1、Introduction,1-1,Chapter 1Introduction,Computer Networking: A Top Down Approach 6th edition Jim Kurose, Keith RossAddison-WesleyMarch 2012,A note on the use of these ppt slides:We’re making these slides freely avai

2、lable 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

3、 a lot of work on our part. In return for use, we only ask the following:,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

4、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 1996-2012 J.F Kurose and K

5、.W. Ross, All Rights Reserved,Chapter 1: Introduction,Our goal: get “feel” and terminologymore depth, detail later in courseapproach:use Internet as example,Introduction,1-2,計算機網絡向用戶提供的最重要的功能,連通性——計算機網絡使上網用戶之間都可以交換信

6、息，好像這些用戶的計算機都可以彼此直接連通一樣。 共享——即資源共享。可以是信息共享、軟件共享，也可以是硬件共享。,世界十大互連網公司(2014),谷歌、臉譜、阿里巴巴、亞馬遜、騰迅、百度、ebay、Priceline、雅虎和京東(2014.9)：,世界六大互連網公司(2017),2017年，全球互聯(lián)網公司市值排名前六位分別是蘋果、谷歌、亞馬遜、Facebook、騰訊和阿里巴巴。放眼全球公司市值，互聯(lián)網公司占據了前五名：,中國十大互連

7、網公司,騰訊、阿里巴巴、百度、京東、網易、新浪、美團點評、攜程、360。其中，騰訊、阿里巴巴、百度連續(xù)五年位居前三。(2017.8),BAT,分別是B百度、A阿里巴巴、T騰訊的首字母,李克強與互連網+,2015年政府工作報告：“制定‘互聯(lián)網+’行動計劃，推動移動互聯(lián)網、云計算、大數據、物聯(lián)網等與現(xiàn)代制造業(yè)結合，促進電子商務、工業(yè)互聯(lián)網和互聯(lián)網金融健康發(fā)展，引導互聯(lián)網企業(yè)拓展國際市場?！?互連網+,互聯(lián)網+XX傳統(tǒng)行業(yè)=互聯(lián)網XX行業(yè)

8、利用信息通信技術以及互聯(lián)網平臺，讓互聯(lián)網與傳統(tǒng)行業(yè)進行深度融合，創(chuàng)造新的發(fā)展生態(tài)。電子商務、互聯(lián)網金融、在線旅游、在線影視、在線房產等行業(yè)都是“互聯(lián)網+”的杰作。,十三五“網絡強國戰(zhàn)略”,構建泛在高效的信息網絡：加快構建高速、移動、安全、泛在的新一代信息基礎設施，推進信息網絡技術廣泛運用，形成萬物互聯(lián)、人機交互、天地一體的網絡空間 發(fā)展現(xiàn)代互聯(lián)網產業(yè)體系：實施“互聯(lián)網+”行動計劃，促進互聯(lián)網深度廣泛應用，帶動生產模式和組

9、織方式變革，形成網絡化、智能化、服務化、協(xié)同化的產業(yè)發(fā)展新形態(tài),國家信息化發(fā)展戰(zhàn)略綱要(2016.7),中國信息化戰(zhàn)略：2025年核心技術不受制于人到2020年3G、4G網絡要覆蓋中國城鄉(xiāng)，5G技術研發(fā)和標準要取得突破性進展；信息消費總額要達到6萬億元(人民幣，下同)，電子商務交易規(guī)模達到38萬億元；核心關鍵技術部分領域要達到國際先進水平；建成中國-東盟信息港，初步建成網上絲綢之路。 到2025年，要實現(xiàn)寬帶網絡無縫覆蓋；信息消費總

10、額達到12萬億元，電子商務交易規(guī)模達到67萬億元；根本改變核心關鍵技術受制于人的局面。,國家信息化發(fā)展戰(zhàn)略綱要(2016.7),屆時，中國還計劃建成四大國際信息通道，連接太平洋、中東歐、西非北非、東南亞、中亞、印巴緬俄等國家和地區(qū)，涌現(xiàn)一批具有強大國際競爭力的大型跨國網信企業(yè)。到本世紀中葉，中國網絡強國地位要日益鞏固，在引領全球信息化發(fā)展方面有更大作為。,計算機和人類潛力將是推動經濟增長的主力,18世紀以前，經濟的驅動力為農耕19

