计算机网络双语课件第一章.ppt

1、Computer Networking: A Top Down Approach Featuring the Internet 3rd edition. Jim Kurose, Keith Ross,Chapter 1 Computer Networks and the Internet,chapter 1-2,Introduction,Chapter goal get context, overview, “feel” of networking more depth, detail later in the course approach: descriptive use Internet

2、 as example, and it is one of outstanding features of the textbook.,chapter 1-3,Chapter 1: roadmap,1.1 What is the Internet? 1.2 Network edge 1.3 Network core 1.4 Network access and physical media 1.5 ISPs and Internet backbones 1.6 Delay email client/server Peer to Peer model(P2P): host interaction

3、 symmetric e.g.: teleconferencing,chapter 1-15,The Network edge: connection-oriented service,handshaking: setup (prepare for) data transfer ahead of time send control packets to each other Hello, hello back human protocol set up “state” in two communicating hosts Why connection-oriented and not just

4、 connection? Connection established between hosts is very loose and only the end systems are aware of the connection. The packet switches are completely oblivious to the connection.,chapter 1-16,The Network edge: TCP,TCP Service reliable, in-order byte-stream data transfer Connection is equivalent t

5、o water pipe loss: acknowledgements and retransmissions flow control: sender wont overwhelm receiver TCP use sender/receiver buffering mechanism Congestion Control senders “slow down sending rate” when network congested,chapter 1-17,The Network edge: connectionless service,Connectionless Service = n

6、o handshaking or setup When one side of an application wants to send packets, simply sends the packets.,UDP - User Datagram Protocol RFC 768: Internets connectionless service unreliable data transfer no flow control no congestion control,Apps using TCP: HTTP (WWW), FTP (file transfer), Telnet (remot

7、e login), SMTP (email) Apps using UDP: streaming media, teleconferencing, Internet telephony,chapter 1-18,Chapter 1: roadmap,1.1 What is the Internet? 1.2 Network edge 1.3 Network core 1.4 Network access and physical media 1.5 ISPs and Internet backbones 1.6 Delay immune to electromagnetic interfere

8、nce,chapter 1-47,Physical media: radio,signal carried in electromagnetic spectrum no physical “wire” bidirectional propagation environment effects: reflection obstruction by objects interference,Radio link types: terrestrial microwave e.g. up to 45 Mbps channels WLAN (e.g., Wifi) 2Mbps, 11Mbps, 54Mb

9、ps wide-area (e.g., cellular) e.g. 3G: hundreds of kbps satellite up to 50Mbps channel (or multiple smaller channels) 250 msec end-end delay,chapter 1-48,Chapter 1: roadmap,1.1 What is the Internet? 1.2 Network edge 1.3 Network core 1.4 Network access and physical media 1.5 ISPs and Internet backbon

10、es 1.6 Delay 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,ten-car caravan,100 km,100 km,chapte

11、r 1-60,Caravan analogy (more),Cars now “propagate” at 1000 km/hr Toll booth now takes 1 min to service a car Q: Will cars arrive to 2nd booth before all cars serviced at 1st booth?,Yes! After 7 min, 1st car at 2nd booth and 3 cars still at 1st booth. 1st bit of packet can arrive at 2nd router before

12、 packet is fully transmitted at 1st router!,ten-car caravan,100 km,100 km,chapter 1-61,Nodal delay,dproc = processing delay typically a few microsecs or less dqueue = queuing delay depends on congestion,A,B,propagation,transmission,nodal processing,queueing,dnodal,dtrans = transmission delay = L/R,

13、significant for low-speed links dprop = propagation delay a few microsecs to hundreds of msecs,chapter 1-62,Queueing delay (revisited),R=link bandwidth (bps) L=packet length (bits) a=average packet arrival rate,traffic intensity = La/R,La/R 0: average queueing delay small La/R - 1: delays become lar

14、ge La/R 1: more “work” arriving than can be serviced, average delay infinite!,chapter 1-63,“Real” Internet delays and routes,What do “real” Internet delay propagation delay : dprop packet length: L bits dend-end= N (dproc+dtrans+dprop),chapter 1-67,Packet loss,queue preceding link in buffer has fini

15、te capacity when packet arrives to full queue, packet is dropped lost packet may be retransmitted by： previous node source end system or not retransmitted at all,chapter 1-68,Chapter 1: roadmap,1.1 What is the Internet? 1.2 Network edge 1.3 Network core 1.4 Network access and physical media 1.5 ISPs

16、 and Internet backbones 1.6 Delay & loss in packet-switched networks 1.7 Protocol layers, service models 1.8 History,chapter 1-69,Protocol “Layers”,Networks are complex! many “pieces”: hosts routers links of various media applications protocols hardware, software,Question: Is there any hope of organ

17、izing structure of network? Or at least our discussion of networks?,chapter 1-70,Layering of airline functionality,Layers: each layer implements a service via its own internal-layer actions relying on services provided by layer(s) below,chapter 1-71,Why layering?,Dealing with complex systems: explic

