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Networks

Networks. Week 2 LBSC 690 Information Technology. Computer Systems. Hardware Types of hardware Storage hierarchy Moore’s law Software Types of software Types of interfaces. Types of Software. Application programs (e.g., Internet Explorer) What you normally think of as a “program”

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Networks

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  1. Networks Week 2 LBSC 690 Information Technology

  2. Computer Systems • Hardware • Types of hardware • Storage hierarchy • Moore’s law • Software • Types of software • Types of interfaces

  3. 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

  4. 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 program’s 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

  5. Discussion Point: What’s a Virus? • Characteristics • Initiation • Behavior • Propagation • Spyware • Detection

  6. Network • Computers and devices connected via • Communication devices • Transmission media

  7. Why Network? • Sharing data • Sharing information • Sharing hardware • Sharing software • Increasing robustness • Facilitating communications • Facilitating commerce

  8. 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

  9. 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!

  10. 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

  11. 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

  12. 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

  13. 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

  14. Local Area Networks CSS www rac2 rac3 rac4 ttclass PLS sam kim ann dove joe HBK

  15. 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)

  16. http://www.tss.northwestern.edu/wireless/faq_wir.html

  17. 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?

  18. Between a LAN and a WAN

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

  20. Maryland’s Campus Network Elsewhere in CSS www rac2 rac3 rac4 ttclass CSS 1410 sam kim ann dove joe HBK

  21. 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)

  22. 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

  23. Overview

  24. http://www.geog.ucl.ac.uk/casa/martin/atlas/isp_maps.html

  25. 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

  26. An Internet Protocol (IP) Address Identifies a LAN IP address:216.183.103.150 Identifies a specific computer

  27. Routing Tables 120.0.0.0 45.0.2.10 121.0.0.0

  28. 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, …

  29. IP Addresses and Domain Names IP address:216.183.103.150 Domain Name:www.howstuffworks.com

  30. 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

  31. 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, …

  32. User Datagram Protocol (UDP) • The Internet’s 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

  33. 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

  34. 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

  35. 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”

  36. 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

  37. HyperText Transfer Protocol (HTTP) • Send request GET /path/file.html HTTP/1.0 From: someuser@jmarshall.com User-Agent: HTTPTool/1.0 • Server response HTTP/1.0 200 OK Date: Fri, 31 Dec 1999 23:59:59 GMT Content-Type: text/html Content-Length: 1354 <html><body> <h1>Happy New Millennium!</h1> … </body> </html>

  38. 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

  39. 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

  40. 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)

  41. Before You Go On a sheet of paper, answer the following (ungraded) question (no names, please): What was the muddiest point in today’s class?

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