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CMU 15441 Computer Networking - Lecture

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Lecture 2 Protocol StacksLast TuesdayToday’s LectureProtocolsSlide 5More on ProtocolsInterfacesToo Many Network ComponentsToo many components 2Looking at protocolsProtocol and Service LevelsA Layered Network ModelOSI MotivationOSI FunctionsExample: Sending a Web PageA TCP / IP / 802.3 PacketMultiplexing and DemultiplexingDifferent Sources of ComponentsLimitations of the Layered ModelThe TCP/IP ModelLocal Area Network ProtocolsInternetworking OptionsThe Internet Protocol SuiteSome History: The Early DaysRecent History: CommercializationStandardizationRelevant Standardization BodiesThe Internet Engineering Task ForceHigher Level StandardsHow do you select protocols?Applications and Application-Layer ProtocolsClient-Server ParadigmWhat Transport Service Does an Application Need?User Datagram Protocol(UDP): An AnalogyTransmission Control Protocol (TCP): An AnalogyTransport Service Requirements of Common ApplicationsServer and ClientReadings1Lecture 2Protocol StacksDavid AndersenSchool of Computer ScienceCarnegie Mellon University15-441 Networking, Spring 2005http://www.cs.cmu.edu/~srini/15-441/S05/2Last TuesdayThe Big Picture»Goals: –Efficiency–“ilities” (scalability, manageability, availability),–Ease of creating applications»Challenges:–Scale–Geography–Heterogeneity (** today’s focus!)A few specific details:»Circuits vs. packets»Little bit about routing»Service model and how to construct services (** today!)3Today’s LectureLast time: “Big picture”Today:»General architectural principles for networks»Introduces a few concrete models & examplesWhere we are going:»Thursday: Application examples (still high level)»After that: Burrowing into the details, ground upToday’s specifics:»What is a protocol.»Protocol stacks.»Some history.»Standards organizations.»Application layer.4ProtocolsRecall goals:»Interoperability»Reuse»Hiding underlying details5ProtocolsAn agreement between parties on who communication should take place.Protocols may have to define many aspects of the communication.Syntax:»Data encoding, language, etc.Semantics:»Error handling, termination, ordering of requests, etc.Protocols at hardware, software, all levels!Example: Buying airline ticket by typing.Syntax: English, ascii, lines delimited by “\n”Friendly greetingMuttered replyDestination?PittsburghThank you6More on ProtocolsProtocols are the key to interoperability.»Networks are very heterogenous:»The hardware/software of communicating parties are often not built by the same vendor»Yet they can communicate because they use the same protocolProtocols exist at many levels.»Application level protocols, e.g. access to mail, distribution of bboards, web access, ..»Protocols at the hardware level allow two boxes to communicate over a link, e.g. the Ethernet protocolComputer: x86Ethernet: 3comRouters: cisco, etc.App: EmailHardwareHardware/linkNetworkApplication7InterfacesEach protocol offers an interface to its users, and expects one from the layers on which it builds»Syntax and semantics strike again–Data formats–Interface characteristics, e.g. IP service modelProtocols build upon each other»Add value–E.g., a reliable protocol running on top of IP»Reuse–E.g., OS provides TCP, so apps don’t have to rewrite8Too Many Network ComponentsApplicationOperating SystemProtocol SoftwareComputerLinksRouter HardwareRouter Software(many protocols)Bridge HW/SWApplicationOperating SystemComputerNetwork Interface9Too many components 2Links: copper, fiber, air, carrier pidgeonRunning ethernet, token ring, SONET, FDDIRouters speaking BGP, OSPF, RIP, …Hosts running FreeBSD, Linux, Windows, MacOS, …People using Mozilla, Explorer, Opera, …and it changes all the timePhew!Protocols hide this stuff with simple abstractions.10Looking at protocolsHop by hop / link protocols»EthernetEnd-to-end protocols»TCP, apps, etc.Management / “control plane” protocols»Routing, etc.–Can be either link or e2e themselves–Definition somewhat vague.Standards»File formats, etc.E.g., JPEG, MPEG, MP3, …Categories not solid / religious, just a way to view things.11Protocol andService LevelsApplicationEnd-to-endCoreNetwork12A Layered Network ModelThe Open Systems Interconnection (OSI) Model.ApplicationApplicationPresentationPresentationSessionSessionTransportTransportNetworkNetworkData linkData linkPhysicalPhysical1234567NetworkNetworkData linkData linkPhysicalPhysicalApplicationApplicationPresentationPresentationSessionSessionTransportTransportNetworkNetworkData linkData linkPhysicalPhysical13OSI MotivationStandard way of breaking up a system in a set of components, but the components are organized as a set of layers.»Only horizontal and vertical communication»Components/layers can be implemented and modified in isolationEach layer offers a service to the higher layer, using the services of the lower layer.“Peer” layers on different systems communicate via a protocol.»higher level protocols (e.g. TCP/IP, Appletalk) can run on multiple lower layers»multiple higher level protocols can share a single physical network“It’s only a model!” - TCP/IP has been crazy successful, and it’s not based on a rigid OSI model. But the OSI model has been very successful at shaping thought.14OSI Functions(1) Physical: transmission of a bit stream.(2) Data link: flow control, framing, error detection.(3) Network: switching and routing.(4) Transport: reliable end to end delivery.(5) Session: managing logical connections.(6) Presentation: data transformations.(7) Application: specific uses, e.g. mail, file transfer, telnet, network management.Multiplexing takes place in multiple layers15Example: Sending a Web PageHttp hdrHttp hdrWeb pageWeb pageTCP headerTCP header. . .ApplicationpayloadApplicationpayloadApplicationApplicationPresentationPresentationSessionSessionTransportTransportNetworkNetworkData linkData linkPhysicalPhysical16Ethernet preambleEthernet preambleA TCP / IP / 802.3 PacketMAC headerMAC headerLLC / SNAP headerLLC / SNAP headerIP headerIP headerTCP headerTCP headerDataDataApplicationApplicationPresentationPresentationSessionSessionTransportTransportNetworkNetworkData linkData linkPhysicalPhysicalHomework explores tradeoffs in header sizes, etc., with different applications17Multiplexing and DemultiplexingThere may be multiple


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