CS 640: Introduction to Computer NetworksToday’s LectureNetwork Communication: Lots of Functions NeededWhat is Layering?Example of LayeringWhat is a Protocol?1. Protocols Offer Interfaces2. Protocols Necessary for InteroperabilityOSI ModelThe OSI Standard: 7 LayersOSI Layers and LocationsThe Reality: TCP/IP ModelThe Thin WaistTCP/IP vs OSITCP/IP LayeringLayers & EncapsulationProtocol DemultiplexingMultiplexing & DemultiplexingLayering vs NotApplications; Application-Layer ProtocolsWriting Applications: Some Design ChoicesClient-Server Paradigm vs. P2PChoosing the Transport ServiceTransmission Control Protocol (TCP)User Datagram Protocol (UDP)Transport Service Requirements of Common Applications1CS 640: Introduction to Computer NetworksAditya AkellaLecture 2Layering, Protocol Stacks,and Standards2Today’s Lecture•Layers and Protocols•A bit about applications3Network Communication:Lots of Functions Needed•Links•Multiplexing •Routing•Addressing/naming (locating peers)•Reliability•Flow control•FragmentationHow do you implement these functions?Key: Layering and protocols4What is Layering?•A way to deal with complexity–Add multiple levels of abstraction–Each level encapsulates some key functionality–And exports an interface to other components–Example?•Layering: Modular approach to implementing network functionality by introducing abstractions•Challenge: how to come up with the “right” abstractions?5Example of Layering•Software and hardware for communication between two hosts•Advantages:–Simplifies design and implementation–Easy to modify/evolveLink hardwareHost-to-host connectivityApplication-to-application channelsApplication semantics6What is a Protocol?•Could be multiple abstractions at a given level –Build on the same lower level–But provide diferent service to higher layers•Protocol: Abstract object or module in layered structureLink hardwareHost-to-host connectivityRequest-ReplyApplicationMessage stream71. Protocols Offer Interfaces•Each protocol offers interfaces –One to higher-level protocols on the same end hosts•Expects one from the layers on which it builds•Interface characteristics, e.g. IP service model–A “peer interface” to a counterpart on destinations•Syntax and semantics of communications•(Assumptions about) data formats•Protocols build upon each other–Adds value, improves functionality overall•E.g., a reliable protocol running on top of IP–Reuse, avoid re-writing•E.g., OS provides TCP, so apps don’t have to rewrite82. Protocols Necessary for Interoperability•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•Actually implementations could be different •But must adhere to same specification•Protocols exist at many levels.–Application level protocols–Protocols at the hardware levelEthernet: 3com, etc.Routers: cisco, juniper etc.App: Email, AIM, IE etc.Hardware/linkNetworkApplication9OSI Model•One of the first standards for layering: OSI•Breaks up network functionality into seven layers•This is a “reference model”–For ease of thinking and implementation•A different model, TCP/IP, used in practice10The OSI Standard: 7 Layers1. Physical: transmit bits (link)2. Data link: collect bits into frames and transmit frames (adaptor/device driver)3. Network: route packets in a packet switched network4. Transport: send messages across processes end2end5. Session: tie related flows together6. Presentation: format of app data (byte ordering, video format)7. Application: application protocols (e.g. FTP)•OSI very successful at shaping thought•TCP/IP standard has been amazingly successful, and it’s not based on a rigid OSI model11OSI Layers and LocationsBridge/SwitchFull fledgedpacket switch:use dst addrto routeRouter/GatewayForward usingnetwork layeraddressesHostHostApplicationTransportNetworkData LinkPresentationSessionPhysicalRepeater/HubSimply copypackets out12The Reality: TCP/IP ModelFTP HTTP TFTPNVTCP UDPIPNET1NET2NETn…Network protocols implemented by a comb of hw and sw.Interconnection of n/w technologies into a single logical n/wTwo transport protocols: provide logical channels to appsApp protocolsNote: No strict layering.App writers can define apps that run on any lower level protocols.13The Thin WaistUDP TCPData LinkPhysicalApplicationsThe Hourglass ModelWaistThe waist: minimal, carefully chosen functions. Facilitates interoperability and rapid evolutionFTP HTTP TFTPNVTCP UDPIPNET1NET2NETn…14TCP/IP vs OSIApplication(pluslibraries)TCP/UDPIPData linkPhysicalApplicationPresentationSessionTransportNetworkData linkPhysical15TCP/IP LayeringBridge/SwitchRouter/GatewayHostHostApplicationTransportNetworkLinkPhysical16Layers & EncapsulationGet index.htmlConnection IDSource/DestinationLink AddressUser A User BHeader17Protocol Demultiplexing•Multiple choices at each layer•How to know which one to pick?FTP HTTP TFTPNVTCP UDPIPNET1NET2NETn…TCP/UDPIPManyNetworks18Multiplexing & Demultiplexing•Multiple implementations of each layer–How does the receiver know what version/module of a layer to use?•Packet header includes a demultiplexing field–Used to identify the right module for next layer–Filled in by the sender–Used by the receiver•Multiplexing occurs at multiple layers. E.g., IP, TCP, …IPTCPIPTCPV/HL TOS LengthID Flags/OffsetTTL Prot. H. ChecksumSource IP addressDestination IP addressOptions..19Layering vs Not•Layer N may duplicate layer N-1 functionality –E.g., error recovery•Layers may need same info (timestamp, MTU)•Strict adherence to layering may hurt performance•Some layers are not always cleanly separated–Inter-layer dependencies in implementations for performance reasons–Many cross-layer assumptions, e.g. buffer management•Layer interfaces are not really standardized.–It would be hard to mix and match layers from independent implementations, e.g., windows network apps on unix (w/o compatibility library)20Applications; Application-Layer Protocols•Application: communicating, distributed processes–Running in network hosts in “user space”–N/w functionality in kernel space–Exchange messages to implement app–e.g., email, file transfer, the Web•Application-layer protocols–One “piece” of an app–Define messages exchanged by apps
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