1、1第一章计算机网络基本原理与机制2提纲网络基本概念Internet简介协议分层和OSI参考模型End-to-End ArgumentsInternet设计原则3提纲网络基本概念Internet简介协议分层和OSI参考模型End-to-End ArgumentsInternet设计原则4Worlds smallest web serverhttp:/www-ccs.cs.umass.edu/shri/iPic.html“Cool”Internet appliancesIP picture framehttp:/ toaster+weather forecaster5Whats a network
2、network edge:millions ofend-system devices:pcs workstations,servers PDAs,phones,toastersrunning network appsnetwork core:routers,switches forwarding data packets:packet switching calls:circuit switchingcommunication links fiber,copper,radio,routerserverlocal netcompanynetworkstationmobileregional ne
3、t6Whats a protocol?a human protocol and a computer network protocol:HiHiGot thetime?2:00TCP connectionreq.TCP connectionreply.Get http:/time7Whats a protocol?human protocols:“whats the time?”“I have a question”Introductionsnetwork protocols:machines ratherthan humansall communicationactivity in Inte
4、rnetgoverned byprotocols specific msgs sent specific actions taken when msgsreceived,or other events8Whats a protocol?协议的组成：语法（syntax)：以二进制形式表示的命令和相应的结构 语义（semantics)：命令请求、完成的动作和回送的响应的具体含义 定时关系（timing)：有关事件顺序的说明protocols define format,order ofmsgs sent and received amongnetwork entities,and actions
5、takenon msg transmission,receipt9A closer look at networkstructurenetwork edge:applications andhostsnetwork core:routers network ofnetworksaccessnetworks,physical media:communicationlinks10The network edgeend systems(hosts)run application programs e.g.,WWW,email at“edge of network”client/server mode
6、l client host requests,receives service fromserver e.g.,WWW client(browser)/server;emailclient/serverpeer-peer model host interactionsymmetric e.g.:BT,Maze,e-Donkey11The network coremesh of interconnectedroutersthe fundamentalquestion:how is datatransferred through net?circuit switching:dedicated ci
7、rcuit percall:telephone net packet switching:data sent thru net indiscrete“chunks”12BroadcastCommunicationNetworkA Taxonomy ofCommunication NetworksCommunication networks can be classifiedbased on the way in which the nodesexchange information:CommunicationNetworkSwitchedCommunicationNetworkCircuit-
8、SwitchedCommunicationNetworkPacket-SwitchedCommunicationNetworkDatagramNetworkVirtual Circuit Network13Broadcast vs.SwitchedCommunication NetworksBroadcast communication networks Information transmitted by one node isreceived by every other node in thenetwork E.g.,Satellite Network,CCTV-Net Problem:
9、coordinate the access of allnodes to the shared communicationmedium(Multiple Access Problem)Switched communication networks Information is transmitted to a sub-set ofdesignated nodes E.g.,WANs(Telephony Network,Internet)Problem:how to forward information tointended node(s)Done by special nodes(e.g.,
10、routers,switches)running routing protocols14A Taxonomy ofCommunication NetworksCommunication networks can be classifiedbased on the way in which the nodesexchange information:CommunicationNetworkSwitchedCommunicationNetworkCircuit-SwitchedCommunicationNetworkBroadcastCommunicationNetworkPacket-Switc
11、hedCommunicationNetworkDatagramNetworkVirtual Circuit Network15Circuit SwitchingThree phases1.circuit establishment2.data transfer3.circuit terminationIf circuit not available:“Busy signal”Examples Telephone networks ISDN(Integrated Services DigitalNetworks)16Timing in Circuit SwitchingCircuitEstabl
12、ishmentHost 1Host 2Node 1Node 2between Host 1and Node 1propagation delaybetween Host 2and Node 1DataTransmissionDATACircuitTerminationprocessing delay at Node 1propagation delay17Circuit SwitchingA node(switch)in a circuitswitching networkincoming linksoutgoing linksNode18Circuit Switching:Multiplex
13、ing/DemultiplexingTime divided in frames and frames divided inslotsRelative slot position inside a frame determineswhich conversation the data belongs toNeeds synchronization between sender andreceiverIn case of non-permanent conversations Needs to dynamic bind a slot to aconservation19A Taxonomy of
