1 Chapter 1 Introduction A note on the use of these ppt slides: We’re making these slides freely available to all (faculty, students, readers). They’re in PowerPoint form so you see the animations; and can add, modify, and delete slides (including this one) and slide content to suit your needs. They obviously represent a lot of work on our part. In return for use, we only ask the following: Computer Networking: A Top Down Approach 6th edition Jim Kurose, Keith Ross Addison-Wesley March 2012 If you use these slides (e.g., in a class) that you mention their source (after all, we’d like people to use our book!) If you post any slides on a www site, that you note that they are adapted from (or perhaps identical to) our slides, and note our copyright of this material. Thanks and enjoy! JFK/KWR All material copyright J.F Kurose and K.W. Ross, All Rights Reserved Introduction 1-1
2 Chapter 1: introductionour goal: get “feel” and terminology more depth, detail later in course approach: use Internet as example overview: what’s the Internet? what’s a protocol? course introduction network edge; hosts, access net, physical media network core: packet/circuit switching, Internet structure performance: loss, delay, throughput security protocol layers, service models history Introduction 1-2
3 Chapter 1: roadmap 1.1 what is the Internet? 1.2 network edgeend systems, access networks, links 1.3 network core packet switching, circuit switching, network structure 1.4 delay, loss, throughput in networks 1.5 protocol layers, service models 1.6 networks under attack: security 1.7 history Introduction 1-3
4 Network： what 一只小昆虫撞在蜘蛛网上,拼命的挣扎,使蜘蛛网发生形变,但它却不能将蜘蛛网撞破,因为蜘蛛网在变形的过程中,可以将运动产生的力分散掉； 搜索引擎蜘蛛； Introduction 1-4 4
5 Network： what Introduction 1-5 5
6 Network： what Introduction 1-6 6
7 Network： what Graph with nodes and links 由节点和连线构成的图，表示研究诸对象及其相互联系 7Introduction 1-7 7
8 Network： How statistics throughput, utilization ratioIf there are p telephone calls in time ∆t， and what is the probability of x calls in time t? Solution: we can divide t into n parts, and then 每个电话呼叫是相互独立的，也就是无后效性，即Markov性； 某个公交站台一个小时内出现了的公交车的数量，用泊松分布表示，期望值是朗姆达，意思就是强度； 某个公交站台任意两辆公交车出现的间隔时间，用指数分布来表示，期望值是朗姆达的倒数； Introduction 1-8 8
9 Network： How statistics throughput, utilization ratio example1暑期补课期间，每天中午就餐时间段，公交中科大西站候车人数服从参数为λ的泊松分布,假设λ=5.2,已知我们班有一位同学在那里候车,求这车站就他一人候车的概率是？ exampl2 结合example1，假设学生均乘坐180路公交到文星餐厅就餐，该公交座位数为20，允许超载50%，平均每10分钟一班，则平均每位同学等车时间是多久？ 请设计一个低成本、高效率的公交运营方案。 旅客到站按每3分钟到达1个人的泊松分布到达汽车站，请问对应的泊松分布表达式; 交通规划的OD矩阵；
10 Network： How statistics Exponential distribution– most commonly used as the inter-arrival time / service time distribution Probability density function (pdf): Cumulative density function (cdf): Mean, variance, standard deviation 每个电话呼叫是相互独立的，也就是无后效性，即Markov性； 某个公交站台一个小时内出现了的公交车的数量，用泊松分布表示，期望值是朗姆达，意思就是强度； 某个公交站台任意两辆公交车出现的间隔时间，用指数分布来表示，期望值是朗姆达的倒数； Introduction 1-10 10
11 Network： How statistics throughput, utilization ratio solvedifferential equations schedule congress control graph theory shortest path algorithm... Introduction 1-11 11
12 Network： How Synchronous basic clock, propagation delay Servicetransport, communications Security correct, protected... And other problems…… Introduction 1-12 12
13 So called computer networksConclusion So called computer networks a telecommunications network which allows computers to exchange data. Networked computing devices pass data to each other along network links (data connections).
