INFO 330 Computer Networking Technology I

1
INFO 330Computer Networking Technology I

• Chapter 1
• Networking Overview
• Glenn Booker

2
Computer Networks

• A network is the structure that allows computer
  applications to communicate with each other
• The applications could be executed by the user,
  or part of the operating system
• Not every computer system is designed to allow
  networking
• Microsoft DOS had no native networking ability
  it was added after the need arose

3
The Internet

• The Internet is the primary model for
  understanding networking concepts because, well,
  nearly every computer and many other things could
  be connected to it
• Key parts of any network include
• Hosts or end systems, which are the computers and
  other things with which most people interact
• End user computers, workstations, and servers are
  all considered hosts

4
The Internet

• Communication links, which are the wired or
  wireless means used to connect to the network
• Packet switches, which help guide information
  between hosts
• Routers and link-layer switches are the primary
  types of packet switches

Graphics are taken from the texts lecture notes
5
The Internet

• The network sends chunks of information called
  packets along a route or path to get from one
  host to another
• The speed at which it does so is the transmission
  rate, typically in bits per second (bps)
• The control over choosing the path is known as
  packet switching
• End systems connect to the Internet through an
  Internet Service Provider (ISP)

6
The Internet

• ISPs provide many levels of service
• Residential or business service, typically from
  56kb dialup to DSL, FIOS, or cable modems
• The packets are defined and handled according to
  protocols, most notably the Transmission Control
  Protocol (TCP) and Internet Protocol (IP)

7
Protocols

• A protocol is a language for communication
• In order for it to work, both parties (e.g.
  hosts, switches, etc.) need to speak the same
  language oder Sie werden einander nicht verstehen
  or they wont understand each other
• Some protocols use a handshake concept
• Like saying Hi as a greeting, special messages
  are defined that request a connection, and reply
  to accept the connection

8
Protocols

• More formally, then, protocols define
• The format of messages (like the spelling of
  words)
• The order of messages (the syntax of sentences,
  or else your messages like Yoda will sound)
• Much of understanding networking is understanding
  how these protocols work

9
Source of Protocols

• Internet protocols are defined by the Internet
  Engineering Task Force (IETF)
• The IETF was created by the Internet Architecture
  Board (IAB) and also reports to the Internet
  Society (ISOC)
• The Request For Comments (RFCs) define the actual
  protocols
• The first RFC was dated April 1969
• As of September 2008, there are over 5300 RFCs

10
Internet vs Intranet

• The Internet (a proper noun, hence is
  capitalized) is the public network of zillions of
  computers, toasters, etc.
• An intranet (not a proper noun) is the generic
  term for a local private network that uses the
  same protocols as the Internet

11
Type of Internet Service

• The Internet runs distributed applications
• The World Wide Web, instant messaging,
  distributed games, etc. are all applications
• There are two choices for the type of service
  provided by an Internet connection
• A connection-oriented, reliable service
• A connection-less, unreliable service
• Neither guarantees how fast a message will get
  from host A to host B

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Connection-oriented, Reliable Service

• This establishes a loose connection between
  client and server, but not to the switches
  between them
• Key traits needed from this are
• Reliable data transfer every little bit counts
• Flow control to keep from overwhelming hosts
• Congestion control to avoid Internet gridlock
• TCP provides this service (see RFC 793)

13
Connection-less, Unreliable Service

• This service has no handshaking it just sends
  packets of data
• Dont know if packets ever got there
• No flow or congestion control
• Handled by User Datagram Protocol (UDP), RFC 768
• Use when speed is critical, such as video
  conferencing or Internet telephone

14
The Edge of the Network

• Now well examine the contents of the Internet
  from the outside in from the edge to the
  core
• Hosts (end systems) can be divided into clients
  and servers
• Clients are computers that request services from
  Servers
• One computer (host) can be multiple clients and
  servers at once (esp. in peer-to-peer
  applications)

15
Circuit vs Packet Switching

• In order to get a packet from host A to host B,
  two major approaches could be used
• Both approaches send packets over communication
  lines
• Circuit switching is what a traditional telephone
  system does
• Reserve a path from A to B which is the circuit
  messages will follow, until the connection is
  closed
• Packet switching is used by the Internet
• Dump packets into the network with no reserved
  path, and make a best effort to get packet to
  destination

