《計算機網(wǎng)絡(luò)-自頂向下方法與Internet特色》幻燈片Lecture11

1、Computer Networks and Communication計算機網(wǎng)絡(luò)與通信（雙語）Professor Chunsheng Li主講：李春生 教授Department of Computer Engineering1Course Overview0.1 What is Computer networks? （什么是計算機網(wǎng)絡(luò)）0.2 What is this course about? (本課程是干什么的）0.3 A top-down approach （一種從上到下的方法）0.4 Contents of this course （本課程的內(nèi)容）20.1 What is Comput

2、er Networks?1Computer network is a communication system to connect at least two computers together for sharing hardware, software, and information resources. (計算機網(wǎng)絡(luò)：用于共享硬件資源、軟件資源和信息資源的連接兩臺或更多臺計算機進行通信的系統(tǒng)。)2Categories of Computer NetworksAccording to the communicating distance：WAN (Wide Area Networks

3、廣域網(wǎng)), MAN (Municipal Area Networks 城域網(wǎng)), LAN (Local Area Networks 局域網(wǎng))According to the topology：Star(星型), Tree(樹型), Ring (環(huán)型), Bus(總線型結(jié)構(gòu))。 30.2 What is this course about?Introductory (first) course in computer networkinglearn principles of computer networkinglearn practice of computer networkingInte

4、rnet architecture/protocols as case studyGoals:learn a lot (not just factoids, but principles and practice)have fun (well, it should be interesting, at least)prerequisites:algorithms, Operating Systems, programming skills40.3 A top-down approach:This course will cover networking top-downend-system a

5、pplications, end-end transportnetwork core: routing, hooking nets togetherlink-level protocols, e.g., Ethernetother stuff: security, mobility, management50.4 Contents of this Course:Part 1: Introduction (3 classes, text: Chapter 1) what is the Internet, What is a protocol? network edge, network core

6、network access, physical media Internet backbones, NAPs and ISPs delay, loss in packet-switched networks protocol layers, service models (Application, Transport, Network, Link, Physical Layers) brief history of networking, Internet6Contents of this Course (Cont.):Part 2: Application Layer (3 classes

7、, text: Ch. 2) principles of application-layer protocols World Wide Web: HTTP file transfer: FTP electronic mail in the Internet the Internets directory service: DNS socket programming with TCP 7Contents of this Course (Cont.):Part 3: Transport Layer (6 classes, text Ch. 3) Transport-layer services

8、and principles Multiplexing and demultiplexing applications Connectionless transport: UDP Principles of reliable of data transfer Connection-Oriented Transport: TCP Principles of congestion control TCP congestion control 8Contents of this Course (Cont.):Part 4: Network Layer (6 classes, text: Ch. 4)

9、 introduction and network service model whats inside a router?routing principles (algorithms)hierarchical routing IP: the Internet Protocol Internet routing: RIP, OSPF, BGP9Contents of this Course (Cont.):Part 5: Link Layer, LANs (3 classes, text: Ch. 5) introduction, services error detection, corre

10、ction multiple access protocols, LANs LAN addresses, ARP Ethernet Interconnection: Hubs and Switches10Chapter 1: IntroductionOur goal: get “feel” and terminologymore depth, detail later in courseapproach:use Internet as exampleOverview:whats the Internetwhats a protocol?network edgenetwork coreacces

11、s net, physical mediaInternet/ISP structureperformance: loss, delayprotocol layers, service models11Chapter 1: roadmap1.1 What is the Internet?1.2 Network edge1.3 Network core1.4 Network access and physical media1.5 Internet structure and ISPs1.6 Delay & loss in packet-switched networks1.7 Protocol

12、layers, service models1.8 History121.1 Whats the InternetTwo ways to describe the Internet:Describe the Nuts-and-Bolts of the Internet, that is, the basic hardware and software components that make up the Internet.Describe the Internet in terms of a networking infrastructure that provide services to

13、 distributed applications.131.1 Whats the Internet: 1. “nuts and bolts” viewmillions of connected computing devices: hosts = end systems (definition p2) running network appscommunication linksfiber, copper, radio, satellitetransmission rate = bandwidthPacket Switches: Routers and Link-layer Switches

14、- forward packets (chunks of data) towards their ultimate destinationsRoute or pathInternet Service Providers (ISPs)-lower-tier ISP & Uper-tier ISPlocal ISPcompanynetworkregional ISProuterworkstationservermobile14“Cool” internet appliancesIP picture frame/Web-enabled toaster +weather forecasterInter

15、net phonesZebraNet (Princeton)Where is your zebra?15Whats the Internet: “nuts and bolts” viewprotocols control sending, receiving of msgse.g., TCP, IP, HTTP, FTP, PPPInternet: “network of networks”loosely hierarchicalpublic Internet versus private intranetInternet standardsIETF: Internet Engineering