14 What’s the Internet: “nuts and bolts” viewsmartphone PC server wireless laptop millions of connected computing devices: hosts = end systems running network apps mobile network global ISP regional ISP home network institutional communication links fiber, copper, radio, satellite transmission rate: bandwidth wired links wireless nuts and bolts 螺母和螺栓 Packet switches: forward packets (chunks of data) routers and switches router Introduction 1-14 14
15 “Fun” internet appliancesWeb-enabled toaster + weather forecaster IP picture frame Tweet-a-watt: monitor energy use Slingbox: watch, control cable TV remotely Internet refrigerator Internet phones Introduction 1-15
16 What’s the Internet: “nuts and bolts” viewmobile network global ISP regional ISP home network institutional Internet: “network of networks” Interconnected ISPs protocols control sending, receiving of msgs e.g., TCP, IP, HTTP, Skype, Internet standards RFC: Request for comments IETF: Internet Engineering Task Force 端系统通过Internet服务提供商接入因特网，端系统、分组交换机和其他因特网部件都要运行控制信息收发的一系列协议如TCP/IP Introduction 1-16
17 What’s the Internet: a service viewmobile network global ISP regional ISP home network institutional Infrastructure that provides services to applications: Web, VoIP, , games, e-commerce, social nets, … provides programming interface to apps hooks that allow sending and receiving app programs to “connect” to Internet provides service options, analogous to postal service Introduction 1-17
18 What’s a protocol? human protocols: network protocols:“what’s the time?” “I have a question” introductions … specific msgs sent … specific actions taken when msgs received, or other events network protocols: machines rather than humans all 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 语法、语义、语用 Introduction 1-18
19 What’s a protocol? a human protocol and a computer network protocol:Hi TCP connection request Hi TCP connection response Got the time? Get 2:00
20 Chapter 1: hyperlink Course introduction of summer 2015 Introduction1-20
21 Chapter 1: roadmap 1.1 what is the Internet? 1.2 network edgeend systems, access networks, links 1.3 network core packet switching, circuit switching, network structure 1.4 delay, loss, throughput in networks 1.5 protocol layers, service models 1.6 networks under attack: security 1.7 history Introduction 1-21
22 A closer look at network structure:network edge: hosts: clients and servers servers often in data centers mobile network global ISP regional ISP home network institutional access networks, physical media: wired, wireless communication links network core: interconnected routers network of networks Introduction 1-22
23 Access networks and physical mediaQ: How to connect end systems to edge router? residential access nets institutional access networks (school, company) mobile access networks keep in mind: bandwidth (bits per second) of access network? shared or dedicated? Introduction 1-23
24 Access net: digital subscriber line (DSL)central office telephone network voice, data transmitted at different frequencies over dedicated line to central office DSL modem splitter DSLAM DSL access multiplexer ISP use existing telephone line to central office DSLAM data over DSL phone line goes to Internet voice over DSL phone line goes to telephone net < 2.5 Mbps upstream transmission rate (typically < 1 Mbps) < 24 Mbps downstream transmission rate (typically < 10 Mbps) subscriber 用户 Introduction 1-24
25 DMT （Discrete Multi-Tone）Spectrum Uplink Downlink telephone … … Freq. (kHz) 4 ~40 ~138 ~1100 Introduction 1-25
26 DMT and DTMF DMT a kind of FSK technologyDTMF （Dual Tone Multi Frequency） Introduction 1-26
27 Access net: cable networkcable headend … cable modem splitter Channels V I D E O A T C N R L 1 2 3 4 5 6 7 8 9 frequency division multiplexing: different channels transmitted in different frequency bands Introduction 1-27
28 Access net: cable networkcable headend … data, TV transmitted at different frequencies over shared cable distribution network cable modem splitter cable modem termination system CMTS ISP HFC: hybrid fiber coax asymmetric: up to 30Mbps downstream transmission rate, 2 Mbps upstream transmission rate network of cable, fiber attaches homes to ISP router homes share access network to cable headend unlike DSL, which has dedicated access to central office HFC:光纤同轴电缆混合网 Introduction 1-28
