大二上专业课考试题英文networks and the internet

1、1Computer Networking4rd edition. AndrewS.Tanenbaum , March 2003. Computer NetworksLectured by：Hai YUSoftware CollegeNortheastern UniversityEmail： Phone: +86 24 83688871 12课程介绍：一、本课程时间短任务重二、课程内容 1、网络概况 2、以太网 3、TCP/IP协议、网络层和传输层 4、网络互联3三、学习方法 1、广泛阅读资料 2、大量参考案例 3、可自学四、参考资料 1 道格拉斯.科默,用TCP/IP进行网际互连,电子工业出版

2、社 2 唐纳德.斯蒂文,TCP/IP协议祥解,电子工业出版社 3 潘爱民译,计算机网络(第4版),清华大学出版社 4 谢希仁,计算机网络(第5版),电子工业出版社 4五、成绩评定 1、出勤率（15%） 2、随堂测试（20%） 3、作业（15%） 4、开卷考试（50%）六、其他 1、BB平台 2、国家级教学团队博客： 5Chapter 1: IntroductionOur goal: get “feel” and terminologymore depth, detail later in courseapproach:use Internet as exampleOverview:whats

3、the Internetwhats a protocol?network edgenetwork coreaccess net, physical mediaInternet/ISP structureperformance: loss, delayprotocol layers, service modelsnetwork modeling6Terms in chapter 1LAN局域网WAN广域网Router路由器Switch交换机Protocol协议Packet数据包/报文/分组ISP因特网服务提供商TCP传输控制协议UDP用户数据报协议Intranet内网Bandwidth 带宽in

4、frastructure 基础设施Ethernet 以太网Multiplexing 多路复用Encapsulation 封装Stack 栈FDM 频分复用TDM 时分复用congestion 拥挤拥塞Extranet 外网7Chapter 1: Roadmap1.1 What is the Internet?1.2 Internet History 1.3 Network edge1.4 Network core1.5 Network access and physical media1.6 Internet structure and ISPs1.7 Delay & loss in pack

5、et-switched networks1.8 Protocol layers, service models8Category of Computer NetworksComputer NetworksA computer network is composed of multiple computers connected together using a munication system for the purpose of sharing data, resources and communication.计算机网络一般地说，将分散的多台计算机、终端和外部设备用通信线路互联起来，彼此

6、间实现互相通信，并且计算机的硬件、软件和数据资源大家都可以共同使用，实现资源共享的整个系统就叫做计算机网络。1.1 What is the Internet?9Computer network connects two or more autonomous computers.The computers can be geographically located anywhere.10Composition of Computer NetworkHardwareEnd Systems: Host, PC, Mainframe, Client, Workstation, ServerInterm

7、ediate Systems: Communications: Switch, RouterInterface: Network interface card(NIC), ModemMedium: Twisted pair, Coaxial cable, Fiber, Wireless SoftwareProtocol: CSMA/CD, TCP/IP, UDP, PPP, ATM Applications: HTTP, SMTP, FTP, Telnet11Applications of NetworksResource SharingHardware (computing resource

8、s, disks, printers)Software (application software)Information SharingEasy accessibility from anywhere (files, databases)Search Capability (WWW)CommunicationEmailMessage broadcastRemote computingDistributed processing (GRID Computing)12ApplicationsE-mailSearchable Data (Web Sites) merceNews GroupsInt

9、ernet Telephony (VoIP)Video ConferencingChat GroupsInstant Messengers Internet Radio13“Cool” internet appliancesWorlds smallest web server IP picture frame Web-enabled toaster +weather forecasterInternet phones14Category of Computer NetworksClassified by scaleLAN(MAN)A local area network (LAN) is a

10、group of computers and associated devices that share a common communications line or wireless link within a small geographic area.WANA wide area network (WAN) is a geographically dispersed munications network. The term distinguishes a broader munication structure from a local area network (LAN). Int

11、erneta worldwide system of computer networks - a network of networks in which users at any one computer can, if they have permission, get information from any other computer, even use the remote computers. 15Category of Computer NetworksClassified by boundaryIntranet （Private Networks）An intranet is

