東北石油大學計算機與信息技術學院計算機網(wǎng)絡與通信英文課件 chapter2

1、 Introduction1-1 Chapter 1 Introduction Computer Networking: A Top Down Approach Featuring the Internet, 3rd edition. Jim Kurose, Keith Ross Addison-Wesley, July 2004. All material copyright 1996-2006 J.F Kurose and K.W. Ross, All Rights Reserved Introduction1-2 Chapter 1: Introduction Our goal: qge

2、t “feel” and terminology qmore depth, detail later in course qapproach: vuse Internet as example Overview: qwhats the Internet qwhats a protocol? qnetwork edge qnetwork core qaccess net, physical media qInternet/ISP structure qperformance: loss, delay qprotocol layers, service models Introduction1-3

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 Internet structure and ISPs 1.6 Delay email client/server qpeer-peer model: v minimal (or no) use of dedicated servers ve.g. Skype, BitTorrent, KaZaA Macintosh PICT image format

4、is not supported M a c in to s h P IC T im a g e fo rm a t is n o t s u p p o rte d Macintosh PICT image format is not supported M a c in to s h P IC T im a g e fo rm a t is n o t s u p p o rte d Macintosh PICT image format is not supported Macintosh PICT image format is not supported Macintosh PICT

5、 image format is not supported Macintosh PICT image format is not supported M a c in to s h P IC T im a g e fo rm a t is n o t s u p p o rte d M a c in to s h P IC T im a g e fo rm a t is n o t s u p p o rte d Macintosh PICT image format is not supported Macintosh PICT image format is not supported

6、Macintosh PICT image format is not supported Macintosh PICT image format is not supported Macintosh PICT image format is not supported Introduction1-13 Network edge: connection-oriented service Goal: data transfer between end systems qhandshaking: setup (prepare for) data transfer ahead of time vHel

7、lo, hello back human protocol vset up “state” in two communicating hosts qTCP - Transmission Control Protocol vInternets connection- oriented service TCP service RFC 793 qreliable, in-order byte- stream data transfer vloss: acknowledgements and retransmissions qflow control: vsender wont overwhelm r

8、eceiver qcongestion control: vsenders “slow down sending rate” when network congested Introduction1-14 Network edge: connectionless service Goal: data transfer between end systems vsame as before! qUDP - User Datagram Protocol RFC 768: vconnectionless vunreliable data transfer vno flow control vno c

9、ongestion control Apps using TCP: qHTTP (Web), FTP (file transfer), Telnet (remote login), SMTP (email) Apps using UDP: qstreaming media, teleconferencing, DNS, Internet telephony Introduction1-15 Chapter 1: roadmap 1.1 What is the Internet? 1.2 Network edge 1.3 Network core 1.4 Network access and p

10、hysical media 1.5 Internet structure and ISPs 1.6 Delay & loss in packet-switched networks 1.7 Protocol layers, service models 1.8 History Introduction1-16 The Network Core qmesh of interconnected routers qthe fundamental question: how is data transferred through net? vcircuit switching: dedicated c

11、ircuit per call: telephone net vpacket-switching: data sent thru net in discrete “chunks” Macintosh PICT image format is not supported M a c in to s h P IC T im a g e fo rm a t is n o t s u p p o rte d Macintosh PICT image format is not supported M a c in to s h P IC T im a g e fo rm a t is n o t s

12、u p p o rte d Macintosh PICT image format is not supported Macintosh PICT image format is not supported Macintosh PICT image format is not supported Macintosh PICT image format is not supported M a c in to s h P IC T im a g e fo rm a t is n o t s u p p o rte d M a c in to s h P IC T im a g e fo rm a

13、 t is n o t s u p p o rte d Macintosh PICT image format is not supported Macintosh PICT image format is not supported Macintosh PICT image format is not supported Macintosh PICT image format is not supported Macintosh PICT image format is not supported Introduction1-17 Network Core: Circuit Switchin

14、g End-end resources reserved for “call” qlink bandwidth, switch capacity qdedicated resources: no sharing qcircuit-like (guaranteed) performance qcall setup required Macintosh PICT image format is not supported M acintosh P IC T im age form at is not supported Macintosh PICT image format is not supp

15、orted M acintosh P IC T im age form at is not supported Macintosh PICT image format is not supported Macintosh PICT image format is not supported Macintosh PICT image format is not supported Macintosh PICT im age format is not supported M a c in to s h P IC T im a g e fo rm a t is n o t s u p p o rt

16、e d M a c in to s h P IC T im a g e fo rm a t is n o t s u p p o rte d M acintosh PIC T im age form at is not supported M acin to sh P IC T im ag e fo rm at is n o t su p p o rted M acintosh PIC T im age form at is not supported M acin to sh P IC T im ag e fo rm at is n o t su p p o rted M acintosh

