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”

• 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

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

Discussion Point: What’s a Virus?

• Characteristics– Initiation– Behavior– Propagation

• Spyware

• Detection

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, …

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

Network

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

Why Network?

• Sharing data

• Sharing information

• Sharing hardware

• Sharing software

• Increasing robustness

• Facilitating communications

• Facilitating commerce

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

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!

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

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

– Every host broadcasts everything to all others• Collisions limit throughput to about 50% utilization

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

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

Local Area Networks

sam

kim

joe

ann

dove

raven

rac4rac3www rac2 ttclass

HBK

PLS

CSS

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 (most common)– IEEE 802.11g: 54Mbps (now becoming available)

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

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?

An example of WAN

Ring Network

• Unidirectional transmission– Used mostly for WANs

• Very high bandwidth– No collisions

– Simple routing policies

• Complex management– Changes must be coordinated

Maryland’s Campus Network

sam

kim

joe

ann

dove

raven

rac4rac3www rac2 ttclass

HBK

PLS

CSS

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)

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

Overview

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

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

An Internet Protocol (IP) Address

IP address: 216.183.103.150

Identifies a LAN

Identifies a specific computer

Routing Tables

IP Prefix Next Router Estimated Delay

216.141.xxx.xxx 120.0.0.0 18 ms

216.xxx.xxx.xxx 121.0.0.0 34 ms

101.42.224.xxx 120.0.0.0 21 ms

xxx.xxx.xxx.xxx 121.0.0.0 250 ms

45.0.2.10120.0.0.0 121.0.0.0

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, …

IP Addresses and Domain Names

Domain Name: www.howstuffworks.com

IP address: 216.183.103.150

Hands-on: Learn About Your IP Address

• Start a command window– Select “start” on the taskbar, then “Run”– Type in “cmd” and click “OK”

• Find your IP address– Type “ipconfig /all” (and press enter)

• See who “owns” that address– Use http://www.checkdomain.com

• See how packets get to your computer– Use http://www.traceroute.org

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, FTP, SSH, …

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

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

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

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”

Hands On: FTP• Start a cmd window• Type “ftp ftp.umiacs.umd.edu”• Login in anonymously with

– User: anonymous– Password: your email address

• Go to directory lbsc690– Type “cd pub/gina/lbsc690/”

• Get file “hwOne.ppt”– Type “get hwOne.ppt”

• Exit– Type “quit”

HyperText Transfer Protocol (HTTP)• Send request

GET /path/file.html HTTP/1.0 From: someuser@jmarshall.com User-Agent: HTTPTool/1.0

• Server responseHTTP/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>

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)

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

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

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?