计算机网络体系结构与Internet.ppt

北京大学 研究生课程,1,计算机网络体系结构与Internet,清华大学计算机系,北京大学 研究生课程,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设计原则基本机制回顾,北京大学 研究生课程,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设计原则基本机制回顾,北京大学 研究生课程,4,“Cool”Internet appliances,Worlds smallest web serverhttp:/www-ccs.cs.umass.edu/shri/iPic.html,IP picture framehttp:/,Web-enabled toaster+weather forecaster,北京大学 研究生课程,5,Whats a network,network edge:millions of end-system devices:pcs workstations,serversPDAs,phones,toastersrunning network appsnetwork core:routers,switches forwarding datapackets:packet switchingcalls:circuit switchingcommunication linksfiber,copper,radio,北京大学 研究生课程,6,Whats a protocol?,a human protocol and a computer network protocol:,北京大学 研究生课程,7,Whats a protocol?,human protocols:“whats the time?”“I have a question”Introductions,network protocols:machines rather than humansall communication activity in Internet governed by protocols,specific msgs sent specific actions taken when msgs received,or other events,北京大学 研究生课程,8,Whats a protocol?,protocols define format,order of msgs sent and received among network entities,and actions taken on msg transmission,receipt,北京大学 研究生课程,9,A closer look at network structure,network edge:applications and hostsnetwork core:routersnetwork of networksaccess networks,physical media:communication links,北京大学 研究生课程,10,The network edge,end systems(hosts)run application programse.g.,WWW,emailat“edge of network”client/server modelclient host requests,receives service from servere.g.,WWW client(browser)/server;email client/serverpeer-peer modelhost interaction symmetrice.g.:BT,Maze,e-Donkey,北京大学 研究生课程,11,The network core,mesh of interconnected routersthe fundamental question:how is data transferred through net?circuit switching:dedicated circuit per call:telephone netpacket switching:data sent thru net in discrete“chunks”,北京大学 研究生课程,12,Communication networks can be classified based on the way in which the nodes exchange information:,A Taxonomy of Communication Networks,北京大学 研究生课程,13,Broadcast communication networksInformation transmitted by any node is received by every other node in the networkE.g.,LANs(Ethernet,Wavelan)Problem:coordinate the access of all nodes to the shared communication medium(Multiple Access Problem)Switched communication networksInformation is transmitted to a sub-set of designated nodesE.g.,WANs(Telephony Network,Internet)Problem:how to forward information to intended node(s)Done by special nodes(e.g.,routers,switches)running routing protocols,Broadcast vs.Switched Communication Networks,北京大学 研究生课程,14,Communication networks can be classified based on the way in which the nodes exchange information:,A Taxonomy of Communication Networks,Circuit-SwitchedCommunication Network,北京大学 研究生课程,15,Circuit Switching,Three phasescircuit establishmentdata transfercircuit terminationIf circuit not available:“Busy signal”ExamplesTelephone networksISDN(Integrated Services Digital Networks),北京大学 研究生课程,16,Timing in Circuit Switching,DATA,Host 1,Host 2,Node 1,Node 2,propagation delay between Host 1 and Node 1,propagation delay between Host 2 and Node 1,processing delay at Node 1,北京大学 研究生课程,17,Circuit Switching,A node(switch)in a circuit switching network,incoming links,outgoing links,Node,北京大学 研究生课程,18,Circuit Switching:Multiplexing/Demultiplexing,Time divided in frames and frames divided in slotsRelative slot position inside a frame determines which conversation the data belongs toNeeds synchronization between sender and receiverIn case of non-permanent conversationsNeeds to dynamic bind a slot to a conservation,北京大学 研究生课程,19,Communication networks can be classified based on the way in which the nodes exchange information:,A Taxonomy of