17、PIC T im age form at is not supported Introduction1-18 Network Core: Circuit Switching network resources (e.g., bandwidth) divided into “pieces” qpieces allocated to calls qresource piece idle if not used by owning call (no sharing) qdividing link bandwidth into “pieces” vfrequency division vtime di

18、vision Introduction1-19 Circuit Switching: FDM and TDM FDM frequency time TDM frequency time 4 users Example: Introduction1-20 Numerical example qHow long does it take to send a file of 640,000 bits from host A to host B over a circuit-switched network? vAll links are 1.536 Mbps vEach link uses TDM

19、with 24 slots/sec v500 msec to establish end-to-end circuit Lets work it out! Introduction1-21 Network Core: Packet Switching each end-end data stream divided into packets quser A, B packets share network resources qeach packet uses full link bandwidth qresources used as needed resource contention:

20、qaggregate resource demand can exceed amount available qcongestion: packets queue, wait for link use qstore and forward: packets move one hop at a time vNode receives complete packet before forwarding Bandwidth division into “pieces” Dedicated allocation Resource reservation Introduction1-22 Packet

21、Switching: Statistical Multiplexing Sequence of A & B packets does not have fixed pattern, shared on demand statistical multiplexing. TDM: each host gets same slot in revolving TDM frame. Macintosh PICT image format is not supported Macintosh PICT image format is not supported Macintosh PICT image f

22、ormat is not supportedA B C 100 Mb/s Ethernet 1.5 Mb/s Macintosh PICT image format is not supported Macintosh PICT image format is not supported D E statistical multiplexing queue of packets waiting for output link Introduction1-23 Packet-switching: store-and-forward qTakes L/R seconds to transmit (

23、push out) packet of L bits on to link or R bps qEntire packet must arrive at router before it can be transmitted on next link: store and forward qdelay = 3L/R (assuming zero propagation delay) Example: qL = 7.5 Mbits qR = 1.5 Mbps qdelay = 15 sec Macintosh PICT image format is not supported Macintos

24、h PICT image format is not supported RRR L more on delay shortly Introduction1-24 Packet switching versus circuit switching q1 Mb/s link qeach user: v100 kb/s when “active” vactive 10% of time qcircuit-switching: v10 users qpacket switching: vwith 35 users, probability 10 active less than .0004 Pack

25、et switching allows more users to use network! N users 1 Mbps link Q: how did we get value 0.0004? Introduction1-25 Packet switching versus circuit switching qGreat for bursty data vresource sharing vsimpler, no call setup qExcessive congestion: packet delay and loss vprotocols needed for reliable d

26、ata transfer, congestion control qQ: How to provide circuit-like behavior? vbandwidth guarantees needed for audio/video apps vstill an unsolved problem (chapter 7) Is packet switching a “slam dunk winner?” Q: human analogies of reserved resources (circuit switching) versus on-demand allocation (pack

27、et-switching)? Introduction1-26 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 History Introduction1-27 Access ne

28、tworks and physical media Q: How to connect end systems to edge router? qresidential access nets qinstitutional access networks (school, company) qmobile access networks Keep in mind: qbandwidth (bits per second) of access network? qshared or dedicated? M acintosh PICT im age form at is not supporte

29、d M a c in to s h P IC T im a g e fo r m a t is n o t s u p p o r te d M acintosh PICT im age form at is not supported M a c in to s h P IC T im a g e fo r m a t is n o t s u p p o r te d M acintosh PICT im age form at is not supported M acintosh PICT im age form at is not supported M acintosh PICT

30、im age form at is not supported M acintosh PICT im age form at is not supported M a c in to s h P IC T im a g e fo rm a t is n o t s u p p o rte d M a c in to s h P IC T im a g e fo rm a t is n o t s u p p o rte d M acintosh P IC T im age form at is not supported Macintosh PICT image format is not s

31、upported M acintosh P IC T im age form at is not supported Macintosh PICT image format is not supported M acintosh P IC T im age form at is not supported Introduction1-28 Residential access: point to point access qDialup via modem vup to 56Kbps direct access to router (often less) vCant surf and pho

32、ne at same time: cant be “always on” Macintosh PICT image format is not supported Macintosh PICT image format is not supported Macintosh PICT image format is not supported Macintosh PICT image format is not supported qADSL: asymmetric digital subscriber line vup to 1 Mbps upstream (today typically 2

33、56 kbps) vup to 8 Mbps downstream (today typically 1: delays become large qLa/R 1: more “work” arriving than can be serviced, average delay infinite! Introduction1-55 “Real” Internet delays and routes qWhat do “real” Internet delay & loss look like? qTraceroute program: provides delay measurement fr