12、 a private network that is contained within an enterprise. It may consist of many interlinked local area networks and also use leased lines in the wide area network. Extranet （Public Networks）An extranet is a private network that uses Internet technology and the public munication system to securely

13、share part of a businesss information or operations with suppliers, vendors, partners, customers, or other businesses. An extranet can be viewed as part of a companys intranet that is extended to users outside the company. It has also been described as a state of mind in which the Internet is percei

14、ved as a way to do business with other companies as well as to sell products to customers. Classified by TopologyThe network topology defines the way in which computers, printers, and other devices are connected. A network topology describes the layout of the wire and devices as well as the paths us

15、ed by data transmissions. Category of Computer NetworksBus TopologyCommonly referred to as a linear bus, all the devices on a bus topology are connected by one single cable.Star & Tree TopologyThe star topology is the most commonly used architecture in Ethernet LANs. When installed, the star topolog

16、y resembles spokes in a bicycle wheel.Larger networks use the extended star topology also called tree topology. When used with network devices that filter frames or packets, like bridges, switches, and routers, this topology significantly reduces the traffic on the wires by sending packets only to t

17、he wires of the destination host.Ring TopologyA frame travels around the ring, stopping at each node. If a node wants to transmit data, it adds the data as well as the destination address to the frame. The frame then continues around the ring until it finds the destination node, which takes the data

18、 out of the frame. Single ring All the devices on the network share a single cable Dual ring The dual ring topology allows data to be sent in both directions. 20Whats the Internet?millions of connected computing devices: hosts = end systems running network appscommunication linksfiber, copper, radio

19、, satellitetransmission rate = bandwidthrouters: forward packets (chunks of data)“nuts and bolts” viewlocal ISPcompanynetworkregional ISProuterworkstationservermobileHome21Whats the Internet: “nuts and bolts” viewprotocols control sending, receiving of msgse.g., TCP, IP, HTTP, FTP, PPPInternet: “net

20、work of networks”loosely hierarchicalpublic Internet versus private intranetInternet standardsRFC: Request for commentsIETF: Internet Engineering Task Forcelocal ISPcompanynetworkregional ISP22Whats the Internet: a service viewcommunication infrastructure enables distributed applications:Web, email,

21、 games, merce, file sharingcommunication services provided to apps:Connectionless unreliableconnection-oriented reliableQuestion：other sevices？23Whats a protocol?human protocols:“whats the time?”“I have a question”introductions specific msgs sent specific actions taken when msgs received, or other e

22、ventsnetwork protocols:machines rather than humansall communication activity in Internet governed by protocolsprotocols define format, order of msgs sent and received among network entities, and actions taken on msg transmission, receipt 24Whats a protocol?a human protocol and a computer network pro

23、tocol:Q: Other human protocols? HiHiGot thetime?2:00TCP connection requestTCP connectionresponseGet time251.2 Internet History1961: Kleinrock - queueing theory shows effectiveness of packet-switching1964: Baran - packet-switching in military nets1967: ARPAnet conceived by Advanced Research Projects

24、Agency1969: first ARPAnet node operational1972: ARPAnet public demonstrationNCP (Network Control Protocol) first host-host protocol first e-mail programARPAnet has 15 nodes1961-1972: Early packet-switching principles261970: ALOHAnet satellite network in Hawaii1974: Cerf and Kahn - architecture for i

25、nterconnecting networks1976: Ethernet at Xerox PARCate70s: proprietary architectures: DECnet, SNA, XNAlate 70s: switching fixed length packets (ATM precursor)1979: ARPAnet has 200 nodesCerf and Kahns internetworking principles:minimalism, autonomy - no internal changes required to interconnect netwo

26、rksbest effort service modelstateless routersdecentralized controldefine todays Internet architecture1972-1980: Internetworking, new and proprietary nets27Early 1990s: ARPAnet missioned1991: NSF lifts restrictions on commercial use of NSFnet ( missioned, 1995)early 1990s: Webhypertext Bush 1945, Nel

27、son 1960sHTML, HTTP: Berners-Lee1994: Mosaic, later Netscapelate 1990s: commercialization of the WebLate 1990s 2000s:more killer apps: instant messaging, P2P file sharingnetwork security to forefrontest. 50 million host, 100 million+ usersbackbone links running at Gbps1990, 2000s: commercialization,

