1、计算机网络体系结构与Internet史长琼长沙理工大学提纲u网络基本概念uInternet简介u协议分层和OSI参考模型提纲u网络基本概念网络基本概念uInternet简介u协议分层和OSI参考模型Whats a networkunetwork edge:millions of end-system devices:pcs workstations,servers PDAs,phones,toastersrunning network appsunetwork core:routers,switches forwarding datapackets:packet switchingcalls:
2、circuit switchingucommunication links fiber,copper,radio,local netcompanynetregional netrouterworkstationservermobile“Cool”Internet appliancesWorlds smallest web serverhttp:/www-ccs.cs.umass.edu/shri/iPic.htmlIP picture framehttp:/ toaster+weather forecasterWhats a protocol?a human protocol and a co
3、mputer network protocol:HiHiGot thetime?2:00TCP connection req.TCP connectionreply.Get http:/ a protocol?human protocols:u“whats the time?”u“I have a question”uIntroductionsnetwork protocols:umachines rather than humansuall communication activity in Internet governed by protocols specific msgs sent
4、specific actions taken when msgs received,or other eventsWhats a protocol?uprotocols define format,order of msgs sent and received among network entities,and actions taken on msg transmission,receipt A closer look at network structureunetwork edge:applications and hostsunetwork core:routers network
5、of networksuaccess networks,physical media:communication linksThe network edgeuend systems(hosts)run application programs e.g.,WWW,email at“edge of network”uclient/server model client host requests,receives service from server e.g.,WWW client(browser)/server;email client/serverupeer-peer model host
6、interaction symmetric e.g.:BT,Maze,e-Donkey The network coreumesh of interconnected routersuthe fundamental question:how is data transferred through net?circuit switching:dedicated circuit per call:telephone net packet-switching:data sent thru net in discrete“chunks”uCommunication networks can be cl
7、assified based on the way in which the nodes exchange information:A Taxonomy of Communication NetworksCommunication Communication NetworkNetworkSwitchedSwitchedCommunication Communication NetworkNetworkBroadcastBroadcastCommunication Communication NetworkNetworkCircuit-SwitchedCircuit-SwitchedCommun
8、ication Communication NetworkNetworkPacket-SwitchedPacket-SwitchedCommunication Communication NetworkNetworkDatagramDatagram Network NetworkVirtual Circuit NetworkVirtual Circuit NetworkuBroadcast communication networks Information transmitted by any node is received by every other node in the netwo
9、rk E.g.,LANs(Ethernet,Wavelan)Problem:coordinate the access of all nodes to the shared communication medium(Multiple Access Problem)uSwitched communication networks Information is transmitted to a sub-set of designated nodes E.g.,WANs(Telephony Network,Internet)Problem:how to forward information to
10、intended node(s)Done by special nodes(e.g.,routers,switches)running routing protocols Broadcast vs.Switched Communication Networks uCommunication networks can be classified based on the way in which the nodes exchange information:A Taxonomy of Communication NetworksCircuit-SwitchedCircuit-SwitchedCo
11、mmunication Communication NetworkNetworkCommunication Communication NetworkNetworkSwitchedSwitchedCommunication Communication NetworkNetworkBroadcastBroadcastCommunication Communication NetworkNetworkPacket-SwitchedPacket-SwitchedCommunication Communication NetworkNetworkDatagramDatagram Network Net
12、workVirtual Circuit NetworkVirtual Circuit NetworkCircuit SwitchinguThree phases1.circuit establishment2.data transfer3.circuit terminationuIf circuit not available:“Busy signal”uExamples Telephone networks ISDN(Integrated Services Digital Networks)Timing in Circuit SwitchingDATACircuit Establishmen
13、t Data Transmission Circuit Termination Host 1Host 2Node 1Node 2propagation delay between Host 1 and Node 1propagation delay between Host 2 and Node 1processing delay at Node 1Circuit SwitchinguA node(switch)in a circuit switching networkincoming linksoutgoing linksNodeuCommunication networks can be