14、Communication NetworksCommunication networks can be classifiedbased on the way in which the nodesexchange information:CommunicationNetworkBroadcastCommunicationNetworkSwitchedCommunicationNetworkCircuit-SwitchedCommunicationNetworkPacket-SwitchedCommunicationNetworkDatagramNetworkVirtual Circuit Net
15、work20Packet SwitchingData are sent as formatted bit-sequences,so-called packetsPackets have the following structure:Header and Trailer carry controlinformation(e.g.,destination address,checksum)Each packet is passed through the networkfrom node to node along some path(Routing)At each node the entir
16、e packet is received,stored briefly,and then forwarded to the nextnode(Store-and-Forward Networks)HeaderDataTrailer21Packet SwitchingA node in a packet switchingnetworkincoming linksoutgoing linksNodeMemory22Packet Switching:Multiplexing/DemultiplexingData can be transmitted at anygiven timeHow to t
17、ell them apart?Use meta-data(header)todescribe data23BroadcastCommunicationNetworkA Taxonomy ofCommunication NetworksCommunication networks can be classifiedbased on the way in which the nodesexchange information:CommunicationNetworkSwitchedCommunicationNetworkCircuit-SwitchedCommunicationNetworkPac
18、ket-SwitchedCommunicationNetworkDatagramNetworkVirtual Circuit Network24Datagram PacketSwitchingEach packet is independentlyswitched Each packet header containsdestination addressNo resources are pre-allocated(reserved)in advanceExample:IP networks25Host AHost BHost EHost DDatagram PacketSwitchingHo
19、st CNode 1Node 3Node 4Node 6Node 2Node 5Node 726How do loss and delay occur?router buffers is fullpacket arrival rate to link exceeds outputlink capacityOutput link(loss)ABdropped(loss)if no free buffers27Four sources of packet delay1.nodal processing check bit errors determine outputlink(?)ABtransm
20、issionnodalprocessingpropagationqueueing2.queueing time waiting at outputlink for transmission depends on congestionlevel of router28Four sources of packet delay3.Transmission delay:R=link bandwidth(bps)L=packet length(bits)time to send bits intolink=L/RABtransmissionnodalprocessingpropagationqueuei
21、ng4.Propagation delay:d=length of physical links=propagation speed inmedium(2x108 m/sec)propagation delay=d/sNote:s and R are verydifferent quantities!29BroadcastCommunicationNetworkA Taxonomy ofCommunication NetworksCommunication networks can be classifiedbased on the way in which the nodesexchange
22、 information:CommunicationNetworkSwitchedCommunicationNetworkCircuit-SwitchedCommunicationNetworkPacket-SwitchedCommunicationNetworkDatagramNetworkVirtual Circuit Network30Virtual-Circuit PacketSwitchingHybrid of circuit switching andpacket switching Data is transmitted as packets All packets from o
23、ne packet stream aresent along a pre-established path(=virtual circuit)Guarantees in-sequence delivery ofpacketsHowever:Packets from differentvirtual circuits may be interleavedExample:ATM networks31Virtual-Circuit PacketSwitchingCommunication with virtualcircuits takes place in threephases1.VC esta
24、blishment2.data transfer3.VC disconnectNote:packet headers dont needto contain the full destinationaddress of the packet32Packet 1Packet 1Packet 2Packet 3Timing of Virtual-CircuitPacket SwitchingPacket 1Packet 2Packet 3Host 1Host 2Node 1Node 2propagation delaybetween Host 1and Node 1VCestablishmentP
25、acket 2Packet 3VCterminationDatatransfer33Host AHost BHost ENode 1Node 3Node 4Node 2Node 5Node 6Node 7Host DVirtual-Circuit PacketSwitchingHost C34Packet-Switching vs.Circuit-SwitchingMost important advantage ofpacket-switching over circuitswitching:ability to exploitstatistical multiplexing Efficie