18、it structure allows identification, relationship of complex systems pieces layered reference model for discussion modularization eases maintenance, updating of system change of implementation of layers service transparent to rest of system e.g., change in gate procedure doesnt affect rest of system,

19、chapter 1-72,Internet protocol stack,application: supporting network applications FTP, SMTP, HTTP transport: host-host data transfer TCP, UDP network: routing of datagrams from source to destination IP, routing protocols link: data transfer between neighboring network elements PPP, Ethernet physical

20、: bits “on the wire”,chapter 1-73,Encapsulation,chapter 1-74,Chapter 1: roadmap,1.1 What is the Internet? 1.2 Network edge 1.3 Network core 1.4 Network access and physical media 1.5 Internet structure and ISPs 1.6 Delay & loss in packet-switched networks 1.7 Protocol layers, service models 1.8 Histo

21、ry,chapter 1-75,Internet History,1961: 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 demonstrated publicly NCP (Networ

22、k Control Protocol) first host-host protocol first e-mail program ARPAnet has 15 nodes,1961-1972: Early packet-switching principles,chapter 1-76,Internet History,1970: ALOHAnet satellite network in Hawaii 1973: Metcalfes PhD thesis proposes Ethernet 1974: Cerf and Kahn - architecture for interconnec

23、ting networks late70s: proprietary architectures: DECnet, SNA, XNA late 70s: switching fixed length packets (ATM precursor) 1979: ARPAnet has 200 nodes,Cerf and Kahns internetworking principles: minimalism, autonomy - no internal changes required to interconnect networks best effort service model st

24、ateless routers decentralized control define todays Internet architecture,1972-1980: Internetworking, new and proprietary nets,chapter 1-77,Internet History,Early 1990s: ARPAnet decommissioned 1991: NSF lifts restrictions on commercial use of NSFnet (decommissioned, 1995) early 1990s: Web hypertext

25、Bush 1945, Nelson 1960s HTML, HTTP: Berners-Lee 1994: Mosaic, later Netscape late 1990s: commercialization of the Web,Late 1990s 2000s: more killer apps: instant messaging, P2P file sharing network security to forefront est. 50 million host, 100 million+ users backbone links running at Gbps,1990, 20

26、00s: commercialization, the Web, new apps,chapter 1-78,Introduction: Summary,Covered a “ton” of material! Internet overview whats a protocol? network edge, core, access network packet-switching versus circuit-switching Internet/ISP structure performance: loss, delay layering and service models histo

27、ry,You now have: context, overview, “feel” of networking more depth, detail to follow!,chapter 1-79,Problems,P63: 2,3,4,5 P64: 7 P65: 14 P66: 15,16 Due date: 2 weeks Deadline means deadline Representatives collect homework and send to my office before the deadline.,chapter 1-80,Chapter 1 :problems,P

28、roblem 2 a) 电路交换网络更适合这种应用。 应用的持续时间很长，并且其对带宽的需求比较平稳，突发很小，因此可以利用资源预留预先为应用在传输路径的每一条链路上预留带宽，而不会招致多大的浪费。 另外，由于应用持续时间较长，建立和拆除连接的开销相对变小。 b) 在如题目所属的条件下，分组交换网络中也没有必要进行拥塞控制。 在最坏情况下，即，所有应用同时传输数据时，链路也有足够的带宽予以容纳，因此，也不会产生拥塞。,chapter 1-81,Chapter 1 :problems,Problem 3,a) We can n connections between each of the fo

29、ur pairs of adjacent switches. This gives a maximum of 4n connections. b) We can n connections passing through the switch in the upper-right-hand corner and another n connections passing through the switch in the lower-left-hand corner, giving a total of 2n connections.,chapter 1-82,Chapter 1 :probl

30、ems,P63.4 Tollbooths are 100 km apart, and the cars propagate at 100km/hr. A tollbooth services a car at a rate of one car every 12 seconds. There are ten cars. a) It takes 120 seconds, or two minutes, for the first tollbooth to service the 10 cars. Each of these cars have a propagation delay of 60

31、minutes before arriving at the second tollbooth. Thus, all the cars are lined up before the second tollbooth after 62 minutes. The whole process repeats itself for traveling between the second and third tollbooths. Thus the total delay is 124 minutes. b) Delay between tollbooths is 7*12 seconds plus

32、 60 minutes, i.e., 61 minutes and 24 seconds. The total delay is twice this amount, i.e., 122 minutes and 48 seconds.,chapter 1-83,Chapter 1 :problems (contd.),P63. 5 a) Packet Switched Virtual-Circuit Networks The time to transmit one packet onto a link is . The time to deliver the packet over Q links is. Thus the total latency is . b) Packet Switched Networks c) Circuit-Switched Networks Because there is no store-and-forward delays at the links, the total delay