计算机网络第6版课件：Chapter 1 Introduction

1、Lecture on Computer NetworkFor 09Spring 2011Introduct by Weifeng SunComputer network-Introduction2/88Open questions What is computer network? How to solve the network problems? How do you feel the Computer network?Introduct by Weifeng SunComputer network-Introduction3/88Course Staff Instructors Weif

2、eng Sun , Office 405 87571607; 81648638 Teaching assistants: ?Introduct by Weifeng SunComputer network-Introduction4/88Important info. About this Unit To pass this unit:o Final Exam 70%o Assignment 2015%o Attendance 1015% Dont fail any component.Introduct by Weifeng SunComputer network-Introduction5

3、/88Course Goals Become familiar with the principles and practice of data networking. Routing, transport protocols, naming, . Learn how to write networked applications: C/S model TCP/UDP Get some understanding about network internals in a hands on way. TCP-style congestion controlIntroduct by Weifeng

4、 SunComputer network-Introduction6/88Reference Computer Networking: A Top Down Approach Featuring the Internet, 3rd edition. Jim Kurose, Keith RossAddison-Wesley, July 2004. 计算机网络（第 4 版）电子工业出版社,潘爱民译 TCP/IP详解（卷1） 数据通信与网络 TCP/IP协议族 More Books have people gone to the bookstore? How many copies? Should

5、be thereIntroduct by Weifeng Sunmeterials http:/ http:/ cn_Computer network-Introduction7/88Introduct by Weifeng SunComputer network-Introduction8/88Challenges for Networks Geographic scope The Internet vs. Andrew, etc. Scale The Internet vs. your home network Application types Email vs. Videoconfer

6、encing Trust and Administration Corporate network one network “provider” Internet 17,000 network providers SecurityIntroduct by Weifeng SunComputer network-Introduction9/88“Cool” internet appliances(2006)Worlds smallest web server/shri/iPic.htmlIP picture framehttp:/ toaste

7、r +weather forecasterInternet phonesIntroduct by Weifeng SunComputer network-Introduction10/88How to study? Study vs. learn Novel Our own way For computer network or other lessonsIntroduct by Weifeng SunComputer network-Introduction11/88How to Draw a NetworkNodeLinkNodeIntroduct by Weifeng SunComput

8、er network-Introduction12/88Building block: The Links Electrical questions Voltage, frequency, Wired or wireless? Link-layer issues: How to send data? When to talk can everyone talk at once? What to say low-level format? Stay tuned for lecture 5 Okay what about more nodes?NodeLinkNodeIntroduct by We

9、ifeng SunComputer network-Introduction13/88 But what if we want more hosts? Scalability?!One wireWires for everybody!Introduct by Weifeng SunComputer network-Introduction14/88Multiplexing! Need to share network resources How? Switched network Party “A” gets resources sometimes Party “B” gets them so

10、metimes Interior nodes act as “Routers” or “Switches” What mechanisms can share resources?Introduct by Weifeng SunComputer network-Introduction15/88Packet Switching (our emphasis) Source sends information as self-contained packets that have an address. Source may have to break up single message in m

11、ultiple Each packet travels independently to the destination host. Routers and switches use the address in the packet to determine how to forward the packets Analogy: a letter in surface mail.Introduct by Weifeng SunComputer network-Introduction16/88Chapter 1: IntroductionOur goal: get “feel” and te

12、rminology more depth, detail later in course approach: use Internet as exampleOverview: whats the Internet whats a protocol? network edge network core access net, physical media Internet/ISP structure performance: loss, delay protocol layers, service models network modelingIntroduct by Weifeng SunCo

13、mputer network-Introduction17/88Chapter 1: roadmap1.1 What is the Internet?1.2 Network edgeq end systems, access networks, links1.3 Network coreq circuit switching, packet switching, network structure1.4 Delay, loss and throughput in packet-switched networks1.5 Protocol layers, service models1.6 Net

14、works under attack: security1.7 HistoryIntroduct by Weifeng SunComputer network-Introduction18/88“The Internet” An inter-net: a network of networks. A set of networks that are connected with each other Networks are connected using routers that support communication in a hierarchical fashion Often ne

15、ed other special devices at the boundaries for security, accounting, . The Internet: the interconnected set of networks of the Internet Service Providers (ISPs) providing data communications services. About 17,000 different networks make up the Internet In order to inter-operate, all participating n