29 Spectrum of HFC Downlink Uplink Original Analogy TV Digital SignalReserved Freq.(MHz) Introduction 1-29
30 Cable Modems Typical details of the upstream and downstream channels in North America. 机顶盒(set-top box) Introduction 1-30
31 to/from headend or central officeAccess net: home network wireless devices to/from headend or central office often combined in single box wireless access point (54 Mbps) router, firewall, NAT cable or DSL modem wired Ethernet (100 Mbps) Introduction 1-31
32 Enterprise access networks (Ethernet)institutional link to ISP (Internet) institutional router Ethernet switch institutional mail, web servers typically used in companies, universities, etc 10 Mbps, 100Mbps, 1Gbps, 10Gbps transmission rates today, end systems typically connect into Ethernet switch Introduction 1-32
33 USTCsz’
34 Wireless access networksshared wireless access network connects end system to router via base station aka “access point” wide-area wireless access provided by telco (cellular) operator, 10’s km between 1 and 10 Mbps 3G, 4G: LTE wireless LANs: within building (100 ft) 802.11b/g (WiFi): 11, 54 Mbps transmission rate to Internet to Internet 1-34
35 Wireless access networksshared wireless access network connects end system to router 1-35
36 Host: sends packets of datahost sending function: takes application message breaks into smaller chunks, known as packets, of length L bits transmits packet into access network at transmission rate R link transmission rate, aka link capacity, aka link bandwidth two packets, L bits each 2 1 R: link transmission rate host Aka， Also known as packet transmission delay time needed to transmit L-bit packet into link L (bits) R (bits/sec) = = 1-36
37 Physical media bit: propagates between transmitter/receiver pairsphysical link: what lies between transmitter & receiver guided media: signals propagate in solid media: copper, fiber, coax unguided media: signals propagate freely, e.g., radio twisted pair (TP) two insulated copper wires Category 5: 100 Mbps, 1 Gpbs Ethernet Category 6: 10Gbps Category 5 5类双绞线 ADSL的极限传输距离与数据率以及用户线的线径都有很大的关系（用户线越细，信号传输时的衰减就越大），而所能得到的最高数据传输速率与实际的用户线上的信噪比密切相关。 例如，0.5 毫米线径的用户线，传输速率为 1.5 ~ 2.0 Mb/s 时可传送 5.5 公里，但当传输速率提高到 6.1 Mb/s 时，传输距离就缩短为 3.7 公里。如果把用户线的线径减小到0.4毫米，那么在6.1 Mb/s的传输速率下就只能传送2.7公里 Introduction 1-37
38 Discuss： How to determine the media’s bandwidth？Inter-Symbol Interference 码间串扰 a form of distortion of a signal in which one symbol interferes with subsequent symbols Multipath propagation Band limited channels Power loss， crosstalk 码间干扰是一种乘性的干扰,1924 年，奈奎斯特(Nyquist) 给出了在假定的理想条件下，为了避免码间串扰，码元的传输速率的上限值。在任何信道中，码元传输的速率是有上限的，否则就会出现码间串扰的问题，使接收端对码元的判决（即识别）成为不可能。如果信道的频带越宽，也就是能够通过的信号高频分量越多，那么就可以用更高的速率传送码元而不出现码间串扰。 香农(Shannon)用信息论的理论推导出了带宽受限且有高斯白噪声干扰的信道的极限、无差错的信息传输速率。 信道的极限信息传输速率 C 可表达为 C = W log2(1+S/N) b/s W 为信道的带宽（以 Hz 为单位）； S 为信道内所传信号的平均功率； N 为信道内部的高斯噪声功率。 Introduction 1-38
39 Discuss： How to determine the media’s bandwidth？Inter-Symbol Interference 码间串扰 a form of distortion of a signal in which one symbol interferes with subsequent symbols Multipath propagation Band limited channels eye patterns for test ISI 香农(Shannon)用信息论的理论推导出了带宽受限且有高斯白噪声干扰的信道的极限、无差错的信息传输速率。 信道的极限信息传输速率 C 可表达为 C = W log2(1+S/N) b/s W 为信道的带宽（以 Hz 为单位）； S 为信道内所传信号的平均功率； N 为信道内部的高斯噪声功率。 Introduction 1-39
40 Discuss： How to determine the media’s bandwidth？ Shannon's TheoremShannon's Theorem gives an upper bound to the capacity of a link, in bits per second (bps), as a function of the available bandwidth and the signal-to-noise ratio of the link. The Theorem can be stated as: C = B * log2(1+ S/N) where C is the achievable channel capacity, B is the bandwidth of the line, S is the average signal power and N is the average noise power. example: For a typical telephone line with a signal-to-noise ratio of 30dB and an audio bandwidth of 3kHz, we get a maximum data rate of: C = 3000 * log2(1001) which is a little less than 30 kbps. 香农(Shannon)用信息论的理论推导出了带宽受限且有高斯白噪声干扰的信道的极限、无差错的信息传输速率。 信道的极限信息传输速率 C 可表达为 C = W log2(1+S/N) b/s W 为信道的带宽（以 Hz 为单位）； S 为信道内所传信号的平均功率； N 为信道内部的高斯噪声功率。 Reference L L Peterson and B S Davie, Computer Networks:a systems approach (Morgan Kaufmann), ISBN: (Paperback ISBN: ) pp Introduction 1-40
