COS 125

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COS 125

• DAY 2

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Agenda

• Questions from last Class??
• Review
• ISOC presentation on Internet History
• Todays topics
• Circuit versus Packet switching
• TCP/IP
• Software Structure of The Internet
• Internet Addresses and Names
• How IP Routers work
• As promised, Assignment 1 is posted to WebCt
• Due Monday, Jan 24

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Circuit Switching

• This is how Phone Networks operate
• For Alice to talk to Bob there must a dedicated
  Connection (wire) from Alice to Bob
• If there is no connection path available than
  circuit is said to be busy
• Connection is dedicated for entire length of
  conversation
• Wasteful

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Circuit Switching
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Packet Switching

• TCP/IP (and the Internet) uses Packet Switched
  networks
• Large files are broken in smaller packets
• Each packets finds its way across Internet
• DEMO
• Allows for Multiplexing
• More efficient
• Causes problems for data that requires specific
  timing
• Audio, Video

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Packet Switching
1. Break message into Smaller packets (also known
as frames)
Original Message
Packet Switch A
B
C
Computer X
Packet
Switching Decision
Computer Y
F
E
D
2. Route packets individually Packet switches
along the way Make decisions about the packet
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TCP/IP

• Two protocols that are part of the Networking
  Stack
• Transmission Control Protocol
• Computer to Computer
• Breaks down Files into Packets and reassemble
• Internet Protocol
• Internet Device to Internet Device
• Ensures packets are delivered to right destination

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TCP/IP Stack
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Connecting to Internet

• Two ways
• LANS
• Direct connection
• Just like in this lab
• 24/7/365
• Modems
• Cable
• DSL
• Telephone
• Use two different protocols
• SLIP or PPP

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Internet Software Structure

• Client/Server
• Clients (PCs) ask for stuff
• Servers (large computers) deliver stuff
• In case of WWW
• Uses HTTP
• Browsers (Internet explorer) is the client
• Web Server (www.umfk.maine.edu) is the server

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Client/Server Architecture
Usually, Two Types of Stations Clients and Servers
Server
Client PC
Service
Network
Clients Receive Services
Servers Provide Services
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Internet Address and Domains

• The Heart of the Internet is DNS
• Domain Name System
• Translate names to addresses
• Sort of an automatic phone book
• www.umfk.maine.edu -gt 130.111.185.92
• Use nslookup at the command prompt (2000, XP,
  Mac OSX, UNIX)
• The name (www.umfk.maine.edu) is a URL or Uniform
  Resource Locator
• 130.111.185.92 is an IP address (like a phone
  number)

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Domain names

• www.umfk.maine.edu
• Computer.subdomain.minordomain.majordomain
• Major Domains
• edu, com, net, org mil
• Minor domains
• Maine, yahoo, nasa
• Sub domains (could have more than one)
• Umfk
• Computer names
• www, tgauvin, nb11

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Domain name organization
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Name servers

• DNS Names Server covert names to IP address
• No ONE name server could know all names and all
  addresses
• more than 4 billion possibilities
• Names ltgt ip address tables are distributed
• Each minor domain is responsible for running its
  own Name Server(s)
• 13 Root Servers (one per major domain) maintain
  lists of all the name servers responsible of the
  minor domains

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Distributed Name Resolution
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Root Servers
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Static versus Dynamic IP Addresses

• Every computer connected to the Internet MUST
  have an IP address
• xxx.xxx.xxx.xxx
• 0.0.0.0 ltgt 255.255.255.255
• If the address for a computer never changes then
  it is static
• Else it is dynamic

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Why use Dynamic Addressing

• There is not enough address to go around
• 4.2 billion possibilities
• Actually only about 3 billion due to allocation
  schemes
• Not all computer are connected 24/7
• If an ISP has only 24 modems that its customers
  connect to than why use more than 24 addresses
  even though it may have 200 or more customers
• Dynamic IPs became possible with DHCP around
  1995

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DHCP
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How routers work

• Traffic cops of the Internet
• Ensure all IP packets get to where the are
  supposed to go
• Look at destination IP address of any packet
  coming into the router on any of its ports
  (connections)
• Looks up ip address in routing table
• Decides where to send packet
• Another port

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Routing Table for a router
network Next Hop
10.2.1.56 10.3.4.56
10.2.0.0 10.3.2.1
10.3.0.0 Deliver directly
10.1.0.0 10.3.1.1
10.5.0.0 Deliver directly
10.6.0.0 10.5.6.1
0.0.0.0 10.3.2.1
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Routing
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Routing
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For next week

• Read HITW Chaps 7-13 (page 85)
• Assignment 1
• Due next Monday, January 24, 2004 at beginning of
  class