28、 the Web, new apps28Vinton Cerf291983: deployment of TCP/IP1982: smtp e-mail protocol defined 1983: DNS defined for name-to-IP-address translation1985: ftp protocol defined1988: TCP congestion controlnew national networks: Csnet, BITnet, NSFnet, Minitel100,000 hosts connected to confederation of net

29、works1980-1990: new protocols, a proliferation (增殖)of networks301961 MIT的Leonard Kleinrock发表Information Flow in Large Communication Nets，(7月) 第一篇有关包交换(PS)的论文。 1962 MIT的J.C.R. Licklider和W. Clark发表On-Line Man Computer Communication，(8月) 包含有分布式社交行为的全球网络概念。 1964 RAND公司的Paul Baran发表On Distributed Communi

30、cations Networks。 包交换网络；不存在出口。 1974 Vinton Cerf和Bob Kahn发表了论文A Protocol for Packet Network Interconnection，文中对TCP协议的设计作了详细的描述。IEEE Trans Comm Internet History311972 BBN的Ray Tomlinson为ARPANET修改了email程序，这个程序变得非常热门。Tomlinson的33型电传打字机选用作为代表在的含义的标点符号(3月) 1972 在ICCC大会期间，精神科病人PARRY(在Stanford)与医生(在BBN)第一次使用

31、计算机-计算机间聊天的形式讨论了病情。 1973 Harvard大学Bob Metcalfe的博士论文首先提出了以太网的概念。他的概念在Xerox公司的PARC的Alto计算机上进行了测试，第一个以太网叫做Alto Aloha System(5月)。 321973 圣诞节死锁 - Harvard的IMP硬件故障导致它向所有的ARPANET节点发出了长度为0的广播信息，造成所有其他的IMP都将它们的通信转向Harvard(12月25日) 。 1979 Kevin MacKenzie向MsgGroup发出email，建议在email的枯燥单调文字中加入一些表情符号，比如-)表示伸出舌头。他的建议多

32、次引起争论，最后被广泛应用(4月12日) 。 1980 由于一种状态信息病毒出人意料的自我繁殖，ARPANET完全停止运行(10月27日) 。1987 在德国和中国间采用CSNET协议建立了email连接，9月20日从中国发出了第一封信。 331988 Internet蠕虫在Internet上蔓延，全部60,000个节点中的大约6,000个节点受到影响(11月2日 ) 。 莫立斯蠕虫事件促使DARPA建立了CERT(计算机危机快速反应小组)以应付此类事件。蠕虫是CERT年内受到咨询的唯一的一件事情。 1988 Jarkko Oikarinen开发了Internet网上聊天(IRC)。 1994

33、 新的蠕虫在Internet上发现他们的生存空间 - 出现了WWW蠕虫(W4)，接着出现了蜘蛛、漫游者、爬虫和蛇等. Internet开始引起商业界和新闻媒体的注意。通过Hut online可直接订购比萨饼。 341995 香港警方为了搜捕一个计算机“黑客(hacker)”，除了本地的一个Internet供应商外，关闭了所有的Internet供应商，使10,000人无法使用网络。1996 由于一个黑客使用黑客杂志上描述的方法，不断地使用SYN攻击，纽约的公共存取网络公司(PANIX)不得不关机。 1996 WWW浏览器之间的战争爆发，主要是在Netscape和Microsoft之间展开，这带来

34、了软件开发的新时代，现在Internet用户急于测试即将发布的软件，使得每个季度都有新版软件发布。353637IETF (Internet Engineering Task Force), Internet Society, IAB(Internet Architecture Board), W3C (The world Wide Web Consortium), ACM (Association for Computing Machine), IEEE, M, sigcommIEEE Communications Society, IEEE Computer Society, Data Co

35、mmunications tutorials, Media History Project, 38A closer look at network structure:network edge: applications and hostsnetwork core: routersnetwork of networksaccess networks, physical media: communication links1.3 Network edge39The network edge:end systems (hosts):run application programse.g. Web,

36、 emailat “edge of network”client/server modelclient host requests, receives service from always-on servere.g. Web browser/server; email client/serverpeer-peer model: minimal (or no) use of dedicated serverse.g. Gnutella, KaZaA, Skype40Network edge: connection-oriented serviceconnection-oriented serv