11、世紀至20世紀，制造和工業(yè)成為經濟發(fā)展的主力進入21世紀，計算機和人類潛力將是推動經濟增長的主力。,Introduction,1-15,什么是網絡 --- 從端系統(tǒng)的角度看(1),網絡提供的服務: 信息傳遞信鴿、烽火、信使 、卡車、電報、電話、Internet…,什么是網絡 --- 從端系統(tǒng)的角度看(2),不同的網絡以什么為區(qū)分所提供的服務服務以什么為區(qū)分延遲、帶寬、丟失率、端節(jié)點數目、服務接口、可靠性, 單播/多播，實時

12、，消息/字節(jié)流,什么是網絡 --- 從網絡核心的角度看,電子、光子等作為傳輸介質鏈路：光纖、電纜和衛(wèi)星交換節(jié)點：機械/電/光協(xié)議: TCP/IP, ATM, MPLS, SONET, Ethernet, PPP, X.25, FrameRelay, AppleTalk, IPX, SNA功能: 路由，差錯控制、擁塞控制、服務質量(QoS)應用：FTP、HTTP、X windows,Introduction,1-19,Chapt

13、er 1: roadmap,1.1 What is the Internet?1.2 Network edge end systems, access networks, links1.3 Network core circuit switching, packet switching, network structure1.4 Delay, loss and throughput in packet-switched net

14、works1.5 Protocol layers, service models1.6 Networks under attack: security1.7 History,概述,1-20,1.1.1 具體構成描述,終端設備：稱 主機（host）或端系統(tǒng)（ end system） 運行網絡應用程序,通信鏈路：光纖，銅線，射頻等傳輸速率常稱為帶寬（bandwidth）,分組交換機: 轉發(fā)分組（packet）路由器和鏈路層

15、交換機,具體構成描述,路由（route, path）：分組從發(fā)送終端到接收終端經過的通信鏈路及分組交換機的序列。Internet Service Provider（ISP）：由分組交換機和通信鏈路組成的網絡為終端提供接入因特網的服務,Introduction,1-22,具體構成描述,協(xié)議規(guī)定了設備之間通信的規(guī)則： 如 TCP, IP, HTTP, Skype因特網標準：以RFC（Request for comments

16、）文檔的形式發(fā)布由IETF（ Internet Engineering Task Force）組織統(tǒng)一管理,具體構成描述,因特網是“網絡的網絡”:按松散的層次結構組織、并且遵循TCP/IP協(xié)議的ISP集合。（描述一）,Introduction,1-24,1.1.2 服務描述,因特網是為分布式應用提供通信服務的基礎設施：（描述二）應用程序運行在端系統(tǒng)上端系統(tǒng)為應用程序提供使用因特網服務的API因特網提供的通信服務包括：源主機

17、到目的主機的可靠數據交付盡力而為的（不可靠的）數據交付,Introduction,1-25,1.1.3 什么是協(xié)議,一個人類協(xié)議 一個計算機網絡協(xié)議,,Hi,,Hi,,,TCP connection request,,,,,,1-26,網絡協(xié)議,human protocols:specific msgs sentspecific actions taken when msgs rec

18、eived, or other events,network protocols:machines rather than humansall communication activity in Internet governed by protocols,協(xié)議定義了通信實體之間交換的報文的格式和次序，以及在報文發(fā)送、接收或收到其它事件后采取的動作。掌握計算機網絡知識的過程，就是理解網絡協(xié)議構成、原理和工作的過程。,Introdu

19、ction,1-27,Chapter 1: roadmap,1.1 What is the Internet?1.2 Network edge end systems, access networks, links1.3 Network core circuit switching, packet switching, network structure1.4 Delay, loss and throughput in pac