Communication Networks,Communication Network,SwitchedCommunication Network,BroadcastCommunication Network,Circuit-SwitchedCommunication Network,Packet-SwitchedCommunication Network,Datagram Network,Virtual Circuit Network,北京大学 研究生课程,20,Packet Switching,Data are sent as formatted bit-sequences,so-called packetsPackets have the following structure:Header and Trailer carry control information(e.g.,destination address,checksum)Each packet is passed through the network from node to node along some path(Routing)At each node the entire packet is received,stored briefly,and then forwarded to the next node(Store-and-Forward Networks)Typically no capacity is allocated for packets,北京大学 研究生课程,21,Packet Switching,A node in a packet switching network,incoming links,outgoing links,Node,Memory,北京大学 研究生课程,22,Packet Switching:Multiplexing/Demultiplexing,Data can be transmitted at any given timeHow to tell them apart?Use meta-data(header)to describe data,北京大学 研究生课程,23,Communication networks can be classified based on the way in which the nodes exchange information:,A Taxonomy of Communication Networks,北京大学 研究生课程,24,Datagram Packet Switching,Each packet is independently switchedEach packet header contains destination addressNo resources are pre-allocated(reserved)in advanceExample:IP networks,北京大学 研究生课程,25,Datagram Packet Switching,Host A,Host B,Host E,Host D,Host C,Node 1,Node 2,Node 3,Node 4,Node 5,Node 6,Node 7,北京大学 研究生课程,26,How do loss and delay occur?,packets queue in router buffers packet arrival rate to link exceeds output link capacitypackets queue,wait for turn,A,B,北京大学 研究生课程,27,Four sources of packet delay,1.nodal processing check bit errorsdetermine output link,2.queueingtime waiting at output link for transmission depends on congestion level of router,北京大学 研究生课程,28,Four sources of packet delay,3.Transmission delay:R=link bandwidth(bps)L=packet length(bits)time to send bits into link=L/R,4.Propagation delay:d=length of physical links=propagation speed in medium(2x108 m/sec)propagation delay=d/s,Note:s and R are very different quantities!,北京大学 研究生课程,29,Communication networks can be classified based on the way in which the nodes exchange information:,A Taxonomy of Communication Networks,北京大学 研究生课程,30,Virtual-Circuit Packet Switching,Hybrid of circuit switching and packet switchingData is transmitted as packetsAll packets from one packet stream are sent along a pre-established path(=virtual circuit)Guarantees in-sequence delivery of packetsHowever:Packets from different virtual circuits may be interleavedExample:ATM networks,北京大学 研究生课程,31,Virtual-Circuit Packet Switching,Communication with virtual circuits takes place in three phases VC establishmentdata transferVC disconnectNote:packet headers dont need to contain the full destination address of the packet,北京大学 研究生课程,32,Timing of Virtual-Circuit Packet Switching,Host 1,Host 2,Node 1,Node 2,propagation delay between Host 1 and Node 1,VCestablishment,VCtermination,Datatransfer,北京大学 研究生课程,33,Virtual-Circuit Packet Switching,Host A,Host B,Host E,Host D,Host C,Node 1,Node 2,Node 3,Node 4,Node 5,Node 6,Node 7,北京大学 研究生课程,34,Packet-Switching vs.Circuit-Switching,Most important advantage of packet-switching over circuit switching:ability to exploit statistical multiplexingEfficient bandwidth usageratio between peek and average rate is 3:1 for audio,and 15:1 for data traffic,北京大学 研究生课程,35,Packet-Switching vs.Circuit-Switching,However,packet-switching needs to deal with congestionMore complex routersHarder to provide good network services(e.g.,delay and bandwidth guarantees)In practice they are combinedIP over SONET,IP over Frame Relay,北京大学 研究生课程,36,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,北京大学 研究生课程,37,The