计算机网络英文版课件：ch1-Introduction

1、1 IntroductionTwo Advances in TechnologyDevelopment of Powerful MicroprocessorsMoores Law: The performance of semiconductors was growing exponentially and would continue to. In 1975, Moore stated that the number of transistors doubled about every 18 months. Gordon Bells Law (1975) SystemPrice=5x3x.0

2、4xMemorySize/1.26(t-1972) K$ 5x: Memory is 20% of cost3x:DEC markup.04x: $ per byteHigh-speed computer networksHigh-speed LANS and WANsDistributed systemDistributed systemA collection of independent computers appears to its users as a single coherent systemA single model or paradigm that it presents

3、 to the usersA layer of software on top of the operating system, called middleware, is responsible for implementing this modelComputer NetworksComputer Networks A collection of autonomous computers interconnected by a single technology Users are exposed to the actual machinesIf the machines have dif

4、ferent hardware and different operating systems, that is fully visible to the users1.2 Network HardwareTransmission technology and scaleTransmission technologyBroadcast networks Sending a packet to all destinations, each machine checks the address fieldIf the packet is intended for the receiving mac

5、hine, that machine processes the packetIf the packet is intended for some other machine, it is just ignored. UnicastingBroadcasting Multicasting (subscribe)Point-to-point networks To go from the source to the destination, a packet may visit one or more intermediate machines (Often multiple routes ar

6、e possible, finding good ones is important) Classification by scale Local Area NetworksTopology Two broadcast networks(a) Bus(b) RingChannel allocationStatic & Dynamic A typical static allocationDivide time into discrete intervals and use a round-robin algorithm, allowing each machine to broadcast o

7、nly when its time slot comes upDynamic allocation methods CentralizedDecentralizedMAN and WANMetropolitan Area NetworksA metropolitan area network based on cable TV.Topology: subnet,LAN,hostRelation between hosts on LANs and the subnetCommunication subnet Transmission lines Switching elements (Route

8、r)Store-and-forward (packet-switched) Routing decisions: Routing algorithm Message PathSingle-path vs. Multi-pathBest route must be usedRouting decisions are made locallyInternetworksInternet & internetA collection of interconnected networks is called an internetwork or internetGateway: provide the

9、necessary translation, both in terms of hardware and software The worldwide Internet (which is one specific internet)Subnet, Network, internet Subnet The collection of routers and communication lines NetworkThe combination of a subnet and its hosts forms a networkinternetworkAn internetwork is forme

10、d when distinct networks are interconnected1.3 Network SoftwareProtocol HierarchiesNetwork architecture Networks are organized as a stack of layersReduce design complexityThe purpose of each layer is to offer certain services to the higher layers, shielding those layers from the details of how the o

11、ffered services are actually implementedEach layer is a kind of virtual machine, offering certain services to the layer above itObject-oriented programmingA particular piece of software (or hardware) provides a service to its users but keeps the details of its internal state and algorithms hidden fr

12、om themLayers, Peers, Protocols, and InterfacesLayers, protocols, and interfacesProtocolAn agreement between the communicating parties on how communication is to proceedPeers The entities comprising the corresponding layers on different machines are called peersThe peers may be processes, hardware d

13、evicesPhysical medium Interface Defines which primitive operations and services the lower layer makes available to the upper oneNetwork architecture A set of layers and protocols Neither the details of the implementation nor the specification of the interfaces is part of the architecture Philosopher

14、-Translator-SecretaryInterfacesProtocolsTranslator: A neutral language known to both of them Secretary: communication methodInformation flowExample information flow supporting virtual communication in layer 5.Virtual Communication of Layer 5 Peers(1)Layer 5A message, M, is produced by an application

15、 process and given to layer 4 for transmissionLayer 4Puts a header in front of the message to identify the message and passes the result to layer 3Header: control information, such as sequence numbers (In some layers, headers can also contain sizes, times, and other control fields)Virtual Communicat

16、ion of Layer 5 Peers(2)Layer 3There is limit to the size of messages transmitted in the layer 3 protocolBreak up the incoming messages into smaller units (packets)Prepend a layer 3 header to each packetDecides which of the outgoing lines to use and passes the packets to layer 2Layer 2Adds a header &

17、 trailer to each pieceGives the resulting unit to layer 1 Layer 1Physical transmissionDesign Issues for the LayersDesign Issues for the Layers (1)Addressing: each layer needs a mechanism for identifying senders and receiversSender & Receiver (processes on machine)Transfer ruleSimplex, Half-duplex, F

18、ull-duplexNumber of logical channels and their priorities Many networks provide at least two logical channels per connection, one for normal data and one for urgent dataDesign Issues for the Layers (2)Error ControlPhysical lines are not perfect (bit error)Error-detecting & Error-correcting codesHow