37、ice,面向连接服务What is Connection？所谓连接，就是两个对等实体进行数据通信而进行的一种结合面向连接服务具有连接建立、数据传输和连接释放这三个阶段(交换机交换机交换机交换机用户线用户线中继线中继线BDCA41Network edge: connection-oriented serviceGoal: data transfer between end systemshandshaking: setup (prepare for) data transfer ahead of timeHello, hello back human protocolset up “state”

38、 in two communicating hostsTCP - Transmission Control Protocol Internets connection-oriented serviceTCP service RFC 793reliable, in-order byte-stream data transferloss: acknowledgements and retransmissionsflow control（流控制） sender wont overwhelm receivercongestion control: （拥塞控制）senders “slow down se

39、nding rate” when network congested如果数据丢失了，怎么办？TCP协议是Internet中的核心协议之一我们会在第三章详细介绍。42Network edge: connectionless serviceconnectionless service，无连接服务两个实体之间的通信不需要事先建立好一个连接其下层的相关资源不需要预定保留，而是在数据传输时动态分配 两个实体并不一定同时是活跃的没有流控制和拥塞控制best effort delivery，尽最大努力交付不能防止报文的丢失、重复或失序，是一种不可靠的服务如手机短信通信：43Network edge: con

40、nectionless serviceGoal: data transfer between end systemssame as before!UDP - User Datagram Protocol RFC 768: connectionless unreliable data transferno flow controlno congestion controlApps using TCP: HTTP (Web), FTP (file transfer), Telnet (remote login), SMTP (email)Apps using UDP:streaming media

41、, teleconferencing, DNS, Internet telephonyQQ441.4 The Network Coremesh(有机组合) of interconnected routersthe fundamental question: how is data transferred through net?circuit switching: 电路交换dedicated circuit per call: telephone netpacket-switching: 分组交换data sent thru net in discrete “chunks”45Network

42、Core: Circuit SwitchingEnd-end resources reserved for “call”link bandwidth, switch capacitydedicated resources: no sharingcircuit-like (guaranteed) performancecall setup required46Network Core: Circuit Switching电路交换必须是面向连接的在通话（通信）的全部时间内，通话的两个用户始终占用端到端的固定传输带宽使用电路交换来传输计算机数据时，线路的传输效率往往很低WHY？电信网电信网向用户电话

43、机提供可靠交付47Network Core: Circuit Switchingnetwork resources (e.g., bandwidth) divided into “pieces”pieces allocated to callsresource piece idle（空闲） if not used by owning call (no sharing)Multiplexing:多工，复用 dividing link bandwidth into “pieces”How to divide？frequency divisionFDMtime divisionTDMcode div

44、isionCDMA48FDM的实例调频广播：频段：88108MHz带宽？被多个广播台使用每个广播台使用某段（个）频率49Circuit Switching: FDM and TDMFDMfrequencytimeTDMfrequencytime4 usersExample:50TDM 频率时间BCDBCDBCDBCDAAAA在 TDM 帧中的位置不变TDM 帧TDM 帧TDM 帧TDM 帧TDM 帧51TDM 频率时间CDCDCDAAAABBBBCD在 TDM 帧中的位置不变TDM 帧TDM 帧TDM 帧TDM 帧TDM 帧52TDM 频率时间BDBDBDAAAABCCCCD在 TDM 帧中的

45、位置不变TDM 帧TDM 帧TDM 帧TDM 帧TDM 帧53TDM 频率时间BCBCBCAAAABCDDDD在 TDM 帧中的位置不变TDM 帧TDM 帧TDM 帧TDM 帧TDM 帧54TDM of E12.048 Mb/s传输线路CH0CH16CH17CH15CH15CH16CH17CH31CH31CH0CH1CH1时分复用帧T CH0 CH1 CH2 CH15 CH16 CH17 CH30CH31CH08 bitt时分复用帧时分复用帧T = 125 ms15 个话路15 个话路55时分复用可能会造成线路资源的浪费 ABCDaabbcdbcattttt4 个时分复用帧#1acbcd时分复

