Introduction 1-1
COMP 561: “Computer Networks”
Qian ZhangSpring 2008
HKUST
Introduction 1-2
Ice-Breaking
Introduction 1-3
Course Info
Instructor: Qian Zhang www.cs.ust.hk/~qianzh
Course web sitehttp://www.cs.ust.hk/~qianzh/comp561/spr2008/index.htmlcontains all notes, announcements, etc. Check it regularly!Lecture schedule
Tuesday/Thursday 13:30-14:50 Rm 3598
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Course Info
Textbook: James Kurose and Keith RossComputer Networking: A Top Down Approach, 4th ed. Addison Wesley, 2007http://wps.aw.com/aw_kurose_network_4/ with useful resource material
The reading materials online for paper reading and student presentationExperience networking research through team projects (1-2 students)
Understand what is good researchHands-on experience in networking researchAppreciate team work / collaborations
Introduction 1-5
Course Info
Grading schemeHomework (2) 20 pointsProject (1) 25 pointsPresentation 20 pointsFinal Exam 35 points
Paper presentationEveryone reviews and presents 1 paperEmail me ids of 3 papers that you’d like to present by Feb. 28Submit a review for one paper of your choice before you present the paper (1 page)
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Course Schedule
Introduction of computer networkingApplication layerTransport layerNetworking layerMulticast and peer-to-peerMultimedia networking Mobile and wireless computing Self-organized wireless networks Student presentation
Introduction 1-7
Chapter 1Introduction
Computer Networking: A Top Down Approach ,4th edition. Jim Kurose, Keith RossAddison-Wesley, July 2007.
A note on the use of these ppt slides:The notes used in this course are substantially based on powerpoint slides developed and copyrighted by J.F. Kurose and K.W. Ross, 2007
Introduction 1-8
Chapter 1: IntroductionOur goal:
Get “feel” and terminologyMore depth, detail later in courseApproach:
use Internet as example
Overview: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
Introduction 1-9
Chapter 1: roadmap
1.1 What is the Internet?1.2 Network edge
End systems, access networks, links1.3 Network core
Circuit 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
Introduction 1-10
What’s the Internet: “nuts and bolts” view
Millions of connected computing devices: hosts = end systems
Running network apps Home network
Institutional network
Mobile network
Global ISP
Regional ISP
router
PC
server
wirelesslaptopcellular handheld
wiredlinks
access points
Communication linksFiber, copper, radio, satelliteTransmission rate = bandwidth
Routers: forward packets (chunks of data)
Introduction 1-11
What’s the Internet: “nuts and bolts” view
Protocols control sending, receiving of msgs
E.g., TCP, IP, HTTP, Skype, Ethernet
Internet: “network of networks”
Loosely hierarchicalPublic Internet versus private intranet
Internet standardsRFC: Request for commentsIETF: Internet Engineering Task Force
Home network
Institutional network
Mobile network
Global ISP
Regional ISP
Introduction 1-12
What’s the Internet: A Service ViewCommunication infrastructure enables distributed applications:
Web, VoIP, email, games, e-commerce, file sharing
Communication services provided to apps:
Reliable data delivery from source to destination“Best effort” (unreliable) data delivery
Introduction 1-13
What’s a Protocol?Human protocols:
“What’s the time?”“I have a question”Introductions
… specific msgs sent… specific actions taken
when msgs received, or other events
Network protocols:Machines rather than humansAll communication activity in Internet governed by protocols
Protocols define format, order of msgs sent and received among network
entities, and actions taken on msg
transmission, receipt
Introduction 1-14
What’s a Protocol?A human protocol and a computer network protocol:
Q: Other human protocols?
Hi
Hi
Got thetime?
