1、Introduction,1-1,The Internet,A global network of computers who runs it? who pays for it? who invented the internet? How do the computers communicate? There are many levels of abstraction, just like speech,Introduction,1-2,Whats a protocol?,Hi,Hi,TCP connectionrequest,Introduction,1-3,Whats a protoc
2、ol?,protocols define format, order of msgs sent and received among network entities, and actions taken on msg transmission, receipt,Introduction,1-4,A closer look at network structure:,network edge: applications and hosts network core: routers network of networks access networks, physical media: com
3、munication links,Introduction,1-5,Distributed Applications,end systems (hosts): run application programs e.g. Web, email at “edge of network” client/server model client host requests, receives service from always-on server e.g. Web browser/server; email client/server peer-peer model:minimal (or no)
4、use of dedicated servers e.g. Skype, BitTorrent, KaZaA,Introduction,1-6,Connection-oriented Vs Connectionless Services,TCP: supports connection oriented services UDP: supports connectionless services whats the difference? Reliability Whats needed to make this difference? flow control: slows down whe
5、n receiver is slow congestion control: slows down when network is slow Trade-off? UDP is faster,Introduction,1-7,Connnection vs Connectionless services,Apps using TCP: HTTP (Web), FTP (file transfer), Telnet (remote login), SMTP (email)Apps using UDP: streaming media, teleconferencing, DNS, Internet
6、 telephony,Introduction,1-8,The Network Core,mesh of interconnected links,Introduction,1-9,Switching,How to transfer data between these four nodes? Reserved bandwidth On-demand bandwidth,Introduction,1-10,Circuit Switching,End-end resources reserved for “call” dedicated resources: no sharing circuit
7、-like (guaranteed) performance call setup required,Introduction,1-11,Circuit Switching: FDM and TDM,network resources (e.g., bandwidth) divided into “pieces”,Introduction,1-12,Numerical example,How long does it take to send a file of 640,000 bits from host A to host B over a circuit-switched network
8、? All links are 1.536 Mbps Each link uses TDM with 24 slots/sec 500 msec to establish end-to-end circuit1,536,000/24 = 64000 640,000/64,000=10 seconds 10 + .5 = 1-.5seconds,Introduction,1-13,Packet Switching,each end-end data stream divided into packets user A, B packets share network resources each
9、 packet uses full link bandwidth resources used as needed,disadvantages: aggregate resource demand can exceed amount available store and forward delay: packets move one hop at a time Node receives complete packet before forwarding Queue delay: must wait for link use,Introduction,1-14,Packet Switchin
10、g vs TDM,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.,A,B,C,100 Mb/s Ethernet,1.5 Mb/s,statistical multiplexing,queue of packets waiting for output link,Introduction,1-15,Propagation delay,Do w
11、e need to receive entire packet before forwarding? Takes L/R seconds to transmit (push out) packet of L bits on to link or R bps Entire packet must arrive at router before it can be transmitted on next link: store and forward delay = 3L/R (assuming zero propagation delay),Example: L = 640,000 bits R
12、 = 1.5 Mbps delay = 3*640,000/1.5=1.28s,R,R,R,L,Introduction,1-16,Propagation delay,1.28 is much faster than 10.5 seconds for circuit switched routing What if there were other users? -revisit this question later,R,R,R,L,Introduction,1-17,Packet switching versus circuit switching,Great for bursty dat
13、a resource sharing simpler, no call setup Excessive congestion: packet delay and loss protocols needed for reliable data transfer, congestion control Q: How to provide circuit-like behavior? bandwidth guarantees needed for audio/video apps still an unsolved problem (chapter 7),Is packet switching a
14、“slam dunk winner?”,Q: human analogies of reserved resources (circuit switching) versus on-demand allocation (packet-switching)?,Introduction,1-18,Chapter 1: roadmap,1.1 What is the Internet? 1.2 Distributed applications 1.3 Routing 1.4 Physical media 1.5 Internet structure 1.6 Delay & loss 1.7 Prot
15、ocol layers 1.8 History,Introduction,1-19,Access networks and physical media,Q: How to connect end systems together?,Introduction,1-20,Home networks,Typical home network components: ADSL or cable modem router/firewall/NAT Ethernet wireless accesspoint,wireless access point,wireless laptops,router/ f