14、 classified based on the way in which the nodes exchange information:A Taxonomy of Communication NetworksCommunication Communication NetworkNetworkSwitchedSwitchedCommunication Communication NetworkNetworkBroadcastBroadcastCommunication Communication NetworkNetworkCircuit-SwitchedCircuit-SwitchedCom
15、munication Communication NetworkNetworkPacket-SwitchedPacket-SwitchedCommunication Communication NetworkNetworkDatagramDatagram Network NetworkVirtual Circuit NetworkVirtual Circuit NetworkPacket SwitchinguData are sent as formatted bit-sequences,so-called packetsuPackets have the following structur
16、e:Header and Trailer carry control information(e.g.,destination address,check sum)uEach packet is passed through the network from node to node along some path(RoutingRouting)uAt each node the entire packet is received,stored briefly,and then forwarded to the next node(Store-and-ForwardStore-and-Forw
17、ard Networks Networks)uTypically no capacity is allocated for packetsHeaderDataTrailerPacket SwitchinguA node in a packet switching networkincoming linksoutgoing linksNodeMemoryHow do loss and delay occur?packets queue in router buffers upacket arrival rate to link exceeds output link capacityupacke
18、ts queue,wait for turnABpacket being transmitted(delay)packets queueing(delay)free(available)buffers:arriving packets dropped(loss)if no free buffers提纲u网络基本概念uInternetInternet简介简介u协议分层和OSI参考模型The InternetuGlobal scale,general purpose,heterogeneous-technologies,public,computer networkuInternet Protoc
19、ol Open standard:Internet Engineering Task Force(IETF)as standard body(http:/www.ietf.org)Technical basis for other types of networks Intranet:enterprise IP networkuDeveloped by the research communityGrowth of the InternetuNumber of Hosts on Number of Hosts on the Internet:the Internet:Aug.1981 213O
20、ct.1984 1,024Dec.1987 28,174 Oct.1990 313,000 Oct.1993 2,056,000Apr.1995 5,706,000Jan.1997 16,146,000Jan.1999 56,218,000Jan.2001 109,374,000Jan.2003 171,638,297Jan.2006 394,991,609 Data available at:http:/www.isc.org/Services Provided by the InternetuShared access to computing resources Telnet(1970s
21、)uShared access to data/files FTP,NFS,AFS(1980s)uCommunication medium over which people interact Email(1980s),on-line chat rooms(1990s)Instant messaging,IP Telephony(2000s)Services Provided by the InternetuA medium for information disseminationUSENET (1980s)WWW(1990s)Replacing newspaper,magazine?Aud
22、io,video(2000s)Replacing radio,CD,TVInternet structure:network of networksuroughly hierarchicaluat center:“tier-1”ISPs(e.g.,UUNet,BBN/Genuity,Sprint,AT&T),national/international coverage treat each other as equalsTier 1 ISPTier 1 ISPTier 1 ISPTier-1 providers interconnect(peer)privatelyNAPTier-1 pro
23、viders also interconnect at public network access points(NAPs)Tier-1 ISP:e.g.,SprintSprint US backbone networkSeattleAtlantaChicagoRoachdaleStocktonSan JoseAnaheimFort WorthOrlandoKansas CityCheyenneNew YorkPennsaukenRelayWash.DCTacomaDS3(45 Mbps)OC3(155 Mbps)OC12(622 Mbps)OC48(2.4 Gbps)to/from cust
24、omerspeering to/from backbonePOP:point-of-presenceInternet structure:network of networksu“Tier-2”ISPs:smaller(often regional)ISPs Connect to one or more tier-1 ISPs,possibly other tier-2 ISPsTier 1 ISPTier 1 ISPTier 1 ISPNAPTier-2 ISPTier-2 ISPTier-2 ISPTier-2 ISPTier-2 ISPTier-2 ISP pays tier-1 ISP
25、 for connectivity to rest of Internetq tier-2 ISP is customer oftier-1 providerTier-2 ISPs also peer privately with each other,interconnect at NAPInternet structure:network of networksu“Tier-3”ISPs and local ISPs last hop(“access”)network(closest to end systems)Tier 1 ISPTier 1 ISPTier 1 ISPNAPTier-