34、om source to router along end-end Internet path towards destination. For all i: vsends three packets that will reach router i on path towards destination vrouter i will return packets to sender vsender times interval between transmission and reply. Macintosh PICT image format is not supported Macint

35、osh PICT image format is not supported 3 probes 3 probes 3 probes Introduction1-56 “Real” Internet delays and routes 1 cs-gw (54) 1 ms 1 ms 2 ms 2 (45) 1 ms 1 ms 2 ms 3 (30) 6 ms 5 ms 5 ms 4 jn1-at1-0-0- (204.14

36、7.132.129) 16 ms 11 ms 13 ms 5 jn1-so7-0-0- (36) 21 ms 18 ms 18 ms 6 () 22 ms 18 ms 22 ms 7 (6) 22 ms 22 ms 22 ms 8 53 (53) 104 ms 109 ms 106 ms 9 de2- (29) 109 ms 102 ms 104

37、ms 10 (0) 113 ms 121 ms 114 ms 11 renater- (4) 112 ms 114 ms 112 ms 12 nio-n2.cssi.renater.fr (3) 111 ms 114 ms 116 ms 13 nice.cssi.renater.fr (02) 123 ms 125 ms 124 ms 14 r3t2-nice.cssi.renater.fr (10) 126 ms 126 ms 124 ms 15 eurecom- (193.48

38、.50.54) 135 ms 128 ms 133 ms 16 5 (5) 126 ms 128 ms 126 ms 17 * * * 18 * * * 19 fantasia.eurecom.fr (42) 132 ms 128 ms 136 ms traceroute: to eurecom.fr Three delay measurements from to * means no response (p

39、robe lost, router not replying) trans-oceanic link Introduction1-57 Packet loss qqueue (aka buffer) preceding link in buffer has finite capacity qwhen packet arrives to full queue, packet is dropped (aka lost) qlost packet may be retransmitted by previous node, by source end system, or not retransmi

40、tted at all Introduction1-58 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 History Introduction1-59 Protocol “La

41、yers” Networks are complex! qmany “pieces”: vhosts vrouters vlinks of various media vapplications vprotocols vhardware, software Question: Is there any hope of organizing structure of network? Or at least our discussion of networks? Introduction1-60 Organization of air travel qa series of steps tick

42、et (purchase) baggage (check) gates (load) runway takeoff airplane routing ticket (complain) baggage (claim) gates (unload) runway landing airplane routing airplane routing Introduction1-61 ticket (purchase) baggage (check) gates (load) runway (takeoff) airplane routing departure airport arrival air

43、port intermediate air-traffic control centers Maci nt osh PI CT i mage f or mat i s not suppor t ed airplane routingairplane routing ticket (complain) baggage (claim gates (unload) runway (land) airplane routing ticket baggage gate takeoff/landing airplane routing Layering of airline functionality L

44、ayers: each layer implements a service vvia its own internal-layer actions vrelying on services provided by layer below Introduction1-62 Why layering? Dealing with complex systems: qexplicit structure allows identification, relationship of complex systems pieces vlayered reference model for discussi

45、on qmodularization eases maintenance, updating of system vchange of implementation of layers service transparent to rest of system ve.g., change in gate procedure doesnt affect rest of system qlayering considered harmful? Introduction1-63 Internet protocol stack qapplication: supporting network appl

46、ications vFTP, SMTP, HTTP qtransport: process-process data transfer vTCP, UDP qnetwork: routing of datagrams from source to destination vIP, routing protocols qlink: data transfer between neighboring network elements vPPP, Ethernet qphysical: bits “on the wire” application transport network link phy

47、sical Introduction1-64 source Macintosh PICT image format is not supported application transport network link physical HtHn M segment Ht datagram destination Macintosh PICT image format is not supported application transport network link physical HtHnHl M HtHn M Ht M M network link physical link phy

48、sical HtHnHl M HtHn M HtHn M HtHnHl M router switch Encapsulation messageM Ht M Hn frame Introduction1-65 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

49、1.7 Protocol layers, service models 1.8 History Introduction1-66 Internet History q1961: Kleinrock - queueing theory shows effectiveness of packet- switching q1964: Baran - packet- switching in military nets q1967: ARPAnet conceived by Advanced Research Projects Agency q1969: first ARPAnet node oper

50、ational q1972: vARPAnet public demonstration vNCP (Network Control Protocol) first host-host protocol vfirst e-mail program vARPAnet has 15 nodes 1961-1972: Early packet-switching principles Introduction1-67 Internet History q1970: ALOHAnet satellite network in Hawaii q1974: Cerf and Kahn - architecture for interconnecting networks q1976: Ethernet at Xerox PARC qate70s