2 Chapter 1 Introduction All material copyright 1996-2009 J.F Kurose and K.W. Ross, All Rights Reserved5th edition Jim Kurose, Keith Ross Addison-Wesley, April 2009
3 Chapter 1: Introduction Goal:Get “feel” and terminologyMore depth, detail later in courseApproach:use Internet as exampleOverview:What’s the Internet?What’s a protocol?Network edge; hosts, access net, physical mediaNetwork core: packet/circuit switching, Internet structurePerformance: loss, delay, throughputSecurityProtocol layers, service modelshistory
4 Chapter 1: Roadmap 1.1 What is the Internet? 1.2 Network edge end systems, access networks, links1.3 Network corecircuit switching, packet switching, network structure1.4 Delay, loss and throughput in packet-switched networks1.5 Protocol layers, service models1.6 Networks under attack: security1.7 History
5 What’s the Internet: “Nuts and Bolts” view PCserverwirelesslaptopcellularhandheldMillions of connected computing devices: hosts = end systemsrunning network appsHome networkInstitutional networkMobile networkGlobal ISPRegional ISPCommunication linksfiber, copper, radio, satellitetransmission rate = bandwidthwiredlinksaccesspointsRouters: forward packets (chunks of data)router
6 “Cool” Internet Appliances Web-enabled toaster +weather forecasterIP picture frameWorld’s smallest Web serverInternet phones
7 What’s the Internet: “nuts and bolts” view Protocols control sending, receiving of msgse.g., TCP, IP, HTTP, Skype, EthernetInternet: “network of networks”loosely hierarchicalInternet standardsDeveloped by IETF: Internet Engineering Task ForceIETF produces RFCs: Request for commentsMore than 5000!IEEE for links (e.g )Home networkInstitutional networkMobile networkGlobal ISPRegional ISP
8 What’s the Internet: a Service View Communication infrastructure enables distributed applications:Web, VoIP, , games, e-commerce, file sharingCommunication services provided to apps:Reliable data delivery from source to destination“best effort” (unreliable) data delivery
9 What’s a Protocol? Human protocols: “What’s the time?” Say “hi” first“I have a question”Raise hand first… specific msgs sent… specific actions taken when msgs received, or other eventsNetwork protocols:Machines rather than humansAll communication activity in Internet governed by protocolsProtocols: 1) define format, 2) order of msgs sent and received among network entities, and 3) actions taken on msg transmission, receipt
10 What’s a Protocol? A human protocol and a computer network protocol: HiTCP connectionrequestHiTCP connectionresponseGot thetime?Get2:00<file>timeQ: Other human protocols? Other network protocols?
11 Chapter 1: Roadmap 1.1 What is the Internet? 1.2 Network edge end systems, access networks, links1.3 Network corecircuit switching, packet switching, network structure1.4 Delay, loss and throughput in packet-switched networks1.5 Protocol layers, service models1.6 Networks under attack: security1.7 History
12 A Closer Look at Network Structure Network edge: applications and hostsAccess networks, physical media: wired, wireless communication linksNetwork coreinterconnected routersnetwork of networks
13 The Network Edge End systems (hosts): Client/server model run application programse.g. Web,at “edge of network”peer-peerclient/serverClient/server modelclient host requests, receives service from always-on servere.g. Web browser/server; client/serverPeer-peer model:minimal (or no) use of dedicated serverse.g. Skype, BitTorrent