16、 Task ForceRFC: Request for commentslocal ISPcompanynetworkregional ISProuterworkstationservermobile161.1 Whats the Internet: 2. a service viewcommunication infrastructure enables distributed applications:Web, email, games, e-commerce, file sharingcommunication services provided to applications:Conn

17、ectionless unreliableconnection-oriented reliableThe internet does not provide a service that promises about how long it will take to deliver the data from sender to receiver173. Whats a protocol?human protocols:“whats the time?”“I have a question”introductions There are specific msgs we send, and s

18、pecific actions we take in response to the received reply msgs or other eventsnetwork 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 transmissio

19、n, receipt 18Whats a protocol?a human protocol and a computer network protocol:HiHiGot thetime?2:00TCP connection requestTCP connectionresponseGet http:/index.htmtime19Chapter 1: roadmap1.1 What is the Internet?1.2 Network edge1.3 Network core1.4 Network access and physical media1.5 Internet structu

20、re and ISPs 1.6 Delay & loss in packet-switched networks1.7 Protocol layers, service models1.8 History20A closer look at network structure:network edge: applications and hostsnetwork core: routersnetwork of networksaccess networks, physical media: communication links211.2 The network edge: 1. end sy

21、stems, Clients, servers end systems (hosts):run application programse.g. Web, emailat “edge of network”(Definitions in P9 )client/server modelclient host requests, receives service from always-on servere.g. Web browser/server; email client/server(Two kinds of definitions in P9 )peer-peer model: mini

22、mal (or no) use of dedicated serverse.g. Skype, BitTorrent, KaZaA221.2 Network edge: 2. connection-oriented service (two types of services of Internet)Goal: data transfer between end systemshandshaking: setup (prepare for) data transfer ahead of timeHello, hello back human protocolset up “state” in

23、two communicating hostsTCP - Transmission Control Protocol Internets connection-oriented serviceTCP services RFC 793reliable, in-order byte-stream data transferloss: acknowledgements and retransmissionsflow control: sender wont overwhelm receivercongestion control: senders “slow down sending rate” w

24、hen network congested231.2 Network edge: 3. connectionless 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 log

25、in), SMTP (email)Apps using UDP:streaming media, teleconferencing, DNS, Internet telephony24Chapter 1: roadmap1.1 What is the Internet?1.2 Network edge1.3 Network core1.4 Network access and physical media1.5 Internet structure and ISPs 1.6 Delay & loss in packet-switched networks1.7 Protocol layers,

26、 service models1.8 History251.3 The Network CoreMeaning of the network core: mesh of routers that interconnect the Internets end systems.There are two ways to construct the network core:circuit switching: dedicated circuit per call: telephone netpacket-switching: data sent thru net in discrete “chun

27、ks”261.3 Network Core: 1. Circuit Switching The resources needed for a path to provide for communication between the end systems are reserved for the duration of the communication session.link bandwidth, switch capacitydedicated resources: no sharingcircuit-like (guaranteed) performancecall setup re

28、quired271.3 Network Core: 1. Circuit Switchingnetwork 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 division: FDM - Frequency-Division Multiplexingtime division: TDM

29、- Time-Division Multiplexing281. Circuit Switching: 1) FDM and TDMFDMfrequencytimeTDMfrequencytime4 usersExample:FrameSlot292) 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 TDM with 2

30、4 slots500 msec to establish end-to-end circuitLets work it out!Each circuit has transmission rate of 1.536mbps/24=64kbpsIt takes 640000bits/64kbps=10seconds to transmit the fileTotal time = 10+0.5 = 10.5 seconds301.3 Network Core: 2. Packet Switchingeach end-end data stream (message ) divided into

31、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 available (queue grows)congestion: packets queue, wait for link usestore and forward: packets move one hop at a timeNode rec

32、eives complete packet before forwardingBandwidth division into “pieces”Dedicated allocationResource reservation312. Packet Switching: 1) Statistical MultiplexingSequence of A & B packets does not have fixed pattern, shared on demand On-demand sharing of resources is called statistical multiplexing.T

33、DM: each host gets same slot in revolving TDM frame.ABC10 Mb/sEthernet1.5 Mb/sDEstatistical multiplexingqueue of packetswaiting for outputlink322. Packet-switching: 2) store-and-forwardTakes L/R seconds to transmit (push out) packet of L bits on to link or R bpsEntire packet must arrive at router be

34、fore 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 secRRRL333. Packet switching versus 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 .0004 =10 active more than 0.9996Packet switching allows more users to use network!N users1 Mbps linkQ: how did we get value 0.0004?34