46、用#2#3#4用户56 STDM:统计时分复用用户ABCDabcdttttt3 个 STDM 帧#1acbabbcacd#2#3统计时分复用57Numerical exampleHow long does it take to send a file of 640K bits from host A to host B over a circuit-switched network?All links are 1.536 MbpsEach link uses TDM with 24 slots/sec500 msec to establish end-to-end circuitLets wo

47、rk it out!传输速度：1.536Mbps24=64Kbps传输时间：640K64Kbps=10s创建时间：0.5s发送文件时间10.5s58Another numerical exampleHow long does it take to send a file of 640,000 bits from host A to host B over a circuit-switched network?All links are 1.536 MbpsEach link uses FDM with 24 channels/frequencies500 msec to establish e

48、nd-to-end circuitLets work it out!传输速度：1.536Mbps2464Kbps传输时间：640K64Kbps=10s创建时间：0.5s发送文件时间=10.5s59Network Core: Packet Switchingeach 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: ag

49、gregate resource demand can exceed amount availablecongestion: packets queue, wait for link usestore and forward: packets move one hop at a timeNode receives complete packet before forwardingBandwidth division into “pieces”Dedicated allocationResource reservation60Network Core: Packet Switching61Pac

50、ket Switching: Statistical MultiplexingSequence of A & B packets does not have fixed pattern, shared on demand statistical multiplexing.TDM: each host gets same slot in revolving TDM frame.ABC10 Mb/sEthernet1.5 Mb/sDEstatistical multiplexingqueue of packetswaiting for outputlink62Packet switching ve

51、rsus circuit switching1 Mb/s linkeach user: 100 kb/s when “active”active 10% of timecircuit-switching: 10 userspacket switching: with 35 users, probability 10 active less than .0004Packet switching allows more users to use network!N users1 Mbps linkQ: how did we get value 0.0004?63Packet switching v

52、ersus circuit switchingGreat for bursty dataresource sharingsimpler, no 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 proble

53、m (chapter 7)Is packet switching a “slam dunk winner?”Q: human analogies of reserved resources (circuit switching) versus on-demand allocation (packet-switching)?64Packet-switching: store-and-forwardTakes L/R seconds to transmit (push out) packet of L bits on to link or R bpsEntire packet must arriv

54、e at router before it can be transmitted on next link: store and forwarddelay = 3L/R (assuming zero propagation delay)Example:L = 7.5 MbitsR = 1.5 Mbpsdelay = 15 secRRRLmore on delay shortly 65Packet-switching: store-and-forwardTakes L/R seconds to transmit (push out) packet of L bits on to link or

55、R bpsEntire packet must arrive at router before it can be transmitted on next link: store and forwarddelay = 3L/RExample:L = 7.5 MbitsR = 1.5 Mbpsdelay = 15 secRRRLmore on delay shortly L=7.5Ml(packet)=1.5KDelay=?66Packet-switched networks: forwardingGoal: move packets through routers from source to

56、 destinationwell study several path selection (i.e. routing) algorithms (chapter 4)datagram network: destination address in packet determines next hoproutes may change during sessionanalogy: driving, asking directions virtual circuit network: each packet carries tag (virtual circuit ID), tag determi

57、nes next hopfixed path determined at call setup time, remains fixed thru callrouters maintain per-call state67Network Taxonomy Datagram network is not either connection-oriented or connectionless. Internet provides both connection-oriented (TCP) and connectionless services (UDP) to apps.Whats the di

58、fference between TDM and FDM? municationnetworksCircuit-switchednetworksFDMTDMPacket-switchednetworksNetworkswith VCsDatagramNetworksCDM68Access networks and physical mediaQ: How to connect end systems to edge router?residential access netsinstitutional access networks (school, company)mobile access

59、 networksKeep in mind: bandwidth (bits per second) of access network?shared or dedicated?where are you going to access the networks?which ISP?1.4 Network access and physical media69Residential access: point to point accessDialup via modemup to 56Kbps direct access to router (often less)Cant surf and

60、 phone at same time: cant be “always on”ADSL: asymmetric(非对称) digital subscriber lineup to 1 Mbps upstream (today typically 256 kbps)up to 8 Mbps downstream (today typically 1: delays e largeLa/R 1: more “work” arriving than can be serviced, average delay infinite!119“Real” Internet delays and route