20、ket-switched networks1.5 Protocol layers, service models1.6 Networks under attack: security1.7 History,Introduction,1-28,A closer look at network structure:,end systems: 終端,access networks：將終端連接到其邊緣路由器的物理鏈路,networ

21、k core: 路由器和通信鏈路組成的網狀網絡,Introduction,1-29,1.2.1 接入網,Q: How to connect end systems to edge router?住宅接入公司接入（學校，公司）移動接入Keep in mind: 接入網的帶寬（bits per second) 是多少？共享還是專用？,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,

22、,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,Introduction,家庭接入: 數字用戶線（DSL）,,,,,,,

23、central office,,telephonenetwork,DSLAM,,,,使用已有的數字電話線：數據送往因特網聲音送往電話網上行速率< 2.5 Mbps (典型地 < 1 Mbps)下行速率< 24 Mbps (典型地 < 10 Mbps),,,DSLmodem,splitter,,,,,,,1-30,Introduction,家庭接入: cable network,,,cablemode

24、m,splitter,,,,,,…,,,,,,,cable headend,,,,,,,,頻分復用: 將電纜劃分成不相重疊的頻段，每個頻段傳輸一路信號,,1-31,Introduction,,,cablemodem,splitter,,,,,,…,,,,,,,cable headend,,,,,,,CMTS,,,混合光纖同軸電纜HFC: hybrid fiber coax下行速率最高 30Mbps上行速率最高 2 Mbps光纖、

25、電纜、cable modem、CMTS構成接入網從住宅到電纜頭端的接入網部分是共享的,家庭接入: cable network,,1-32,Introduction,企業(yè)（和家庭）接入：Ethernet,主要用于公司、學校等傳輸速率：10 Mbps, 100Mbps, 1Gbps, 10Gbps以太網交換機及鏈路構成接入網,,,,,,,,,,Ethernet switch,,institutional mail,web serv

26、ers,institutional router,,,,,institutional link to ISP (Internet),,,,,,,1-33,Introduction,1-34,企業(yè)（和家庭）接入：無線局域網,公司或個人提供基站（接入點），主要將移動終端連接到有線網絡：終端與基站相距幾十米內基站通常位于有線網絡上傳輸速率（Wifi）：11 or 54Mbps無線局域網是共享的,,,,,,,basestation,

27、mobilehosts,,router,Introduction,一個典型的家庭網絡,,to/from headend or central office,,,,wirelessdevices,,,,,1-35,Introduction,廣域無線接入：3G/4G,由電信公司提供，使用現(xiàn)有的蜂窩電話網絡基站可為數萬米半徑內的用戶提供無線接入服務傳輸速率：3G：最大（靜止）2Mbps4G：下行100Mbps，上行20Mbps,t

28、o Internet,1-36,Introduction,1-37,1.2.2 物理媒體,設備之間通過物理媒體相連在一條路徑上，每對設備之間的物理媒體可以不同導引型媒體: 信號沿固體媒體傳播，如銅線，光纖非導引型媒體: 信號在空間自由傳播，如無線電,雙絞線：兩條絕緣的銅導線：3類線: 10 Mbps5類線: 100Mbps~1Gbps6類線：10Gbps,Introduction,1-38,同軸電纜，光纖,同軸電纜:

29、兩根同心的絕緣銅導體基帶同軸電纜:single channel on cablelegacy Ethernet寬帶同軸電纜 multiple channels on cable HFC,光纖:能引導光脈沖的玻璃纖維，每個脈沖傳輸一比特。點-點傳輸速率:數十甚至數百Gb/s低誤碼率: 長距離傳輸; 抗電磁干擾,Introduction,1-39,射頻（radio）,電磁信號影響傳輸速率的因素:距離：路徑損耗環(huán)境