16、irewall,cable modem,to/from cable headend,Ethernet,Introduction,1-21,Physical Media,Bit: propagates between transmitter/rcvr pairs physical link: what lies between transmitter & receiver guided media: signals propagate in solid media: copper, fiber, coax unguided media: signals propagate freely, e.g
17、., radio,Twisted Pair (TP) two insulated copper wires Category 3: traditional phone wires, 10 Mbps Ethernet Category 5: 100Mbps Ethernet,Introduction,1-22,Sharing the Medium,home,cable headend,cable distribution network (simplified),Ethernet protocol: Listen If channel is clear, send If collision, b
18、ack off,Introduction,1-23,Getting more data on the wire,home,cable headend,cable distribution network,FDM:,Introduction,1-24,Chapter 1: roadmap,1.1 What is the Internet? 1.2 Distributed Applications 1.3 Routing 1.4 Physical media 1.5 Internet structure 1.6 Delay & loss 1.7 Protocol layers 1.8 Histor
19、y,Introduction,1-25,Internet structure: network of networks,roughly hierarchical at center: “tier-1” ISPs (e.g., MCI, Sprint, AT&T, Cable and Wireless), national/international coverage treat each other as equals,Tier 1 ISP,Tier 1 ISP,Tier 1 ISP,Introduction,1-26,Tier-1 ISP: e.g., Sprint,Sprint US ba
20、ckbone network,Introduction,1-27,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 1 ISP,Introduction,1-28,Internet structure: network of networks,“Tier-3” ISPs and local ISPs
21、 last hop (“access”) network (closest to end systems),Tier 1 ISP,Tier 1 ISP,Tier 1 ISP,Introduction,1-29,Internet structure: network of networks,a packet passes through many networks!,Tier 1 ISP,Tier 1 ISP,Tier 1 ISP,Introduction,1-30,Chapter 1: roadmap,1.1 What is the Internet? 1.2 Network edge 1.3
22、 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,Introduction,1-31,How do loss and delay occur?,packets queue in router buffers packet arrival rate to link exceeds output l
23、ink capacity packets queue, wait for turn,A,B,Introduction,1-32,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,Introduction,1-33,Delay in packet-switched networks,
24、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 link s = propagation speed in medium (2x108 m/sec) propagation delay = d/s,Note: s and R are very different quantities!,Introduction,1-34,Caravan analog
25、y,Cars “propagate” at 100 km/hr Toll booth takes 12 sec to service a car (transmission time) carbit; caravan packet Q: How long until caravan is lined up before 2nd toll booth?,Time to “push” entire caravan through toll booth onto highway = 12*10 = 120 sec Time for last car to propagate from 1st to
26、2nd toll both: 100km/(100km/hr)= 1 hr A: 62 minutes,ten-car caravan,100 km,100 km,Introduction,1-35,Caravan analogy (more),Cars now “propagate” at 1000 km/hr Toll booth now takes 1 min to service a car Q: Will cars arrive to 2nd booth before all cars serviced at 1st booth?,Yes! After 7 min, 1st car
27、at 2nd booth and 3 cars still at 1st booth. 1st bit of packet can arrive at 2nd router before packet is fully transmitted at 1st router! See Ethernet applet at AWL Web site,ten-car caravan,100 km,100 km,Introduction,1-36,Nodal delay,dproc = processing delay typically a few microsecs or less dqueue =
28、 queuing delay depends on congestion dtrans = transmission delay = L/R, significant for low-speed links dprop = propagation delay a few microsecs to hundreds of msecs,Introduction,1-37,Queueing delay (revisited),R=link bandwidth (bps) L=packet length (bits) a=average packet arrival rate,traffic inte
29、nsity = La/R,La/R 0: average queueing delay small La/R - 1: delays become large La/R 1: more “work” arriving than can be serviced, average delay infinite!,Introduction,1-38,“Real” Internet delays and routes,What do “real” Internet delay & loss look like? Traceroute program: provides delay measuremen
30、t from source to router along end-end Internet path towards destination. For all i: sends three packets that will reach router i on path towards destination router i will return packets to sender sender times interval between transmission and reply.,3 probes,3 probes,3 probes,Introduction,1-39,“Real
31、” Internet delays and routes,1 cs-gw (128.119.240.254) 1 ms 1 ms 2 ms 2 border1-rt-fa5-1-0.gw.umass.edu (128.119.3.145) 1 ms 1 ms 2 ms 3 cht-vbns.gw.umass.edu (128.119.3.130) 6 ms 5 ms 5 ms 4 jn1-at1-0-0- (204.147.132.129) 16 ms 11 ms 13 ms 5 jn1-so7-0-0- (204.147.136.136) 21 ms 18 ms 18 ms 6 abilen
32、e-vbns.abilene.ucaid.edu (198.32.11.9) 22 ms 18 ms 22 ms 7 nycm-wash.abilene.ucaid.edu (198.32.8.46) 22 ms 22 ms 22 ms 8 62.40.103.253 (62.40.103.253) 104 ms 109 ms 106 ms 9 de2- (62.40.96.129) 109 ms 102 ms 104 ms 10 (62.40.96.50) 113 ms 121 ms 114 ms 11 renater- (62.40.103.54) 112 ms 114 ms 112 m