14 Access Networks and Physical Media 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?
15 Dial-up Modem Uses existing telephony infrastructure telephonenetworkInternethome dial-upmodemISP modem(e.g., AOL)homePCcentral officeUses existing telephony infrastructureHome is connected to central officeup to 56Kbps direct access to router (often less)Can’t surf and phone at same time: not “always on”
16 Digital Subscriber Line (DSL) telephonenetworkDSLmodemhomePCphoneInternetDSLAMExisting phone line: 0-4KHz phone; 4-50KHz upstream data; 50KHz-1MHz downstream datasplittercentralofficeAlso uses existing telephone infrastrutureup to 1 Mbps upstream (today typically < 256 kbps)up to 8 Mbps downstream (today typically < 1 Mbps)dedicated physical line to telephone central office
17 Cable Modems Does not use telephone infrastructure Instead uses cable TV infrastructureHFC: hybrid fiber coaxasymmetric: up to 30 Mbps downstream, 2 Mbps upstreamNetwork of cable and fiber attaches homes to ISP routerHomes share access to router (500 to 5,000 homes)Unlike DSL, which has dedicated accesscable headendhomecable distributionnetwork (simplified)
18 Cable Network Architecture: Overview Typically 500 to 5,000 homescable headendhomecable distributionnetwork (simplified)
22 Fiber to the Home OLT Optical links from central office to the home ONTOLTcentral officeopticalsplitteroptical fiberoptical fibersInternetOptical links from central office to the homeMuch higher Internet rates; fiber also carries television and phone services
23 Ethernet Internet access 100 Mbps1 GbpsserverEthernetswitchInstitutionalrouterTo Institution’s ISPTypically used in companies, universities, etc10 Mbps, 100 Mbps, 1 Gbps, 10 Gbps EthernetToday, end-systems typically connect into Ethernet switch
24 Wireless Access Networks Shared wireless access network connects end system to routervia base station, aka “access point” (AP)Wireless LANs:802.11b/g (WiFi): 11 or 54 MbpsWider-area wireless accessProvided by telco operator~1Mbps over cell phone system (EVDO, HSDPA)next up (?): WiMAX (10’s Mbps) over wide arearouterbasestationmobilehosts
25 Home Networks Typical home network components: DSL or Cable modem Router/firewall/NATEthernetWireless accesspointwirelesslaptops/devicesto/fromcableheadendcablemodemrouter/firewallwirelessaccesspointEthernet
26 Physical Media: Twisted Pair Twisted Pair (TP)Two insulated copper wiresCategory 3: traditional phone wires, 10 Mbps EthernetCategory 5: 100 Mbps EthernetBit: propagates between transmitter/rcvr pairsPhysical link: what lies between transmitter & receiverGuided media:signals propagate in solid media: copper, fiber, coaxUnguided media:signals propagate freely, e.g., radio
27 Physical Media: Coax, Fiber Fiber optic cable:glass fiber carrying light pulses, each pulse a bithigh-speed operation:high-speed point-to-point transmission (e.g., 10’s-100’s Gps)low error rate: repeaters spaced far apart ; immune to electromagnetic noiseCoaxial cable:Two concentric copper conductorsBidirectionalBaseband:single channel on cablelegacy EthernetBroadband:multiple channels on cableHFC
28 Physical Media: Radio Radio link types: terrestrial microwaveup to 45 Mbps channelsLAN (e.g., Wifi)802.11b - 11 Mbps801.11g - 54 MbpsWide-area (e.g., cellular)3G cellular ~ 1 MbpsSatelliteKbps to 45Mbps channel (or multiple smaller channels)270 msec end-end delaygeosynchronous versus low altitudeSignal carried in electromagnetic spectrumNo physical “wire”BidirectionalPropagation environment effects:reflectionobstruction by objectsinterference
29 Chapter 1: Roadmap 1.1 What is the Internet? 1.2 Network edge end systems, access networks, links1.3 Network corecircuit switching, packet switching, network structure1.4 Delay, loss and throughput in packet-switched networks1.5 Protocol layers, service models1.6 Networks under attack: security1.7 History
30 The Network Core Mesh of interconnected routers The fundamental question: how is data transferred through net?circuit switching: dedicated circuit per callE.g. telephonepacket-switching: data sent thru net in discrete “chunks”E.g. postal mailQ: human analogies of reserved resources (circuit switching) versus on-demand allocation (packet-switching)?
31 Network Core: Circuit Switching End-end resources reserved for “call”link bandwidth, switch capacitydedicated resources: no sharingcircuit-like (guaranteed) performancecall setup required
32 Network Core: Circuit Switching network resources (e.g., bandwidth) divided into “pieces”pieces allocated to callsresource piece idle if not used by owning call (no sharing)dividing link bandwidth into “pieces”frequency division multiplexing (FDM)Radio frequency MHzPhone 4kHztime division multiplexing (TDM)
33 Circuit Switching: FDM and TDM 4 usersExample:FDMfrequencytimeTDMfrequencytimeTwo simple multiple access control techniques.Each mobile’s share of the bandwidth is divided into portions for the uplink and the downlink. Also, possibly, out of band signaling.As we will see, used in AMPS, GSM, IS-54/136
34 Numerical ExampleHow long does it take to send a file of 80 Kbytes from host A to host B over a circuit-switched network?All links are MbpsEach link uses TDM with 24 slots/sec500 msec to establish end-to-end circuitLet’s work it out!