41 Physical Guided Media Introduction 1-41
42 Physical media: coax, fibercoaxial cable: two concentric copper conductors bidirectional broadband: multiple channels on cable HFC fiber optic cable: glass fiber carrying light pulses, each pulse a bit high-speed operation: high-speed point-to-point transmission (e.g., 10’s-100’s Gpbs transmission rate) low error rate: repeaters spaced far apart immune to electromagnetic noise Introduction 1-42
43 Physical Guided Media coaxial cable fiber BNC connector Introduction1-43
44 Physical media: radio radio link types:terrestrial microwave e.g. up to 45 Mbps channels LAN (e.g., WiFi) 11Mbps, 54 Mbps wide-area (e.g., cellular) 3G cellular: ~ few Mbps satellite Kbps to 45Mbps channel (or multiple smaller channels) 270 msec end-end delay geosynchronous versus low altitude signal carried in electromagnetic spectrum no physical “wire” bidirectional propagation environment effects: reflection obstruction by objects interference Introduction 1-44
45 Chapter 1: roadmap 1.1 what is the Internet? 1.2 network edgeend systems, access networks, links 1.3 network core packet switching, circuit switching, network structure 1.4 delay, loss, throughput in networks 1.5 protocol layers, service models 1.6 networks under attack: security 1.7 history Introduction 1-45
46 The network core mesh of interconnected routerspacket-switching: hosts break application-layer messages into packets forward packets from one router to the next, across links on path from source to destination each packet transmitted at full link capacity Introduction 1-46
47 Packet-switching: store-and-forwardL bits per packet 3 2 1 source destination R bps R bps takes L/R seconds to transmit (push out) L-bit packet into link at R bps store and forward: entire packet must arrive at router before it can be transmitted on next link one-hop numerical example: L = 7.5 Mbits R = 1.5 Mbps one-hop transmission delay = 5 sec end-end delay = 2L/R (assuming zero propagation delay) more on delay shortly … Introduction 1-47
48 Packet Switching: queuing delay, lossR = 100 Mb/s D R = 1.5 Mb/s B E queue of packets waiting for output link queuing and loss: If arrival rate (in bits) to link exceeds transmission rate of link for a period of time: packets will queue, wait to be transmitted on link packets can be dropped (lost) if memory (buffer) fills up Introduction 1-48
49 Packet Switching: queuing delay, lossA Basic Queue Setup Introduction 1-49
50 Packet Switching: queuing delay, lossElrang distibution (Gamma distribution) Let T1, T2…Tk be independent exponential distribution with parameter λ T=T1+T2+…+Tk, T follows Elrang distribution Probability density function: Mean, variance K=1-> exponential distribution When k is suitably large -> normal distribution Introduction 1-50
51 Packet Switching: queuing delay, lossElrang distibution (Gamma distribution) K=1-> exponential distribution When k is suitably large -> normal distribution Introduction 1-51
52 Two key network-core functionsrouting: determines source-destination route taken by packets routing algorithms forwarding: move packets from router’s input to appropriate router output routing algorithm local forwarding table header value output link 0100 0101 0111 1001 3 2 1 1 2 3 0111 dest address in arriving packet’s header Network Layer 4-52
53 Try yourself：route table
54 Alternative core: circuit switchingend-end resources allocated to, reserved for “call” between source & dest: In diagram, each link has four circuits. call gets 2nd circuit in top link and 1st circuit in right link. dedicated resources: no sharing circuit-like (guaranteed) performance circuit segment idle if not used by call (no sharing) Commonly used in traditional telephone networks 电话公司喜欢电路交换的理由：1. 服务质量 2.记账 X.25 帧中继 ATM Introduction 1-54
55 Circuit switching: FDM versus TDM4 users Example: FDM frequency time TDM frequency time Two simple multiple access control techniques. Each mobile’s share of the bandwidth is divided into portions for the uplink and the downlink. Also, possibly, out of band signaling. As we will see, used in AMPS, GSM, IS-54/136 Introduction 1-55 55