2:00
TCP connectionrequest
TCP connectionresponse
Get http://www.awl.com/kurose-ross
<file>time
Introduction 1-15
Chapter 1: Roadmap
1.1 What is the Internet?1.2 Network edge
End systems, access networks, links1.3 Network core
Circuit 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
Introduction 1-16
A Closer Look at Network Structure:Network edge:applications and hosts
Access networks, physical media:wired, wireless communication links
Network core:Interconnected routersNetwork of networks
Introduction 1-17
The Network Edge:End systems (hosts):
Run application programsE.g. Web, emailAt “edge of network”
client/server
peer-peer
Client/server modelClient host requests, receives service from always-on serverE.g. Web browser/server; email client/server
Peer-peer model:Minimal (or no) use of dedicated serversE.g. Skype, BitTorrent
Introduction 1-18
Network Edge: Reliable Data Transfer Service
Goal: data transfer between end systemsHandshaking: setup (prepare for) data transfer ahead of time
Hello, hello back human protocolSet up “state” in two communicating hosts
TCP - Transmission Control Protocol
Internet’s reliable data transfer service
TCP service [RFC 793]Reliable, in-order byte-stream data transfer
Loss: acknowledgements and retransmissions
Flow control:Sender won’t overwhelm receiver
Congestion control:Senders “slow down sending rate” when network congested
Introduction 1-19
Network Edge: Best Effort (Unreliable) Data Transfer Service
Goal: data transfer between end systems
same as before!UDP - User Datagram Protocol [RFC 768]:
Connectionless Unreliable data transferNo flow controlNo congestion control
App’s using TCP:HTTP (Web), FTP (file transfer), Telnet (remote login), SMTP (email)
App’s using UDP:streaming media, teleconferencing, DNS, Internet telephony
Introduction 1-20
Access Networks and Physical Media
Q: How to connect end systems to edge router?Residential access netsInstitutional access networks (school, company)Mobile access networks
Keep in mind: Bandwidth (bits per second) of access network?Shared or dedicated?
Introduction 1-21
Residential Access: Point to Point Access
Dialup via modemUp to 56Kbps direct access to router (often less)Can’t surf and phone at same time: can’t be “always on”
DSL: digital subscriber linedeployment: telephone company (typically)up to 1 Mbps upstream (today typically < 256 kbps)up to 8 Mbps downstream (today typically < 1 Mbps)dedicated physical line to telephone central office
Introduction 1-22
Residential Access: Cable Modems
HFC: hybrid fiber coaxAsymmetric: up to 30Mbps downstream, 2 Mbps upstreamIs shared broadcast medium
Network of cable and fiber attaches homes to ISP router
Homes share access to router Deployment: available via cable TV companies
Introduction 1-23
Company Access: Local Area Networks
Company/univ local area network (LAN) connects end system to edge routerEthernet:
10 Mbs, 100Mbps, 1Gbps, 10Gbps EthernetModern configuration: end systems connect into Ethernet switch
LANs: chapter 5
Introduction 1-24
Wireless Access NetworksShared wireless access network connects end system to router
Via base station aka “access point”
Wireless LANs:802.11b/g (WiFi): 11 or 54 Mbps
Wider-area wireless accessProvided by telco operator~1Mbps over cellular system (EVDO, HSDPA)Next up (?): WiMAX (10’s Mbps) over wide area
basestation
mobilehosts
router
Introduction 1-25
Wireless Technologies
WWAN (3G,4G?)
WLAN (Wi-Fi)
WPAN
WMAN (Wi-Max)
BluetoothUWBRFID
coverage
Introduction 1-26
Home NetworksTypical home network components:
ADSL or cable modemRouter/firewall/NATEthernetWireless access point
wirelessaccess point
wirelesslaptops
router/firewall
cablemodem
to/fromcable
headend
Ethernet
Introduction 1-27
Physical Media
Bit: propagates betweentransmitter/rcvr pairsPhysical link: what lies between transmitter & receiverGuided media:
Signals propagate in solid media: copper, fiber, coax
Unguided media:Signals propagate freely, e.g., radio
Twisted Pair (TP)Two insulated copper wires
Category 3: traditional phone wires, 10 Mbps EthernetCategory 5: 100Mbps Ethernet
Introduction 1-28
Physical Media: Coax, Fiber
Coaxial cable:Two concentric copper conductorsBidirectionalBaseband:
Single channel on cableLegacy Ethernet
Broadband:Multiple channels on cableHFC
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 noise
Introduction 1-29
Physical Media: Radio
Signal carried in electromagnetic spectrumNo physical “wire”BidirectionalPropagation environment effects:
Reflection Obstruction by objectsInterference
Multipath propagation
Signal at Sender
Signal at Receiver
Introduction 1-30
Physical Media: Radio
Radio link types:Terrestrial microwave
e.g. up to 45 Mbps channelsLAN (e.g., Wifi)
11Mbps, 54 MbpsWide-area (e.g., cellular)
e.g. 3G: hundreds of kbpsSatellite
Kbps to 45Mbps channel (or multiple smaller channels)270 msec end-end delayGeosynchronous versus low altitude
Introduction 1-31
Chapter 1: roadmap
1.1 What is the Internet?1.2 Network edge
End systems, access networks, links1.3 Network core
Circuit 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
Introduction 1-32
The Network CoreMesh of interconnected routers
The fundamental question: how is data transferred through net?