19、the receiver tells the sender which messages have been correctly received and which have notMessages may be out of sequenceHow the sender numbers the messagesHow the receiver deals with the messages arriving out of order Design Issues for the Layers (3)Flow Control: A fast sender may swamp a slow re

20、ceiver with dataMethod 1: some kind of feedback from the receiver to announce its current situationMethod 2: limit the senders transmission rateDisassemble and reassemble: Each layer could have different message sizeLower layer may disassemble upper layers message into several smaller messages reass

21、embled by receiving peerLower layer may gather several small messages into a single large message to improve efficiencyDesign Issues for the Layers (4)MultiplexingThe underlying layer may use the same connection for multiple, unrelated conversationsThe multiplexing and demultiplexing must be done tr

22、ansparently RoutingWhen multiple paths exist, a route must be chosenConnection-Oriented and Connectionless ServicesConnection-oriented Service Establishes a connectionsender, receiver, and subnet conduct a negotiation about parameters to be used, such as maximum message size, quality of service requ

23、ired, and other issuesUses the connectionsender pushes objects (bits) in at one end, receiver takes them out at the other endIn most cases the order is preserved so that the bits arrive in the order they were sent.Releases the connectionConnectionless ServiceEach message (letter) carries the full de

24、stination addresseach one is routed through the system independent of all the othersQuality of Service (QoS)Reliable Connection-oriented Service Message sequencesMessage boundaries are preservedByte streamsConnection is simply a stream of bytes, with no message boundariesReliable Connection-oriented

25、 Service Message sequences Message boundaries are preservedByte streamsConnection is simply a stream of bytes, with no message boundariesService PrimitivesService PrimitivesA set of primitives (operations) available to a user process to access the serviceThese primitives tell the service to perform

26、some action or report on an action taken by a peer entityIf the protocol stack is located in the operating system, the primitives are normally system callsInterfaces and servicesEntity Active elements in a layer, software entity or hardware entity PeersEntities in the same layer on different machine

27、sService provider and service userThe entities in layer n implement a service used by layer n+1, layer n is called the services provider, layer n+1 is called service userSAP: Service Access PointLayer n SAPs are the places where layer n+1 can access the services offeredPDU: Protocol Data UnitInforma

28、tion exchanged between two peersService Primitives: Message StyleA service is formally specified by a set of primitives (operations) available to a user process to access the serviceExampleCONNECT.request - request a connection to be establishedCONNECT.indication - signal the called partyCONNECT.res

29、ponse - used by the called to accept/reject callsCONNECT.confirm tell the caller whether the call was acceptedDATA.request request that data be sent DATA.indication signal the arrival of dataDISCONNECT.request request that a connection be releasedDISCONNECT.indication signal the peer about the reque

30、st A Simple Connection-oriented Service If the protocol stack is located in the operating system, the primitives are normally system calls Five service primitives for implementing a simple connection-oriented service.A Simple Connection-oriented ServicePackets sent in a simple client-server interact

31、ion on a connection-oriented networkThe Relationship of Services to Protocols ServiceDefines what operations the layer is prepared to performRelates to an interface between the service provider and the service userProtocolA set of rules governing the format and meaning of the messages exchanged by t

32、he peer entities Entities use protocols to implement service definitionsService and protocol are completely decoupled They are free to change their protocols provided they do not change the service1.4 Reference ModelsThe OSI Reference ModelThe OSI reference modelLayer PrinciplesEach layer should per

33、form a well-defined functionThe layer boundaries should be chosen to minimize the information flow across the interfacesL1: Physical layerTransmitting raw bits over a communication channelhow many volts should be used to represent 1/0 (electrical)how many nanoseconds a bit lasts (timimg)how the conn

34、ection is established / torn downPhysical transmission mediumL2: Data link layerTransform a raw transmission facility into a logic channel (protocols between each machine and its immediate neighbors, Point-to-Point)Sender break up the input data into data frames and transmit the framesIf the service

35、 is reliable, the receiver sends back an acknowledgement frameFlow control: Keep a fast transmitter from drowning a slow receiver in dataBroadcast networks: how to control access to the shared channelL3: Network layerControls operation of subnetForwardingRouting: How packets are routed from source t

36、o destination (route table)StaticBe determined at start of each conversationHighly dynamicCongestionQoS (Quality of service)delay, jitter, etc.Heterogeneous networks interconnectionL4: Transport LayerTransport Layer: end-to-end layer Determines what type of service to provide to the upper layer an e

