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
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
•	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
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
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)

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?
Between a LAN and a 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
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

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
Routing Tables
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
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
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, …
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 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
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> �c </body> </html> 
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
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)
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?