Circuit-switching:dedicated circuit per call: telephone netPacket-switching: data sent thru net in discrete “chunks”
Introduction 1-33
Network Core: Circuit Switching
End-end resources reserved for “call”Link bandwidth, switch capacityDedicated resources: no sharingCircuit-like (guaranteed) performanceCall setup required
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Network Core: Circuit SwitchingNetwork 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 divisionTime division
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Circuit Switching: FDM and TDM
FDM
frequency
timeTDM
frequency
time
4 usersExample:
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Network Core: Packet SwitchingEach end-end data stream
divided into packetsUser A, B packets sharenetwork resourcesEach packet uses full link bandwidth Resources used as needed
Resource contention:Aggregate resource demand can exceed amount availableCongestion: packets queue, wait for link useStore and forward: packets move one hop at a time
Node receives complete packet before forwarding
Bandwidth division into “pieces”Dedicated allocationResource reservation
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Packet Switching: Statistical Multiplexing
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
C100 Mb/sEthernet
1.5 Mb/s
D E
statistical multiplexing
queue of packetswaiting for output
link
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Packet-Switching: Store-and-Forward
Takes L/R seconds to transmit (push out) packet of L bits on to link or R bpsEntire packet must arrive at router before it can be transmitted on next link: store and forwardDelay = 3L/R (assuming zero propagation delay)
Example:L = 7.5 MbitsR = 1.5 Mbpsdelay = 15 sec
R R RL
more on delay shortly …
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Packet Switching versus Circuit Switching
Great for bursty dataResource sharingSimpler, no call setup
Excessive congestion: packet delay and lossProtocols needed for reliable data transfer, congestion control
Q: How to provide circuit-like behavior?Bandwidth guarantees needed for audio/video appsStill an unsolved problem (chapter 7)
Is packet switching a “slam dunk winner?”
Introduction 1-40
Internet Structure: Network of Networks
Roughly hierarchicalAt center: “tier-1” ISPs (e.g., Verizon, Sprint, AT&T, Cable and Wireless), national/international coverage
Treat each other as equals
Tier 1 ISP
Tier 1 ISP
Tier 1 ISP
Tier-1 providers interconnect (peer) privately
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Tier-1 ISP: e.g., Sprint
…
to/from customers
peering
to/from backbone
….
………
POP: point-of-presence
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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
Tier-2 ISPTier-2 ISP
Tier-2 ISP Tier-2 ISP
Tier-2 ISP
Tier-2 ISP pays tier-1 ISP for connectivity to rest of Internet
Tier-2 ISP is customer oftier-1 provider
Tier-2 ISPs also peer privately with each other.
Introduction 1-43
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
Tier-2 ISPTier-2 ISP
Tier-2 ISP Tier-2 ISP
Tier-2 ISP
localISPlocal
ISPlocalISP
localISP
localISP Tier 3
ISP
localISP
localISP
localISP
Local and tier-3 ISPs are customers ofhigher tier ISPsconnecting them to rest of Internet
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Internet Structure: Network of Networks
A packet passes through many networks!
Tier 1 ISP
Tier 1 ISP
Tier 1 ISP
Tier-2 ISPTier-2 ISP
Tier-2 ISP Tier-2 ISP
Tier-2 ISP
localISPlocal
ISPlocalISP
localISP
localISP Tier 3
ISP
localISP
localISP
localISP
Introduction 1-45
Chapter 1: roadmap
1.1 What is the Internet?1.2 Network edge
End systems, access networks, links1.3 Network core
Circuit 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
Introduction 1-46
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
packet being transmitted (delay)
packets queueing (delay)
free (available) buffers: arriving packets dropped (loss) if no free buffers
Introduction 1-47
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
A
Bnodal
processing queueing
Introduction 1-48
Delay in Packet-Switched Networks3. 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
propagation
transmission
Note: s and R are very different quantities!
