Computer Networking by yaoyufang

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									  Computer Networking

      Yehuda Afek (afek at cs.tau.ac.il)
Adopted the slides from Yishay Mansour
        Teaching Assistant: Yahav Nussbaum


                                       1
      Course Information
 Lectures: Sunday     4–7                  Schreiber 006
 Exercises: Wednesday 11 –12, 12 – 1       Orenstein 103

 Web site: http://www.cs.tau.ac.il/~nuss/comnet08/



  Books:
 •A Top-down Approach to Computer Networking / Kurouse-Ross



1. An Engineering Approach to Computer Networking / Keshav
2. Computer Networks / Tanenbaum
3. Data Networks / Bertsekas and Gallager
                                                             2/60
   Practical Information


Homework assignment:
Mandatory
Both theoretical and programming




Grades:
Final Exam:                60%
theory exercises:          20%
Programming exercises:     20%
                                   3/60
                         Motivation

   Today‟s economy
       manufacturing, distributing, and retailing goods
       but also creating and disseminating information
            publishing
            banking
            film making….
    part of the ‘information economy’
   Future economy is likely to be dominated by
    information!
                                                           4/60
                     Information?
   A representation of knowledge
   Examples:
       books
       bills
       CDs & DVDs
   Can be represented in two ways
       analog (atoms)
       digital (bits)
   the Digital Revolution
       convert information as atoms to information as bits
       use networks to move bits around instead of atoms
                                                          5/60
The Challenges


   represent all types of information as bits.
   move the bits
       In large quantities,
       everywhere,
       cheaply,
       Securely,
       with quality of service,
       ….


                                                  6/60
Today’s Networks are complex!

   hosts
   routers
   links of various media
   applications
   protocols
   hardware, software


Tomorrow‟s will be even more!
                                7/60
            Internet Physical Infrastructure
   Residential access
          Cable
          Fiber
          DSL          ISP         Backbone ISP     ISP
          Wireless




                               The Internet is a network
                                of networks
Campus access,                 Each individually
  e.g.,                         administrated network is
      Ethernet
       Wireless
                                called an Autonomous
                                System (AS)
   
                                                            8/60   8
        This course’s Challenge
   To discuss this complexity in an
    organized way, that will make today‟s
    computer networks (and their
    limitations) more comprehensive.
   identification, and understanding relationship
    of complex system‟s pieces.
   Problems that are beyond a specific
    technology

                                               9/60
Early communications systems
   I.e. telephone
   point-to-point links
   directly connect together the users wishing to
    communicate
   use dedicated communication circuit
   if distance between users increases beyond the
    length of the cable, the connection is formed by a
    number of sections connected end-to-end in series.




                                                  10/60
                Data Networks

   set of interconnected nodes exchange information
   sharing of the transmission circuits= "switching".
   many links allow more than one path between every
    2 nodes.
   network must select an appropriate path for each
    required connection.




                                                    11/60
12/60
           Qwest backbone




http://www.qwest.com/largebusiness/enterprisesolutions/networkMaps/preloader.swf 13/60
        Networking Issues - Telephone

Addressing - identify the end user

phone number 1-201-222-2673 = country code + city code +
exchange + number

   Routing - How to get from source to destination.
Telephone circuit switching: Based on the phone number.

   Information Units - How is information sent
telephone Samples @ Fixed sampling rate. not self
descriptive! have to know where and when a sample came
                                                       14/60
           Networking Issues - Internet
   Addressing - identify the end user
IP addresses 132.66.48.37, Refer to a host interface =
   network number + host number

   Routing- How to get from source to destination
Packet switching: move packets (chunks) of data among
  routers from source to destination independently.