37、rror-free end-to-end channel transporting of isolated messages, with no guarantee about the order of deliverybroadcasting of messages to multiple destinations(Layer 1-2: p-to-p)(Layer 4-7: e-to-e)Session Layer Allows users on different machines to establish sessionsDialog control keeping track of wh

38、ose turn it is to transmitToken management preventing two parties from attempting the same critical operation at the same timeSynchronization checkpointing long transmissions to allow them to continue from where they were after a crashPresentation Layer Syntax and semantics of the information transm

39、ittedabstract data structures (semantics)a standard encoding (syntax)Application Layer A variety of protocols that are commonly needed by usersFile transferElectronic mailNetwork news TelnetHTTP (HyperText Transfer Protocol).The TCP/IP Reference ModelThe TCP/IP Reference ModelThe Internet Layer A pa

40、cket-switching network based on a connectionless internetwork layerDefines an official packet format and protocol called IP Deliver IP packetsBest-effort deliveringPacket routing Avoiding congestionThe Transport Layer: TCPTCP (Transmission Control Protocol)Reliable connection-oriented byte stream Fr

41、agments incoming byte stream into discrete messagesReassembles the received messages into the output streamFlow controlThe Transport Layer: UDPUDP (User Datagram Protocol)Unreliable, connectionless protocolFor applications that wish to imply their own reliability NFSrequest/reply queriesDNSFor appli

42、cations in which prompt delivery is more important than accurate deliverytransmitting speech or video, DNSProtocols & networks in the TCP/IP Comparing OSI and TCP/IP Models3 Concepts are Central to OSI Model Services (semantics)What the layer does, not how entities above it access it or how the laye

43、r worksInterfaces (syntax)Tells the processes above it how to access itProtocols (implementation)A Critique of OSI Model & ProtocolsWhy OSI did not take over the world?Bad timing: too lateBad technologyThe choice was more political than technical Layer 5&6 are nearly empty, layer 2&3 are overfullSer

44、vice definitions and protocols are extraordinarily complex and difficult to implement and inefficient in operationFlow control and error control, reappear again and again in each layer Bad implementations: poor qualityBad politics: bureaucrats of European & U.S. GovernmentA Critique of the TCP/IP Re

45、ference ModelService, interface, and protocol not distinguishedHost-to-network “layer” not really a layerNo mention of physical and data link layersMinor protocols deeply entrenched, hard to replaceHybrid ModelThe hybrid reference model to be used in this book1.5 Example NetworksInternet1960: Barans

46、 Design(a) Structure of the telephone system(b) About 1960, Barans design:highly distributed, fault-tolerant digital packet-switching technology1968: Original ARPANET DesignSubnet: first electronic store-and-forward packet-switching network (Larry Roberts, Wesley Clark )IMP: Honeywell minicomputer (

47、12KB mem, no disk)56 kbps transmission linesIMP is connected to at least 2 other IMPs19691972: Growth of the ARPANET(a) December 1969 (b) July 1970 (c) March 1971 (d) April 1972. (e) September 1972. 1979: NSFNETHardware (subnet)Fuzzball (LSI-11 microcomputer)56kbps leased linesSoftware: The first TC

48、P/IP WANNSFNET backbone in 1988. In 1990, ANS take over NSFNETInternet UsageTraditional applications (1970 1990) E-mailNewsRemote loginFile transfer1990-WWW (World Wide Web) brought millions of nonacademic users to the netArchitecture of the InternetTraffic exchange across an IXPIXP: Internet exchan

49、ge point (IXP list) PeeringDE-CIXOver 450 customers from52 countries (DE-CIX)maximum throughput: 4029.5 Gbit/sDellForce 10TeraScale E1200 switches5-year graph Dell Force10(a United Statescompany) 10 Gigabitand40 Gigabit EthernetswitchesZ series: 10 and 40 Gbit/s EthernetE series: 1 and 10 Gbit/s FTO

50、S (Force10 Operating System)usesNetBSDas the underlying operating systemPower consumption (perGbpsthroughput)Force10 E1200i: 477WattCisco Nexus 7000: 928 Watt Juniper EX8216: 615 WattConnection-Oriented NetworksConnection-Oriented Networks:X.25 & FR X.25 (2.4K64K)a computer first established a conne

51、ction to the remote computer (Virtual Circuit)ReliabilityData packet consists of a 3-byte header (VC#/seq#/ACK#) and up to 128 bytes of dataFrame Relay (9.6K2M)a connection-oriented networkno error control (no retransmission)no flow control Connection-Oriented Networks: ATMATM (155M/622M)Transmit information in fixed-size packets called cellsThe cells are 53 bytes long, of which 5 bytes are header and 48 bytes are payloadCell routing is done in hardware at high speedHardware can be set up to copy one incoming cell to multiple output lines (multicast, broadcast)ATM Vi