30、：多徑衰落（信號反射） 遮擋衰落（障礙物）干擾,射頻鏈路類型:短距離：如藍牙局域：如Wifi廣域：如3G衛(wèi)星280ms端到端延遲同步衛(wèi)星，近地軌道衛(wèi)星,Introduction,1-40,Chapter 1: roadmap,1.1 What is the Internet?1.2 Network edge end systems, access networks, links1.3 Network core

31、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 History,Introduction,1-41,網絡核心

32、,由路由器和通信鏈路形成的網狀網絡任務：將數據從發(fā)送終端的邊緣路由器傳輸到接收終端的邊緣路由器基本問題: 數據如何在網絡中傳遞?電路交換分組交換,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,Introduction,主機將應用報文劃分成分組在從源終端到目的終端的路

33、徑上，分組從一個設備發(fā)送到下一個設備交換機僅在接收到整個分組后，才可以開始轉發(fā)（存儲-轉發(fā)）,1.3.1 分組交換（packet switching）,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,1-42,Introduction,存儲轉發(fā)引入延遲,假設分組長度為L比特，鏈路速率為R b/s將一個分組全部推送到一條鏈路上，耗時 L/R 秒將一個分組從源發(fā)送到目的，總耗時 = 2 L/R （不考慮信號傳

34、播時間）,3個分組從源終端發(fā)送到目的終端，總耗時=？4 L/R問題：P個分組經過N條鏈路的總耗時是多少？（P+N-1）L/R,1-43,source,,,R bps,,destination,1,2,3,L bitsper packet,R bps,,Introduction,排隊延時和分組丟失,,,,,,,,,,,,,,,,,,,A,B,C,R = 100 Mb/s,R = 1.5 Mb/s,,,,D,E,queue of

35、packetswaiting for output link,,,,,,1-44,排隊和丟包: 當分組到達速率大于鏈路輸出速率時:分組在對應鏈路的輸出緩存中排隊，等待被發(fā)送到鏈路上，產生排隊延遲 若鏈路的輸出緩存滿，分組被丟棄，產生丟包,Network Layer,1-45,網絡核心的兩個重要功能,轉發(fā)（forwarding）: 按照轉發(fā)表，將分組從輸入鏈路移動到輸出鏈路,選路(routing): 確定分組所走的路徑，生成轉

36、發(fā)表,,分組交換網絡原理總結,存儲轉發(fā)動態(tài)路由(包括每個分組自帶源地址、目的地址，拓撲發(fā)現(xiàn)、路由選擇)出錯交由端系統(tǒng)處理,Introduction,1.3.2 電路交換（circuit switching）,電話網采用電路交換通信前預留好端-端資源（對比：分組交換不預留資源）資源獨占：保證性能（帶寬，延遲）在通信的靜默期，資源被閑置,1-47,Introduction,1-48,頻分復用FDM ，時分復用TDM,,,,,In

37、troduction,1-49,Numerical 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 MbpsEach link uses TDM with 24 slots/sec500 msec to esta

38、blish end-to-end circuitLet’s work it out!,Introduction,1-50,分組交換 vs 電路交換,1 Mb/s linkeach user: 100 kb/s when “active”active 10% of time電路交換（固定分配）10 users分組交換（按需分配）with 35 users, probability > 10 active at sa

39、me time is less than .0004,分組交換允許更多用戶使用網絡!,,,,,N users,1 Mbps link,,Introduction,1-51,時分復用 vs 統(tǒng)計復用,Sequence of A & B packets does not have fixed pattern, bandwidth shared on demand ? statistical multiplexing.TDM: 每個

40、終端在周而復始的TDM幀中占有固定的時隙.,,,,,,,,,,,,,,,,,,,,,,,,,,,,A,B,C,100 Mb/sEthernet,1.5 Mb/s,,,,,statistical multiplexing,queue of packetswaiting for outputlink,,Introduction,1-52,分組交換 vs 電路交換,適合突發(fā)數據簡單，不需建立電路可能產生嚴重擁塞: 延遲，丟包需要有