56 Packet switching versus circuit switchingpacket switching allows more users to use network! example: 1 Mb/s link each user: 100 kb/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 * ….. N users 1 Mbps link Q: how did we get value ? Q: what happens if > 35 users ? * Check out the online interactive exercises for more examples Introduction 1-56
57 Packet switching versus circuit switchingis packet switching a “slam dunk winner?” great for bursty data resource sharing simpler, no call setup excessive congestion possible: packet delay and loss protocols needed for reliable data transfer, congestion control Q: How to provide circuit-like behavior? bandwidth guarantees needed for audio/video apps still an unsolved problem (chapter 7) 分组交换的优势：更多带宽，比电路交换更简单有效； 劣势：排队时延的变动和不可预测； slam dunk 灌篮高手 Q: human analogies of reserved resources (circuit switching) versus on-demand allocation (packet-switching)? Introduction 1-57
58 See the flash demonstrationPacket switching versus circuit switching See the flash demonstration 分组交换的优势：更多带宽，比电路交换更简单有效； 劣势：排队时延的变动和不可预测； Introduction 1-58
59 Internet structure: network of networksEnd systems connect to Internet via access ISPs (Internet Service Providers) Residential, company and university ISPs Access ISPs in turn must be interconnected. So that any two hosts can send packets to each other Resulting network of networks is very complex Evolution was driven by economics and national policies Let’s take a stepwise approach to describe current Internet structure
60 Internet structure: network of networksQuestion: given millions of access ISPs, how to connect them together? access net …
61 Internet structure: network of networksOption: connect each access ISP to every other access ISP? access net … … connecting each access ISP to each other directly doesn’t scale: O(N2) connections.
62 Internet structure: network of networksOption: connect each access ISP to a global transit ISP? Customer and provider ISPs have economic agreement. access net … global ISP
63 Internet structure: network of networksBut if one global ISP is viable business, there will be competitors …. access net … ISP A ISP B ISP C
64 Internet structure: network of networksBut if one global ISP is viable business, there will be competitors …. which must be interconnected Internet exchange point access net … ISP A IXP IXP ISP B ISP C peering link
65 Internet structure: network of networks… and regional networks may arise to connect access nets to ISPS access net … ISP A IXP IXP ISP B ISP C regional net
66 Internet structure: network of networks… and content provider networks (e.g., Google, Microsoft, Akamai ) may run their own network, to bring services, content close to end users access net … ISP A IXP Content provider network IXP ISP B ISP B regional net
67 Internet structure: network of networksaccess ISP Regional ISP IXP Tier 1 ISP Google at center: small # of well-connected large networks “tier-1” commercial ISPs (e.g., Level 3, Sprint, AT&T, NTT), national & international coverage content provider network (e.g, Google): private network that connects it data centers to Internet, often bypassing tier-1, regional ISPs Introduction 1-67
68 Tier-1 ISP: e.g., Sprint … to/from backbone peering to/from customersPOP: point-of-presence Introduction 1-68
69 Chapter 1: roadmap 1.1 what is the Internet? 1.2 network edgeend systems, access networks, links 1.3 network core packet switching, circuit switching, network structure 1.4 delay, loss, throughput in networks 1.5 protocol layers, service models 1.6 networks under attack: security 1.7 history Introduction 1-69
70 How do loss and delay occur?packets queue in router buffers packet arrival rate to link (temporarily) exceeds output link capacity packets queue, wait for turn packet being transmitted (delay) A free (available) buffers: arriving packets dropped (loss) if no free buffers B packets queueing (delay) Introduction 1-70
71 Four sources of packet delaytransmission A propagation B nodal processing queueing dnodal = dproc + dqueue + dtrans + dprop dproc: nodal processing check bit errors determine output link typically < msec dqueue: queueing delay time waiting at output link for transmission depends on congestion level of router Introduction 1-71