Internet,Global scale,general purpose,heterogeneous-technologies,public,computer networkInternet ProtocolOpen standard:Internet Engineering Task Force(IETF)as standard body http:/www.ietf.orgTechnical basis for other types of networksIntranet:enterprise IP networkDeveloped by the research community,北京大学 研究生课程,38,Internet History,1961:Kleinrock-queueing theory shows effectiveness of packet-switching1964:Baran-packet-switching in military nets1967:ARPAnet conceived by Advanced Research Projects Agency Larry Roberts1969:first ARPAnet node operational,1972:ARPAnet demonstrated publiclyNCP(Network Control Protocol)first host-host protocol first e-mail programARPAnet has 15 nodes,1961-1972:Early packet-switching principles,北京大学 研究生课程,39,Internet History,1970:ALOHAnet satellite network in Hawaii1973:Metcalfes PhD thesis proposes Ethernet1974:Cerf and Kahn-architecture for interconnecting networkslate 70s:proprietary architectures:DECnet,SNA,XNAlate 70s:switching fixed length packets(ATM precursor)1979:ARPAnet has 200 nodes,Cerf and Kahns internetworking principles:minimalism,autonomyno internal changes required to interconnect networksbest effort service modelstateless routersdecentralized controldefine todays Internet architecture,1972-1980:Internetworking,new and proprietary nets,北京大学 研究生课程,40,Internet History,1982:SMTP e-mail protocol defined1983:deployment of TCP/IP1983:DNS defined for name-to-IP-address translation1985:FTP protocol defined1988:TCP congestion control,new national networks:Csnet,BITnet,NSFnet,Minitel100,000 hosts connected to confederation of networks,1980-1990:new protocols,a proliferation of networks,北京大学 研究生课程,41,Internet History,Early 1990s:ARPAnet decommissioned1991:NSF lifts restrictions on commercial use of NSFnet(decommissioned,1995)early 1990s:Webhypertext Bush 1945,Nelson 1960sHTML,HTTP:Berners-Lee1994:Mosaic,later Netscapelate 1990s:commercialization of the Web,Late 1990s 2000s:more killer apps:instant messaging,peer2peer file sharing(e.g.,BT,Napster)network security to forefrontest.50 million host,100 million+usersbackbone links running at Gbps,1990,2000s:commercialization,the Web,new apps,北京大学 研究生课程,42,Time Line of the Internet,Source:Internet Society,北京大学 研究生课程,43,Growth of the Internet,Number of Hosts on the Internet:Aug.1981 213Oct.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 394,991,609July 2007 489,774,269Jan.2009 625,226,456,Data available at:http:/www.isc.org/,北京大学 研究生课程,44,Growth of the Internet,北京大学 研究生课程,45,Who is Who in the Internet?,Internet Society(ISOC)ISOC is a professional membership society with more than 100 organization and over 20,000 individual members in over 180 countriesIt provides leadership in addressing issues of the Internet,and is the organization home for the groups responsible for Internet infrastructure standards,including IETF and IAB,北京大学 研究生课程,46,Who is Who in the Internet?,Internet Engineering Task Force(IETF)The IETF is the protocol engineering and development arm of the InternetSubdivided into many working groups,which specify Request For Comments or RFCs,北京大学 研究生课程,47,Who is Who in the Internet?,IRTF(Internet Research Task Force)The Internet Research Task Force is composed of a number of focused,long-term and small Research GroupsInternet Architecture Board(IAB)The IAB is responsible for defining the overall architecture of the Internet,providing guidance and broad direction to the IETF,北京大学 研究生课程,48,Who is Who in the Internet?,The Internet Engineering Steering Group(IESG)The IESG is responsible for technical management of IETF activities and the Internet standards processComposed of the Area Directors of the IETF working groups,北京大学 研究生课程,49,Who is Who in the Internet?,IETF and IESG