26、2 ISPTier-2 ISPTier-2 ISPTier-2 ISPTier-2 ISPlocalISPlocalISPlocalISPlocalISPlocalISPTier 3ISPlocalISPlocalISPlocalISPLocal and tier-3 ISPs are customers ofhigher tier ISPsconnecting them to rest of InternetInternet structure:network of networksua packet passes through many networks!Tier 1 ISPTier 1
27、 ISPTier 1 ISPNAPTier-2 ISPTier-2 ISPTier-2 ISPTier-2 ISPTier-2 ISPlocalISPlocalISPlocalISPlocalISPlocalISPTier 3ISPlocalISPlocalISPlocalISPTry a traceroute!徐闻徐闻 长春长春 哈尔滨哈尔滨 乌鲁木齐乌鲁木齐 拉萨拉萨 西宁西宁 兰州兰州 银川银川 呼和浩特呼和浩特 台北台北 沈阳沈阳 南昌南昌 西安西安 徐州徐州 武汉武汉 合肥合肥 郑州郑州 石家庄石家庄 北京北京 南宁南宁 广州广州 福州福州 杭州杭州 上海上海 南京南京 天津天津 贵
28、阳贵阳 海口海口 三亚三亚 湛江湛江 无锡无锡 大连大连 太原太原 济南济南 烟台烟台 成都成都 长沙长沙 重庆重庆 黄梅黄梅 九江九江 昆明昆明 青岛青岛 汕头汕头 唐山唐山 汉中汉中 宜昌宜昌 珠海珠海 深圳深圳 惠州惠州 柳州柳州 百色百色 厦门厦门 主干网 地区网 GigaPop Pop 桂林桂林 深圳深圳 Internet GraphAccess NetworksQ:How to connect end systems to edge router?uresidential access netsuinstitutional access networks(school,compan
29、y)umobile access networksKeep in mind:ubandwidth(bits per second)of access network?ushared or dedicated?Residential access:point to point accessuDialup via modem up to 56Kbps direct access to router(often less)uISDN:integrated services digital network 128kbps+regular phone lineuADSL:asymmetric digit
30、al subscriber line up to 1 Mbps upstream(today typically 256 kbps)up to 8 Mbps downstream(today typically 1 Mbps)FDM:50 kHz-1 MHz for downstreamResidential access:cable modemsuHFC:hybrid fiber coax asymmetric:up to 10Mbps upstream,30 Mbps downstreamunetwork of cable and fiber attaches homes to ISP r
31、outer shared access to router among home issues:congestion,dimensioning udeployment:available via cable companies,e.g.,ComCastResidential access:cable modemsDiagram:http:/ Network Architecture:Overviewhomecable headendcable distributionnetwork(simplified)Typically 500 to 5,000 homesCable Network Arc
32、hitecture:Overviewhomecable headendcable distributionnetworkserver(s)Cable Network Architecture:Overviewhomecable headendcable distributionnetwork(simplified)Cable Network Architecture:Overviewhomecable headendcable distributionnetworkChannelsVIDEOVIDEOVIDEOVIDEOVIDEOVIDEODATADATACONTROL123456789FDM
33、:Company access:local area networksucompany/univ local area network(LAN)connects end system to edge routeruEthernet:shared or dedicated link(switched Ethernet)connects end system and router 10 Mbs,100Mbps,Gigabit Ethernetudeployment:institutions,home LANs happening nowTo/From ISPWireless access netw
34、orksushared wireless access network connects end system to router via base station aka“access point”uwireless LANs:802.11b:11 Mbps 802.11g:54 Mbpsuwider-area wireless access provided by telecom operator 3G 384 kbps Will it happen?WAP/GPRS in Europe GPRS/CDMA in ChinabasestationmobilehostsrouterHome
35、networksTypical home network components:uADSL or cable modemurouter/firewall/NATuEthernetuwireless access pointwirelessaccess pointwirelesslaptopsrouter/firewallcablemodemto/fromcableheadendEthernet(switched)提纲u网络基本概念uInternet简介u协议分层和协议分层和OSIOSI参考模型参考模型What is Layering?uA technique to organize a net
36、work system into a succession of logically distinct entities,such that the service provided by one entity is solely based on the service provided by the previous(lower level)entityWhy Layering?uNo layering:each new application has to be re-implemented for every network technology!Telnet FTPNFSPacket
37、radioCoaxial cableFiberopticApplicationTransmissionMediaHTTPWhy Layering?uSolution:introduce an intermediate layer that provides a unique abstraction for various network technologiesTelnet FTPNFSPacketradioCoaxial cableFiberopticApplicationTransmissionMediaHTTPIntermediate layerLayeringuAdvantages M
38、odularity protocols easier to manage and maintain Abstract functionality lower layer can be changed without affecting the upper layer Reuse upper layer can reuse the functionality provided by lower layer uDisadvantages Information hiding inefficient implementations LayeringuLayer N software on the d