72 Four sources of packet delaytransmission A propagation B nodal processing queueing dnodal = dproc + dqueue + dtrans + dprop dtrans: transmission delay: L: packet length (bits) R: link bandwidth (bps) dtrans = L/R dprop: propagation delay: d: length of physical link s: propagation speed in medium (~2x108 m/sec) dprop = d/s dtrans and dprop very different * Check out the Java applet for an interactive animation on trans vs. prop delay Introduction 1-72
73 Caravan analogy cars “propagate” at 100 km/hrtoll booth ten-car caravan 100 km cars “propagate” at 100 km/hr toll booth takes 12 sec to service car (bit transmission time) car~bit; caravan ~ packet Q: How long until caravan is lined up before 2nd toll booth? time to “push” entire caravan through toll booth onto highway = 12*10 = 120 sec time for last car to propagate from 1st to 2nd toll both: 100km/(100km/hr)= 1 hr A: 62 minutes Introduction 1-73
74 Caravan analogy (more)toll booth ten-car caravan 100 km suppose cars now “propagate” at 1000 km/hr and suppose toll booth now takes one min to service a car Q: Will cars arrive to 2nd booth before all cars serviced at first booth? A: Yes! after 7 min, 1st car arrives at second booth; three cars still at 1st booth. Introduction 1-74
75 Queueing delay (revisited)R: link bandwidth (bps) L: packet length (bits) a: average packet arrival rate average queueing delay traffic intensity = La/R La/R ~ 0: avg. queueing delay small La/R -> 1: avg. queueing delay large La/R > 1: more “work” arriving than can be serviced, average delay infinite! La/R ~ 0 * Check out the Java applet for an interactive animation on queuing and loss La/R -> 1 Introduction 1-75
76 “Real” Internet delays and routeswhat do “real” Internet delay & loss look like? traceroute program: provides delay measurement from source to router along end-end Internet path towards destination. For all i: sends three packets that will reach router i on path towards destination router i will return packets to sender sender times interval between transmission and reply. 1.Traceroute程序使用ICMP报文和IP首部中的TTL字段，每个处理数据报的路由器都需要把TTL的值减1或减去数据报在路由器中停留的秒数，因为大部分转发时延都小于1秒钟，因此TTL最终成为一个跳站的计数器；RFC1009； 2.对于目的主机，即使TTL为1，也不会丢弃该数据包并产生一个超时ICMP报文，但Traceroute会选择一个不可能的UDP端口号，使目的主机产生一份“端口不可达”ICMP报文，源主机由此判断程序结束。 3 probes 3 probes 3 probes Introduction 1-76
77 “Real” Internet delays, routestraceroute: gaia.cs.umass.edu to 3 delay measurements from gaia.cs.umass.edu to cs-gw.cs.umass.edu 1 cs-gw ( ) 1 ms 1 ms 2 ms 2 border1-rt-fa5-1-0.gw.umass.edu ( ) 1 ms 1 ms 2 ms 3 cht-vbns.gw.umass.edu ( ) 6 ms 5 ms 5 ms 4 jn1-at wor.vbns.net ( ) 16 ms 11 ms 13 ms 5 jn1-so wae.vbns.net ( ) 21 ms 18 ms 18 ms 6 abilene-vbns.abilene.ucaid.edu ( ) 22 ms 18 ms 22 ms 7 nycm-wash.abilene.ucaid.edu ( ) 22 ms 22 ms 22 ms ( ) 104 ms 109 ms 106 ms 9 de2-1.de1.de.geant.net ( ) 109 ms 102 ms 104 ms 10 de.fr1.fr.geant.net ( ) 113 ms 121 ms 114 ms 11 renater-gw.fr1.fr.geant.net ( ) 112 ms 114 ms 112 ms 12 nio-n2.cssi.renater.fr ( ) 111 ms 114 ms 116 ms 13 nice.cssi.renater.fr ( ) 123 ms 125 ms 124 ms 14 r3t2-nice.cssi.renater.fr ( ) 126 ms 126 ms 124 ms 15 eurecom-valbonne.r3t2.ft.net ( ) 135 ms 128 ms 133 ms ( ) 126 ms 128 ms 126 ms 17 * * * 18 * * * 19 fantasia.eurecom.fr ( ) 132 ms 128 ms 136 ms trans-oceanic link 如果在5秒钟内没有收到三份数据报的任意一份响应，则打印一个星号，继续下一份数据报； * means no response (probe lost, router not replying) * Do some traceroutes from exotic countries at Introduction 1-77
78 example
79 Packet loss queue (aka buffer) preceding link in buffer has finite capacity packet arriving to full queue dropped (aka lost) lost packet may be retransmitted by previous node, by source end system, or not at all buffer (waiting area) packet being transmitted A B packet arriving to full buffer is lost * Check out the Java applet for an interactive animation on queuing and loss Introduction 1-79
80 Throughput throughput: rate (bits/time unit) at which bits transferred between sender/receiver instantaneous: rate at given point in time average: rate over longer period of time server, with file of F bits to send to client link capacity Rs bits/sec server sends bits (fluid) into pipe pipe that can carry fluid at rate Rs bits/sec) Rc bits/sec) link capacity Rc bits/sec Introduction 1-80
81 Throughput (more) Rs < Rc What is average end-end throughput?Rc bits/sec Rs bits/sec Rs > Rc What is average end-end throughput? Rs bits/sec Rc bits/sec link on end-end path that constrains end-end throughput bottleneck link Introduction 1-81