35 Numerical Example: Solution 80 Kbytes is 640,000 KbitsNOTE: networks in bits, end systems in bytesNOTE: 1 Kbyte = 1024 bytes, 1Kbit = 1000 bitsEach circuit has a rate of / 24 = 64 KbpsSo, it takes 640,000 bits / 64 Kbps = 10 seconds to transmit the fileNeed to add the circuit establishment time (½ second)So, 10.5 seconds
36 Network Core: Packet Switching Each end-end data stream divided into packetsuser A, B packets share network resourceseach packet uses full link bandwidthresources used as neededResource contention:aggregate resource demand can exceed amount availablecongestion: packets queue, wait for link usestore and forward: packets move one hop at a timeNode receives complete packet before forwardingBandwidth division into “pieces”Dedicated allocationResource reservation
37 Packet Switching: Statistical Multiplexing 100 Mb/sEthernetCAstatistical multiplexing1.5 Mb/sBqueue of packetswaiting for outputlinkDESequence of A & B packets does not have fixed pattern, bandwidth shared on demand statistical multiplexing.
38 Packet-switching: Store-and-Forward LRRRTakes L/R seconds to transmit (push out) packet of L bits on to link at R bpsStore and forward: entire packet must arrive at router before it can be transmitted on next linkdelay = 3 L/R (assuming zero propagation delay)Example:3 hops (end plus 2 routers)L = 7.5 MbitsR = 1.5 Mbpstransmission delay= 3 * 7.5 / 1.5= 15 secmore on delay shortly …
39 Packet Switching versus Circuit Switching Packet switching can allow more users to use network!1 Mb/s linkEach user:100 kb/s when “active”active 10% of timecircuit-switching:10 userspacket switching:With 35 users, probability 10+ active at same time is less than .0004N users1 Mbps linkQ: how did we get value ?
40 Packet Switching versus Circuit Switching Packet switching can give individual users better performance!Consider3 users,TDM with 1000 bit slot, 1 slot per 10 msecUsers quiet, then one user kbit packetsWith TDM, will take 10 seconds to transmitWith packet switch, user can take all (1 Mbps) and transmit in about 1 second
41 Packet Switching versus Circuit Switching Is packet switching a “slam dunk” winner?Great for bursty dataResource sharingSimpler, no call setupBut… can have excessive congestion: packet delay and lossProtocols needed for reliable data transfer, congestion controlQ: How to provide circuit-like behavior?Bandwidth guarantees needed for audio/video appsStill an unsolved problem (chapter 7)
42 Internet structure: Network of Networks Roughly hierarchicalAt center: “tier-1” ISPs (e.g., Verizon, Sprint, AT&T, Cable and Wireless), national/international coveragetreat each other as equalsTier 1 ISPTier-1 providers interconnect (peer) privatelyTier 1 ISPTier 1 ISP
44 Internet structure: network of networks “Tier-2” ISPs: smaller (often regional) ISPsConnect to one or more tier-1 ISPs, possibly other tier-2 ISPsTier-2 ISPs also peer privately with each other.Tier-2 ISPTier-2 ISP pays tier-1 ISP for connectivity to rest of Internettier-2 ISP is customer oftier-1 providerTier 1 ISPTier 1 ISPTier 1 ISP
45 Internet structure: network of networks “Tier-3” ISPs and local ISPslast hop (“access”) network (closest to end systems)localISPTier 3Local and tier- 3 ISPs are customers ofhigher tier ISPsconnecting them to rest of InternetTier-2 ISPTier 1 ISPTier 1 ISPTier 1 ISP
46 Internet structure: network of networks a packet passes through many networks!Tier 1 ISPTier-2 ISPlocalISPTier 3
47 Chapter 1: Roadmap 1.1 What is the Internet? 1.2 Network edge end systems, access networks, links1.3 Network corecircuit switching, packet switching, network structure1.4 Delay, loss and throughput in packet-switched networks1.5 Protocol layers, service models1.6 Networks under attack: security1.7 History