26、nt bandwidth usage ratio between peek and average rate is3:1 for audio,and 15:1 for data traffic35Packet-Switching vs.Circuit-SwitchingHowever,packet-switching needsto deal with congestion More complex routers Harder to provide good networkservices(e.g.,delay and bandwidthguarantees)In practice they
27、 are combined IP over SONET,IP over Frame Relay36提纲网络基本概念Internet简介协议分层和OSI参考模型End-to-End ArgumentsInternet设计原则基本机制回顾37The InternetGlobal scale,general purpose,heterogeneous-technologies,public,computer networkInternet Protocol Open standard:Internet Engineering TaskForce(IETF)as standard bodyhttp:/
28、www.ietf.org Technical basis for other types ofnetworks Intranet:enterprise IP networkDeveloped by the research community381961:Kleinrock-queueing theory showseffectiveness of packet-switching1964:Paul Baran-packet-switching inmilitary nets1967:ARPAnetconceived by AdvancedResearch ProjectsAgency Lar
29、ry Roberts1969:first ARPAnet nodeoperational1972:ARPAnetdemonstratedpublicly NCP(NetworkControl Protocol)first host-hostprotocol first e-mail program ARPAnet has 15nodesInternet History1961-1972:Early packet-switchingprinciples39Internet History1972-1980:Internetworking,new and proprietary1970:ALOHA
30、net satellitenetwork in Hawaiiprinciples:no internal changesrequired tointerconnectlate 70s:proprietary networksarchitectures:DECnet,SNA,best effort serviceXNA modellate 70s:switching fixed stateless routerslength packets(ATMprecursor)1979:ARPAnet has 200nodes401982:SMTP e-mailprotocol defined1983:d
31、eployment ofTCP/IP1983:DNS definedfor name-to-IP-address translation1985:FTP protocoldefined1988:TCPcongestion controlnew national networks:Csnet,BITnet,NSFnet,Minitel100,000 hostsconnected toconfederation ofnetworksInternet History1980-1990:new protocols,aproliferation of networks41Internet History
32、1990,2000s:commercialization,theWeb,new appsEarly 1990s:ARPAnetdecommissioned1991:NSF lifts restrictions on Late 1990s 2000s:commercial use of NSFnet more killer apps:instant(decommissioned,1995)messaging,peer2peer filesharing(e.g.,BT,Napster)network security toforefrontest.50 million host,100millio
33、n+users 1994:Mosaic,later backbone links running atNetscape Gbps late 1990s:commercialization of theWeb42Time Line of the InternetSource:Internet Society43Growth of the InternetNumber of Hosts on theInternet:Aug.1981 213Oct.1984 1,024Dec.1987 28,174Oct.1990 313,000Oct.1993 2,056,000Apr.1995 5,706,00
34、0Jan.1997 16,146,000Jan.1999 56,218,000Jan.2001 109,374,000Jan.2003 171,638,297Jan.2006 394,991,609July 2007 489,774,269Jan.2009 625,226,456July.2009 681,064,561Data available at:http:/www.isc.org/44Growth of the Internet45Who is Who in the Internet?Internet Society(ISOC)ISOC is a professional membe
35、rshipsociety with more than 100 organizationand over 20,000 individual members inover 180 countries It provides leadership in addressing issuesof the Internet,and is the organizationhome for the groups responsible forInternet infrastructure standards,including IETF and IAB46Who is Who in the Interne
36、t?Internet Engineering Task Force(IETF)The IETF is the protocol engineeringand development arm of the Internet Subdivided into many workinggroups,which specify Request ForComments or RFCs47Who is Who in the Internet?IRTF(Internet Research Task Force)The Internet Research Task Force iscomposed of a n
37、umber of focused,long-term and small Research GroupsInternet Architecture Board(IAB)The IAB is responsible for defining theoverall architecture of the Internet,providing guidance and broad direction tothe IETF48Who is Who in the Internet?The Internet EngineeringSteering Group(IESG)The IESG is respon
38、sible fortechnical management of IETFactivities and the Internet standardsprocess Composed of the Area Directors ofthe IETF working groups49Who is Who in the Internet?IETF and IESG Chair Russ Housley,VigilSecurity,LLCApplications Area(app)Lisa Dusseault,OpenSource ApplicationsFoundation Alexey Melni