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Circuit Switching

• To link host A and host B, each link between
  switches along the way must be reserved for the
  duration of that connection or circuit
• There are two ways to share links with many
  circuits
• Frequency-division multiplexing (FDM)
• Time-division multiplexing (TDM)

17
FDM and TDM

• FDM acts like FM radio it divides the link by
  frequency ranges, and assigns a frequency range
  to each circuit
• Typical frequency range, or bandwidth, is 4 kHz
• This way one link can handle many circuits
• TDM breaks the link into some number (n) of
  slots in a frame
• Each slot is dedicated to one circuit, so that
  circuit has full attention of the link 100/n
  percent of the time

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Bits and Bytes

• To review basic computer units
• A bit is a binary digit a 0 or 1
• Typically eight bits are a byte, the shortest
  word
• Old ASCII text files may use seven bits per byte,
  so there are 27 128 ASCII characters
• Transmission rate of data is given in bits per
  second (bps), or thousands or millions or
  billions of bits per second (kbps, Mbps, Gbps)
• Data transfer rate time
• Which has units of bits bits/sec sec

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TDM Example

• Suppose you have a 1.536 Mbps TDM connection, and
  want to send a 1 Mb (megabit) file the
  connection has 12 links
• How long does it take?
• Your transmission speed is 1/12 of the 1.536
  Mbps, or 0.128 Mbps
• Time data / rate 1 Mb / 0.128 Mbps 7.8125
  seconds
• This doesnt include time to make the connection

20
Packet Switching

• Messages are divided into packets before going
  into the network
• Most packet switches must receive an entire
  packet before forwarding it to the next switch
• This store-and-forward transmission introduces
  delays while the switch waits for the entire
  packet to get there
• If a packet size is L, and the transmission rate
  is R, the delay to receive one full packet is L/R

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Store and Forward Delay

• Assume 1) no queuing delay, 2) no time to make a
  connection, and 3) no delay to propagate packets
• Send a packet of L bits across a packet-switched
  network with Q links, all of which have a
  transmission rate of R bps
• For each link, the store and forward delay of L/R
  seconds this occurs Q times, for a total delay
  of QL/R seconds

22
Packet Switching

• Each switch typically connects to many links
• For each link, there is an output buffer (or
  output queue) to hold packets waiting to go on
  that link
• This introduces queuing delays, while a packet
  waits its turn
• If the buffer is full, the packet can be lost
  packet loss isnt good!

23
Statistical Multiplexing

• Compare circuit to packet switching
• Suppose users are active 10 of the time, sending
  100 kbps of data, and not using the connection
  the other 90 of the time
• If theres a 1 Mbps connection available
• TDM circuit switching would need 10 slots to
  allow each user 100 kbps

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Statistical Multiplexing

• Packet switching could handle 35 users total
  because the total number of active users will be
  11 or more only 0.04 of the time (look up the
  binomial distribution for details)
• The remaining 99.96 of the time, the total data
  rate is less than the 1 Mbps capacity of the
  connection
• Hence sharing resources on demand (which is
  statistical multiplexing) allows the same
  performance 99.96 of the time, for over three
  times the number of users!

25
Packet-Switched Networks

• There are two major kinds of packet-switched
  networks datagram networks and virtual-circuit
  networks
• A datagram network forwards packets according to
  the host destination address
• Hence the Internet is a datagram network
• Routers forward packets to make a best effort to
  get them to the destination address

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Virtual Circuit Networks

• A virtual circuit network forwards packets
  according to virtual circuit numbers
• A virtual circuit (VC) is an imaginary connection
  between the source and destination hosts
• Examples are X.25, frame relay, and asynchronous
  transfer mode (ATM)
• Each packet has a VC identifier (VC ID)
• Each packet switch indexes its VC translation
  table, and forwards the packet to the right
  outbound link

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Virtual Circuit Networks

• A key difference between datagram and VC networks
  is that VC networks have to maintain state
  information about connections
• Each new VC means a new entry has to be added to
  the VC translation table, and then is removed
  when the connection is ended
• It also needs to keep a table to map VC numbers
  to output interface numbers

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Datagram Networks

• Datagram networks are like the post office
• The contents of a message (like a letter or box)
  are only seen by the sender and recipient (we
  hope), and in between them, the postal service
  only looks at the recipients address, e.g. my
  address here is
• 306 Rush Hall3141 Chestnut StPhiladelphia, PA
  19104 USA