48 How do Loss and Delay Occur? Packets queue in router bufferspacket arrival rate to link exceeds output link capacitypackets queue, wait for turn (delay)Queue is full, packets are dropped (loss)packet being transmitted (delay)Afree (available) buffers: arriving packetsdropped (loss) if no free bufferspackets queueing (delay)B
49 Four Sources of Packet Delay 1. Nodal processing:check bit errorsdetermine output link2. Queueingtime waiting at output link for transmissiondepends on congestion level of routerABpropagationtransmissionnodalprocessingqueueing
50 Delay in Packet-switched Networks 3. Transmission delay:R=link bandwidth (bps)L=packet length (bits)time to send bits into link = L/R4. Propagation delay:d = length of physical links = propagation speed in medium (~2x108 m/sec)propagation delay = d/sNote: s and R are very different quantities!ABpropagationtransmissionnodalprocessingqueueing
51 Caravan Analogytollboothten-carcaravan100 kmCar~bit; caravan~packetCars “propagate” at 100 km/hrToll booth takes 12 sec to service car (transmission time)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 secTime for last car to propagate from 1st to 2nd toll both: 100km/(100km/hr)= 1 hrA: 62 minutes
52 Caravan Analogy (more) tollboothten-carcaravan100 kmCars now “propagate” at km/hrToll booth now takes 1 min to service a carQ: Will cars arrive to 2nd booth before all cars serviced at 1st booth?Yes! After 7 min, 1st car 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 book Web site)
53 Nodal delay dproc = processing delay dqueue = queuing delay typically a few microsecs or lessdqueue = queuing delaydepends on congestiondtrans = transmission delay= L/R, significant for low-speed linksdprop = propagation delaya few microsecs to hundreds of msecs
54 Queueing Delay (revisited) R=link bandwidth (bps)L=packet length (bits)a=average packet arrival ratetraffic intensity = La/RLa/R ~ 0: average queueing delay smallLa/R -> 1: delays become largeLa/R > 1: more “work” arriving than can be serviced, average delay infinite!
55 “Real” Internet Delays and Routes What do “real” Internet delay & loss look like?Traceroute program: provides delay measurement 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 destinationrouter i will return packets to sendersender times interval between transmission and reply.3 probes3 probes3 probes
56 “Real” Internet Delays and Routes traceroute: gaia.cs.umass.edu toThree delay measurements fromgaia.cs.umass.edu to cs-gw.cs.umass.edu1 cs-gw ( ) 1 ms 1 ms 2 ms2 border1-rt-fa5-1-0.gw.umass.edu ( ) 1 ms 1 ms 2 ms3 cht-vbns.gw.umass.edu ( ) 6 ms 5 ms 5 ms4 jn1-at wor.vbns.net ( ) 16 ms 11 ms 13 ms5 jn1-so wae.vbns.net ( ) 21 ms 18 ms 18 ms6 abilene-vbns.abilene.ucaid.edu ( ) 22 ms 18 ms 22 ms7 nycm-wash.abilene.ucaid.edu ( ) 22 ms 22 ms 22 ms( ) 104 ms 109 ms 106 ms9 de2-1.de1.de.geant.net ( ) 109 ms 102 ms 104 ms10 de.fr1.fr.geant.net ( ) 113 ms 121 ms 114 ms11 renater-gw.fr1.fr.geant.net ( ) 112 ms 114 ms 112 ms12 nio-n2.cssi.renater.fr ( ) 111 ms 114 ms 116 ms13 nice.cssi.renater.fr ( ) 123 ms 125 ms 124 ms14 r3t2-nice.cssi.renater.fr ( ) 126 ms 126 ms 124 ms15 eurecom-valbonne.r3t2.ft.net ( ) 135 ms 128 ms 133 ms( ) 126 ms 128 ms 126 ms17 * * *18 * * *19 fantasia.eurecom.fr ( ) 132 ms 128 ms 136 mstrans-oceaniclink* means no response (probe lost, router not replying)
57 Packet LossQueue (aka buffer) preceding link in buffer has finite capacityPacket arriving to full queue dropped (aka lost)Lost packet may be retransmitted by previous node, by source end system, or not at allbuffer(waiting area)packet being transmittedABpacket arriving tofull buffer is lost
58 ThroughputThroughput: rate (bits/time unit) at which bits transferred between sender/receiverInstantaneous: rate at given point in timeAverage: rate over longer period of timepipe that can carryfluid at rateRc bits/sec)pipe that can carryfluid at rateRs bits/sec)link capacityRs bits/seclink capacityRc bits/secserver sends bits(fluid) into pipeserver, withfile of F bitsto send to client