A
Bnodal
processing queueing
Introduction 1-49
Caravan Analogy
Cars “propagate” at 100 km/hrToll booth takes 12 sec to service a car (transmission time)car~bit; caravan ~ packetQ: 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
toll booth
toll booth
ten-car caravan
100 km 100 km
Introduction 1-50
Caravan Analogy (more)
Cars now “propagate” at 1000 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 AWL Web site
toll booth
toll booth
ten-car caravan
100 km 100 km
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Nodal Delay
dproc = processing delayTypically a few microsecs or less
dqueue = queuing delayDepends on congestion level in the router
dtrans = transmission delay= L/R, significant for low-speed links
dprop = propagation delayA few microsecs to hundreds of msecs
proptransqueueprocnodal ddddd +++=
Introduction 1-52
Queueing Delay (revisited)
R=link bandwidth (bps)L=packet length (bits)a=average packet arrival rate
traffic intensity = La/R
La/R ~ 0: average queueing delay smallLa/R -> 1: delays become largeLa/R > 1: more “work” arriving than can be serviced, average delay infinite!
Introduction 1-53
“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 probes
3 probes
3 probes
Introduction 1-54
Packet Loss
Queue (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 all
A
B
packet being transmitted
packet arriving tofull buffer is lost
buffer (waiting area)
Introduction 1-55
ThroughputThroughput: rate (bits/time unit) at which bits transferred between sender/receiver
Instantaneous: rate at given point in timeAverage: rate over long(er) period of time
server, withfile of F bits
to send to client
link capacityRs bits/sec
link capacityRc bits/sec
pipe that can carryfluid at rateRs bits/sec)
pipe that can carryfluid at rateRc bits/sec)
server sends bits (fluid) into pipe
Introduction 1-56
Throughput (more)Rs < Rc What is average end-end throughput?
Rs bits/sec Rc bits/sec
Rs > Rc What is average end-end throughput?
Rs bits/sec Rc bits/sec
link on end-end path that constrains end-end throughputbottleneck link
Introduction 1-57
Throughput: Internet Scenario
10 connections (fairly) share backbone bottleneck link R bits/sec
Rs
RsRs
Rc
Rc
Rc
R
Per-connection end-end throughput: min(Rc,Rs,R/10)In practice: Rc or Rs is often bottleneck
Introduction 1-58
Chapter 1: roadmap
1.1 What is the Internet?1.2 Network edge
End systems, access networks, links1.3 Network core
Circuit 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
Introduction 1-59
Protocol “Layers”Networks are complex!
Many “pieces”:HostsRoutersLinks of various mediaApplicationsProtocolsHardware and software
Question:Is there any hope of organizing structure of
network?
Or at least our discussion of networks?
Introduction 1-60
Why Layering?Dealing with complex systems:
Explicit structure allows identification, relationship of complex system’s pieces
Layered reference model for discussionModularization eases maintenance, updating of system
Change of implementation of layer’s service transparent to rest of systemE.g., change in gate procedure doesn’t affect rest of system
Layering considered harmful?
Introduction 1-61
Internet Protocol StackApplication: supporting network applications
FTP, SMTP, HTTPTransport: process-process data transfer
TCP, UDPNetwork: routing of datagrams from source to destination
IP, routing protocolsLink: data transfer between neighboring network elements
PPP, EthernetPhysical: bits “on the wire”
application
transport
network
link
physical
Introduction 1-62
Chapter 1: roadmap
1.1 What is the Internet?1.2 Network edge
End systems, access networks, links1.3 Network core
Circuit 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
Introduction 1-63
Network SecurityAttacks on Internet infrastructure:
Infecting/attacking hosts: malware, spyware, worms, unauthorized access (data stealing, user accounts)Denial of service: deny access to resources (servers, link bandwidth)
Internet not originally designed with (much) security in mind
Original vision: “a group of mutually trusting users attached to a transparent network” ☺Internet protocol designers playing “catch-up”Security considerations in all layers!