16、etworks have to follow a common set of rules.Introduct by Weifeng SunComputer network-Introduction19/88What is Computer network? A serials of computers.大公司地区 ISP网络接入点NAP（对等点）公司校园网主干服务提供者校园网校园网校园网校园网本地 ISP地区 ISP地区 ISP地区 ISP本地 ISP本地 ISP大公司大公司网络接入点NAP（对等点）Introduct by Weifeng SunComputer network-Introd

17、uction20/88Whats the Internet: “nuts and bolts” view millions of connected computing devices: hosts = end systems running network appsHome networkInstitutional networkMobile networkGlobal ISPRegional ISProuterPCserverwirelesslaptopcellular handheldwiredlinksaccess points communication links fiber, c

18、opper, radio, satellite transmission rate = bandwidth routers: forward packets (chunks of data)Introduct by Weifeng SunComputer network-Introduction21/88Whats the Internet: “nuts and bolts” view protocols control sending, receiving of msgs e.g., TCP, IP, HTTP, Skype, Ethernet Internet: “network of n

19、etworks” loosely hierarchical public Internet versus private intranet Internet standards RFC: Request for comments IETF: Internet Engineering Task ForceHome networkInstitutional networkMobile networkGlobal ISPRegional ISPIntroduct by Weifeng SunComputer network-Introduction22/88Whats a protocol?huma

20、n protocols: “whats the time?” “I have a question” introductions specific msgs sent specific actions taken when msgs received, or other eventsnetwork protocols: machines rather than humans all communication activity in Internet governed by protocolsprotocols define format, order of msgs sent and rec

21、eived among network entities, and actions taken on msg transmission, receipt Introduct by Weifeng SunComputer network-Introduction23/88Whats a protocol?a human protocol and a computer network protocol:Q: Other human protocols? HiHiGot thetime?2:00TCP connection requestTCP connectionresponseGet http:

22、/ by Weifeng SunComputer network-Introduction24/88Chapter 1: roadmap1.1 What is the Internet?1.2 Network edgeq end systems, access networks, links1.3 Network coreq circuit switching, packet switching, network structure1.4 Delay, loss and throughput in packet-switched networks1.5 Protocol layers, ser

23、vice models1.6 Networks under attack: security1.7 HistoryIntroduct by Weifeng SunComputer network-Introduction25/88A closer look at network structure: network edge: applications and hosts access networks, physical media: wired, wireless communication links network core: interconnected routers networ

24、k of networksIntroduct by Weifeng SunComputer network-Introduction26/88The network edge end systems (hosts):run application programse.g. Web, emailat “edge of network”client/serverpeer-peer client/server modelclient host requests, receives service from always-on servere.g. Web browser/server; email

25、client/server peer-peer model: minimal (or no) use of dedicated serverse.g. Skype, BitTorrentIntroduct by Weifeng SunComputer network-Introduction27/88Access networks and physical mediaQ: How to connect end systems to edge router? residential access nets institutional access networks (school, compan

26、y) mobile access networksKeep in mind: bandwidth (bits per second) of access network? shared or dedicated?Introduct by Weifeng SunComputer network-Introduction28/88Residential : How to access? Residential access: point to point access Dialup via modem DSL: digital subscriber line HFC: hybrid fiber c

27、oax network of cable and fiber attaches homes to ISP router deployment: available via cable TV companieshomecable headendcable distributionnetworkIntroduct by Weifeng SunComputer network-Introduction29/88Residential access: cable modemsIntroduct by Weifeng SunComputer network-Introduction30/88Compan

28、y access: local area networks company/univ local area network (LAN) connects end system to edge router Ethernet: 10 Mbs, 100Mbps, 1Gbps, 10Gbps Ethernet modern configuration: end systems connect into Ethernet switch LANs: chapter 5Introduct by Weifeng SunComputer network-Introduction31/88Home networ

29、ksTypical home network components: DSL or cable modem router/firewall/NAT Ethernet wireless access pointwirelessaccess pointwirelesslaptopsrouter/firewallcablemodemto/fromcableheadendEthernetIntroduct by Weifeng SunComputer network-Introduction32/88Physical media: radio signal carried in electromagn

30、etic spectrum no physical “wire” bidirectional propagation environment effects: reflection obstruction by objects interferenceRadio link types: terrestrial microwave e.g. up to 45 Mbps channels LAN (e.g., Wifi) 11Mbps, 54 Mbps wide-area (e.g., cellular) e.g. 3G: hundreds of kbps satellite Kbps to 45