   Information Units - How is information sent.
Self-descriptive data: packet = data + metadata (header).
                                                         15/60
Telephone networks support a single, end-to-
  end quality of service but is expensive to boot

Internet supports no quality of service but is
flexible and cheap


Future networks will have to support a wide
range of service qualities at a reasonable cost
                                                 16/60
                           History
       1961-1972: Early packet-switching principles

1961: Kleinrock - queuing theory shows effectiveness of
  packet-switching
1964: Baran - packet-switching in military networks
1967: ARPAnet – conceived by Advanced Research Projects
  Agency
1969: first ARPAnet node operational
1972: ARPAnet demonstrated publicly
    NCP (Network Control Protocol) first host-host

     protocol
    first e-mail program

    ARPAnet has 15 nodes
                                                      17/60
                           History
          1972-1980: Internetworking, new and
                    proprietary nets


1970: ALOHAnet satellite network in Hawaii
1973: Metcalfe‟s PhD thesis proposes Ethernet
1974: Cerf and Kahn - architecture for interconnecting
   networks
late70‟s: proprietary architectures: DECnet, SNA, XNA
late 70‟s: switching fixed length packets (ATM precursor)
1979: ARPAnet has 200 nodes




                                                      18/60
Cerf and Kahn‟s internetworking principles:


   minimalism, autonomy - no internal
    changes required to interconnect networks
   best effort service model
   stateless routers
   decentralized control


Defines today‟s Internet architecture
                                         19/60
                        History
              1980-1990: new protocols,
               proliferation of networks


1983:   deployment of TCP/IP
1982:   SMTP e-mail protocol defined
1983:   DNS defined for name-to-IP-address translation
1985:   FTP protocol defined
1988:   TCP congestion control


new national networks: CSnet, BITnet, NSFnet, Minitel
100,000 hosts connected to confederation of networks

                                                    20/60
                          History
         1990 - : commercialization and WWW


early 1990‟s: ARPAnet decomissioned
1991: NSF lifts restrictions on commercial use of NSFnet
  (decommissioned, 1995)
early 1990s: WWW
   hypertext [Bush 1945, Nelson 1960‟s]
   HTML, http: Berners-Lee
   1994: Mosaic, later Netscape
   late 1990‟s: commercialization of WWW


                                                      21/60
    Demand and Supply

   Huge growth in users
       The introduction of the web
   Faster home access
       Better user experience.
   Infrastructure
       Significant portion of telecommunication.
   New evolving industries
       Although, sometimes temporary setbacks
                                                22/60
                  Internet: Users
                1500
                1300
Million users




                1100
                 900
                 700
                 500
                 300
                 100
                -100
                    1995 1997 1999 2001 2003 2005 2007 2009
                                    year                 23/60
      Penetration around the Globe (2008)
80                               %Population     %Penetration               USA+Canada

70
                                                                                                          Australia
60                      Asia/Pacific

                                                Europe
50
40
30                                                                                       Latin America
                                                              Middle East
20      Africa               Asia/Pacific
                                            Europe
                                                                                    Latin America
10             Africa
                                                         Middle East
                                                                       USA+Canada
                                                                                                    Australia
 0




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          http://www.internetworldstats.com/stats.htm                                                    26/60
                                                              U
  Users around the Globe (2002/5/8)
700
             Asia/Pacific
600
                                                                                 2008
500
                            Europe
                                                                                 2005
400
300                                                                              2002
                                              USA+Canada

200            Asia                        Latin America
              Pacific Europe Middle     USA
100                                              Latin
      Africa               Middle East Canada
       Africa                   East          America    Australia
                                                         Australia
  0




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                                                                                 27/60
Technology: Modem speed

       100000

        80000
                                                56000
        60000
 bps




        40000                               33600
                                        28800
        20000                       14400
                                9600
                300 1200 2400
            0
             79

             80

             84

             87

             91

             93

             95

             97

             08
           19

           19

           19

           19

           19

           19

           19

           19

           20
                                   year
                                                        28/60
Today‟s options

   Modem: 56 K
   ISDN: 64K – 128K       OBSOLETE

   Frame Relay: 56K ++
   Today High Speed Connections
       Cable, ADSL, Satellite.
       All are available at 5Mb (2005)



