全套英文版《计算机网络》ppt电子课件教案-part i introduction

1: Introduction,1,Part I: Introduction,Chapter goal: get context, overview, “feel” of networkingmore depth, detail later in courseapproach:descriptiveuse Internet as example,Overview:whats the Internetwhats a protocol?network edgenetwork coreaccess net, physical mediaperformance: loss, delayprotocol layers, service modelsbackbones, NAPs, ISPshistoryATM network,1: Introduction,2,Whats the Internet: “nuts and bolts” view,millions of connected computing devices: hosts, end-systemspcs workstations, serversPDAs phones, toastersrunning network appscommunication linksfiber, copper, radio, satelliterouters: forward packets (chunks) of data thru network,1: Introduction,3,Whats the Internet: “nuts and bolts” view,protocols: control sending, receiving of msgse.g., TCP, IP, HTTP, FTP, PPPInternet: “network of networks”loosely hierarchicalpublic Internet versus private intranetInternet standardsRFC: Request for commentsIETF: Internet Engineering Task Force,local ISP,companynetwork,regional ISP,router,workstation,server,mobile,1: Introduction,4,Whats the Internet: a service view,communication infrastructure enables distributed applications:WWW, email, games, e-commerce, database., voting, more?communication services provided:connectionlessconnection-orientedcyberspace Gibson:“a consensual hallucination experienced daily by billions of operators, in every nation, .,1: Introduction,5,Whats a protocol?,human protocols:“whats the time?”“I have a question”introductions specific msgs sent specific actions taken when msgs received, or other events,network protocols:machines rather than humansall communication activity in Internet governed by protocols,protocols define format, order of msgs sent and received among network entities, and actions taken on msg transmission, receipt,1: Introduction,6,Whats a protocol?,a human protocol and a computer network protocol:,Q: Other human protocol?,Hi,Hi,TCP connection req.,1: Introduction,7,A closer look at network structure:,network edge: applications and hostsnetwork core: routersnetwork of networksaccess networks, physical media: communication links,1: Introduction,8,The network edge:,end systems (hosts):run application programse.g., WWW, emailat “edge of network”client/server modelclient host requests, receives service from servere.g., WWW client (browser)/ server; email client/serverpeer-peer model: host interaction symmetrice.g.: teleconferencing,1: Introduction,9,Network edge: connection-oriented service,Goal: data transfer between end sys.handshaking: setup (prepare for) data transfer ahead of timeHello, hello back human protocolset up “state” in two communicating hostsTCP - Transmission Control Protocol Internets connection-oriented service,TCP service RFC 793reliable, in-order byte-stream data transferloss: acknowledgements and retransmissionsflow control: sender wont overwhelm receivercongestion control: senders “slow down sending rate” when network congested,1: Introduction,10,Network edge: connectionless service,Goal: data transfer between end systemssame as before!UDP - User Datagram Protocol RFC 768: Internets connectionless serviceunreliable data transferno flow controlno congestion control,Apps using TCP: HTTP (WWW), FTP (file transfer), Telnet (remote login), SMTP (email)Apps using UDP:streaming media, teleconferencing, Internet telephony,1: Introduction,11,The Network Core,mesh of interconnected routersthe fundamental question: how is data transferred through net?circuit switching: dedicated circuit per call: telephone netpacket-switching: data sent thru net in discrete “chunks”,1: Introduction,12,Network Core: Circuit Switching,End-end resources reserved for “call”link bandwidth, switch capacitydedicated resources: no sharingcircuit-like (guaranteed) performancecall setup required,1: Introduction,13,Network Core: Circuit Switching,network resources (e.g., bandwidth) divided into “pieces”pieces allocated to callsresource piece idle if not used by owning call (no sharing)dividing link bandwidth into “pieces”frequency divisiontime division,1: Introduction,14,Network Core: Packet Switching,each end-end data stream divided into packetsuser A, B packets share network resources each packet uses full link bandwidth resources used as needed,resource contention: aggregate resource demand can exceed amount availablecongestion: packets queue, wait for link usestore and forward: packets move one hop at a timetransmit over linkwait turn at next link,1: Introduction,15,Network Core: Packet Switching,Packet-switching versus circuit switching: human restaurant analogyother human analogies?,A,B,C,10 MbsEthernet,1.5 Mbs,45 Mbs,statistical multiplexing,queue of packetswaiting for outputlink,1: Introduction,16,Network Core: Packet Switching,Packet-switching: store and forward behavior,1: Introduction,17,Packet switching versus circuit switching,1 Mbit linkeach user: 100Kbps when “active”active 10% of timecircuit-switching: 10 userspacket switching: with 35 users, probability 10 active less that .004,Packet switching allows more users to use network!,N users,1 Mbps link,1: Introduction,18,Packet switching versus circuit switching,Great for bursty dataresource sharingno call setupExcessive congestion: packet delay and lossprotocols needed for reliable data transfer, congestion controlQ: How to provide circuit-like behavior?bandwidth guarantees needed for audio/video appsstill an unsolved problem (chapter 6),Is packet switching a “slam dunk winner?”,1: Introduction,19,Packet-switched networks: routing,Goal: move packets among routers from source to destinationwell study several path selection algorithms (chapter 4)datagram network: destination address determines next hoproutes may change during sessionanalogy: driving, asking directions virtual circuit network: each packet carries tag (virtual circuit ID), tag determines next hopfixed path determined at call setup time, remains fixed thru callrouters maintain per-call state,1: Introduction,20,Access networks and physical media,Q: How to connection end systems to edge router?residential access netsinstitutional access networks (school, company)mobile access networksKeep in mind: bandwidth (bits per second) of access network?shared or dedicated?,1: Introduction,21,Residential access: point to point access,Dialup via modemup to 56Kbps direct access to router (conceptually)ISDN: intergrated services digital network: 128Kbps all-digital connect to routerADSL: asymmetric digital subscriber lineup to 1 Mbps home-to-routerup to 8 Mbps router-to-homeADSL deployment: UPDATE THIS,1: Introduction,22,Residential access: cable modems,HFC: hybrid fiber coaxasymmetric: up to 10Mbps upstream, 1 Mbps downstreamnetwork of cable and fiber attaches homes to ISP routershared access to router among homeissues: congestion, dimensioning deployment: available via cable companies, e.g., MediaOne,1: Introduction,23,Institutional access: local area networks,company/univ local area network (LAN) connects end system to edge routerEthernet: shared or dedicated cable connects end system and router10 Mbs, 100Mbps, Gigabit Ethernetdeployment: institutions, home LANs soonLANs: chapter 5,1: Introduction,24,Wireless access networks,shared wireless access network connects end system to routerwireless LANs:radio spectrum replaces wiree.g., Lucent Wavelan 10 Mbpswider-area wireless accessCDPD: wireless access to ISP router via cellular network,1: Introduction,25,Physical Media,physical link: transmitted data bit propagates across linkguided media: signals propagate in solid media: copper, fiberunguided media: signals propagate freelye.g., radio,Twisted Pair (TP)two insulated copper wiresCategory 3: traditional phone wires, 10 Mbps ethernetCategory 5 TP: 100Mbps ethernet,1: Introduction,26,Physical Media: coax, fiber,Coaxial cable:wire (signal carrier) within a wire (shield)baseband: single channel on cablebroadband: multiple channel on cablebidirectionalcommon use in 10Mbs Ethernet,Fiber optic cable:glass fiber carrying light pulseshigh-speed operation:100Mbps Ethernethigh-speed point-to-point transmission (e.g., 5 Gps)low error rate,1: Introduction,27,Physical media: radio,signal carried in electromagnetic spectrumno physical “wire”bidirectionalpropagation environment effects:reflection obstruction by objectsinterference,Radio link types:microwavee.g. up to 45 Mbps channelsLAN (e.g., waveLAN)2Mbps, 11Mbpswide-area (e.g., cellular)e.g. CDPD, 10s Kbpssatelliteup to 50Mbps channel (or multiple smaller channels)270 Msec end-end delaygeosynchronous versus LEOS,1: Introduction,28,Delay in packet-switched networks,packets experience delay on end-to-end pathfour sources of delay at each hop,nodal processing: check bit errorsdetermine output linkqueueingtime waiting at output link for transmission depends on congestion level of router,1: Introduction,29,Delay in packet-switched networks,Transmission delay:R=link bandwidth (bps)L=packet length (bits)time to send bits into link = L/R,Propagation delay:d = length of physical links = propagation