39、estination computer must receive exactly the message sent by layer N software on the sending computeruMathematically,if the sender applies a transformation T,the receiver must apply the inverse T-1uRecall the concept of“protocol”ISO OSI Reference ModeluISO International Standard OrganizationuOSI Ope
40、n System InterconnectionuStarted to 1978;first standard 1979 ARPANET started in 1969;TCP/IP protocols ready by 1974uGoal:a general open standard Allow vendors to enter the market by using their own implementation and protocols ISO OSI Reference ModeluSeven layers Lower three layers are peer-to-peer
41、Next four layers are end-to-endApplicationPresentationSessionTransportNetworkDatalinkPhysicalApplicationPresentationSessionTransportNetworkDatalinkPhysicalNetworkDatalinkPhysicalPhysical mediumData TransmissionuA layer can use only the service provided by the layer immediate below ituEach layer may
42、change and add a header to data packetdatadatadatadatadatadatadatadatadatadatadatadatadatadataOSI Model ConceptsuService says what a layer doesuInterface says how to access the service uProtocol says how is the service implemented A set of rules and formats that govern the communication between two
43、peersPhysical MediauBit:propagates betweentransmitter/rcvr pairsuphysical link:what lies between transmitter&receiveruguided media:signals propagate in solid media:copper,fiber,coaxuunguided media:signals propagate freely,e.g.,radioTwisted Pair(TP)utwo insulated copper wires Category 3:traditional p
44、hone wires,10 Mbps Ethernet Category 5 TP:100Mbps EthernetPhysical Media:coax,fiberCoaxial cable:utwo concentric copper conductorsubidirectionalubaseband:single channel on cable legacy Ethernetubroadband:multiple channel on cableFiber optic cable:uglass fiber carrying light pulses,each pulse a bituh
45、igh-speed operation:high-speed point-to-point transmission(e.g.,10 Gbps)ulow error rate:repeaters spaced far apart;immune to electromagnetic noisePhysical media:radiousignal carried in electromagnetic spectrumuno physical“wire”ubidirectionalupropagation environment effects:reflection obstruction by
46、objects interferenceRadio link types:uterrestrial microwave e.g.up to 45 Mbps channelsuLAN(e.g.,WaveLAN)2Mbps,11Mbps,55Mbpsuwide-area(e.g.,cellular)e.g.3G:hundreds of kbpsusatellite up to 50Mbps channel(or multiple smaller channels)270 msec end-end delay geosynchronous versus LEOSPhysical media:radi
47、oPhysical LayeruService:move the information between two systems connected by a physical linkuInterface:specifies how to send a bit uProtocol:coding scheme used to represent a bit,voltage levels,duration of a bituExamples:coaxial cable,optical fiber links;transmitters,receivers 物理层的四个重要特性u机械特性(mecha
48、nical characteristics)主要定义物理连接的边界点，即接插装置 规定物理连接所采用的规格、引脚的数量和排列u电气特性(electrical characteristics)规定传输二进制位时，线路上信号的电压高低、阻抗匹配、传输速率和距离限制u功能特性(functional characteristics)主要定义各条物理线路的功能u规程特性(procedural characteristics)主要定义各条物理线路的工作规程和时序关系Datalink Layer(2)uService:Framing,i.e.,attach frames separator Send dat
49、a frames between peers attached to the same physical media Others(optional):Arbitrate the access to common physical media Ensure reliable transmission Provide flow controluInterface:send a data unit(packet)to a machine connected to the same physical mediauProtocol:layer addresses,implement Medium Ac
50、cess Control(MAC)(e.g.,CSMA/CD)Framingu将比特流分成离散的帧，并计算每个帧的校验和u成帧方法：字符计数法 在帧头中用一个域来表示整个帧的字符个数 缺点：若计数出错，对本帧和后面的帧有影响 带字符填充的标记字符定界法 标记字符FLAG 字符填充 缺点：局限于8位字符和ASCII字符传送FramingFraming 带位填充的首尾标记定界法 帧的起始和结束都用一个特殊的位串“01111110”，称为标记(flag)“0”比特插入删除技术，连续5个1后面加一个0Framing 物理层编码违例法 802 LAN 曼彻斯特编码或差分曼彻斯特编码用high-low p