82 Throughput: Internet scenarioper-connection end-end throughput: min(Rc,Rs,R/10) in practice: Rc or Rs is often bottleneck Rs Rs Rs R Rc Rc Rc 10 connections (fairly) share backbone bottleneck link R bits/sec Introduction 1-82
83 Chapter 1: roadmap 1.1 what is the Internet? 1.2 network edgeend systems, access networks, links 1.3 network core packet switching, circuit switching, network structure 1.4 delay, loss, throughput in networks 1.5 protocol layers, service models 1.6 networks under attack: security 1.7 history Introduction 1-83
84 Protocol “layers” Question: Networks are complex, with many “pieces”:hosts routers links of various media applications protocols hardware, software Question: is there any hope of organizing structure of network? …. or at least our discussion of networks? Introduction 1-84
85 Organization of air travelticket (purchase) baggage (check) gates (load) runway takeoff airplane routing ticket (complain) baggage (claim) gates (unload) runway landing 哲学家—翻译---秘书结构 a series of steps Introduction 1-85
86 Layering of airline functionalityticket (purchase) baggage (check) gates (load) runway (takeoff) airplane routing departure airport arrival intermediate air-traffic control centers ticket (complain) baggage (claim gates (unload) runway (land) ticket baggage gate takeoff/landing layers: each layer implements a service via its own internal-layer actions relying on services provided by layer below Introduction 1-86
87 Why layering? dealing with complex systems:explicit structure allows identification, relationship of complex system’s pieces layered reference model for discussion modularization eases maintenance, updating of system change of implementation of layer’s service transparent to rest of system e.g., change in gate procedure doesn’t affect rest of system layering considered harmful? Introduction 1-87
88 Internet protocol stackapplication: supporting network applications FTP, SMTP, HTTP transport: process-process data transfer TCP, UDP network: routing of datagrams from source to destination IP, routing protocols link: data transfer between neighboring network elements Ethernet, (WiFi), PPP physical: bits “on the wire” application transport network link physical Introduction 1-88
89 ISO/OSI reference modelpresentation: allow applications to interpret meaning of data, e.g., encryption, compression, machine-specific conventions session: synchronization, checkpointing, recovery of data exchange Internet stack “missing” these layers! these services, if needed, must be implemented in application needed? application presentation session transport network link physical Introduction 1-89
90 Encapsulation source destination application transport network linkmessage M application transport network link physical segment Ht M Ht datagram Ht Hn M Hn frame Ht Hn Hl M link physical switch destination network link physical Ht Hn M Ht Hn Hl M M application transport network link physical Ht Hn M Ht M Ht Hn M router Ht Hn Hl M Introduction 1-90
91 Chapter 1: roadmap 1.1 what is the Internet? 1.2 network edgeend systems, access networks, links 1.3 network core packet switching, circuit switching, network structure 1.4 delay, loss, throughput in networks 1.5 protocol layers, service models 1.6 networks under attack: security 1.7 history Introduction 1-91
92 Network security field of network security:how bad guys can attack computer networks how we can defend networks against attacks how to design architectures that are immune to attacks Internet not originally designed with (much) security in mind original vision: “a group of mutually trusting users attached to a transparent network” Internet protocol designers playing “catch-up” security considerations in all layers! Introduction 1-92
93 Bad guys: put malware into hosts via Internetmalware can get in host from: virus: self-replicating infection by receiving/executing object (e.g., attachment) worm: self-replicating infection by passively receiving object that gets itself executed spyware malware can record keystrokes, web sites visited, upload info to collection site infected host can be enrolled in botnet, used for spam. DDoS attacks Malware:恶意软件 Introduction 1-93