Introduction 1-64
What Can Bad Guys Do: Malware?
Spyware:Infection by downloading web page with spywareRecords keystrokes, web sites visited, upload info to collection site
VirusInfection by receiving object (e.g., e-mail attachment), actively executingSelf-replicating: propagate itself to other hosts, users
Worm:Infection by passively receiving object that gets itself executedSelf-replicating: propagates to other hosts, usersSapphire Worm: aggregate scans/sec
in first 5 minutes of outbreak (CAIDA, UWisc data)
Introduction 1-65
Denial of Service AttacksAttackers make resources (server, bandwidth) unavailable to legitimate traffic by overwhelming resource with bogus traffic
1. Select target2. Break into hosts
around the network (see malware)
3. Send packets toward target from compromised hosts
target
Introduction 1-66
Sniff, Modify, Delete Your PacketsPacket sniffing:
Broadcast media (shared Ethernet, wireless)Promiscuous network interface reads/records all packets (e.g., including passwords!) passing by
A
B
C
src:B dest:A payload
Ethereal software used for end-of-chapter labs is a (free) packet-snifferMore on modification, deletion later
Introduction 1-67
Masquerade as youIP spoofing: send packet with false source address
A
B
C
src:B dest:A payload
Introduction 1-68
Masquerade as youIP spoofing: send packet with false source addressRecord-and-playback: sniff sensitive info (e.g., password), and use later
Password holder is that user from system point of view
A
B
C
src:B dest:A user: B; password: foo
Introduction 1-69
Masquerade as youIP spoofing: send packet with false source addressRecord-and-playback: sniff sensitive info (e.g., password), and use later
Password holder is that user from system point of view
A
B
later ….. C
src:B dest:A user: B; password: foo
Introduction 1-70
Chapter 1: roadmap
1.1 What is the Internet?1.2 Network edge
end systems, access networks, links1.3 Network core
circuit 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
Introduction 1-71
Internet History
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 operational
1961-1972: Early packet-switching principles
The first link in theInternet backbone
What was the first message ever sent on the Internet? (LOGIN)
We sent an “L” - did you get the “L”? YEP!We sent an “O” - did you get the “O”? YEP!We sent a “G” - did you get the “G”?
UCLA
SRI
crash
The first message: LO!
Introduction 1-72
Internet History
1972:ARPAnet public demonstrationNCP (Network Control Protocol) first host-host protocol First e-mail programARPAnet has 15 nodes
1961-1972: Early packet-switching principles
The Internet is Born!at UCLA on October 29, 1969What it looked like at the end of 1969
Introduction 1-73
Internet History
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 nodes
Cerf and Kahn’s internetworking principles:
minimalism, autonomy -no internal changes required to interconnect networksbest effort service modelstateless routersdecentralized control
define today’s Internet architecture
1972-1980: Internetworking, new and proprietary nets
Introduction 1-74
Internet History
1983: deployment of TCP/IP1982: SMTP e-mail protocol defined 1983: 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
Introduction 1-75
Internet History
Early 1990’s: ARPAnet decommissioned1991: NSF lifts restrictions on commercial use of NSFnet (decommissioned, 1995)Early 1990s: Web
Hypertext [Bush 1945, Nelson 1960’s]HTML, HTTP: Berners-Lee1994: Mosaic, later NetscapeLate 1990’s: commercialization of the Web
Late 1990’s – 2000’s:More killer apps: instant messaging, P2P file sharingNetwork security to forefrontEst. 50 million host, 100 million+ usersBackbone links running at Gbps
1990, 2000’s: commercialization, the Web, new apps
Introduction 1-76
Internet History
2007:~500 million hostsVoice, Video over IPP2P applications: BitTorrent (file sharing), Skype (VoIP), PPLive (video)More applications: YouTube, gamingWireless, mobility
Introduction 1-77
Introduction: SummaryCovered a “ton” of material!
Internet overviewWhat’s a protocol?Network edge, core, access network
Packet-switching versus circuit-switchingInternet structure
Performance: loss, delay, throughputLayering, service modelsSecurityHistory
You now have:Context, overview, “feel” of networkingMore depth, detail to follow!
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