CPS 1 - Spring, 1998 - Ramm 1/19 #2

Chapt 11. Computer Communications

  • Computer Communications is one of the Great Ideas

  • Modes of Communications
    • Computer and humans communicate
      • Early "time sharing"
    • Computers talk to each other
      • ARPA Net
    • Now Humans use computers to talk to each other
      • Email, Chat, etc.
      • News; started at Duke CS
      • IRC: 6000 people, 1000 channels
      • Voice
      • Video

  • Internet Still "Free"
    • Institutions do Pay (e.g. DU)
    • Not on usage basis
    • Not likely to stay this way long

  • Like Most of Computing: Layers upon Layers
    • Hardware
    • Software layers

  • Basic Communications
    • In binary
    • Represented by something over some medium by something
    • started with terminals connected to computers
    • Communicated by voltages on wires
    • Then modems allowed communication over phone lines
      • Turn binary representing voltages into audio tones
      • Dial up computer communications
      • Time sharing systems
      • Bulletin boards
      • USENET (started at Duke, UNC, Bell Labs)

  • Configuration/Methodology -- Different Views -- Basic Vocabulary

  • Basic Communications
    • Fixed/Dial Up
    • Topography
      • Star
      • Point to Point
      • Ring
      • Bus
      • Network
    • Connection Mode
      • Circuit Switched
        • connection made
        • e.g. Telephone System
        • exclusive use of part of medium
        • interactive use possible
      • Message Switched
        • store and forward
        • batch
        • shared facilities
      • Packet Switched
        • shared medium
        • interactive use possible

TCP/IP

  • Ethernet (Bus Example)
    • Also called LAN
    • Many nodes (hosts, stations, ...) on same bus
    • Each has address (unique!)
    • All messages "broadcast" with dest and src address
    • Everyone listens for their address
    • Potential security problem
    • Collisions possible
    • ethernet.gif, ethernet.ps

  • Internet -- a network of LANs that are interconnected

  • Packets -- the currency of the Internet

  • The Layers
    • The Physical Layer
      • Hardware (e.g. Ethernet)
      • The Ethernet Packet
    • The IP (Internet Protocol) Layer
      • The IP Packet
      • Hardware Independence
    • The TCP (Task Control Protocol) Layer
      • The TCP Packet
    • The Application Layer
      • E.g. Email, News, ...

  • Packets Within Packets

  • Reliability

  • Addressing (Layers Again!)
    • Hardware Address (Ethernet Address)
      • unique 6 octet (8 bit -- byte) number
      • usually 6 groups of two hex digits: b3 fe 13 21 a5 41
      • centrally administered
    • IP Address
      • 4 octet number: e.g. 128.109.40.1
      • usually 4 decimal number separated by periods
      • network part ;host part
      • Centrally Administered
    • Berkeley Internet Name Domain
      • hierarchical system
      • root: edu, com, mil, net, country
      • hostname; suborg; organization
      • e.g. oberon.cs.duke.edu
      • person on host is e.g. ramm@oberon.cs.duke.edu

  • Applications
    • email
    • news
    • talk
    • ftp
      • anonymous
      • archives
      • free software
      • weather servers
      • forecasts
      • weather maps
      • finger
      • telephone
    • telnet
    • rlogin
    • irc
    • information services
      • WWW
      • gopher
      • WAIS

  • Client/Server
    • WWW

  • Computer Security
    • Password Security
      • Promiscuous Listener
      • Trojan Horses
      • Cracking
    • Spoofing

  • Confidentiality

  • Encryption
    • Basic Ideas
    • Public Key Encryption
    • RSA
    • PGP

  • Kerberos
    • MIT Athena Project

  • Liability