31、Mbps channel (or multiple smaller channels) 270 msec end-end delay geosynchronous versus low altitudeIntroduct by Weifeng SunComputer network-Introduction33/88Physical Media Bit: propagates betweentransmitter/rcvr pairs physical link: what lies between transmitter & receiver guided media: signal

32、s propagate in solid media: copper, fiber, coax unguided media: signals propagate freely, e.g., radioTwisted Pair (TP) two insulated copper wires Category 3: traditional phone wires, 10 Mbps Ethernet Category 5: 100Mbps EthernetIntroduct by Weifeng SunComputer network-Introduction34/88Physical Media

33、: coax, fiberCoaxial cable: two concentric copper conductors bidirectional baseband: single channel on cable legacy Ethernet broadband: multiple channels on cable HFCFiber optic cable: glass fiber carrying light pulses, each pulse a bit high-speed operation: high-speed point-to-point transmission (e

34、.g., 10s-100s Gps) low error rate: repeaters spaced far apart ; immune to electromagnetic noiseIntroduct by Weifeng SunComputer network-Introduction35/88Network edge: connection-oriented serviceGoal: data transfer between end systemshandshaking: setup (prepare for) data transfer ahead of timeHello,

35、hello back human protocolset up “state” in two communicating hostsTCP - Transmission Control Protocol Internets connection-oriented serviceTCP service RFC 793 reliable, in-order byte-stream data transfer loss: acknowledgements and retransmissions flow control: sender wont overwhelm receiver congesti

36、on control: senders “slow down sending rate” when network congestedIntroduct by Weifeng SunComputer network-Introduction36/88Network edge: connectionless serviceGoal: data transfer between end systems same as before! UDP - User Datagram Protocol RFC 768: connectionless unreliable data transfer no fl

37、ow control no congestion controlApps using TCP: HTTP (Web), FTP (file transfer), Telnet (remote login), SMTP (email)Apps using UDP: streaming media, teleconferencing, DNS, Internet telephonyIntroduct by Weifeng SunComputer network-Introduction37/88Chapter 1: roadmap1.1 What is the Internet?1.2 Netwo

38、rk edgeq end systems, access networks, links1.3 Network coreq circuit switching, packet switching, network structure1.4 Delay, loss and throughput in packet-switched networks1.5 Protocol layers, service models1.6 Networks under attack: security1.7 HistoryIntroduct by Weifeng SunComputer network-Intr

39、oduction38/88The Network Core mesh of interconnected routers the fundamental question: how is data transferred through net? circuit switching: dedicated circuit per call: telephone net packet-switching: data sent thru net in discrete “chunks”Introduct by Weifeng SunComputer network-Introduction39/88

40、Network Core: Circuit SwitchingEnd-end resources reserved for “call” link bandwidth, switch capacity dedicated resources: no sharing circuit-like (guaranteed) performance call setup requiredIntroduct by Weifeng SunComputer network-Introduction40/88Network Core: Circuit Switchingnetwork resources (e.

41、g., bandwidth) divided into “pieces” pieces allocated to calls resource piece idle if not used by owning call (no sharing) dividing link bandwidth into “pieces” frequency division time divisionIntroduct by Weifeng SunComputer network-Introduction41/88Circuit Switching: FDM and TDMFDMfrequencytimeTDM

42、frequencytime4 usersExample:Introduct by Weifeng SunComputer network-Introduction42/88Numerical example How 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 Mbps Each link uses TDM with 24 slots/sec 500 msec to establish end-

43、to-end circuitLets work it out!Introduct by Weifeng SunComputer network-Introduction43/88Switching network 交换机Introduct by Weifeng SunComputer network-Introduction44/88Network Core: Packet Switchingeach end-end data stream divided into packets user A, B packets share network resources each packet us

44、es full link bandwidth resources used as needed resource contention: aggregate resource demand can exceed amount available congestion: packets queue, wait for link use store and forward: packets move one hop at a timeNode receives complete packet before forwardingBandwidth division into “pieces”Dedi

45、cated allocationResource reservationIntroduct by Weifeng SunComputer network-Introduction45/88Packet 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.ABC100 Mb/

46、sEthernet1.5 Mb/sDEstatistical multiplexingqueue of packetswaiting for outputlinkIntroduct by Weifeng SunComputer network-Introduction46/88Packet-switching: store-and-forward Takes L/R seconds to transmit (push out) packet of L bits on to link or R bps Entire packet must arrive at router before it c