59 Throughput (more) Rs < Rc What is average end-end throughput? Rc bits/secRs bits/secRs > Rc What is average end-end throughput?Rs bits/secRc bits/seclink on end-end path that constrains end-end throughputbottleneck link
60 Throughput: Internet Scenario RsPer-connection end-end throughput: min(Rc,Rs,R/10)In practice: Rc or Rs is often bottleneck“last mile” connectionRsRsRRcRcRc10 connections (fairly) share backbone bottleneck link R bits/sec
61 Chapter 1: Roadmap 1.1 What is the Internet? 1.2 Network edge end systems, access networks, links1.3 Network corecircuit switching, packet switching, network structure1.4 Delay, loss and throughput in packet-switched networks1.5 Protocol layers, service models1.6 Networks under attack: security1.7 History
62 Protocol “Layers” Question: Networks are complex! Many “pieces”: hosts routerslinks of various mediaapplicationsprotocolshardware, softwareQuestion:Is there any hope of organizing structure of network?Or at least our discussion of networks?
63 Organization of air travel ticket (purchase)baggage (check)gates (load)runway takeoffairplane routingticket (complain)baggage (claim)gates (unload)runway landinga series of steps
64 Layering of Airline Functionality ticket (purchase)baggage (check)gates (load)runway (takeoff)airplane routingdepartureairportarrivalintermediate air-trafficcontrol centersticket (complain)baggage (claimgates (unload)runway (land)ticketbaggagegatetakeoff/landingLayers: each layer implements a servicevia its own internal-layer actionsrelying on services provided by layer below
65 Why Layering? Dealing with complex systems: explicit structure allows identification, relationship of complex system’s pieceslayered reference model for discussionmodularization eases maintenance, updating of systemchange of implementation of layer’s service transparent to rest of systeme.g., change in gate procedure doesn’t affect rest of systemlayering considered harmful?May need info from other layer (e.g. rate)May be redundant functions (e.g. error check)
66 Internet Protocol Stack application: supporting network applicationsFTP, SMTP, HTTPtransport: process-process data transfer (segment)TCP, UDPnetwork: routing of datagrams from source to destinationIP, routing protocolslink: data transfer between neighboring network elements (frames)PPP, Ethernetphysical: bits “on the wire”applicationtransportnetworklinkphysical
67 ISO/OSI Reference Model presentation: allow applications to interpret meaning of data, e.g., encryption, compression, machine-specific conventionssession: synchronization, checkpointing, recovery of data exchangeInternet stack “missing” these layers!these services, if needed, must be implemented in applicationneeded?applicationpresentationsessiontransportnetworklinkphysical
68 Encapsulation source destination application transport network link messageMapplicationtransportnetworklinkphysicalsegmentHtMHtdatagramHtHnMHnframeHtHnHlMlinkphysicalswitchdestinationnetworklinkphysicalHtHnMHtHnHlMMapplicationtransportnetworklinkphysicalHtHnMHtMHtHnMrouterHtHnHlM
69 Chapter 1: Roadmap 1.1 What is the Internet? 1.2 Network edge end systems, access networks, links1.3 Network corecircuit switching, packet switching, network structure1.4 Delay, loss and throughput in packet-switched networks1.5 Protocol layers, service models1.6 Networks under attack: security1.7 History
70 Network Security The field of network security is about: how bad guys can attack computer networkshow we can defend networks against attackshow to design architectures that are immune to attacksInternet not originally designed with (much) security in mindoriginal vision: “a group of mutually trusting users attached to a transparent network” Internet protocol designers playing “catch-up”Security considerations in all layers!