speed in medium (2x108 m/sec)propagation delay = d/s,Note: s and R are very different quantitites!,1: Introduction,30,Queueing delay (revisited),R=link bandwidth (bps)L=packet length (bits)a=average packet arrival rate,traffic intensity = La/R,La/R 0: average queueing delay smallLa/R - 1: delays become largeLa/R 1: more “work” arriving than can be serviced, average delay infinite!,1: Introduction,31,Protocol “Layers”,Networks are complex! many “pieces”:hostsrouterslinks of various mediaapplicationsprotocolshardware, software,Question: Is there any hope of organizing structure of network?Or at least our discussion of networks?,1: Introduction,32,Organization of air travel,a series of steps,1: Introduction,33,Organization of air travel: a different view,Layers: each layer implements a servicevia its own internal-layer actionsrelying on services provided by layer below,1: Introduction,34,Layered air travel: services,Counter-to-counter delivery of person+bagsbaggage-claim-to-baggage-claim deliverypeople transfer: loading gate to arrival gaterunway-to-runway delivery of plane,airplane routing from source to destination,1: Introduction,35,Distributed implementation of layer functionality,ticket (purchase)baggage (check)gates (load)runway takeoffairplane routing,ticket (complain)baggage (claim)gates (unload)runway landingairplane routing,Departing airport,arriving airport,intermediate air traffic sites,1: Introduction,36,Why layering?,Dealing with complex systems:explicit structure allows identification, relationship of complex systems pieceslayered reference model for discussionmodularization eases maintenance, updating of systemchange of implementation of layers service transparent to rest of systeme.g., change in gate procedure doesnt affect rest of systemlayering considered harmful?,1: Introduction,37,Internet protocol stack,application: supporting network applicationsftp, smtp, httptransport: host-host data transfertcp, udpnetwork: routing of datagrams from source to destinationip, routing protocolslink: data transfer between neighboring network elementsppp, ethernetphysical: bits “on the wire”,1: Introduction,38,Layering: logical communication,Each layer:distributed“entities” implement layer functions at each nodeentities perform actions, exchange messages with peers,1: Introduction,39,Layering: logical communication,E.g.: transporttake data from appadd addressing, reliability check info to form “datagram”send datagram to peerwait for peer to ack receiptanalogy: post office,transport,transport,1: Introduction,40,Layering: physical communication,1: Introduction,41,Protocol layering and data,Each layer takes data from aboveadds header information to create new data unitpasses new data unit to layer below,source,destination,message,segment,datagram,frame,1: Introduction,42,Internet structure: network of networks,roughly hierarchicalnational/international backbone providers (NBPs)e.g. BBN/GTE, Sprint, AT&T, IBM, UUNetinterconnect (peer) with each other privately, or at public Network Access Point (NAPs) regional ISPsconnect into NBPslocal ISP, companyconnect into regional ISPs,NBP A,NBP B,regional ISP,regional ISP,1: Introduction,43,National Backbone Provider,e.g. BBN/GTE US backbone network,1: Introduction,44,Internet History,1961: Kleinrock - queueing theory shows effectiveness of packet-switching1964: Baran - packet-switching in military nets1967: ARPAnet conceived by Advanced Reearch Projects Agency1969: first ARPAnet node operational,1972: ARPAnet demonstrated publiclyNCP (Network Control Protocol) first host-host protocol first e-mail programARPAnet has 15 nodes,1961-1972: Early packet-switching principles,1: Introduction,45,Internet History,1970: ALOHAnet satellite network in Hawaii1973: Metcalfes PhD thesis proposes Ethernet1974: Cerf and Kahn - architecture for interconnecting networkslate70s: proprietary architectures: DECnet, SNA, XNAlate 70s: switching fixed length packets (ATM precursor)1979: ARPAnet has 200 nodes,Cerf and Kahns internetworking principles:minimalism, autonomy - no internal changes required to interconnect networksbest effort service modelstateless routersdecentralized controldefine todays Internet architecture,1972-1980: Internetworking, new and proprietary nets,1: Introduction,46,Internet History,1983: deployment of TCP/IP1982: smtp e-mail protocol defined 1983: DNS defined for name-to-IP-address translation1985: ftp protocol defined1988: TCP congestion control,new national networks: Csnet, BITnet, NSFnet, Minitel100,000 ho