                                          29/60
Coming soon   (1999)




                       30/60
Today (2005)




               31/60
      Why do we need Standards

   Networks (and other media) support
    communication between different entities



   Need agreement to ensure correct, efficient
    and meaningful communication


                                           32/60
      Various Organizations Issue Standards



   IEEE (Institute for Electrical and Electronic Engineers)

   IETF (Internet Engineering Task Force)

   ITU (International Telecommunications Union)

   ISO (International Organization for Standardization)

   W3C (World Wide Web Consortium)

                                                     33/60
    Protocol Layers


   A way for organizing structure of network


 … Or at least our discussion of networks


   The idea: a series of steps

                                                34/60
        Protocol Layering
   Necessary because communication is complex
   Intended primarily for protocol designers
   Divides the problem into intellectually manageable pieces
   Provides a conceptual framework that can help us understand
    protocols
   Think of layering as a guideline, not a rigid specification
   Understand that optimizations may violate strict layering
   Should be invisible to users

                                                             35/60
 Mail system functionality



          QuickTime™ and a
TIFF (Un compressed) decompressor
   are neede d to see this picture.




                                      36/60
            How do we Communicate?

   Send a mail from Alice to Bob                                 Bob
         Alice in Champaign, Bob in Hollywood
   Example:
         US Postal Service


    Alice




                                                                  Hollywood, California

             Champaign, Illinois

         8/29/07                   UIUC - CS/ECE 438, Fall 2007               37/60
    What does Alice do?

           Alice
           200 Cornfield Rd.
           Champaign, IL 61820



                         Bob
                         100 Santa Monica Blvd.
                         Hollywood, CA 90028

   Bob‟s address (to a mailbox)
   Bob‟s name – in case people share mailbox
   Postage – have to pay!
   Alice‟s own name and address – in case Bob wants to return a
    message

8/29/07                  UIUC - CS/ECE 438, Fall 2007              38/60
    What does Bob do?

          Alice
          200 Cornfield Rd.
          Champaign, IL 61820



                          Bob
                          100 Santa Monica Blvd.
                          Hollywood, CA 90028


   Install a mailbox
   Receive the mail
   Get rid of envelope
   Read the message

8/29/07                   UIUC - CS/ECE 438, Fall 2007   39/60
       Layers:

       Person delivery of parcel

       Post office counter handling

       Ground transfer: loading on trucks            Peer entities

       Airport transfer: loading on airplane

       Airplane routing from source to destination


each layer implements a service
   via its own internal-layer actions
   relying on services provided by layer below             42/60
      Advantages of Layering

   explicit structure allows identification &
    relationship of complex system‟s pieces
      layered reference model for discussion

   modularization eases maintenance &
    updating of system
      change of implementation of layer‟s

       service transparent to rest of system


                                                 43/60
    Protocols

   A protocol is a set of rules and formats
    that govern the communication
    between communicating peers
       set of valid messages
       meaning of each message


   Necessary for any function that requires
    cooperation between peers
                                           44/60
    Protocols

   A protocol provides a service
       For example: the post office protocol for reliable
        parcel transfer service


   Peer entities use a protocol to provide a
    service to a higher-level peer entity
       for example, truck drivers use a protocol to
        present post offices with the abstraction of an
        unreliable parcel transfer service

                                                     45/60
        Protocol Layers

   A network that provides many services needs
    many protocols
   Some services are independent, But others
    depend on each other
   A Protocol may use another protocol as a step in
    its execution
       for example, ground transfer is one step in the
        execution of the example reliable parcel transfer
        protocol
   This form of dependency is called layering
       Post office handling is layered above parcel ground
        transfer protocol.                                46/60
        Open protocols and systems

   A set of protocols is open if
       protocol details are publicly available
       changes are managed by an organization whose
        membership and transactions are open to the public
   A system that implements open protocols is
    called an open system
   International Organization for Standards (ISO)
    prescribes a standard to connect open systems
       open system interconnect (OSI)
   Has greatly influenced thinking on protocol
    stacks                                            47/60
     The seven Layers
               There are only 5 !!