71 Bad guys can put malware into hosts via Internet Malware can get in host from a virus, worm, or trojan horse.(More on next slide)Spyware malware can record keystrokes, Web sites visited, upload info to collection siteInfected host can be enrolled in a botnet, used for spam and DDoS attacksMalware is often self-replicating: from an infected host, seeks entry into other hosts
72 Bad guys can put malware into hosts via Internet Trojan horseHidden part of some otherwise useful softwareToday often on a Web page (Active-X, plugin)Virusinfection by receiving object (e.g., attachment), actively executingself-replicating: propagate itself to other hosts, usersWorm:infection by passively receiving object that gets itself executedself- replicating: propagates to other hosts, usersSapphire Worm: aggregate scans/secin first 5 minutes of outbreak (CAIDA, UWisc data)
73 Bad guys can attack servers and network infrastructure Denial of service (DoS): attackers make resources (server, bandwidth) unavailable to legitimate traffic by overwhelming resource with bogus traffictargetselect targetbreak into hosts around the network (see botnet)send packets toward target from compromised hosts
74 The bad guys can sniff packets Packet sniffing:broadcast media (shared Ethernet, wireless)promiscuous network interface reads/records all packets (e.g., including passwords!) passing byACsrc:B dest:A payloadBWireshark software used for end-of-chapter labs is a (free) packet-sniffer
75 The bad guys can use false source addresses IP spoofing: send packet with false source addressACsrc:B dest:A payloadB
76 The bad guys can record and playback record-and-playback: sniff sensitive info (e.g., password), and use laterpassword holder is that user from system point of viewCAsrc:B dest:A user: B; password: fooB
77 Network SecurityA bit in this courseChapter 8: focus on security
78 Chapter 1: roadmap 1.1 What is the Internet? 1.2 Network edge end systems, access networks, links1.3 Network corecircuit switching, packet switching, network structure1.4 Delay, loss and throughput in packet-switched networks1.5 Protocol layers, service models1.6 Networks under attack: security1.7 History
79 Internet History 1961-1972: Early packet-switching principles 1961: Kleinrock - queueing theory shows effectiveness of packet-switching1964: Baran - packet-switching in military nets1967: ARPAnet conceived by Advanced Research Projects Agency1969: first ARPAnet node operational1972:ARPAnet public demonstrationNCP (Network Control Protocol) first host-host protocolfirst programARPAnet has 15 nodes
80 Internet History 1972-1980: Internetworking, new and proprietary nets 1970: ALOHAnet satellite network in Hawaii1974: Cerf and Kahn - architecture for interconnecting networks1976: Ethernet at Xerox PARCate70’s: proprietary architectures: DECnet, SNA, XNAlate 70’s: switching fixed length packets (ATM precursor)1979: ARPAnet has 200 nodesCerf and Kahn’s internetworking principles:minimalism, autonomy - no internal changes required to interconnect networksbest effort service modelstateless routersdecentralized controldefine today’s Internet architecture
81 Internet History 1980-1990: new protocols, a proliferation of networks 1983: deployment of TCP/IP1982: smtp protocol defined1983: DNS defined for name-to-IP-address translation1985: ftp protocol defined1988: TCP congestion controlnew national networks: Csnet, BITnet, NSFnet, Minitel100,000 hosts connected to confederation of networks
82 Internet History 1990, 2000’s: commercialization, the Web, new apps Early 1990’s: ARPAnet decommissioned1991: NSF lifts restrictions on commercial use of NSFnet (decommissioned, 1995)early 1990s: Webhypertext [Bush 1945, Nelson 1960’s]HTML, HTTP: Berners-Lee1994: Mosaic, later Netscapelate 1990’s: commercialization of the WebLate 1990’s – 2000’s:more killer apps: instant messaging, P2P file sharingnetwork security to forefrontest. 50 million hosts, 100 million+ usersbackbone links running at Gbps
83 Internet History 2007: ~500 million hosts Voice, Video over IP P2P applications: BitTorrent (file sharing) Skype (VoIP), PPLive (video)more applications: YouTube, gamingwireless, mobility
84 Introduction: Summary Covered a “ton” of material!Internet overviewwhat’s a protocol?network edge, core, access networkpacket-switching versus circuit-switchingInternet structureperformance: loss, delay, throughputlayering, service modelssecurityhistoryYou now have:context, overview, “feel” of networkingmore depth, detail to follow!