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1、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 Arguments Internet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 Arguments Internet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 server http:/www-ccs.cs
2、.umass.edu/shri/iPic.html,IP picture frame http:/ 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, servers PDAs, phones, toasters running network apps network core: routers, switches forwarding data packets: packet switching call
3、s: circuit switching communication links fiber, 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 humans all comm
4、unication 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
5、程,9,A closer look at network structure,network edge: applications and hosts network core: routers network of networks access networks, physical media: communication links,北京大学 研究生课程,10,The network edge,end systems (hosts) run application programs e.g., WWW, email at “edge of network” client/server m
6、odel client host requests, receives service from server e.g., WWW client (browser)/ server; email client/server peer-peer model host interaction symmetric e.g.: BT, Maze, e-Donkey,北京大学 研究生课程,11,The network core,mesh of interconnected routers the fundamental question: how is data transferred through
7、net? circuit switching: dedicated circuit per call: telephone net packet 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 co
8、mmunication networks Information transmitted by any node is received by every other node in the network E.g., LANs (Ethernet, Wavelan) Problem: coordinate the access of all nodes to the shared communication medium (Multiple Access Problem) Switched communication networks Information is transmitted t
9、o a sub-set of designated nodes E.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 classif
10、ied based on the way in which the nodes exchange information:,A Taxonomy of Communication Networks,Circuit-Switched Communication Network,北京大学 研究生课程,15,Circuit Switching,Three phases circuit establishment data transfer circuit termination If circuit not available: “Busy signal” Examples Telephone ne
11、tworks ISDN (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 circui
12、t 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 slots Relative slot position inside a frame determines which conversation the data belongs to Needs synchronization between sender and rece
13、iver In case of non-permanent conversations Needs 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,Switched Communication Network,Broadc
14、ast Communication Network,Circuit-Switched Communication Network,Packet-Switched Communication 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 packets Packets have the following structure: Header and Trailer carry co
15、ntrol 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 allocate
16、d 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 time How to tell them apart? Use meta-data (header) to describe data,北京大学 研究生课程
17、,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 switched Each packet header contains destination address No resources are pre-allocated (re
18、served) in advance Example: 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 capacity pack
19、ets 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 errors determine output link,2. queueing time 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 b
20、andwidth (bps) L=packet length (bits) time to send bits into link = L/R,4. Propagation delay: d = length of physical link s = 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
21、 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 switching Data is transmitted as packets All packets from one packet stream are sent along a pre-established path (=virtual cir
22、cuit) Guarantees in-sequence delivery of packets However: Packets from different virtual circuits may be interleaved Example: 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 establishment data transfer VC disconnect Note:
23、 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,VC establishment,VC termination,Data transfer,北京大学 研究生课程,33,Virtual-Circuit Packet Switchin
24、g,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 multiplexing Efficient bandwidth usage ratio between peek and
25、 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 congestion More complex routers Harder to provide good network services (e.g., delay and bandwidth guarantees) In practice they are combined IP
26、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 Arguments Internet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 network Internet Protocol Open standard: Internet Engineering Task Force (IE
27、TF) as standard body http:/www.ietf.org Technical basis for other types of networks Intranet: enterprise IP network Developed by the research community,北京大学 研究生课程,38,Internet History,1961: Kleinrock - queueing theory shows effectiveness of packet-switching 1964: Baran - packet-switching in military
28、nets 1967: ARPAnet conceived by Advanced Research Projects Agency Larry Roberts 1969: first ARPAnet node operational,1972: ARPAnet demonstrated publicly NCP (Network Control Protocol) first host-host protocol first e-mail program ARPAnet has 15 nodes,1961-1972: Early packet-switching principles,北京大学
29、 研究生课程,39,Internet History,1970: ALOHAnet satellite network in Hawaii 1973: Metcalfes PhD thesis proposes Ethernet 1974: Cerf and Kahn - architecture for interconnecting networks late 70s: proprietary architectures: DECnet, SNA, XNA late 70s: switching fixed length packets (ATM precursor) 1979: ARPA
30、net has 200 nodes,Cerf and Kahns internetworking principles: minimalism, autonomy no internal changes required to interconnect networks best effort service model stateless routers decentralized control define todays Internet architecture,1972-1980: Internetworking, new and proprietary nets,北京大学 研究生课
31、程,40,Internet History,1982: SMTP e-mail protocol defined 1983: deployment of TCP/IP 1983: DNS defined for name-to-IP-address translation 1985: FTP protocol defined 1988: TCP congestion control,new national networks: Csnet, BITnet, NSFnet, Minitel 100,000 hosts connected to confederation of networks,
32、1980-1990: new protocols, a proliferation of networks,北京大学 研究生课程,41,Internet History,Early 1990s: ARPAnet decommissioned 1991: NSF lifts restrictions on commercial use of NSFnet (decommissioned, 1995) early 1990s: Web hypertext Bush 1945, Nelson 1960s HTML, HTTP: Berners-Lee 1994: Mosaic, later Nets
33、cape late 1990s: commercialization of the Web,Late 1990s 2000s: more killer apps: instant messaging, peer2peer file sharing (e.g., BT, Napster) network security to forefront est. 50 million host, 100 million+ users backbone links running at Gbps,1990, 2000s: commercialization, the Web, new apps,北京大学
34、 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 213 Oct. 1984 1,024 Dec. 1987 28,174 Oct. 1990 313,000 Oct. 1993 2,056,000 Apr. 1995 5,706,000 Jan. 1997 16,146,000 Jan. 1999 56,218,000 Jan. 2001 109,374,000
35、Jan. 2003 171,638,297 Jan. 2006 394,991,609 July 2007 489,774,269 Jan. 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 organizat
36、ion and over 20,000 individual members in over 180 countries It 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 Engineer
37、ing Task Force (IETF) The IETF is the protocol engineering and development arm of the Internet Subdivided 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 o
38、f a number of focused, long-term and small Research Groups Internet 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 G
39、roup (IESG) The IESG is responsible for technical management of IETF activities and the Internet standards process Composed 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 D
40、usseault, 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, Cis
41、co 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,I
42、nternet 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.org A typical (but not only) way of standardization is: BOF (Birds of a feather) Internet Drafts RFC Proposed Standard Draft St
43、andard (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 acce
44、ss to computing resources Telnet (1970s) Shared access to data/files FTP, NFS, AFS (1980s) Communication medium over which people interact Email (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 dissem
45、ination USENET (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 hierarchical at center: “tier-1” ISPs (e.g., UUNet, BBN/Genuity, Sprint, AT&T), national/international coverage treat each othe
46、r 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) ISPs Connect to one or more tier-1 ISPs, possibly other tier-2 ISPs,Tier 1 ISP,Tier 1 ISP,Tier
47、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
48、,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 nets institutional access networks (school, company) mobile access networks Keep in mind: bandwidth (bits per second) of access network? shared or dedicated?,北京大学 研究生课程,65,Residenti