on explicit congestion notification•Binary or multi-valued feedback•Fairness is based on Max-Min Fair Sharing.(small demands are satisfied, unsatisfied demands share equally)Lecture #19: 11-08-01 18UBR Challenges•Cell loss results in packet loss.•Cell from middle of packet: lost packet•EOF cell: lost two packets•Even low cell loss rate can result in high packet loss rate.•E.g. 0.2% cell loss -> 2 % packet loss•Disaster for TCP•Solution: drop remainder of the packet, i.e. until EOF cell.•Helps a lot: dropping useless cells reduces bandwidth and lowers the chance of later cell drops•Slight violation of layersLecture #19: 11-08-01 19ABR: Max-Min Fair Sharing•Flows are divided in two groups.•Flows that are bottlenecked elsewhere•Flows that are bottlenecked here•The max-min fair share rate Rfair of a network link is defined such that•Flows bottlenecked at the link have rate r = Rfair •Flows


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CMU 15441 Computer Networking - Lecture

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