39、kov,IsodeLimitedInternet Area(int)Jari Arkko,Ericsson Ralph Droms,CiscoOperations andManagement Area(ops)Ronald Bonica,JuniperNetworks Dan Romascanu,AvayaReal-time Applications andInfrastructure Area(rai)Cullen Jennings,CiscoSystems Robert Sparks,TekelecRouting Area(rtg)Ross Callon,Juniper Networks
40、Adrian Farrel,HuaweiSecurity Area(sec)Pasi Eronen,Nokia Tim Polk,National Institute ofStandards and TechnologyTransport Area(tsv)Lars Eggert,Nokia ResearchCenter Magnus Westerlund,Ericsson50Internet StandardizationProcessAll standards of the Internet are publishedas RFC(Request for Comments).But not
41、all RFCs are Internet Standards available:http:/www.ietf.orgA typical(but not only)way ofstandardization is:BOF(Birds of a feather)Internet Drafts RFC Proposed Standard Draft Standard(requires 2 workingimplementation)Internet Standard(declared by IAB)51Internet StandardizationProcess David Clark,MIT
42、,1992:We reject:kings,presidents,and voting.We believe in:rough consensus andrunning code.52Services Provided by theInternetShared access to computing resources Telnet(1970s)Shared access to data/files FTP,NFS,AFS(1980s)Communication medium over whichpeople interact Email(1980s),on-line chat rooms(1
43、990s)Instant messaging,IP Telephony(2000s)53Services Provided by theInternetA medium for informationdissemination USENET (1980s)WWW(1990s)Replacing newspaper,magazine?Audio,video(2000s)Replacing radio,CD,TV54Internet structure:network ofnetworksroughly hierarchicalat center:“tier-1”ISPs(e.g.,UUNet,B
44、BN/Genuity,Sprint,AT&T),national/international coverage treat each other as equalsTier-1providersinterconnect(peer)privatelyTier 1 ISPNAPTier 1 ISP Tier 1 ISPTier-1 providersalso interconnectat public networkaccess points(NAPs)55Tier-1 ISP:e.g.,SprintSprint US backbone networkChicagoRoachdaleStockto
45、nSan JoseNew YorkPennsaukenRelayWash.DCSeattleTacomaDS3(45 Mbps)OC3(155 Mbps)OC12(622 Mbps)OC48(2.5 Gbps)AnaheimAtlantato/from customersOrlandoCheyennepeeringKansas CityPOP:point-of-presenceto/from backbone56Internet structure:network ofnetworks“Tier-2”ISPs:smaller(often regional)ISPs Connect to one
46、 or more tier-1 ISPs,possibly othertier-2 ISPsTier-2 ISP Tier-2 ISPTier 1 ISPNAPTier 1 ISP Tier 1 ISPTier-2 ISPTier-2 ISPTier-2 ISP paystier-1 ISP forconnectivity torest of Internettier-2 ISP iscustomer oftier-1 providerTier-2 ISPsalso peerprivately witheach other,interconnectat NAPTier-2 ISPISPloca
47、llocalISPISPTier-2 ISPTier-2 ISPTier 1 ISPTier 1 ISPISPISP57Internet structure:network ofnetworks“Tier-3”ISPs and local ISPs last hop(“access”)network(closest to end systems)Tier 3 localISP ISPTier-2 ISPlocalISPcustomers of Tier 1 ISPlocalLocal and tier-3 ISPs arehigher tier NAPISPsconnectingthem to
48、 restof InternetTier-2 ISP Tier-2 ISPlocal local localISPlocalISP58Internet structure:network ofnetworksa packet passes through many networks!Tier 1 ISPTier 1 ISPTier 1 ISPNAPTier-2 ISPlocalISPTier-2 ISPlocalISPlocalISPlocalISPTier 3ISPTier-2 ISPTier-2 ISPlocalISPTier-2 ISPlocalISPlocalISPTry atrace
49、route!59606162Access NetworksQ:How to connect endsystems to edge router?residential access netsinstitutional accessnetworks(school,company)mobile access networksKeep in mind:bandwidth(bits persecond)of accessnetwork?shared or dedicated?63Residential access:point to point accessDialup via modem up to
50、 56Kbps direct access torouter(often less)ISDN:integrated services digitalnetwork 128kbps+regular phone lineADSL:asymmetric digital subscriber line up to 1 Mbps upstream(today typically 256 kbps)up to 8 Mbps downstream(today typically 5 Mbps)FDM:50 kHz-1 MHz for downstream64Residential access:point