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Datagram Networks

• If a letter is mailed to me from outside the USA,
  the first thing they need to know is that the
  letter needs to go to America
• Then a machine reader finds the zip code, and
  knows the letter needs to go to Philadelphia,
  since 19104 is entirely within Philly
• The local letter carrier recognizes 3141 Chestnut
  St as the central location for all Drexel mail
• Someone within Drexel knows where 306 Rush Hall
  is, and carries the letter there

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Datagram Networks

• And the receptionist in 306 Rush Hall knows that
  Im full time faculty, and puts the letter in my
  mailbox
• Each step along the way, the letter is routed
  essentially by reading the address backward (USA
  - 19104 Philadelphia, PA 3141 Chestnut St
  306 Rush Hall Glenn Booker)
• Datagram networks do the same thing a packet of
  data is wrapped in layers of addresses, which are
  used by routers

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Datagram Networks

• Notice that datagram networks do not maintain
  state information about any packet they only
  read the address and decide where to send it
  based on that address
• Traceroute (in Windows, tracert) is an
  application that shows you the details of how a
  packet gets from one host to another

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Traceroute Output

• FROM www.adelphiacom.net TO www.nero.com.
• traceroute to www.nero.com (62.93.192.11), 64
  hops max, 44 byte packets
• 1 i0.chi75.adelphiacom.net (66.109.10.17)
  0.554 ms 0.420 ms 0.360 ms
• 2 g1-01-02-00.a0.chi75.adelphiacom.net
  (66.109.3.17) 0.561 ms 0.873 ms 0.313 ms
• 3 a1-00-00-00.c0.chi75.adelphiacom.net
  (66.109.3.1) 0.372 ms 0.355 ms 0.317 ms
• 4 so-00-01-00.c1.dca91.adelphiacom.net
  (66.109.0.82) 16.992 ms 16.940 ms 16.925 ms
• 5 p3-05-00-00.p0.dca90.adelphiacom.net
  (66.109.1.142) 17.748 ms 17.743 ms 17.740 ms
• 6 so-4-0-0.mpr2.iad5.us.above.net
  (64.124.11.225) 17.817 ms 17.812 ms 20.384 ms
• 7 so-7-0-0.mpr2.iad1.us.above.net
  (64.125.28.13) 17.832 ms 17.917 ms 17.798 ms
• 8 so-6-0-0.cr2.dca2.us.above.net
  (64.125.27.210) 18.178 ms 18.202 ms 18.211 ms
• 9 so-6-0-0.cr2.lhr3.uk.above.net
  (64.125.27.166) 90.064 ms 90.101 ms 97.132 ms
• 10 64.125.27.221.available.above.net
  (64.125.27.221) 107.404 ms 107.474 ms 107.519
  ms
• 11 pos-9-1.mpr2.fra1.de.above.net
  (64.125.23.253) 113.379 ms 113.830 ms 113.340
  ms
• 12 ge-9-7.er2a.fra1.de.above.net (64.125.23.186)
  154.871 ms 117.584 ms 117.607 ms
• 13 62.93.192.11.insoft.fra2.de.mfnx.net
  (62.93.192.11) 113.757 ms 113.659 ms 113.576 ms

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Traceroute Output

• Each line of output gives you
• The hop number (1, 2, )
• The name of the server its passing through
• The IP address of that server (e.g. 66.109.1.142)
• And times of three attempts to ping that server
  (say Hi to it), given in milliseconds (ms)
• Notice the example goes through servers in the UK
  and Germany (uk, de), and the ping times go over
  a hundred milliseconds

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Summary of Network Types
35
Access Networks

• To get from a host to a distant part of the
  Internet, you need to pass through the access
  network
• Access networks get residential, business, and
  wireless users connected
• Types of connections include
• 56 kbps dial-up modem, an analog connection over
  a voice phone line
• Typically get 40-42 kbps due to line noise

36
Access Networks

• Digital subscriber line (DSL) gives a dedicated
  connection, with different upstream and
  downstream rates
• DSL uses FDM
• Downstream/upstream rates are typically values
  like 768k/128k, 3.0M/768k, etc.
• Business connections may use dedicated T1 lines
  (1.536 Mbps), ISDN connections, and other options