33、s 12 nio-n2.cssi.renater.fr (193.51.206.13) 111 ms 114 ms 116 ms 13 nice.cssi.renater.fr (195.220.98.102) 123 ms 125 ms 124 ms 14 r3t2-nice.cssi.renater.fr (195.220.98.110) 126 ms 126 ms 124 ms 15 eurecom- (193.48.50.54) 135 ms 128 ms 133 ms 16 194.214.211.25 (194.214.211.25) 126 ms 128 ms 126 ms 17
34、 * * * 18 * * * 19 fantasia.eurecom.fr (193.55.113.142) 132 ms 128 ms 136 ms,traceroute: gaia.cs.umass.edu to www.eurecom.fr,Three delay measurements from gaia.cs.umass.edu to cs-gw.cs.umass.edu,* means no response (probe lost, router not replying),trans-oceanic link,Introduction,1-40,Packet loss,qu
35、eue (aka buffer) preceding link in buffer has finite capacity when packet arrives to full queue, packet is dropped (aka lost) lost packet may be retransmitted by previous node, by source end system, or not retransmitted at all,Introduction,1-41,Chapter 1: roadmap,1.1 What is the Internet? 1.2 Networ
36、k 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,Introduction,1-42,Protocol “Layers”,Networks are complex! many “pieces”: hosts routers links of various media app
37、lications protocols hardware, software,Question: Is there any hope of organizing structure of network?Or at least our discussion of networks?,Introduction,1-43,Organization of air travel,a series of steps,Introduction,1-44,Layering of airline functionality,Layers: each layer implements a service via
38、 its own internal-layer actions relying on services provided by layer below,Introduction,1-45,Why layering?,Dealing with complex systems: explicit structure allows identification, relationship of complex systems pieces layered reference model for discussion modularization eases maintenance, updating
39、 of system change of implementation of layers service transparent to rest of system e.g., change in gate procedure doesnt affect rest of system layering considered harmful?,Introduction,1-46,Internet protocol stack,application: supporting network applications FTP, SMTP, HTTP transport: process-proce
40、ss data transfer TCP, UDP network: routing of datagrams from source to destination IP, routing protocols link: data transfer between neighboring network elements PPP, Ethernet physical: bits “on the wire”,Introduction,1-47,source,application transport network link physical,segment,datagram,destinati
41、on,application transport network link physical,router,switch,Encapsulation,message,frame,Introduction,1-48,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
42、 1.7 Protocol layers, service models 1.8 History,Introduction,1-49,Internet History,1961: Kleinrock - queueing theory shows effectiveness of packet-switching 1964: Baran - packet-switching in military nets 1967: ARPAnet conceived by Advanced Research Projects Agency 1969: first ARPAnet node operatio
43、nal,1972: ARPAnet public demonstration NCP (Network Control Protocol) first host-host protocol first e-mail program ARPAnet has 15 nodes,1961-1972: Early packet-switching principles,Introduction,1-50,Internet History,1970: ALOHAnet satellite network in Hawaii 1974: Cerf and Kahn - architecture for i
44、nterconnecting networks 1976: Ethernet at Xerox PARC ate70s: proprietary architectures: DECnet, SNA, XNA late 70s: switching fixed length packets (ATM precursor) 1979: ARPAnet has 200 nodes,Cerf and Kahns internetworking principles: minimalism, autonomy - no internal changes required to interconnect
45、 networks best effort service model stateless routers decentralized control define todays Internet architecture,1972-1980: Internetworking, new and proprietary nets,Introduction,1-51,Internet History,1983: deployment of TCP/IP 1982: smtp e-mail protocol defined 1983: DNS defined for name-to-IP-addre
46、ss translation 1985: ftp protocol defined 1988: TCP congestion control,new national networks: Csnet, BITnet, NSFnet, Minitel 100,000 hosts connected to confederation of networks,1980-1990: new protocols, a proliferation of networks,Introduction,1-52,Internet History,Early 1990s: ARPAnet decommission
47、ed 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 Netscape late 1990s: commercialization of the Web,Late 1990s 2000s: more killer apps: instant messaging, P2P file sharing
48、network security to forefront est. 50 million host, 100 million+ users backbone links running at Gbps,1990, 2000s: commercialization, the Web, new apps,Introduction,1-53,Introduction: Summary,Covered a “ton” of material! Internet overview whats a protocol? network edge, core, access network packet-switching versus circuit-switching Internet/ISP structure performance: loss, delay layering and service models history,