Chair Russ Housley,Vigil Security,LLC Applications Area(app)Lisa Dusseault,Open Source Applications Foundation Alexey Melnikov,Isode Limited Internet Area(int)Jari Arkko,Ericsson Ralph Droms,Cisco Operations and Management Area(ops)Ronald Bonica,Juniper Networks Dan Romascanu,Avaya,Real-time Applications and Infrastructure Area(rai)Cullen Jennings,Cisco Systems Robert Sparks,Tekelec Routing Area(rtg)Ross Callon,Juniper Networks Adrian Farrel,Huawei Security Area(sec)Pasi Eronen,Nokia Tim Polk,National Institute of Standards and Technology Transport Area(tsv)Lars Eggert,Nokia Research Center Magnus Westerlund,Ericsson,北京大学 研究生课程,50,Internet Standardization Process,All standards of the Internet are published as RFC(Request for Comments).But not all RFCs are Internet Standards available:http:/www.ietf.orgA typical(but not only)way of standardization is:BOF(Birds of a feather)Internet DraftsRFCProposed Standard Draft Standard(requires 2 working implementation)Internet Standard(declared by IAB),北京大学 研究生课程,51,Internet Standardization Process,David Clark,MIT,1992:We reject:kings,presidents,and voting.We believe in:rough consensus and running code.,北京大学 研究生课程,52,Services Provided by the Internet,Shared access to computing resourcesTelnet(1970s)Shared access to data/filesFTP,NFS,AFS(1980s)Communication medium over which people interactEmail(1980s),on-line chat rooms(1990s)Instant messaging,IP Telephony(2000s),北京大学 研究生课程,53,Services Provided by the Internet,A medium for information disseminationUSENET(1980s)WWW(1990s)Replacing newspaper,magazine?Audio,video(2000s)Replacing radio,CD,TV,北京大学 研究生课程,54,Internet structure:network of networks,roughly hierarchicalat center:“tier-1”ISPs(e.g.,UUNet,BBN/Genuity,Sprint,AT&T),national/international coveragetreat each other as equals,Tier 1 ISP,Tier 1 ISP,Tier 1 ISP,北京大学 研究生课程,55,Tier-1 ISP:e.g.,Sprint,Sprint US backbone network,北京大学 研究生课程,56,Internet structure:network of networks,“Tier-2”ISPs:smaller(often regional)ISPsConnect to one or more tier-1 ISPs,possibly other tier-2 ISPs,Tier 1 ISP,Tier 1 ISP,Tier 1 ISP,北京大学 研究生课程,57,Internet structure:network of networks,“Tier-3”ISPs and local ISPs last hop(“access”)network(closest to end systems),Tier 1 ISP,Tier 1 ISP,Tier 1 ISP,北京大学 研究生课程,58,Internet structure:network of networks,a packet passes through many networks!,Tier 1 ISP,Tier 1 ISP,Tier 1 ISP,Try a traceroute!,北京大学 研究生课程,59,北京大学 研究生课程,60,北京大学 研究生课程,61,北京大学 研究生课程,62,北京大学 研究生课程,63,北京大学 研究生课程,64,Access Networks,Q:How to connect end systems to edge router?residential access netsinstitutional access networks(school,company)mobile access networksKeep in mind:bandwidth(bits per second)of access network?shared or dedicated?,北京大学 研究生课程,65,Residential access:point to point access,Dialup via modemup to 56Kbps direct access to router(often less)ISDN:integrated services digital network128kbps+regular phone line,ADSL:asymmetric digital subscriber lineup to 1 Mbps upstream(today typically 256 kbps)up to 8 Mbps downstream(today typically 1 Mbps)FDM:50 kHz-1 MHz for downstream,北京大学 研究生课程,66,Residential access:point to point access,ADSL:asymmetric digital subscriber lineADSL充分利用了双绞铜线的频谱(1)传统话音业务频段，约为4带宽(2)ADSL上行双工低速信道，位于话音频谱之上(3)ADSL下行单工高速信道，位于高频部分频分多路复用(FDM)或回波抵消(Echo Cancellation)技术在回波抵消ADSL系统中，下行信道与上行信道有重叠(主要是利用低频部分信号衰减小的特点)与频分复用技术相比，回波抵消技术消除了因频率叠加所带来的干涉(如近端串音)，可使ADSL系统在性能指标上有较大的提高，复杂度较高,北京大学 研究生课程,67,Residential access:cable modems,HFC:hybrid fiber coaxasymmetric:up to 10Mbps upstream,30 Mbps downstreamnetwork of cable and fiber attaches homes to ISP routershared access to router among homeissues:congestion,dimensioning deployment:available via cable companies,e.g.,ComCast,北京大学 研究生课程,68,Residential access:cable modems,Diagram:http:/