41、保證可靠傳輸和擁塞控制的協(xié)議Q: How to provide circuit-like behavior?音/視頻應用需要保證帶寬至今未解決的問題 (chapter 7),Is packet switching a “slam dunk winner?”,1.3.3 網絡的網絡,端系統(tǒng)通過一個接入ISP與因特網相連接入ISP必須相互連接 受經濟因素和國家政策的影響，因特網的結構非常復雜,網絡的網絡,Question: g

42、iven millions of access ISPs, how to connect them together?,網絡的網絡：樸素的方法,Option: connect each access ISP to every other access ISP?,connecting each access ISP to each other directly doesn’t scale: O(N2) connections.,網絡的網絡

43、：單一的全球ISP,Option: connect each access ISP to a global transit ISP? Customer and provider ISPs have economic agreement.,,globalISP,網絡的網絡：多個全球ISP,But if one global ISP is viable business, there will be competitors ….,網絡的網

44、絡：多個全球ISP,But if one global ISP is viable business, there will be competitors …. which must be interconnected,peering link,Internet exchange point,網絡的網絡：多層結構,… and regional networks may arise to connect access nets to I

45、SPS,,regional net,,因特網生態(tài)系統(tǒng),接入ISP區(qū)域ISP第一層ISP存在點PoP：低層ISP接入高層ISP的地方多宿：一個ISP可以與兩個或多個上層ISP連接對等：相同層次上的一對ISP直接相連，不進行結算因特網交換點IXP：多個ISP在這里共同對等,網絡的網絡：內容提供商網絡,… and content provider networks (e.g., Google, Microsoft, Akam

46、ai ) may run their own network, to bring services, content close to end users,,regional net,,,Content provider network,Introduction,網絡的網絡：今天的因特網結構,at center: small # of well-connected large networks“tier-1” commercial I

47、SPs ：national & international coveragecontent provider network : private network that connects it data centers to Internet, often bypassing tier-1, regional ISPs,1-62,Introduction,1-63,Chapter 1: roadmap,1.1 What is

48、 the Internet?1.2 Network edge end systems, access networks, links1.3 Network core circuit switching, packet switching, network structure1.4 Delay, loss and throughput in packet-switched networks1.5 Protocol layers

49、, service models1.6 Networks under attack: security1.7 History,Introduction,1-64,分組延遲的來源,1. 節(jié)點處理: 檢查比特錯誤確定輸出鏈路,2. 排隊在輸出緩存等待傳輸 時間長短取決于鏈路擁塞程度,Introduction,1-65,分組延遲的來源,3. 傳輸延遲:R=link bandwidth (bps)L=packet length

50、(bits)將分組發(fā)送到鏈路上的時間 = L/R,4. 傳播延遲:d = length of physical links = propagation speed in medium (~2x108 m/sec)propagation delay = d/s,Note: s and R are very different quantities!,,Introduction,1-66,節(jié)點延遲,dproc = 處理延遲典型地為

51、幾個微秒或更低dqueue = 排隊延遲取決于擁塞程度dtrans = 傳輸延遲微秒～毫秒, 低速鏈路上較大dprop = 傳播延遲幾微秒～幾百毫秒，長距離鏈路上較大,Introduction,1-67,排隊延遲與流量強度,R=link bandwidth (bps)L=packet length (bits)a=average packet arrival rate,traffic intensity = La/R,L

52、a/R ~ 0: average queueing delay smallLa/R -> 1: delays become largeLa/R > 1: more “work” arriving than can be serviced, average delay infinite!,Introduction,1-68,排隊與丟包,輸出隊列的容量是有限的隊列滿時，新到達的分組被丟棄,,,,,,,,,,,,,,,A,B

53、,packet being transmitted,,,,,,,,,packet arriving tofull buffer is lost,buffer (waiting area),,Introduction,1-69,,端到端吞吐量,發(fā)送端和接收端之間的比特傳輸速率：瞬時吞吐量: 給定時刻的傳輸速度平均吞吐量: 較長時間內的傳輸速度,server, withfile of F bits to send to clie

