Networks

1
Networks

• Week 3
• LBSC 690
• Information Technology

2
System Architecture
Keyboard
Mouse
Video Card
Sound Card
Input Controller
Bus
Hard Drive
CD/ DVD
Floppy
RAM
CPU
Cache
Mass Storage
Motherboard
3
The Storage Hierarchy
fast
expensive
cheap
slow
4
The Storage Hierarchy

• The problem
• Fast memory devices are expensive
• So large memory devices are slow!
• But fast access to large memories is needed
• The solution
• Keep what you need often in small (fast) places
• Keep the rest in large (slow) places
• Get things to the fast place before you need them

5
Computer Systems

• Hardware
• Types of hardware
• Storage hierarchy
• Moores law
• Software
• Types of software
• Types of interfaces

6
Types of Software

• Application programs (e.g., Internet Explorer)
• What you normally think of as a program
• Compilers and interpreters (e.g., JavaScript)
• Allow programmers to create new behavior
• Operating system (e.g., Windows XP)
• Moves data between disk and RAM (lots more!)
• Embedded program (e.g., BIOS)
• Permanent software inside some device

7
Installing Applications

• Copy to a permanent place on your hard drive
• From a CD, the Internet,
• Installs any other required programs
• DLL files can be shared by several applications
• Register the programs location
• Associates icons/start menu items with it
• Configures the uninstaller for later removal
• Configure it for your system
• Where to find data files and other programs

8
Discussion Point Whats a Virus?

• Characteristics
• Initiation
• Behavior
• Propagation
• Spyware
• Detection

9
Graphical User Interfaces

• Easy way to perform simple tasks
• Used to start programs, manage files,
• Relies on a physical metaphor (e.g., a desktop)
• Built into most modern operating systems
• Windows XP, Mac System 10, Unix X-windows
• Application programs include similar ideas
• Point-and-click, drag and drop,

10
Cursor-based Interfaces

• Useful for specifying complex operations
• Available in most operating systems
• SSH connection to WAM
• Command window in Windows XP
• Used when graphical display is difficult
• Dial-in access from older computers

11
Network

• Computers and devices connected via
• Communication devices
• Transmission media

12
Why Network?

• Sharing data
• Sharing information
• Sharing hardware
• Sharing software
• Increasing robustness
• Facilitating communications
• Facilitating commerce

13
Packet vs. Circuit Networks

• Telephone system (circuit-switched)
• Fixed connection between caller and called
• High network load results in busy signals
• Internet (packet-switched)
• Each transmission is routed separately
• High network load results in long delays

14
Packet Switching

• Break long messages into short packets
• Keeps one user from hogging a line
• Route each packet separately
• Number them for easy reconstruction
• Request retransmission for lost packets
• Unless the first packet is lost!

15
Networks of Networks

• Local Area Networks (LAN)
• Connections within a room, or perhaps a building
• Wide Area Networks (WAN)
• Provide connections between LANs
• Internet
• Collection of WANs across multiple organizations

16
Local Area Networks

• Within a campus or an office complex
• Short-distance lines are fast and cheap
• Fast communications makes routing simple
• Ethernet is a common LAN technology
• All computers are connected to the same cable
• Ordinary phone lines can carry 10 Mb/sec
• 100 Mb/s connections require special cables
• 1 Gb/s connections require special switches
• Every host broadcasts everything to all others
• Collisions limit throughput to about 50
  utilization

17
Shared Network

• All attach to the same cable
• Ethernet and cable modems
• Transmit anytime
• Collision detection
• Automatic retransmission
• Inexpensive and flexible
• Easy to add new machines
• Robust to computer failure
• Practical for short distances
• Half the bandwidth is wasted

18
Switched (Star) Network

• All attach directly to a hub
• Switched Ethernet
• Digital Subscriber Lines (DSL)
• Higher cost
• Line from hub to each machine
• Hub must handle every packet
• Hub requires backup power
• Much higher bandwidth
• No sharing, no collisions
• Allows disks to be centralized

19
Local Area Networks
CSS
rac4
rac3
www
rac2
ttclass
PLS
sam
kim
ann
dove
joe
HBK
20
Wireless Networks

• Radio-based Ethernet
• Effective for a few rooms within buildings
• Access Point gateways to wired networks
• Available throughout most of the Maryland campus
• Commercial providers offer hot spots in
  airports, etc.
• Available in two speeds
• IEEE 802.11b 10Mbps (good enough for most uses)
• IEEE 802.11g 54Mbps (required for wireless video)

21
Wide Area Networks

• Campus, regional, national, or global scale
• Expensive communications must be used well
• Limiting to two hosts allows 100 utilization
• Routing is complex with point-to-point circuits
• Which path is shortest? Which is least busy?
• Internet routers exchange routing tables
• Which routes seem fast, which seem slow?

22
Ring Network

• Unidirectional transmission
• Used mostly for WANs
• Very high bandwidth
• No collisions
• Simple routing policies
• Complex management
• Changes must be coordinated

23
Marylands Campus Network
Elsewhere in CSS
rac4
rac3
www
rac2
ttclass
CSS 1410
sam
kim
ann
dove
joe
HBK
24
The Internet

• Global collection of public IP networks
• Private networks are often called intranets
• Independent
• Each organization maintains its own network
• Cooperating
• Internet Protocol (IP) address blocks
• Domain names
• World-Wide Web Consortium (W3C)
• Computer Emergency Response Team (CERT)

25
A Short History of the Internet

• 1969 Origins in government research
• Advanced Research Projects Agency (ARPAnet)
• Key standards UDP, TCP, DNS
• 1983 Design adopted by other agencies
• Created a need for inter-network connections
• Key standards IP
• 1991 World-Wide Web added point-and-click
• Now 150 million Internet hosts
• Key standards HTTP, URL, HTML, XML