Application                            Application
Presentation     Application           Presentation
Session                                Session
Transport                              Transport
Network                Network         Network
Data Link              Data Link       Data Link
Physical               Physical        Physical

End system              Intermediate    End system
                         system
                                               49/60
       The seven Layers - protocol stack

                                                            data


  Application                                      AH       data   Application
  Presentation                           PH             data       Presentation
  Session                           SH             data            Session
  Transport                    TH             data                 Transport
  Network               Network               NH        data       Network
  Data Link             Data Link             DH+data+DT           Data Link
  Physical              Physical                     bits          Physical

Session   and presentation layers are not so important, and are often ignored
                                                                      50/60
                     ‫עיקרון השכבות‬
Source            ‫ מתקבלת הודעה‬X ‫בשכבה‬       Destination
                  ‫זהה להודעה ששכבה‬
    Application   ‫ מסרה בצד המקור‬X   Application
                      Identical message

    Transport                             Transport
                      Identical message

     Network                              Network
                      Identical message

    Data-Link                             Data-Link


                     Network
                                                      51/60
Postal network

   Application: people using the postal system
   Session and presentation: chief clerk sends some
    priority mail, and some by regular mail ;
    translator translates letters going abroad.
   mail clerk sends a message, retransmits if not acked
   postal system computes a route and forwards the
    letters
   datalink layer: letters carried by planes, trains,
    automobiles
   physical layer: the letter itself
                                                   52/60
        Internet protocol stack

   application: supporting network applications   application
       ftp, smtp, http

   transport: host-host data transfer             transport
       tcp, udp

   network: routing of datagrams from source       network
    to destination
       ip, routing protocols                         link
   link: data transfer between neighboring
    network elements                                physical
       ppp, ethernet

   physical: bits “on the wire”                        53/60
             Protocol layering and data


                source      destination
        M     application   application           M    message
     Ht M      transport     transport         Ht M    segment
   Hn Ht M      network       network        Hn Ht M   datagram
Hl Hn Ht M        Link          Link      Hl Hn Ht M   frame
                physical      physical




                                                        54/60
             Physical layer                  L1

   Moves bits between physically connected
    end-systems
   Standard prescribes
       coding scheme to represent a bit
       shapes and sizes of connectors
       bit-level synchronization
   Internet
       technology to move bits on a wire, wireless link, satellite
        channel etc.

                                                            55/60
        Datalink layer                     L2

   (Reliable) communication over a single link.
   Introduces the notion of a frame
       set of bits that belong together
   Idle markers tell us that a link is not carrying a
    frame
   Begin and end markers delimit a frame
   Internet
       a variety of datalink layer protocols
       most common is Ethernet
       others are FDDI, SONET, HDLC
                                                   56/60
              Datalink layer (contd.)

       Ethernet (broadcast link)
             end-system must receive only bits meant for it
             need datalink-layer address
             also need to decide who gets to speak next
             these functions are provided by Medium ACcess sublayer (MAC)


       Datalink layer protocols are the first layer of software
       Very dependent on underlying physical link properties
       Usually bundle both physical and datalink in hardware.

                                                                       57/60
     Network layer                      L3

   Carries data from source to destination.
   Logically concatenates a set of links to form the
    abstraction of an end-to-end link
   Allows an end-system to communicate with any other
    end-system by computing a route between them
   Hides idiosyncrasies of datalink layer
   Provides unique network-wide addresses
   Found both in end-systems and in intermediate systems



                                                     58/60
        Network layer types

   In datagram networks
       provides both routing and data forwarding
   In connection-oriented network
       separate data plane and control plane
       data plane only forwards and schedules data
        (touches every byte)
       control plane responsible for routing, call-
        establishment, call-teardown (doesn‟t touch data
        bytes)


                                                           59/60
        Network layer (contd.)