37
Access Networks

• Cable modems use hybrid fiber-coaxial cable (HFC)
  to connect to special cable modems
• HFC is a variant on the same cable used for cable
  TV service
• HFC is a shared medium if all your neighbors
  are online, your connection speed will suffer!
• Dial-up connections are only present when needed
  DSL and cable modems are always on (we hope)

38
Wireless Access

• Wireless devices connect through wireless access
  points (base station) on a LAN
• Then the LAN uses some other access connection to
  get to the Internet
• Wireless devices use the IEEE 802.11 family of
  technologies
• 802.11a supports up to 54 Mbps _at_ 5 GHz
• 802.11b supports 5.5 and 11 Mbps _at_ 2.4 GHz
• 802.11g supports up to 54 Mbps _at_ 2.4 GHz

39
Why Does Frequency Matter?

• Wireless signals can be interfered with by other
  devices when that occurs, they detune their
  speed
• 802.11a has seven (48, 36, 24, 18, 12, 9, and 6
  Mbps)
• 802.11b has three lower data rates (5.5, 2, and 1
  Mbps)
• 802.11g has a range of lower speeds
• The 802.11b and 802.11g standards use the 2.4 GHz
  (gigahertz) frequency range
• This frequency range is used by other networking
  technologies, microwave ovens, 2.4GHz cordless
  phones (a huge market), and Bluetooth devices
• The 5 GHz frequency range for 802.11a is
  relatively clear, so its less likely to have
  interference (so far)

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Wireless Network Example
41
Physical Media

• Physical media used for connecting networks can
  be guided or unguided
• Guided media use something solid wires, coaxial
  cable, fiber-optic cable, etc.
• Unguided media use electromagnetic waves of some
  kind wireless LAN signals, satellite channels,
  etc.

42
Physical Media

• Specific kinds of physical media include
• Twisted pair copper wire
• Coaxial cable
• Fiber optics
• Terrestrial radio channels
• Satellite radio channels

43
Twisted pair copper wire

• Most common physical medium, has multiple coated
  wires wrapped around each other
• Includes phone lines, which have four thin wires
  with RJ-11 plugs on the end
• Ethernet cables have eight wires, and RJ-45 plugs
  on the end, so theyre wider than phone plugs
• Can handle several Mbps speeds over distances of
  a few hundred yards

44
Coaxial cable

• Coaxial (coax) cable has a copper wire core, and
  a copper cylinder around it they share the same
  axis of rotation, hence the name
• Handles multiple Mbps speeds for miles
• There are only two conductors, which is why its
  a shared medium everyone shares the same
  resources

45
Fiber optics

• Fiber optics use hollow fibers to guide light
  pulses
• Handles hundreds of Gbps speeds up to 100 km
• Most international phone lines, and the Internet
  backbone, are fiber optic cables
• Used on high speed LANs 1 to 10 Gbps

46
Terrestrial radio channels

• These include the wireless network channels
  discussed previously, plus radio signals used to
  beam networks between buildings
• Can reach long distances with the latter, but
  signals can be intercepted, bounce, fade, and
  have interference from other signals

47
Satellite radio channels

• Consist of geostationary satellites and
  low-altitude satellites
• Geostationary satellites hover 24,000 miles above
  the Earths surface, and are used to relay TV
  channels and parts of the Internet backbone
• Low altitude satellites (LEO, low-Earth orbiting)
  orbit much faster, so you need several to be able
  to find one at any given time are not used for
  networks

48
Psst what Internet Backbone?

• The Internet is a network of many networks
• It was designed that way to be redundant in the
  event of war if one part of it was no longer
  usable (nice euphemism!), the rest of the network
  would still work
• At its heart are many Tier-1 ISPs
• Sprint, MCI, WorldCom, ATT, etc. are all Tier-1
• They run extremely fast backbone connections
  (622 Mbps to 10 Gbps)

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Internet Backbone

• The Tier-2 ISPs are regional or national in
  scope, and connect to Tier-1 and Tier-2 ISPs
• Points where ISPs connect to each other are
  Points Of Presence (POPs)
• Dont confuse with Post Office Protocol (POP)
• They may also connect at Network Access Points
  (NAPs) to local telecom companies or Tier 1 ISPs

50
Internet Backbone

• There are thousands of lower level ISPs, Tier-3,
  probably including your local ISP
• For a packet to get from one host to another, it
  may pass through a variety of Tier-1, Tier-2, and
  Tier-3 ISPs, NAPs, POPs, etc.