94 Bad guys: attack server, network infrastructureDenial of Service (DoS): attackers make resources (server, bandwidth) unavailable to legitimate traffic by overwhelming resource with bogus traffic 1. select target 2. break into hosts around the network (see botnet) target 3. send packets to target from compromised hosts Introduction 1-94
95 Bad guys can sniff packetspacket “sniffing”: broadcast media (shared ethernet, wireless) promiscuous network interface reads/records all packets (e.g., including passwords!) passing by A C src:B dest:A payload B wireshark software used for end-of-chapter labs is a (free) packet-sniffer Introduction 1-95
96 Bad guys can use fake addressesIP spoofing: send packet with false source address A C src:B dest:A payload B … lots more on security (throughout, Chapter 8) Introduction 1-96
97 Chapter 1: roadmap 1.1 what is the Internet? 1.2 network edgeend systems, access networks, links 1.3 network core packet switching, circuit switching, network structure 1.4 delay, loss, throughput in networks 1.5 protocol layers, service models 1.6 networks under attack: security 1.7 history Introduction 1-97
98 Internet history 1961-1972: Early packet-switching principles1961: Kleinrock - queueing theory shows effectiveness of packet-switching 1964: Baran - packet-switching in military nets 1967: ARPAnet conceived by Advanced Research Projects Agency 1969: first ARPAnet node operational 1972: ARPAnet public demo NCP (Network Control Protocol) first host-host protocol ,RFC001 first program ARPAnet has 15 nodes ARPA，1967年，田纳西州的ACM Introduction 1-98
99 Internet history 1972-1980: Internetworking, new and proprietary nets1970: ALOHAnet satellite network in Hawaii 1974: Cerf and Kahn - architecture for interconnecting networks 1976: Ethernet at Xerox PARC late70’s: proprietary architectures: DECnet, SNA, XNA late 70’s: switching fixed length packets (ATM precursor) 1979: ARPAnet has 200 nodes Cerf and Kahn’s internetworking principles: minimalism, autonomy - no internal changes required to interconnect networks best effort service model stateless routers decentralized control define today’s Internet architecture 简单化，网络自治，网络之间不需要数据变换 Introduction 1-99
100 Internet history 1980-1990: new protocols, a proliferation of networks1983: deployment of TCP/IP 1982: smtp protocol defined 1983: DNS defined for name-to-IP-address translation 1985: ftp protocol defined 1988: TCP congestion control(Host/End) new national networks: Csnet, BITnet, NSFnet, Minitel 100,000 hosts connected to confederation of networks NCP-TCP Minitel,based on X.25(virtual circuit) Introduction 1-100
101 Internet history 1990, 2000’s: commercialization, the Web, new appsearly 1990’s: ARPAnet decommissioned 1991: NSF lifts restrictions on commercial use of NSFnet (decommissioned, 1995) early 1990s: Web hypertext [Bush 1945, Nelson 1960’s] HTML, HTTP: Berners-Lee 1994: Mosaic, later Netscape late 1990’s: commercialization of the Web late 1990’s – 2000’s: more killer apps: instant messaging, P2P file sharing network security to forefront est. 50 million host, 100 million+ users backbone links running at Gbps Introduction 1-101
102 Internet history 2005-present ~750 million hostsSmartphones and tablets Aggressive deployment of broadband access Increasing ubiquity of high-speed wireless access Emergence of online social networks: Facebook: soon one billion users Service providers (Google, Microsoft) create their own networks Bypass Internet, providing “instantaneous” access to search, emai, etc. E-commerce, universities, enterprises running their services in “cloud” (eg, Amazon EC2) Introduction 1-102
103 Introduction: summarycovered a “ton” of material! Internet overview what’s a protocol? network edge, core, access network packet-switching versus circuit-switching Internet structure performance: loss, delay, throughput layering, service models security history you now have: context, overview, “feel” of networking more depth, detail to follow! Introduction 1-103
104 homework 1. Give your comprehension of binomial， Poisson ，exponential ,Elrang and normal distribution. Forth edition 中文版 Review of chapter1 P45： Exercise 1,7,22 P49：Discussion 9 Introduction 1-104