   Internet
       network layer is provided by Internet Protocol (IP)
       found in all end-systems and intermediate systems
       provides abstraction of end-to-end link
       segmentation and reassembly
       packet-forwarding, routing, scheduling
       unique IP addresses
       can be layered over anything, but only best-effort
        service

                                                         60/60
         Network layer (contd.)

   At end-systems
        primarily hides details of datalink layer
        segments and reassemble
        detects errors
   At intermediate systems
      participates in routing protocol to create routing

       tables
      responsible for forwarding packets

      schedules the transmission order of packets

      chooses which packets to drop
                                                            61/60
         Transport layer                                L4

   Reliable end-to-end communication.
   creates the abstraction of an error-controlled,
    flow-controlled and multiplexed end-to-end link
    (Network layer provides only a „raw‟ end-to-end service)
   Some transport layers provide fewer services
        e.g. simple error detection, no flow control, and no retransmission

   Internet
        TCP provides error control, flow control, multiplexing
        UDP provides only multiplexing
                                                                     62/60
        Transport layer (contd.)

   Error control
       GOAL: message will reach destination despite packet loss,
        corruption and duplication
       ACTIONS: retransmit lost packets; detect, discard, and
        retransmit corrupted packets; detect and discard duplicated
        packets
   Flow control
       match transmission rate to rate currently sustainable on the path
        to destination, and at the destination itself
   Multiplexes multiple applications to the same
    end-to-end connection
       adds an application-specific identifier (port number) so that
        receiving end-system can hand in incoming packet to the correct
        application                                                 63/60
        Session layer


   Not common
   Provides full-duplex service, expedited data
    delivery, and session synchronization
   Internet
       doesn‟t have a standard session layer



                                                64/60
        Session layer (cont.)

   Duplex
       if transport layer is simplex, concatenates two transport
        endpoints together
   Expedited data delivery
       allows some messages to skip ahead in end-system queues,
        by using a separate low-delay transport layer endpoint
   Synchronization
       allows users to place marks in data stream and to roll back
        to a prespecified mark



                                                                    65/60
        Presentation layer

   Usually ad hoc
   Touches the application data
(Unlike other layers which deal with headers)
   Hides data representation differences between
    applications
       characters (ASCII, unicode, EBCDIC.)
   Can also encrypt data
   Internet
       no standard presentation layer
       only defines network byte order for 2- and 4-byte
        integers                                         66/60
      Application layer

   The set of applications that use the network
   Doesn‟t provide services to any other layer




                                                   67/60
                         ‫עיקרון השכבות‬
Source                                           Destination

 3 ‫אפליק‬       2 ‫אפליק‬          1 ‫אפליק‬   Application

  UDP                 TCP                 Transport

   Network (IPv4)                          Network

Modem      Ethernet      WiFi             Data-Link


                         Network
                                                        68/60
                            ‫עיקרון השכבות‬
 Source                                                 Destination

 3 ‫אפליק‬   2 ‫אפליק‬     1 ‫אפליק‬         3 ‫אפליק‬       2 ‫אפליק‬          1

  UDP          TCP                      UDP                 TCP

 Network (IPv4)                          Network (IPv4)

Modem Ethernet       WiFi             Modem      Ethernet      WiFi


                            Network
                                                            69/60
        Discussion

   Layers break a complex problem into
    smaller, simpler pieces.
   Why seven layers?
       Need a top and a bottom  2
       Need to hide physical link; so need datalink  3
       Need both end-to-end and hop-by-hop actions; so
        need at least the network and transport layers  5



                                                       70/60
        Course outline
1    Introduction and Layering
2    Data Link: Multi Access
3    Hubs, Bridges and Routers
4    Scheduling and Buffer Management
5    Switching Fabrics
6    Routing
7    Reliable Data Transfer
8    End to End Window Based Protocols
9    Flow Control
10   Multimedia and QoS
11   Network Security
12   Distributed Algorithms              71

								
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