51
Delays and Losses

• Weve hinted at several kinds of things that can
  delay a packet or make it get lost now well
  examine those concepts in more detail
• After a packet leaves the host, it typically
  passes through several routers before getting to
  its destination
• Each router examines the packets header to
  determine which outbound link it needs to
  follow, and puts it in a queue for that link

52
Delays and Losses

• Four main causes of delay at each router
• Nodal processing delay
• Queuing delay
• Transmission delay
• Propagation delay

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Delays and Losses

• Nodal processing delay is the time needed for the
  router to examine the packets header and choose
  the right outbound link
• Also may include time for error checking the
  packet
• Typically in microseconds for good routers
• Queuing delay is the time for a packet waiting to
  be transmitted across the outbound link
• Depends mostly on how much traffic got to the
  router which is waiting for the same link
• Could be microseconds or milliseconds in duration

54
Delays and Losses

• Transmission delay is like the store-and-forward
  delay mentioned earlier its the time to
  transmit the packet onto the link
• The entire packet has to be pushed onto the link
  by the router, so the transmission delay is L/R,
  or (packet size)/(transmission speed)
• Propagation delay is the time for the packet to
  get to the next router
• Distance speed time, so the propagation delay
  is distance/speed, where speed is 2 or 3x108
  m/sec (the speed of light is 3x108 m/s)

55
Delays and Losses

• You might think of Transmission delay and
  Propagation delay as being like leaving for a
  trip transmission delay is the time to pack the
  car (time to get out of the house), and
  propagation delay is the time to drive to your
  destination (travel time)
• Or ignore this analogy if it doesnt help ?

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Delays and Losses

• So the delay at one node, the nodal delay, is
  given bydnodal dproc dqueue dtrans dprop
• Where dproc Nodal processing delaydqueue
  Queuing delaydtrans Transmission delaydprop
  Propagation delay
• The relative magnitude of these terms can vary
  widely, depending on the circumstances

57
Traffic Intensity

• Consider if all packets were the same size L
  bits, and arrive at a router at a rate of a
  packets per second
• The rate of data arriving at the router is La
  bits per second
• The output rate from the router is its
  transmission rate, R bits per second
• The traffic intensity is La/R
• Want traffic intensity lt 1 why?

58
Traffic Intensity

• Average queuing delay grows exponentially as
  traffic intensity approaches one
• This is the router equivalent of gridlock!
• It was assumed that the router could hold an
  infinite amount of packets in its queue
• A dropped or lost packet occurs when a packet
  arrives at a router with its outbound link queue
  full
• Fraction of lost packets is a key measure

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End-to-end Delay

• So far we focused on one router
• Now consider the total delays getting from host
  to host the end-to-end delay
• If we assume
• 1) there are N-1 routers between hosts,
• 2) queuing delays are negligible, and
• 3) processing delays are the same for each router
  and the source host,
• 4) transmission rates are all R bits/sec, and
• 5) propagation delays are all equal

60
End-to-end Delay

• Then the total delay from source host to
  destination host isdend-end N(dproc dtrans
  dprop)
• And dtrans is L/R, with L the packet size
• So why is it N instead of (N-1)?

61
Internet Throughput

• Much of the Internet core has more capacity than
  currently needed (it is over-provisioned)
• As a result, the limit of getting data through
  the Internet is the speed of your access link
  (ISP connection) and your destinations access
  link

62
Layered Architecture

• As hinted at in the syllabus, the layers of
  networking are a key concept
• Why use layers?
• To solve a big problem, break it into little
  problems
• Each layer has a small, focused amount of work it
  needs to accomplish each layer provides services
  to the layer above it
• Disadvantages are possible duplication of work
  (error recovery on multiple layers), and
  violating the scope of a layers services

63
Layered Architecture

• The layers are seen at right
• The application layer is where user-visible
  software exists HTTP, SMTP, FTP, etc. protocols
• The transport layer is home to the TCP and UDP
  protocols
• The network layer is home to the Internet
  Protocol, IP, and the protocols used by routers

64
Layered Architecture

• The link layer is key for local routing includes
  Ethernet and Point-to-Point Protocol (PPP)
• The physical layer moves the bits of data
  (frames, as well see shortly) across the guided
  or unguided media discussed earlier
• Each medium has protocols for how data is
  encoded and decoded

65
But Wait Professor Booker!

• Arent we missing the Presentation and Session
  layers?!?
• Yes, the OSI reference model has them between the
  application and transport layers, but they arent
  directly relevant here
• The presentation layer includes coding and
  conversion functions that are applied to
  application layer data such as MPEG, QuickTime,
  JPG, GIF, TIFF
• The session layer opens and closes communication
  sessions AppleTalk is a familiar protocol here

66
Layered Architecture

• To make it more confusing, the packet weve been
  discussing has different names as it descends the
  layers
• Terms may vary from vendor to vendor
• A packet becomes
• A message in the application layer
• A segment in the transport layer
• A dataframe in the network layer
• A frame in the link and physical layers

67
Layered Architecture

• With each layer, headers are added to the message
  to describe the address information needed by
  that layer
• This process is called encapsulation, as we put
  the message in bigger and bigger boxes
• Routers and switches typically look at the link
  or network layer information
• Like a letter carrier, they dont read your mail

68
Layered Architecture
69
Network Security

• While security is covered in detail in INFO 331,
  well mention a couple of key concepts
• Malware is a generic term for software that does
  harm (malicious software)
• It could enroll your computer in a botnet, where
  it helps distribute spam or help attack other
  computers
• Much malware is self-replicating, so it can
  spread very quickly

70
Network Security

• Viruses are malware that require the user to
  activate it somehow, but it could be disguised as
  a web link
• Worms can enter your computer without user
  activation
• Trojan horses enter via a legitimate application,
  such as a simple game

71
Network Security

• Threats can keep a host from getting legitimate
  network traffic this is a denial of service
  (DoS) attack
• Types of DoS attacks include exploiting a
  vulnerability in the OS or an application,
  flooding the bandwidth leading to the host, or
  making the host establish phony network
  connections
• Herds of computers can participate in a
  distributed DoS attack (DDoS)

72
Network Security

• Network data can be read using packet sniffers
• Well use one for our labs, WireShark
• Or people can fake who they are on the network,
  and impersonate you (IP spoofing) or intercept a
  network connection (man in the middle attack)

73
A Little History

• The concept of packet switching was developed in
  the early 1960s by MIT and the Rand Institute,
  in order to make it possible to share really
  expensive computer time efficiently
• The first packet switches were called interface
  message processors (IMPs)
• ARPAnet, the Internet predecessor, was proposed
  in 1967

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A Little History

• By 1969, four computers were on ARPAnet, and RFCs
  were being published
• By 1972 there were 15 nodes on ARPAnet, and it
  was first seen publicly
• The first email program was written in 1972
• A microwave network was developed in Hawaii, and
  various packet switching networks were developed
  by the mid 1970s

75
A Little History

• As the number of similar networks grew,
  connecting them to aid researchers became an
  obvious direction
• Vint Cerf helped establish the core Internet
  protocols by the end of the 1970s TCP, IP, and
  UDP
• Robert Metcalfe defined Ethernet in 1976
• By 1983, ARPAnet switched to TCP/IP

76
A Little History

• The French installed Minitel, a public
  packet-switched network, in the early 1980s, a
  decade before the US caught on to the Internet
• DNS wasnt invented until the late 1980s (RFC
  1034)
• The World Wide Web was invented between 1989 and
  1991 by Tim Berners-Lee, based on work as far
  back as 1945

77
A Little History

• At the end of 1992 there were 200 web servers in
  the world
• In 1994 Mosaic was formed, later known as
  Netscape, and much of the world was introduced to
  the Internet
• By the late 1990s, peer-to-peer file sharing,
  instant messaging, email, and the Web formed the
  killer apps that launched the world we see today

78
A Little History

• The dot-Com bubble burst by 2001, but a few
  companies survived
• Through the 1990s, issues such as security and
  handling of streaming video became urgent, as
  e-commerce became as common as a 7-11
• Now more devices are connected phones, PDAs
  and we cant imagine not having the Internet at
  our disposal