26
Overview
27
http//www.geog.ucl.ac.uk/casa/martin/atlas/isp_ma
ps.html
28
vBNS Network
http//www.geog.ucl.ac.uk/casa/martin/atlas/topolo
gy.html
29
Types of Internet Nodes

• Hosts
• Computers that use the network to do something
• Routers
• Specialized computers that route packets
• Gateway
• Routers that connect two networks
• Firewall
• Gateways that pass packets selectively

30
An Internet Protocol (IP) Address
Identifies a LAN
IP address 216.183.103.150
Identifies a specific computer
31
Routing Tables
45.0.2.10
120.0.0.0
121.0.0.0
32
Domain Name Service (DNS)

• Domain names improve usability
• Easier to remember than numeric IP addresses
• DNS coverts between names and numbers
• Written like a postal address general-to-specific
  
• Each name server knows one level of names
• Top level name server knows .edu, .com, .mil,
• .edu name server knows umd, umbc, stanford,
• .umd.edu name server knows wam, glue, ttclass,
• .wam.umd.edu name server knows rac1, rac2,

33
IP Addresses and Domain Names
IP address 216.183.103.150
Domain Name www.howstuffworks.com
34
Hands-on Learn About Your IP Address

• Find your IP address
• Select start on the taskbar, then Run
• Type in cmd and click OK
• Type ipconfig /all (and press enter)
• See who owns that address
• Use http//whois.godaddy.com
• See how packets get to your computer
• Use http//www.traceroute.org

35
Illustration by Catherine Werst
36
http//en.wikipedia.org/wiki/OSI_model
37
TCP/IP layer architecture
Application
Application
Virtual network service
Transport
Transport
Virtual link for end to end packets
Network
Network
Network
Network
Virtual link for packets
Link
Link
Link
Link
Link
Link
Link for bits
Link for bits
Link for bits
38
OSI Model v.s. TCP/IP Model
39
The TCP/IP Protocol Stack

• Link layer moves bits
• Ethernet, cable modem, DSL
• Network layer moves packets
• IP
• Transport layer provides services to applications
• UDP, TCP
• Application layer uses those services
• DNS, SFTP, SSH,

40
User Datagram Protocol (UDP)

• The Internets basic transport service
• Sends every packet immediately
• Passes received packets to the application
• No delivery guarantee
• Collisions can result in packet loss
• Example sending clicks on web browser

41
Transmission Control Protocol (TCP)

• Built on the network-layer version of UDP
• Guarantees delivery all data
• Retransmits missing data
• Guarantees data will be delivered in order
• Buffers subsequent packets if necessary
• No guarantee of delivery time
• Long delays may occur without warning

42
File Transfer Program (FTP)

• Used to move files between machines
• Upload (put) moves from client to server
• Download (get) moves files from server to client
• Available using command line and GUI interfaces
• Normally requires an account on the server
• Userid anonymous provides public access
• Web browsers incorporate anonymous FTP
• Automatically converts end-of-line conventions
• Unless you select binary

43
Hands On FTP

• Start a cmd window
• Type ftp ftp.umiacs.umd.edu
• Login in anonymously with
• User anonymous
• Password your email address
• Go download a file
• Type cd pub/gina/lbsc690/
• Type binary
• Type get hwOne.ppt
• Exit
• Type quit
• Try it again with a graphical FTP program
• WS_FTP, for example

44
Telnet

• Simulates a dial-up connection
• Read data from another machine
• VT-100 protocol allows only text
• The pine email program is designed for VT-100
• X Windows extension adds graphics

45
The World-Wide Web
My Browser
Local copy of Page requested
Page Requested
Proxy Server
Fetch Page
Send Request
Remote Sever
Internet
46
Web Standards

• HTML
• How to write and interpret the information
• URL
• Where to find it
• HTTP
• How to get it

47
HyperText Markup Language (HTML)

• Simple document structure language for Web
• Advantages
• Adapts easily to different display capabilities
• Widely available display software (browsers)
• Disadvantages
• Does not directly control layout

48
Uniform Resource Locator (URL)

• Uniquely identify web pages on the WWW
• Domain name
• Directory path
• File name

Domain name
File name
URL http//www.clis.umd.edu/courses/schedules/fal
l2003.html
Directory path
49
HyperText Transfer Protocol (HTTP)

• Send request
• GET /path/file.html HTTP/1.0
• From someuser_at_jmarshall.com
• User-Agent HTTPTool/1.0
• Server response
• HTTP/1.0 200 OK
• Date Fri, 31 Dec 1999 235959 GMT
• Content-Type text/html
• Content-Length 1354
• Happy New Millennium!
  

50
Encryption

• Secret-key systems (e.g., DES)
• Use the same key to encrypt and decrypt
• Public-key systems (e.g., PGP)
• Public key open, for encryption
• Private key secret, for decryption
• Digital signatures
• Encrypt with private key, decrypt with public key

51
Encrypted Standards

• Secure Shell (SSH)
• Replaces Telnet
• Secure FTP (SFTP)/Secure Copy (SCP)
• Replaces FTP
• Secure HTTP (HTTPS)
• Used for financial and other private data
• Wired Equivalent Protocol (WEP)
• Used on wireless networks

52
Encryption Issues

• Key length
• 128 bits balances speed and protection today
• Trust infrastructure
• How do you prevent bait and switch?
• Who certifies a digital signature is valid?

53
Network Abuse

• Flooding
• Excessive activity, intended to prevent valid
  activity
• Worms
• Like a virus, but self-propagating
• Sniffing
• Monitoring network traffic (e.g., for passwords)

54
Before You Go

• On a sheet of paper, answer the following
  (ungraded) question (no names, please)
• What was the muddiest point in todays class?