47、an be transmitted on next link: store and forward delay = 3L/R (assuming zero propagation delay)Example: L = 7.5 Mbits R = 1.5 Mbps delay = 15 secRRRLmore on delay shortly Introduct by Weifeng SunComputer network-Introduction47/88Packet switching versus circuit switching 1 Mb/s link each user: 100 k

48、b/s when “active” active 10% of time circuit-switching: 10 users packet switching: with 35 users, probability 10 active at same time is less than .0004Packet switching allows more users to use network!N users1 Mbps linkQ: how did we get value 0.0004?Introduct by Weifeng SunComputer network-Introduct

49、ion48/88Packet switching versus circuit switching Great for bursty data resource sharing simpler, no call setup Excessive congestion: packet delay and loss protocols needed for reliable data transfer, congestion control Q: How to provide circuit-like behavior? bandwidth guarantees needed for audio/v

50、ideo apps still an unsolved problemIs packet switching a “slam dunk winner?”Q: human analogies of reserved resources (circuit switching) versus on-demand allocation (packet-switching)?Introduct by Weifeng SunComputer network-Introduction49/88Internet structure and ISPsIntroduct by Weifeng SunCompute

51、r network-Introduction50/88Internet structure: network of networks “Tier-2” ISPs: smaller (often regional) ISPs Connect to one or more tier-1 ISPs, possibly other tier-2 ISPsTier 1 ISPTier 1 ISPTier 1 ISPNAPTier-2 ISPTier-2 ISPTier-2 ISPTier-2 ISPTier-2 ISPTier-2 ISP pays tier-1 ISP for connectivity

52、 to rest of Internetq tier-2 ISP is customer oftier-1 providerTier-2 ISPs also peer privately with each other, interconnect at NAPIntroduct by Weifeng SunComputer network-Introduction51/88Internet structure: network of networks “Tier-3” ISPs and local ISPs last hop (“access”) network (closest to end

53、 systems)Tier 1 ISPTier 1 ISPTier 1 ISPNAPTier-2 ISPTier-2 ISPTier-2 ISPTier-2 ISPTier-2 ISPlocalISPlocalISPlocalISPlocalISPlocalISPTier 3ISPlocalISPlocalISPlocalISPLocal and tier- 3 ISPs are customers ofhigher tier ISPsconnecting them to rest of InternetIntroduct by Weifeng SunComputer network-Intr

54、oduction52/88Chapter 1: roadmap1.1 What is the Internet?1.2 Network edgeq end systems, access networks, links1.3 Network coreq circuit switching, packet switching, network structure1.4 Delay, loss and throughput in packet-switched networks1.5 Protocol layers, service models1.6 Networks under attack:

55、 security1.7 HistoryIntroduct by Weifeng SunComputer network-Introduction53/88How do loss and delay occur?packets queue in router buffers packet arrival rate to link exceeds output link capacity packets queue, wait for turnABpacket being transmitted (delay)packets queueing (delay)free (available) bu

56、ffers: arriving packets dropped (loss) if no free buffersIntroduct by Weifeng SunComputer network-Introduction54/88Four sources of packet delay 1. nodal processing: check bit errors determine output linkABpropagationtransmissionnodalprocessingqueueing 2. queueing time waiting at output link for tran

57、smission depends on congestion level of routerIntroduct by Weifeng SunComputer network-Introduction55/88Delay in packet-switched networks3. Transmission delay: R=link bandwidth (bps) L=packet length (bits) time to send bits into link = L/R4. Propagation delay: d = length of physical link s = propaga

58、tion speed in medium (2x108 m/sec) propagation delay = d/sABpropagationtransmissionnodalprocessingqueueingNote: s and R are very different quantities!Introduct by Weifeng SunComputer network-Introduction56/88Nodal delay dproc = processing delay typically a few microsecs or less dqueue = queuing dela

59、y depends on congestion dtrans = transmission delay = L/R, significant for low-speed links dprop = propagation delay a few microsecs to hundreds of msecsproptransqueueprocnodaldddddIntroduct by Weifeng SunComputer network-Introduction57/88Queueing delay (revisited) R=link bandwidth (bps) L=packet le

60、ngth (bits) a=average packet arrival ratetraffic intensity = La/R La/R 0: average queueing delay small La/R - 1: delays become large La/R 1: more “work” arriving than can be serviced, average delay infinite!Introduct by Weifeng SunComputer network-Introduction58/88“Real” Internet delays and routes1 cs-gw (