CS551Layering and AddressingBill Chenghttp://merlot.usc.edu/cs551-f121 Computer Communications - CSCI 551 Copyright © William C. ChengFormat and order of messagesSet of rules governing communication between networkelements (applications, hosts, routers)2Protocols Computer Communications - CSCI 551 Copyright © William C. ChengProtocols define:Actions taken on receipt of a messageWe need design guidelines!Protocols are hard to designHostLayering: technique to simplify complex systems3Layering Computer Communications - CSCI 551 Copyright © William C. ChengApplicationTransportNetworkLinkTeleconferencingUser A User BPeersHostEach layer relies on services from layer below and exportsservices to layer above4Layering Characteristics Computer Communications - CSCI 551 Copyright © William C. ChengInterface defines interactionHides implementation - layers can change withoutdisturbing other layers (black box)5Layer Encapsulation Computer Communications - CSCI 551 Copyright © William C. ChengPhysical: how to transmit bits6OSI Model: 7 Protocol Layers Computer Communications - CSCI 551 Copyright © William C. ChengData link: how to transmit framesNetwork: how to route packets hop2hopTransport: how to send packets end2endSession: how to tie flows togetherPresentation: byte ordering, securityApplication: everything else!Reliability7Layering General Issues Computer Communications - CSCI 551 Copyright © William C. ChengFlow controlFragmentationMultiplexingConnection setup (handshaking)Addressing/naming (locating peers)First end-to-end layer8Example: Transport Layer Computer Communications - CSCI 551 Copyright © William C. ChengEnd-to-end stateMay provide reliability, flow control, and congestion controlPoint-to-point communication9Example: Network Layer Computer Communications - CSCI 551 Copyright © William C. ChengNetwork and host addressingRoutingLayer N may duplicate lower level functionality (e.g.,error recovery).Sometimes..10Is Layering Harmful? Computer Communications - CSCI 551 Copyright © William C. ChengLayers may need same info (timestamp, MTU).Strict adherence to layering may hurt performance.Naïve layer implementation frequently hurts performance.11Course Focus Computer Communications - CSCI 551 Copyright © William C. ChengIP & TCPIP & TCP12IP Computer Communications - CSCI 551 Copyright © William C. ChengHostApplicationTransportNetworkLinkHostRouter Router0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |Version| IHL |Type of Service| Total Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Identification |Flags| Fragment Offset | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Time to Live | Protocol | Header Checksum | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Source IP Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Destination IP Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Options | Padding | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Example Internet Datagram Header13IP Header Computer Communications - CSCI 551 Copyright © William C. ChengNot used until recentlyType of service14IP Functions Computer Communications - CSCI 551 Copyright © William C. ChengFragmentationIdentification, flags and fragment offsetBounded deliveryTime to live(De)multiplexing higher layer protocols (analogous toport numbers in TCP)ProtocolIP packet length limited to 64KLengthEnsures some degree of header integrityHeader checksumNice if we can send large chunks of dataForwarding costs per packet15Fragmentation Computer Communications - CSCI 551 Copyright © William C. ChengDifferent link-layers have different MTUsFragmentationIntra-networkInter-networkExample of packet just bigger than MTUUses resources poorly16Fragmentation Is Harmful Computer Communications - CSCI 551 Copyright © William C. ChengLoss of a fragmentPoor end-to-end performanceBuffering constraintsReassembly is hardSend message with don’t fragment bitHosts dynamically discover MTU of path17Path MTU Discovery Computer Communications - CSCI 551 Copyright © William C. ChengGet ICMP message indicating sizeIncreasing/decreasing path MTUWhat happens if path changes?Expected that future routing protocols will provide MTUinformationUsually implemented by the transport layerInitialize MTU to MTU to next hopAlgorithm:18Path MTU Computer Communications - CSCI 551 Copyright © William C. ChengSend datagrams with DF bit setIf "datagram too big", decrease MTUPeriodically (>5mins, or >1min after previous increase),increase MTUSome routers will return proper MTUMTU values cached in routing tableIP addresses are names of interfaces19Addressing in IP Computer Communications - CSCI 551 Copyright © William C. ChengDNS names are names of hostsDNS binds host names to interfacesRouting binds interface names to pathsFlexibilityFixed length or variable length?20Addressing Considerations Computer Communications - CSCI 551 Copyright © William C. ChengIssues:Processing costsHeader sizeEngineering choice: IP uses fixed length addressesNeed structure for designing scalable binding frominterface name to route!Structured vs flat21Addressing Considerations Computer Communications - CSCI 551 Copyright © William C. ChengIssuesHow many levels? Fixed? Variable?22Packet Traveling Throughthe Internet Computer Communications - CSCI 551 Copyright © William C. ChengRouters send packetto next closest pointRRRH RRRHHH RHRHR: Hosts: Routers23IP Addressing Hierarchy Computer Communications - CSCI 551 Copyright © William C. ChengReginalsCampus LANsBackbones127.0.0.1: local host (a.k.a. The loopback address.24Some Special IP Addresses Computer Communications - CSCI 551 Copyright © William C. Cheng127.X.X.X: same as above.Host bits all set to 0: network address.Host bits all set to 1: broadcast address.0.0.0.0: this host on this network.Fixed length: 32 bits25IP Addresses Computer Communications - CSCI 551 Copyright © William C. ChengInitial classful structureHigh Order Bits Format Class---------------
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