54、nt,,link capacity Rs bits/sec,link capacity Rc bits/sec,,,,,,,server sends bits (fluid) into pipe,Introduction,1-70,吞吐量 (more),Rs < Rc What is average end-end throughput?,,,Rs bits/sec,,,,Introduction,1-71,Through

55、put: Internet scenario,10 connections (fairly) share backbone bottleneck link R bits/sec,,,,,,,,,,Rs,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,Rs,Rs,,Rc,,,Rc,Rc,R,端到端吞吐量: min(Rc,Rs,R/10)吞吐量與鏈路速率及鏈路上的負載有關,Introd

56、uction,1-72,Chapter 1: roadmap,1.1 What is the Internet?1.2 Network edge end systems, access networks, links1.3 Network core circuit switching, packet switching, network structure1.4 Delay, loss and throughput in pa

57、cket-switched networks1.5 Protocol layers, service models1.6 Networks under attack: security1.7 History,Introduction,1-73,Protocol “Layers”,Networks are complex! many “pieces”:hostsrouterslinks of various mediaap

58、plicationsprotocolshardware, software,Question: Is there any hope of organizing structure of network?Or at least our discussion of networks?,Introduction,1-74,Organization of air travel,a series of steps,Introductio

59、n,1-75,Layering of airline functionality,分層：將系統(tǒng)功能組織成一系列水平的層次，每層實現(xiàn)一個功能（服務）每層通過以下方式提供它的服務：在本層內執(zhí)行一些動作依靠下層提供的服務,Introduction,1-76,為什么分層?,易于處理復雜系統(tǒng):顯式的層次結構易于確定系統(tǒng)的各個部分及其相互關系模塊化簡化了系統(tǒng)的維護和升級改變某層服務的實現(xiàn)對于其它層次是透明的,Introduction,1

60、-77,,Internet協(xié)議棧,application: 支持各種網絡應用E.g., FTP, SMTP, HTTPtransport: 進程-進程的分組傳輸TCP, UDPnetwork: 源主機-目的主機的分組傳輸IP, routing protocolslink: 相鄰網絡設備之間的分組傳輸E.g., PPP, Ethernetphysical: 在物理媒體上傳輸比特,Introduction,1-78,,ISO

61、/OSI reference model,presentation: allow applications to interpret meaning of data, e.g., encryption, compression, machine-specific conventionssession: synchronization, checkpointing, recovery of data exchangeInternet

62、stack “missing” these layers!these services, if needed, must be implemented in applicationneeded?,Introduction,1-79,,,source,,,,,applicationtransportnetworklinkphysical,,,,segment,datagram,destination,,,,,applicati

63、ontransportnetworklinkphysical,,,,,,router,switch,封裝,message,frame,Introduction,1-80,Chapter 1: roadmap,1.1 What is the Internet?1.2 Network edge end systems, access networks, links1.3 Network core circuit switch

64、ing, packet switching, network structure1.4 Delay, loss and throughput in packet-switched networks1.5 Protocol layers, service models1.6 Networks under attack: security1.7 History,Introduction,1-81,Chapter 1: roadmap

65、,1.1 What is the Internet?1.2 Network edge end systems, access networks, links1.3 Network core circuit switching, packet switching, network structure1.4 Delay, loss and throughput in packet-switched networks1.5 Pro

66、tocol layers, service models1.6 Networks under attack: security1.7 History,Introduction,1-82,Internet History,1961: Kleinrock - queueing theory shows effectiveness of packet-switching1964: Baran - packet-switching in

67、military nets1967: ARPAnet conceived by Advanced Research Projects Agency1969: first ARPAnet node operational,1972: ARPAnet public demonstrationNCP (Network Control Protocol) first host-host protocol first e-mail pr

68、ogramARPAnet has 15 nodes,1961-1972: Early packet-switching principles,Introduction,1-83,Internet History,1970: ALOHAnet satellite network in Hawaii1974: Cerf and Kahn - architecture for interconnecting networks1976: