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COSC 465: Networking

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COSC 465: Networking. Week 1. Applications. P2P. napster, gnutella, bit torrent. email ... Overhead required to establish and maintain the dedicated resources ... – PowerPoint PPT presentation

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Title: COSC 465: Networking


1
COSC 465 Networking
  • Week 1

2
Applications
  • P2P
  • napster, gnutella, bit torrent
  • email
  • Web (HTTP)
  • FTP
  • Skype
  • streaming video

3
Hosts on the internet
  • Computers
  • mac / PC / unix
  • Devices
  • PDAs
  • Vending machines
  • Toasters
  • Why is this possible (or even desirable)?

4
Protocols
  • Speakeasies in the 1920s
  • banana pastry
  • Hello / Hi / How are you? / Just fine
  • Hello / Hi / How are you? / Bite me
  • Answering the phone
  • Hello
  • (checks caller ID) Fred, quit calling me!s

5
Switching (draw picture on the board)
  • Circuit-switched network
  • Dedicated connection
  • Telephone
  • Packet-switched network
  • Break transmission into packets
  • Internet
  • Snail-mail

6
Circuit-switched network
  • Dedicated connection between 2 endpoints
  • Telephone
  • landlines
  • Trains
  • Overhead required to establish and maintain the
    dedicated resources for the connection

7
Multiplexing on circuit-switched networks
  • Necessary because we cant have dedicated wires
    between every possible pair of endpoints
  • Frequency-Division Multiplexing (FDM)
  • FM radio
  • Bus lane / HOV lane
  • Time-Division Multiplexing (TDM)
  • Train tracks

8
Inefficiencies of circuit-switched network?
9
Inefficiencies of circuit-switched network?
  • During a pause in telephone conversation
  • Resources are still dedicated
  • Establishing and maintaining dedicated
    connections requires complex software

10
How long does it take
  • 1,600,000 bit file
  • 3.2 Mbps connection
  • 16 slots TDM
  • 1 second to establish the connection
  • (figure out how long it takes)
  • (now how long would it take if we didnt use TDM)

11
Packet-switched network
  • Break messages up into pieces
  • Packets
  • Send packets along links
  • Store-and-forward
  • Must receive entire packet before the first bit
    can be sent out
  • Effects of packet size?

12
Packet switched delays
  • Store and forward delay
  • L bits
  • R bps
  • L/R store-and-forward-delay
  • Queuing delays
  • routers need to buffer packets while links are
    busy
  • If the buffer gets full while the link is
    saturated
  • packets get dropped!
  • Unpredictable

13
Disadvantages of packet-switching
  • variable and unpredictable end-to-end delays
  • Telephones
  • video-conferencing

14
Comparison of circuit and packet switched networks
  • 1 Mbps pipe
  • 10 users
  • each is active only 10 of time (0.10)
  • Circuit-switching
  • 100 kbps per user
  • maximum of 10 users

15
Haha! These look like future exam questions
  • Very possible
  • But the more important issue here is being
    comfortable doing back-of-the-envelope
    calculations to guesstimate stuff
  • Very important skill for a scientist
  • Helps prevent me from making outrageous claims

16
Packet-switching
  • 35 users
  • 0.0004 chance of 11 or more being active at the
    same time
  • Math involved in computing this
  • If we get 11 or more users, performance degrades
    until we get back under 1 Mbps total usage

17
Suppose one user generates a lot of data
  • circuit switching
  • they just have to wait
  • packet switching
  • if others are inactive, then they can use more
    bandwidth

18
Statistical Multiplexing
  • Taking advantage of how often people use the
    system
  • Efficiently use the network
  • Better tolerance for heavier bursts of data

19
Take-home message here
  • Packet-switching is the model predominantly used
    by the internet
  • Important to understand the tradeoffs
  • And that there are other ways of building a
    network

20
Net Neutrality
  • ISPs want to give preference to certain types of
    traffic
  • higher-paying customers get preference for their
    traffic
  • video conferencing
  • virtual private network (VPN)

21
Datagram vs Virtual Circuit (VC) networks
  • VC networks maintain state in the routers
  • Similar to circuit-switched networks
  • X.25
  • frame relay
  • asynchronous transfer mode (ATM)
  • Datagram networks forward packets based only on
    the addresses
  • The internet

22
Datagram networks are stateless inside the network
  • Switches forward packets based on destination
    address only
  • Analogy
  • Snail mail through the post office
  • Advantages?
  • Disadvantages?

23
Residential access
  • Modem
  • Does anyone remember these?
  • DSL
  • FDM of the telephone wire
  • Reserve more space for downstream
  • Cable modem
  • hybrid fiber coaxial cable (HFC)
  • also reserves more bandwidth for downstream
  • broadcasts all data from neighborhood access
    points

24
Tradeoffs between DSL/HFC
  • Both are always on
  • advantage or disadvantage?
  • Cable is a shared broadcast technology

25
Local Area Network
  • College campus
  • Company
  • Big edge router
  • Lots of other machines/devices connected to that
  • You can do interesting things on a LAN
  • iTunes for sharing music
  • networked video games
  • LANs use ethernet
  • broadcast
  • switched

26
Wireless
  • cell phones
  • radio waves
  • WAP (wireless access protocol)
  • Wireless broadband access is better in Japan than
    in the US

27
Wireless access in the developing world
  • Easier to put up cell towers than to wire houses
  • Same for internet
  • Business model
  • long tail
  • The World is Flat

28
Physical layer
  • Two media
  • twisted-pair copper wire
  • fiber and coaxial cable
  • bits get sent over these media
  • electromagnetic waves
  • optical pulses

29
Internet backbone
  • The internet is really a meta-network
  • complicated picture
  • details arent terribly important
  • tier 1
  • international and interconnected
  • exclusive club
  • not really regulated
  • Not all ISPs are created equally
  • tier 2 ISP ! tier 278 ISP

30
Whos in charge of the internet?
  • China does various things to censor internet
    traffic
  • Dont route any packets through Canada
  • IP addresses are not distributed equally

31
Internet Corporation For Assigned Names and
Numbers (ICANN)
  • ICANN is responsible for the global coordination
    of the Internet's system of unique identifiers.
    These include domain names (like .org, .museum
    and country codes like .UK), as well as the
    addresses used in a variety of Internet
    protocols. Computers use these identifiers to
    reach each other over the Internet. Careful
    management of these resources is vital to the
    Internet's operation, so ICANN's global
    stakeholders meet regularly to develop policies
    that ensure the Internet's ongoing security and
    stability
  • ICANN is a California non-profit
  • Function used to be done by the US government
  • ICANN exists under a remit from the US Dept. of
    Commerce
  • ICANN lacks teeth to prevent commercial
    organizations from having .org

32
Total Nodal Delay
  • processing delay
  • queuing delay
  • transmission delay
  • propagation delay

33
processing delay
  • Time to read the packets headers
  • Possibly perform checksums or other
    error-detection algorithms
  • typically very, very fast on modern routers
  • microseconds

34
queuing delay
  • unpredictable
  • based on traffic at that router at any given
    moment
  • depends on what everyone else is doing

35
transmission delay
  • all bits must arrive before the first bit can be
    sent out
  • L bit packet
  • R bits/second link
  • L/R

36
propagation delay
  • depends on the speed of the physical medium
  • fiber, twisted-pair, copper-wire, etc
  • 2 108 to 3 108 m/sec
  • close to the speed of light
  • 2.99 108 m/sec
  • distance between routers / propagation speed
  • d / s
  • millis for Wide-Area Networks (WANs)

37
transmission vs propagation
  • transmission is the time to receive and forward
    the whole packet
  • function of packets length and the speed of the
    outgoing link
  • propagation delay is how long it takes for the
    packet to travel between the routers
  • takes into account distance between routers

38
queuing delays revisited
  • Very active research area
  • a is packets/sec (arrival rate)
  • R is bits/sec (transmission rate)
  • L is packet size (assume all packets are uniform
    size)
  • La/R is the traffic intensity

39
Traffic intensity
  • La/R 1
  • packets will be buffered
  • buffers are finite, thus packets will be lost
  • La/R
  • What does this mean?
  • Think about how the traffic arrives

40
La/R
  • If packets arrive in bursts, there can be
    significant queuing delays
  • Suppose N packets arrive simultaneously every
    (L/RN) seconds
  • first packet?
  • second packet?
  • in general?

  • 41
    Traffic intensity conclusions
    • La/R is helpful, but is not the full story
    • pattern of packet arrival effects queuing delays
    • La/R is a good general guideline
    • 1
    • system will eventually fall down
    • close to 1
    • high traffic intensity
    • packet loss
    • close to 0
    • very low queuing delays
    • not much packet loss

    42
    Who should handle a lost packet?
    • (try to elicit class participation)

    43
    Total End-to-End delays
    • N -1 routers between source and destination
    • N hops
    • d-proc processing delay
    • d-trans transmission delay
    • d-prop propagation delay

    44
    Total End-to-End delays
    • N -1 routers between source and destination
    • N hops
    • d-proc processing delay
    • d-trans transmission delay
    • d-prop propagation delay
    • N -1 ( d-proc d-trans d-prop)
    • Other delays?

    45
    www.traceroute.org
    Route from Colgate to MIT 1
    W92-RTR-1-W92SRV21.MIT.EDU (18.7.21.1) 0.817 ms
    0.273 ms 0.257 ms 2 EXTERNAL-RTR-1-BACKBONE.MIT
    .EDU (18.168.0.18) 112.438 ms 7.047 ms 1.233
    ms 3 ge-6-23.car2.Boston1.Level3.net
    (4.79.2.1) 0.525 ms 0.739 ms 0.508 ms 4
    ae-5-5.ebr1.NewYork1.Level3.net (4.69.132.250)
    5.242 ms 16.197 ms 5 ae-1-100.ebr2.NewYork1
    .Level3.net (4.69.132.26) 9.440 ms 17.718 ms 6
    ae-24-52.car4.NewYork1.Level3.net (4.68.97.51)
    5.535 ms ae-24-54.car4.NewYork1.Level3.net
    (4.68.97.115) 5.755 ms ae-24-56.car4.NewYork1.Lev
    el3.net (4.68.97.179) 5.558 ms 7
    ROADRUNNER.car4.NewYork1.Level3.net (4.78.188.2)
    18.827 ms 4.78.166.234 (4.78.166.234) 12.138 ms
    4.78.166.238 (4.78.166.238) 12.282 ms 8
    pos5-0.syrcnyspp-rtr02.nyroc.rr.com (24.24.7.17)
    12.423 ms 18.396 ms 12.236 ms 9
    rdc-24-24-7-86.nyroc.rr.com (24.24.7.86) 12.713
    ms 18.542 ms 12.094 ms 10 srp2-0.syrcnyrmh-rtr0
    1.nyroc.rr.com (24.92.229.200) 13.812 ms 13.402
    ms 13.853 ms 11 fas0-1.syrcnyhml-swt03.nyroc.rr.
    com (24.92.225.174) 14.832 ms 21.988 ms 15.089
    ms 12 rrcs-72-43-89-2.nys.biz.rr.com
    (72.43.89.2) 15.125 ms 16.403 ms 15.526 ms 13

    46
    Layered architecture
    • Example in the book, page 47
    • draw picture on the board

    47
    Layered architecture of the internet
    48
    Application
    • HTTP
    • FTP
    • Email (SMTP)
    • AIM

    49
    Transport
    • TCP
    • Transmission Control Protocol
    • Connection-oriented
    • UDP
    • User Datagram Protocol
    • connectionless

    50
    Question
    • I thought that the internet doesnt store any
      state?
    • How is TCP able to keep track of connections?

    51
    Network
    • Datagrams
    • Different from UDP
    • Internet Protocol layer
    • IP
    • Everything on the internet uses IP
    • Well, there are routing protocols, but it all
      uses IP

    52
    Link Layer
    • routes IP datagrams from the level above
    • Ethernet
    • PPP
    • link layer packets are called frames

    53
    Physical layer
    • moves bits along the physical medium
    • twisted pair, coaxial cable, fiber optic,
      satellite, smoke signals, whatever

    54
    How this works at the endpoints
    55
    Switches VS Routers
    • I was a little loose with my terminology last
      week
    • switches and router are different
    • switches forward Link-layer packets
    • routers forward packets Link-layer or
      Network-layer packets
    • Draw picture from page 51 of KR textbook

    56
    (ethereal demo, frame 9)
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